Why is gynodioecy common and androdioecy rare?

Why is gynodioecy common and androdioecy rare?

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Gynodioecy is when a population features individuals that produce both male and female gametes and individuals that produce just female gametes. Androdioecy is when a population features individuals that produce both male and female gametes and individuals that produce just male gametes. The former is "not uncommon" and the latter is "virtually unknown."

Charlesworth 1991 states that this is common because cosexual populations would normally evolve a male sterile mutation first, causing gynodioecy, and then female sterility mutation(s) will build up leading to dioecy.

Why should it be so much more common for male sterility mutations to evolve first?

(I think the answer lies within articles cited in the above paper, Charlesworth & Charlesworth 1979 Proc R Soc B, and 1987 Evolution which I'll try to read this weekend, if it is I'll post a self-answer but in the mean time… ).

A model for the evolution of high frequencies of males in an androdioecious plant based on a cross-compatibility advantage of males

Lloyd’s (1975) and Charlesworth & Charlesworth’s (1978) phenotypic selection models for the maintenance of androdioecy predict that males (female-sterile individuals) must have an advantage in fertility (K) of at least two in order to invade a hermaphroditic population, and that their equilibrium frequency (xeq=(K − 2)/2(K − 1)) is always less than 0.5. In this paper, we develop a model in which male fertility is frequency-dependent, a situation not investigated in the previous models, to explore the conditions under which a high frequency of males (i.e. more than 50%) could be maintained at equilibrium. We demonstrate that a gametophytic self-incompatibility (GSI) locus linked to a nuclear sex determination locus can favour rare alleles through male function, by causing frequency-dependent selection. Thus, the spread of a female-sterility allele in a hermaphroditic population may be induced. In contrast with the previous models, our model can explain male frequencies greater than 50% in a functionally androdioecious species, as long as there is (i) dominance of female-sterility at the sex locus, and (ii) a few alleles at the self-incompatibility locus, even if the advantage in fertility of male phenotype is lower than two.


Androdioecy (populations consisting of males and hermaphrodites) is a rare mating system in plants and animals: up to 50 plants and only 36 animals have been described as being androdioecious, with most of the latter being crustaceans. To date, a thorough comparative analysis of androdioecy in animals has not been undertaken. Herein we present such an analysis. Androdioecy has only been extensively surveyed in 2 animal taxa: the nematode Caenorhabditis and the clam shrimp Eulimnadia. The other major taxon having androdioecious species is the Cirripedia (barnacles), but there are only limited studies on androdioecy in this group. In animals, androdioecy is found either in species that have morphologically and ecologically distinct sexes (that is, hermaphrodites and small, “complemental” males) that are derived from hermaphroditic ancestors (that is, the barnacles) or in species that have similarly-sized males and hermaphrodites that have been derived from dioecious ancestors (the remaining androdioecious species). We suggest that the barnacles have evolved a sexual specialization in the form of these complemental males that can more efficiently use the constrained habitats that these barnacles often experience. For the remaining species, we suggest that androdioecy has evolved as a response to reproductive assurance in species that experience episodic low densities. Additionally, we hypothesize that the development of mechanisms allowing reproductive assurance in species with a number of sexually differentiated traits is most likely to result in androdioecy rather than gynodioecy (mixtures of females and hermaphrodites), and that these species may be developmentally constrained to stay androdioecious rather than being capable of evolving into populations solely consisting of efficient, self-compatible hermaphrodites. We conclude by suggesting several areas in need of further study to understand more completely the evolution and distribution of this interesting mating system in animals.


The presence of females in hermaphroditic populations is expected to place selective pressure on hermaphrodites to gain more of their fitness through male function and less through female function. The strength of this selection is predicted to increase with female frequency and the magnitude of their seed fertility advantage over hermaphrodites ( Charlesworth, 1989). Thus before reviewing the evidence for latter stages of the G-D pathway, it is worthwhile to examine the distribution of these features in species at intermediate stages of this pathway. To accomplish this, we performed a literature search. We searched ISI Web of Science using the keywords ‘gyndioec*’, ‘subdioec*’ and ‘trioec*’ for studies published between 2005 and 2010. We chose to focus the number of years examined for the question of female frequency and fertility advantage because of the large number of studies with these keywords (666 between 1945 and the present listed in ISI) earlier work on females' seed fertility advantage has been reviewed in the context of the transition from hermaphroditism to gynodioecy ( Couvet et al., 1990 Shykoff et al., 2003). We included studies in which the species was denoted as ‘trioecious’ (males, females and hermaphrodites) because these systems may also reflect a point along a continuum ( Lloyd, 1976 Webb, 1976 Ross, 1982 Fig. 1), but note that the terms ‘subdioecy’ and ‘trioecy’ have been defined differently or used to indicate distinct systems ( Ross, 1982 Gregorius et al., 1983 Sakai and Weller, 1991 Maurice et al., 1994, 1998 Maurice and Fleming, 1995 Seger and Eckhart, 1996 Sakai and Weller, 1999 Ehlers and Bataillon, 2007). For example, Gregorius et al. (1983), Maurice et al. (1998) and Sakai and Weller (1999) all distinguished trioecy from subdioecy in that the former describes cases in which there are morphologically distinct females with pistillate flowers, males with staminate flowers and bisexuals with perfect flowers, whereas the latter indicates that some unisexuals may be ‘imperfectly differentiated’ ( Gregorius et al., 1983). These terms have also been used to invoke different evolutionary processes, with trioecy denoting a stable evolutionary system and subdioecy as transitional one (e.g. Maurice et al., 1994, 1998 Maurice and Flemming, 1995). Thus, we also recorded for each study how the sexual system was identified by the authors. To ensure that we captured the full range of relative fertilities in species that have been studied, we also examined species included in a recent review by Ehlers and Bataillon (2007, table 1) that were near the dioecy end of the spectrum but that had evidence of ‘fruiting males’ or ‘inconstant males’. We used the ratio of seed fertility of females to hermaphrodites (‘F : H’) (or, where applicable, of ‘fruiting’ or ‘inconstant’ male morphs) reported by the authors or calculated it from the data presented. Where multiple components of female reproduction were reported, we selectively chose one in the following order of preference: seed number per plant, fruit number per plant, proportion fruit-set, proportion seed-set. We also recorded sex ratio (proportion of females) when reported for the study population. When multiple populations (or treatments) were included, we calculated the average fertility or sex ratio. Ultimately, we compiled F : H seed fertility data from 58 studies on 43 species and sex ratio from 51 studies on 38 species ( Supplementary Data , available online). We used a Spearman's rank correlation to assess the relationship between F : H seed fertility and sex ratio.

The results of our search revealed a nearly continuous distribution of relative seed fertilities across species with a modal bin at 1·4 (Fig. 2). Approximately 40 % (23) of the studies demonstrated that females have at least a two-fold advantage over hermaphrodites, great enough for females to persist assuming nuclear control, and even greater than needed under cytoplasmic-nuclear control ( Lewis, 1941 Lloyd, 1974, 1975 Charlesworth and Charlesworth, 1978). These results are consistent with a meta-analysis of earlier published data that demonstrated higher seed fertility of females than hermaphrodites ( Shykoff et al., 2003). Across studies, mean F : H seed (±s.d.) fertility was 3·2 ± 4·5 (adjusted by removing two influential data points over a order magnitude greater than the next highest value 22·1 ± 118·5 including those values). These fertility estimates do not include potential offspring quality differences associated with differences in selfing by the sex morphs and thus potential for inbreeding depression in the progeny therefore, the prevalence and degree of female advantage is likely to be even greater than presented in Fig. 2. The proportion of females ranged from 0·014 to 0·63 (mean 0·32 ± 0·16) across the species reviewed. Consistent with theory on the evolution of gynodioecy ( Lloyd, 1976), female frequency was strongly positively correlated with F : H seed fertility across studies (rs = 0·62, P < 0·0001 Fig. 3). A similar positive slope is found in a review of earlier published data (r = 0·37, P = 0·02 Couvet et al., 1990: data in appendix with one outlier removed). Taken together, these data demonstrate that, for many species, females are frequent and have a pronounced seed fertility advantage over hermaphrodites and thus ought to exert selection for increased investment in male function in hermaphrodites and/or the invasion of pure males. These species provide prime opportunities to investigate the second phase of the G-D pathway.

Histograms showing the distribution of the relative seed fertility of females to hermaphrodites (F : H seed fertility) in the literature reviewed. Different filled bars indicate when species represented in those bins were defined using terms other than ‘gynodioecy’. The category defined as ‘other’ indicates species described as having ‘leaky dioecy’, ‘almost dioecy’ or ‘polygamous’. Note that species described as ‘subdioecious’ and ‘trioecious’ are found across the entire range of the distribution.

Histograms showing the distribution of the relative seed fertility of females to hermaphrodites (F : H seed fertility) in the literature reviewed. Different filled bars indicate when species represented in those bins were defined using terms other than ‘gynodioecy’. The category defined as ‘other’ indicates species described as having ‘leaky dioecy’, ‘almost dioecy’ or ‘polygamous’. Note that species described as ‘subdioecious’ and ‘trioecious’ are found across the entire range of the distribution.

Scatter plot showing the correlation (rs = 0·62, P < 0·0001) between sex ratio (proportion females) and the relative fertility of females to hermaphrodites (F : H seed fertility, log-transformed) across studies reviewed. Different symbols indicate how the original authors defined the species according to sexual system. ‘Other’ indicates species described as having ‘leaky dioecy’, ‘almost dioecy’ or ‘polygamous’.

Scatter plot showing the correlation (rs = 0·62, P < 0·0001) between sex ratio (proportion females) and the relative fertility of females to hermaphrodites (F : H seed fertility, log-transformed) across studies reviewed. Different symbols indicate how the original authors defined the species according to sexual system. ‘Other’ indicates species described as having ‘leaky dioecy’, ‘almost dioecy’ or ‘polygamous’.

It is worth noting that there does not appear to be a clear pattern as to where species classified as ‘subdioecious’ or ‘trioecious’ fall along this continuum (Fig. 2). In fact, the extremities of the range of F : H fertility values are represented by a ‘trioecious’ species (Opuntia robusta Del Castillo and Argueta, 2009) and one classified as ‘subdioecious’ [Silene acaulis subsp. exscapa, which has females, males and males that set fruit (‘H’) Maurice et al., 1998]. This underlines that the use of sexual system labels may obscure where along the G-D pathway a species resides and probably does not inform on the direction of the evolutionary trajectory. Instead, we argue that it is more informative to characterize the relative fertilities and the conditions that influence them.

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Adaptive evolution of sexual systems in pedunculate barnacles

How and why diverse sexual systems evolve are fascinating evolutionary questions, but few empirical studies have dealt with these questions in animals. Pedunculate (gooseneck) barnacles show such diversity, including simultaneous hermaphroditism, coexistence of dwarf males and hermaphrodites (androdioecy), and coexistence of dwarf males and females (dioecy). Here, we report the first phylogenetically controlled test of the hypothesis that the ultimate cause of the diverse sexual systems and presence of dwarf males in this group is limited mating opportunities for non-dwarf individuals, owing to mating in small groups. Within the pedunculate barnacle phylogeny, dwarf males and females have evolved repeatedly. Females are more likely to evolve in androdioecious than hermaphroditic populations, suggesting that evolution of dwarf males has preceded that of females in pedunculates. Both dwarf males and females are associated with a higher proportion of solitary individuals in the population, corroborating the hypothesis that limited mating opportunities have favoured evolution of these diverse sexual systems, which have puzzled biologists since Darwin.

1. Introduction

Diverse sexual systems in organisms and the evolutionary forces driving them have been a major topic of inquiry in evolutionary biology [1–7]. However, few empirical studies have addressed these problems, especially in animals. Interest in sexual systems dates back to Darwin [8], who found that pedunculate barnacles (Thoracica: Pedunculata) show such diversity, including simultaneous hermaphroditism, coexistence of males and hermaphrodites (often called androdioecy), and coexistence of males and females (dioecy figure 1). Darwin also found that barnacle males are always much smaller than mature hermaphrodites or females [9–11] and that these males (referred to as dwarf males) are attached to specific sites of large conspecifics [12]. However, he did not specify the evolutionary force responsible for this sexual diversity and the occurrence of dwarf males. He wrote that ‘the diversity in sexual relations … appears to me eminently curious’ (p. 292 of [8]), but that ‘regarding the final cause … of separation of the sexes … and … of the existence of Complemental males (i.e. dwarf males attached to hermaphrodites), I can throw no light’ (p. 291).

Figure 1. Three scalpellid species representing different sexual systems. (a,b) Scalpellum scalpellum. Androdioecious species attached to hydroids. (a) A solitary hermaphrodite (note the penis upper inset) that depends on dwarf males (lower inset) for reproduction. (b) A small group that can reproduce by cross-fertilization even without dwarf males. (c) Trianguloscalpellum regium. This deep-sea dioecious species depends entirely on the presence of dwarf males for fertilization. The receptacle inside the scutal edge (inset) contains numerous males (asterisk indicates a metamorphosing cyprid). (d) Pollicipes pollicipes. This hermaphroditic species inhabits the upper rocky intertidal zone in dense populations, which ensures cross-fertilization. Scale bar, 1 cm.

Although the ultimate cause of the diverse sexual systems in barnacles has been an enigma for more than a century, recent theoretical studies have suggested that both the evolution of separate sexes and male dwarfing can be understood under sex allocation models [2,13–15]. Since barnacles are sedentary animals that mate with conspecifics within the reach of their penises, male fitness is limited by the number of available eggs produced by neighbouring individuals, following the law of diminishing returns [2,13–15]. Thus, total fitness through both male and female functions is maximized at an intermediate allocation to both functions, making simultaneous hermaphroditism evolutionarily stable. However, as population density decreases, these theories predict that the optimal allocation to male function (sperm production) of each hermaphrodite becomes smaller, and the intensity of sperm competition is reduced. Then dwarf males, even with a small amount of sperm, can expect some fertilization success although they must compete with hermaphrodites for fertilizing eggs. Since dwarf males have an advantage of surviving better to maturity [13,15] and possibly a mating advantage by attaching nearer the fertilization site of conspecifcs [1,16], they may have fitness comparable to that of hermaphrodites, and hence are expected to evolve. The evolutionarily stable proportion of larvae that become dwarf males increases as the population density reduces, but it does not exceed 50 per cent [15]. When density becomes so low that most individuals live solitarily and have no chance to fertilize conspecifics, they should allocate all resources to female function to produce as many eggs as possible and become pure females. These females are expected to be fertilized by dwarf males as most females keep more than one male [12,17]. In fact, in the best-studied pedunculate, Scalpellum scalpellum, although androdioecious, both genetic and environmental factors affect sex determination, and the proportion of larvae that will develop into males does not exceed 50 per cent [18]. However, two to five dwarf males are usually attached to a large hermaphrodite in the field [12], because of much earlier sexual maturity of males (within 10 days after settlement) than hermaphrodites (at least 1 year) [18]. Thus, the sex allocation models on barnacle sexuality [2,13–15] predict that (i) as mating opportunities become more limited, both dwarf males and females are expected to evolve, and (ii) dwarf males will evolve prior to females as the density decreases (i.e. they tend to evolve in hermaphroditic populations, whereas females tend to evolve in androdioecious populations). Gynodioecy (populations with females and hermaphrodites) is predicted to be extremely rare. Although mode of sex determination is largely unknown in barnacles, the above models postulate genetically determined strategies [13–15]. However, sex allocation strategies are usually unrelated to the underlying mechanisms [2] (see §4).

Most shallow-water pedunculate barnacles that live in large groups, such as Pollicipes or Lepas spp., are hermaphroditic [19], whereas dwarf males and pure females tend to occur in deep sea or symbiotic forms [20,21] (figure 1). To critically test the importance of limited mating opportunities on the reproductive mode of barnacles, we collected information on sexual systems and the extent of limitation of mating opportunities in 48 pedunculate barnacle species. The degree of limitation was assessed as the proportion of solitary non-dwarf individuals in a sampled barnacle population. Since species are not independent units of comparison, phylogenetic information is required for testing character evolution and factors affecting it [22,23]. Therefore, molecular phylogenies of these species were constructed using approximately 1400 bp of 18S rDNA from published sources [24,25] and our own sequencing results (electronic supplementary material, table S1).

2. Material and methods

(a) Sexual systems

The sexual system of each species was studied by dissection of samples or coded based on literature data (electronic supplementary material, table S1). Although ‘true’ barnacles (superorder Thoracica) comprise two orders, Pedunculata (pedunculate barnacles) and Sessilia (acorn barnacles), we confined our analyses to the pedunculates because few dwarf males and no females have been observed in acorn barnacles [19,26–30], which may imply phylogenetic constraint. In addition, unlike pedunculates, the penises of some acorn barnacles are lost in non-reproductive seasons [31], rendering the distinction between hermaphrodites and females inaccurate. Large (non-dwarf) individuals were recorded as hermaphrodites if they had a penis and as females if they lacked a penis. Dwarf males were defined as males that mature (as judged by the presence of a developed penis or testis) at a much smaller size (less than half as long) than conspecific hermaphrodites or females [10,11]. No large males are known in barnacles, but dwarf males of several barnacle species continue to grow, and some of them may later become hermaphroditic [11,26,30]. They differ from normal hermaphrodites in that (i) they are always attached to specific sites of conspecifics, (ii) they allocate more resources to male function and (iii) only a few (if any) will become hermaphroditic [11,19]. In this paper, the coexistence of such dwarf males and hermaphrodites is also referred to as androdioecy, although androdioecy normally refers to the coexistence of pure males and hermaphrodites [32,33]. A species was regarded as hermaphroditic (as opposed to androdioecious) if no males were found or reported even after several non-dwarf individuals were investigated, irrespective of the sexual system of the majority of the lineage. For instance, Annandale [34] reported that Arcoscalpellum sociable are ‘so gregarious’ but ‘no males were found’, and that in Trianguloscalpellum balanoides ‘males are believed to be always absent’. Thus, these species are treated as hermaphroditic.

(b) Barnacle distribution pattern

Data on the proportion of solitary individuals were obtained either from our own observations or from the literature (electronic supplementary material, table S1). Although mating group size is usually referred to as a factor that determines male fitness curves [2,13], group size is often difficult to measure in practice (the number of individuals was often simply recorded as ‘several’ or ‘many’ in the literature). A solitary individual was defined as a non-dwarf individual that had no neighbouring conspecifics (except for dwarf males) within the distance that it could fertilize. This distance is approximately twice its total length [35]. When accurate data on the distance between individuals were not available, individuals were regarded as being solitary only when they were attached singly to a specific substratum (such as molluscan shells, pebbles, corals, crabs, etc.).

(c) DNA amplification and sequencing

Total genomic DNA was extracted from muscle tissue of barnacles using the QuickGene DNA tissue kit (Fujifilm). PCR products for the 18S rDNA gene were amplified using the primers 18S 1.2F and 18S b5.0, 18S a0.7 and 18S b2.5, 18S a1.0 and 18S bi, and 18S a3.5 and 18S 9R, as described by Whiting [36], under the following temperature regime: an initial denaturation at 96°C for 3 min, followed by 35 cycles of 95°C for 1 min, 45°C–50°C for 30 s and 72°C for 1 min, followed by an extension at 72°C for 7 min. Amplification products were purified using ExoSAP-IT (USB Corporation), and then sequenced using the ABI Prism BigDye Terminator Cycle Sequencing Ready Reaction Kit on an ABI Prism 310 Genetic Analyzer or an ABI Prism 3130xl Genetic Analyzer. All final sequences were obtained from both strands for verification.

(d) Phylogenetic analysis

Phylogenetic analyses were performed using the sequence data (approx. 1400 bp) from 18S ribosomal DNA (18S rDNA) obtained to date from public genetic databases (GenBank, DDBJ, etc.) and from our own sequencing results (see electronic supplementary material, table S1 for accession numbers). Sequence data for a total of 48 species were used for the phylogenetic analysis. The 48 species represent all four suborders and 10 of 14 families of the order Pedunculata, excluding the small families of Anelasmatidae (consisting of one sp.), Malacolepadidae (two spp.), Microlepadidae (three spp.) and Rhizolepadidae (two spp.). Sequences were initially aligned using C lustal X [37] with default gap penalties, and regions of uncertain homology were removed before phylogenetic analysis. Phylogenetic relationships were analysed by Bayesian method as implemented by M r B ayes v. 3.1.2 [38], and by maximum-likelihood (ML) analyses using the software package PAUP* v. 4.0b10 [39]. Sequences were first analysed with M odeltest software [40] to find the evolutionary model that best fitted the data. The Bayesian analyses were performed by running a Markov chain Monte Carlo (MCMC) algorithm for 10 000 000 cycles, sampling every 2000th generation. The initial 40 per cent of cycles were discarded for tree-building, and the convergence of MCMC runs was confirmed by M r B ayes . The posterior probabilities of the phylogeny were determined by constructing a 50 per cent majority-rule consensus of the remaining 3000 trees. The ML analysis was performed using heuristic searches, with 10 random additional replicates and TBR branch-swapping. One hundred replications, TBR branch-swapping and one random addition replicate were used for bootstrap analysis.

(e) Statistical tests using Bayesian inference

To conduct Bayesian tests, 3000 Bayesian trees for constructing the consensus tree were used. Since B ayes T raits cannot treat polytomies (three or more unresolved branches from a common ancestor), the trees were re-rooted with both Ibla quadrivalvis and Ibla cumingi (Iblomorpha) always designated as the outgroup. The presence (1) and absence (0) of dwarf males and females were both treated as continuous variables [41] since the explanatory variable (the proportion of solitary individuals) was also continuous (B ayes T raits does not allow mixture of continuous and discrete variables). Their relationships were analysed using the Continuous module in B ayes T raits ( [22,23].

An MCMC algorithm was run for 5 050 000 cycles, sampling every 100th generation, and the initial 50 000 cycles were discarded (all default settings of B ayes T raits ). The log-likelihoods of the MCMC samples from the unrestricted model were compared with those of the model that restricts the covariance to 0 [42]. The log Bayes factor, which is two times the difference between the natural logarithms of the harmonic means of the likelihoods of the two models, was used as the test statistic [42] (see also the manual of B ayes T raits , available at the website). The logic is similar to likelihood-ratio test, but it uses the marginal likelihoods of the models rather than MLs. In the log-scale, Bayes factors greater than 2 are considered positive evidence, and values greater than 5 are taken as strong evidence [42,43]. The scaling parameters kappa, delta and lambda for continuous variables (related to the tempo, mode and phylogenetic associations of trait evolution see the discussion in the manual of Continuous) were simultaneously estimated. Other parameters were all set to the default values.

Transition rates among sexual systems were analysed using the Discrete module in B ayes T raits . For this purpose, the presence and absence of dwarf males and females were treated as discrete variables. MCMC was run with RateDev (deviation of the normal distribution, from which changes to the rates are drawn) set at 30 to meet the criterion of the acceptance rate being approximately 0.2. Other parameters were all set to the default values. The harmonic mean of likelihoods of the unrestricted model was compared with that of the model that restricts q12 = q34 or q13 = q24. The likelihood ratio of the log harmonic means was compared to calculate the log Bayes factor. All analyses using Bayesian inference were run at least three times and confirmed that the statistical results did not differ among the runs.

(f) Statistical tests using maximum likelihood

ML [22,23] was also used to test the relationship between sexual systems and the proportion of solitary individuals based on the ML tree (electronic supplementary material, figure S1). Since the ML tree contained several polytomies, 100 trees with randomly resolved polytomies were created using M esquite v. 2.71 [44] with branch lengths of 0.0001 [41,45]. The 100 trees gave very similar log-likelihoods, in the case of both the unrestricted (i.e. the variables were allowed to covary) and the restricted (covariance set to 0) models (with standard errors smaller than 0.001). The ranges of likelihood ratios of these models based on the 100 trees are shown in §3, as well as their p-values under the χ 2 distribution with d.f. = 1 and α = 0.05.

Transition rates among sexual systems were analysed by ML using the Discrete module in B ayes T raits , with the presence or absence of dwarf males and females treated as discrete variables. The log-likelihoods of the unrestricted model and those of the model that restricts q12 = q34 or q13 = q24 were calculated based on the 100 randomly resolved trees. The likelihood ratios of the 100 trees were each tested against the χ 2 distribution with d.f. = 1 and α = 0.05, and their p-values are shown as ranges in §3.

3. Results

The obtained Bayesian (figure 2) and ML trees (electronic supplementary material, figure S1) are generally congruent with each other and with published trees [24,25,46]. The pattern of distribution of sexual systems on the Bayesian tree (figure 2) shows that dwarf males and females have multiple origins in the barnacle phylogeny (i.e. they have evolved independently in several lineages) [19]. Sexual systems differ even between closely related species, such as between Octolasmis angulata (hermaphroditic) and Octolasmis warwickii (androdioecious) [11] or between Koleolepas avis (androdioecious) and Koleolepas sp. (dioecious). Thus, sexuality can change on relatively short evolutionary timescales. The same conclusions were drawn from the ML tree (electronic supplementary material, figure S1).

Figure 2. Sexual system placed on the Bayesian phylogenetic tree of pedunculate barnacles. The consensus tree was constructed with Bayesian inference based on the general time reversible model (GTR + I + G). Ibla cumingi and I. quadrivalvis were used as outgroups. Only confidence values higher than 0.5 are shown in the tree. H, hermaphroditic A, androdioecious (males + hermaphrodites) D, dioecious (males + females).

When Bayesian inference [22,23] was used to test character evolution based on the Bayesian phylogeny (figure 2), there was a strong positive association between the proportion of solitary individuals and the presence of dwarf males (log Bayes factor statistics of correlated evolution = 23.14). Thus, as the proportion of solitary individuals increases, the probability of the presence of dwarf males increases. Likewise, there was also a positive association between the proportion of solitary individuals and the presence of females (log Bayes factor = 17.85). Virtually identical results were obtained if character evolution was tested based on the ML tree (electronic supplementary material, figure S1 likelihood ratio χ 2 of 100 randomly resolved trees = 16.61–17.84, all p < 0.0001 for dwarf males likelihood ratio χ 2 = 18.64–19.55, all p < 0.0001 for females).

In both Bayesian (figure 2) and ML (electronic supplementary material, figure S1) trees, androdioecy occurs commonly, whereas no gynodioecy appears. Thus, the Bayesian inference on character coevolution (figure 3) suggests that evolutionary transition rates from the absence to the presence of females when dwarf males are lacking (i.e. from hermaphroditism to gynodioecy q12 = 6.7) were much lower than the rates for moving from the absence to the presence of females when dwarf males are available (from androdioecy to dioecy q34 = 63.2 log Bayes factor = 5.17). On the other hand, transition rates from the absence to the presence of dwarf males in the absence of females (from hermaphroditism to androdioecy q13 = 26.9) did not differ significantly from the corresponding rates in the presence of females (from gynodioecy to dioecy q24 = 62.9 log Bayes factor = 0.61). Although the evolutionary transition rates estimated by the ML method were different from those estimated using Bayesian inference (electronic supplementary material, figure S2), the above statistical conclusions were also supported (q12 versus q34: likelihood ratio χ 2 of 100 randomly resolved trees = 4.35–4.56 p = 0.033–0.037 q13 versus q24: likelihood ratio χ 2 = 0.65–0.83 p = 0.36–0.42). Taken together, these results suggest that the evolution of dwarf males preceded that of females in the transition of sexual systems from hermaphroditism to dioecy.

Figure 3. Evolutionary transition rates between sexual systems in pedunculate barnacles estimated by Bayesian inference. qij refers to the transition rate from the ith to the jth sexual system. In parentheses, the presence (1) or absence (0) of dwarf males (left) and of females (right) is shown.

4. Discussion

Our results support the hypothesis that limitation of mating opportunities is important for the evolution of dwarf males (i.e. androdioecy) and females (i.e. dioecy) from hermaphrodites in pedunculate barnacles. This perspective contrasts with the widely accepted view that low density promotes hermaphroditism rather than dioecy to facilitate self-fertilization and/or mating with any mates individuals may encounter (low-density model) [1]. In pedunculate barnacles, these merits of being hermaphroditic are small, as they do not usually self-fertilize [47] and encounters with novel individuals are limited owing to their sedentary nature. Instead, as explained, sex allocation theories predict the evolution of hermaphroditism from dioecy (with normal-sized males), then androdioecy with dwarf males, and finally dioecy with dwarf males, as density decreases [2,13–15]. In addition, barnacle larvae (the cyprids) are highly mobile and actively select a suitable site for settlement [48]. This active settlement might have facilitated accurate and prompt settlement by larvae on the specific sites of large conspecifics, hence the evolution of dwarf males. Ghiselin [1] himself suggested that the combination of low density and the low mobility (or even sedentary) nature of large adults, together with highly mobile larvae or young adults, tends to promote the evolution of dwarf males. The likely examples include the sister taxa of ‘true’ (thoracican) barnacles (the parasitic Rhizocephala and the burrowing Acrothoracica), Cycliophora (sessile animals on lobster mouthparts), many parasitic Copepoda, Annelida, parasitic Gastropoda (Enteroxididae) and deep-sea angler fishes [1,10].

Sex allocation models usually postulate genetically determined strategies [2,4,5], and models on barnacle sexual systems are no exceptions [13–15]. Although there is evidence of genetic sex determination in some thoracican barnacles [18,27], the genetic basis of sexual systems is largely unknown and this may differ between barnacle species. However, the evolutionary consequences of sex allocation are largely unrelated to the underlying mechanisms [2,32]. Therefore, we believe that the general trend towards dioecy under low densities should apply also in the case of environmentally determined sexual systems, as conditions for the appearance of dwarf males and females (i.e. small mating groups and being solitary, respectively) are irrespective of whether underlying mechanisms of sexual systems are genetic or environmental. For instance, under plastic sexual expression, a larva that finds a solitary large individual should settle on it and become a dwarf male to fertilize its eggs as soon as possible [1]. However, a larva that settles in the middle of many large hermaphrodites should grow and later become a hermaphrodite to compete with others for fertilizing eggs (if the substratum will last long enough to allow it to grow large) [2,11]. In fact, dwarf males of the pedunculate barnacle O. warwickii [11] and those of the acorn (sessile) barnacle Chelonibia patula [26] are morphologically indistinguishable from conspecific hermaphrodites (except for smaller size and earlier maturation of the testis), and sexual expression appears to be environmentally determined. Dwarf males of O. warwickii tend to occur on solitary rather than aggregating hermaphrodites [11]. This may suggest that the larvae choose to settle on conspecifics to become dwarf males or on a substratum (crab carapaces) to become hermaphrodites in an adaptive way. Likewise, adaptive sex allocation as a phenotypic response (i.e. more female-biased allocation in small mating groups) is known in an acorn barnacle, Catomerus polymerus [49] (but not in another species, Tetraclita rubescens) [19]. Although fragmentary, these pieces of evidence suggest that sexual systems and sex allocation patterns observed in barnacles are in agreement with sex allocation theories even in species with environmentally determined sexual systems.

Previous theories predict that androdioecy is an extremely rare sexual system compared with hermaphroditism, dioecy or gynodioecy [6,7,50]. In fact, in the best-studied taxon, angiosperms, gynodioecy is much more common than androdioecy [6,50]. However, our results suggest that androdioecy is more common than gynodioecy during the course of evolution from hermaphroditism to dioecy in pedunculate barnacles. To explain this apparent discrepancy between theory and empirical patterns in barnacles, or between patterns observed in flowering plants and barnacles, two questions should be addressed: (i) why androdioecy is common in barnacles when compared with other organisms and (ii) why barnacles lack gynodioecy. To address the first question, it is important to distinguish two types of androdioecy: one with normal-sized males and the other with dwarf males [7]. For males to evolve in a hermaphroditic population, they must compensate for the disadvantage of lacking female reproductive function. Normal-sized males must attain at least twofold higher mating success than hermaphrodites through male function to have identical fitness [6,7]. Attaining this is difficult since in hermaphroditic populations, male mating success does not increase proportionally to the amount of resource input [2,13]. Thus, most cases of androdioecy with normal-sized males in animals and plants are considered to have evolved from dioecious populations rather than from hermaphroditic populations, by females becoming self-fertile hermaphrodites under low density to ensure fertilization [6,7]. However, dwarf males suffer less from diminishing returns because of their smaller amount of resources, and possibly they have mating advantages over hermaphrodites by being nearer the fertilization site of conspecifcs [1,16]. More importantly, dwarf males need not maintain the twofold higher mating success because of their survival advantage owing to early maturation [15,26]. In fact, a mathematical model showed that smaller males are more likely to evolve than larger males in hermaphroditic barnacles even with a simple trade-off between survival rate and body size (assuming survival rate multiplied by body size is constant) [15]. The conditions for the occurrence of dwarf males would be even more relaxed if the sexual system is environmentally determined, as in this case, the fitness of dwarf males and that of hermaphrodites need not be equal. Although androdioecy with relatively small males has been little known in other taxa outside of thoracican barnacles [7], it may be more common than previously considered.

Concerning the second question (i.e. the lack of gynodioecy in barnacles), it is important to realize that conditions for the evolution of gynodioecy are not a mirror image of conditions for androdioecy [6,50]. When hermaphrodites self-fertilize, for males to evolve, they must attain more than twofold higher reproductive success than male-acting hermaphrodites, whereas females have only to attain less than twofold higher reproductive success. This discrepancy becomes more exaggerated as the degree of selfing and its resultant inbreeding depression increases. Thus, in angiosperms, gynodioecy may evolve relatively easily as a mechanism to avoid inbreeding depression [6,50]. On the other hand, most barnacles do not self-fertilize, and hence require twofold higher reproductive success for females to evolve. Since the optimal allocation of hermaphrodites is female-biased in small mating groups [2], pure females will not have twofold higher reproductive success when compared with hermaphrodites even if they allocate all resources to female function. In short, outcrossing and female-biased sex allocation in hermaphrodites probably prevent gynodioecy from evolving in barnacles.

Most problems in reproductive biology raised by Darwin, such as sexual selection, variation in sex ratios or cross- versus self-fertilization, have been well addressed under the framework of modern evolutionary biology [1–5], but the sexual systems of barnacles recognized by Darwin [8] remain incompletely understood. This study demonstrates the importance of limited mating opportunity and provides an explanation of hermaphroditism, androdioecy and dioecy, as well as the presence of dwarf males, in pedunculate barnacles. However, the forces driving the evolution of sexual systems may differ among taxa. Phylogenetically controlled tests have identified the environmental factors responsible for the evolution of different sexual systems in other taxa, especially plants [41,51]. Few tests exist in animal taxa with diverse sexual systems comparable to those of barnacles [33], and the evolution of such systems in most cases remains to be clarified. Thus, further study is needed on the relative importance of the evolutionary forces producing the diverse sexual systems observed in animals.


Literature review and GenBank data

Over the past years, I used a combination of web-based literature searches and personal communications to update a previous compilation of dioecious genera ( Renner and Ricklefs, 1995 ). The family assignments of all genera were updated in March 2014 using GenBank (, which was also used to check for phylogenetic studies on the evolution of dioecy. Studies were downloaded from the World Wide Web or obtained from their authors and checked for information about mating systems and changes in generic circumscriptions. Experts on particular families and genera were consulted by e-mail (see acknowledgments).

Distribution of androdioecious and hermaphroditic populations of the mangrove Laguncularia racemosa (Combretaceae) in Florida and the Bahamas

The breeding system of Laguncularia racemosa is variable among populations some populations are androdioecious while other populations lack male plants. To determine whether androdioecy is widespread in L. racemosa , 65 populations were surveyed in Florida and the Bahamas. Fruits are water-dispersed, so the observed distribution of breeding systems was compared to local and regional water currents in order to determine whether dispersal could be important to the maintenance of male plants in androdioecious populations. Twenty-two of the 36 populations surveyed in Florida were androdioecious, with male frequencies that ranged from 1–68%. On the Florida east coast, all populations north of latitude 26°30′ N lacked males while all populations south of this latitude were androdioecious, which suggests that northern populations may lack males due to dispersal limitation. The pattern of distribution on the Florida west coast suggests that males may be maintained in some populations via dispersal. Nine islands in north-central Bahamas were surveyed, and androdioecious populations were found only on San Salvador Island, where male frequencies ranged from 5–28%. Dispersal, fragmentation, and selection hypotheses are suggested to explain the observed pattern of distribution these hypotheses will be tested in future studies.

Ancient androdioecy in the freshwater crustacean Eulimnadia

Among the variety of reproductive mechanisms exhibited by living systems, one permutation—androdioecy (mixtures of males and hermaphrodites)—is distinguished by its rarity. Models of mating system evolution predict that androdioecy should be a brief stage between hermaphroditism and dioecy (separate males and females), or vice versa. Herein we report evidence of widespread and ancient androdioecy in crustaceans in the genus Eulimnadia, based on observations of over 33 000 shrimp from 36 locations from every continent except Antarctica. Using phylogenetic, biogeographical and palaeontological evidence, we infer that androdioecy in Eulimnadia has persisted for 24–180 million years and has been maintained through multiple speciation events. These results suggest that androdioecy is a highly successful aspect of the life history of these freshwater crustaceans, and has persisted for orders of magnitude longer than predicted by current models of this rare breeding system.


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We found that an herbaceous growth form and a temperate distribution were significantly associated with gynodioecy (tables 2, 3). This association has two nonmutually exclusive explanations. First, gynodioecy may be more likely to evolve from hermaphroditism in herbaceous or temperate lineages than in woody or tropical lineages. Second, gynodioecy may be less likely to revert to hermaphroditism or transition to dioecy in herbaceous or temperate lineages than in woody or tropical lineages. These two explanations could be tested by modeling the evolution of gynodioecy within families that are polymorphic for growth form and geographic distribution (for similar analyses of other sexual systems, see Torices et al. 2011 Leslie et al. 2013). If an herbaceous growth form or temperate distribution facilitates the evolution of gynodioecy, then the rate of transitions to the gynodioecious state should be higher for herbaceous or temperate lineages than for woody or tropical lineages. If an herbaceous growth form or temperate distribution facilitates the persistence of gynodioecy, then the rate of transitions away from the gynodioecious state should be lower for herbaceous or temperate lineages than for woody or tropical lineages.

Our data do not allow us to determine the mechanisms by which an herbaceous growth form and a temperate distribution facilitate the evolution and/or persistence of gynodioecy. However, gynodioecy models suggest two possible mechanisms. First, models (Charlesworth and Charlesworth 1978 Charlesworth 1981 Ehlers and Schierup 2008 Dornier and Dufay 2013) predict that self-compatibility favors the evolution and/or persistence of gynodioecy, and self-compatibility is more common in herbaceous and temperate species (Igic et al. 2008). Second, models (Lloyd 1974 Dornier and Dufay 2013) predict that females, and thus gynodioecy, are more likely to persist when seed production is not pollen limited, and pollen limitation is less common in herbaceous and temperate species (Larson and Barrett 2000 Vamosi et al. 2006).

Although a temperate geographic distribution has been anecdotally associated with gynodioecy, our study is the first to statistically test for this association. In contrast, there has been one test of the association between an herbaceous growth form and gynodioecy in the Hawaiian flora, an herbaceous growth form and gynodioecy were not associated (Sakai et al. 1995). One explanation for why gynodioecy was associated with herbaceous growth form in our angiosperm-wide analysis (tables 2, 3), but not in the Hawaiian flora, is that an herbaceous growth form is underrepresented in the Hawaiian flora 34.1% of the Hawaiian flora is herbaceous (Sakai et al. 1995), whereas 55% of all plant species are herbaceous (FitzJohn et al. 2014). More generally, because the species in a local flora such as Hawaii are not a random sample of all plant species, any inferences about the phenotypic traits or ecological factors that are associated with particular sexual systems may not be generalizable across the angiosperms (Renner and Ricklefs 1995 Vamosi et al. 2003). In contrast, because our analysis was angiosperm-wide, it should be more likely to yield generalizable inferences about the phenotypic traits or ecological factors that are associated with gynodioecy.

One limitation of our study is that gynodioecy is likely to be underreported in the scientific literature because some populations of gynodioecious species contain few or no females (e.g., Caruso and Case 2007). Consequently, when species are assigned to sexual systems based on data from only a few populations, gynodioecy can be mistakenly identified as hermaphroditism (Dufay et al. 2014). If gynodioecy is a difficult sexual system to detect, then that could explain why so few gynodioecious species have a tropical distribution (Dufay et al. 2014 Renner 2014). However, monoecy, a sexual system where hermaphrodites produce both staminate and pistillate flowers, is also difficult to detect (Gross 2005) but can be common in tropical floras (e.g., Gross 2005 Chen and Li 2008). Consequently, the difficulty of detecting gynodioecy is unlikely to be the only explanation for why there are so few gynodioecious species in the tropics (Table 1).

Finally, our results suggest that gynodioecy is associated with different phenotypic traits/ecological factors than dioecy gynodioecy is associated with an herbaceous growth form and temperate distribution (tables 2, 3), whereas dioecy is associated with a woody growth form and tropical distribution (Renner 2014). One potential explanation for this difference that also accounts for the phylogenetic association between gynodioecy and dioecy (Dufay et al. 2014) is that dioecy may evolve more frequently via the gynodioecy pathway in herbaceous, temperate lineages but more frequently via other pathways in woody, tropical lineages. In particular, dioecy can also evolve via monoecy (reviewed in Barrett 2002), a sexual system that is associated with some of the same phenotypic traits as dioecy, including a woody growth form (Vary et al. 2011) and a tropical distribution (e.g., Gross 2005 Chen and Li 2008). This could be tested by modeling the evolution of sexual systems within families the rate of transitions from gynodioecy to dioecy should be higher in herbaceous, temperate lineages than in woody, tropical lineages, whereas the rate of transitions from monoecy to dioecy should be higher in woody, tropical lineages than in herbaceous, temperate lineages.

We thank H. Maherali for advice on statistical analysis and K. Brown and R. Rivkin for help with data collection. E. Bothwell, J. Jarvis, L. Jesson, H. Maherali, S. Otten, P. Rekret, R. Rivkin, K. Thompson, R. Torices, and several anonymous reviewers provided comments on an earlier version of the manuscript. This work was funded by grants from the US National Science Foundation (DEB-0842280, awarded to A. L. Case and C. M. Caruso) and the Natural Science and Engineering Research Council of Canada (awarded to C. M. Caruso). C. M. Caruso was supported by sabbatical and short-term fellowships from the National Evolutionary Synthesis Center (NSF EF-0905606).