Examination of boldness traits in sexual and asexual mollies (<Emphasis Type="Italic">Poecilia latipinna</Emphasis>, <Emphasis Type="Italic">P. formosa</Emphasis>)

Considering the high costs of sexual reproduction (e.g., the production of males), its maintenance and predominance throughout the Animal Kingdom remain elusive. Especially the mechanisms allowing for a stable coexistence of closely related sexual and asexual species are still subject to a lively debate. Asexuals should rapidly outnumber sexuals due to higher population growth rates, unless they face some disadvantages. Here, we investigate potential differences in feeding behavior in a system of sexual (sailfin mollies, Poecilia latipinna) and coexisting gynogenetic fishes (Amazon mollies, Poecilia formosa). In two different experiments, we tested for differences in behavioral traits associated with boldness. Bold individuals take higher risks for gains in resources, so shyer individuals should be less competitive. Our study was motivated by the recent finding that P. formosa are less likely to be preyed upon by piscine predators than P. latipinna. We asked whether this result is indicative of low boldness in P. formosa. However, no differences between the two species were detectible in our behavioral experiments measuring (a) time to emerge from shelter to explore a novel environment, (b) latency time until feeding in a novel environment, and (c) recovery time until feeding restarted after a simulated predator attack. Furthermore, different boldness measures were not correlated with each other within individuals.     

Juvenile survival in a unisexual/sexual complex of mollies

Unisexual species like the gynogenetic Amazon molly, Poecilia formosa, enjoy a twofold advantage over sexual species, because they do not produce males. Therefore, unisexuals should be able to outcompete and consequently, replace sexual species. For sperm-dependent (gynogenetic) unisexuals this creates a paradox: they cannot replace their sexual hosts without eradicating themselves. Thus, mechanisms must be in place to stabilize such mating systems. We assessed juvenile survivorship between asexual P. formosa and sexual Poecilia latipinna as a possible factor allowing for persistence and coexistence between the two sympatric species. Offspring of gynogenetic Amazon mollies did not differ significantly in survivorship compared to their sexual host, the Sailfin molly, P. latipinna. The presence of an adult female significantly reduced survival in both species, suggesting that filial cannibalisms operates in this system, but does not appear to play a role in stabilizing mixed sexual/asexual populations. Clark Hubbs, who spent 59 years at the University of Texas and was widely regarded as one of the state’s foremost researchers in the field of ichthyology, the study of fish, passed away February 3rd of 2008 after a long battle with colorectal cancer. He was 86.     

Differential susceptibility to food stress in neonates of sexual and asexual mollies (<Emphasis Type="Italic">Poecilia</Emphasis>, Poeciliidae)

The maintenance of sex is still an evolutionary puzzle given its immediate costs. Stably coexisting complexes of asexually and sexually reproducing forms allow to study mechanisms that balance the costs and benefits of both asexual and sexual reproduction. Here, we tested whether coexisting asexual and sexual fish of the genus Poecilia differed in neonate mortality when exposed to environmental stress in the form of fluctuating temperatures and food deprivation. We find that asexual Amazon mollies, Poecilia formosa, are significantly more sensitive to food stress than their sexual relative Poecilia latipinna, but both are equally unaffected by variable temperatures. Differences in the susceptibility to environmental stress may contribute to diminishing the asexuals’ benefits of a higher intrinsic population growth rate and thus mediate stable coexistence of the two reproductive forms.     

Character displacement in sailfin mollies, <Emphasis Type="Italic">Poecilia latipinna</Emphasis>: allozymes and behavior

Synopsis We analyzed variation in allozymes and mating preferences in 12 populations across much of the range of the sailfin molly, Poecilia latipinna. Sailfin mollies can be sympatric with its sexual parasite Amazon mollies, P. formosa. Amazon mollies must co-exist and mate with bisexual males of closely related species (including sailfin mollies) to induce embryogenesis but inheritance is strictly maternal. Where sailfin and Amazon mollies are sympatric there is evidence of reproductive character displacement as males show a significantly stronger mating preference for sailfin molly females over Amazon mollies compared to preferences of males from allopatric populations. From the allozyme data we found a moderate amount of genetic variation across all populations but this variation did not reveal significant partitioning between sympatric and allopatric populations. Additionally, we found no evidence for isolation by distance as genetic distance was not significantly correlated with geographic distance. While allozyme variation also did not significantly correlate with male mating preferences, there was a significant correlation between male mating preferences and geographic distance. This correlation between mating preferences and geographic distance may have arisen from coevolution with Amazon mollies resulting in reproductive character displacement. Taken together, the distribution of genetic and behavioral variation among sympatric and allopatric populations suggests that behavioral evolution has outpaced evolution at the allozyme loci we examined in P. latipinna.     

Equal fecundity in asexual and sexual mollies (<Emphasis Type="Italic">Poecilia</Emphasis>)

The evolution and maintenance of sexual reproduction is still one of the major unresolved problems in evolutionary biology. Sexual reproduction is fraught with a number of costs as compared to asexual reproduction. For example, sexuals have to produce males, which–given a 1:1 sex ratio—results in a two-fold advantage for asexuals that do not produce males. Consequently, asexuals will outperform and replace sexuals over time assuming everything else is equal. Nonetheless, a few cases of closely related asexuals and sexuals have been documented to coexist stably in natural systems. We investigated the presence of a two-fold cost in a unique system of three closely related fish species: the asexual Amazon Molly (Poecilia formosa), and two sexual species, Sailfin Molly (P. latipinna) and Atlantic Molly (P. mexicana). Amazon Molly reproduce gynogenetically (by sperm dependent parthenogenesis) and always coexist with one of the sexual species, which serves as sperm donor. In the laboratory, we compared reproductive output between P. formosa and P. mexicana as well as P. formosa and P. latipinna. We found no differences in the fecundity in either comparison of a sexual and the asexual species. Under the assumption of a 1:1 sex ratio, the asexual Amazon Molly should consequently have a full two-fold advantage and be able to outcompete sexuals over time. Hence, the coexistence of the species pairs in nature presents a paradox still to be solved.     

Male mating behaviour of a molly, <Emphasis Type="Italic">Poecilia latipunctata</Emphasis>: a third host for the sperm-dependent Amazon molly, <Emphasis Type="Italic">Poecilia formosa</Emphasis>

The Tamesí molly, Poecilia latipunctata, has a very limited biogeographical range in northeast Mexico. This area is nested within the ranges of the Atlantic molly, Poecilia mexicana, and the unisexual Amazon molly, Poecilia formosa. Based on morphology, especially fin shape, the Tamesí molly has been considered to be a "short-fin" molly. We describe the courtship sequence of P. latipunctata. The courtship clearly places the species into the clade of "long-fin" mollies, a finding that corroborates earlier studies based on nuclear DNA and mitochondrial DNA. All three species live together in certain habitats. This renders P. latipunctata a potential host species for the sperm-dependent, unisexual Amazon molly. Using behavioural tests, we demonstrate that P. latipunctata males actually copulate with Amazon mollies, despite a pronounced preference for conspecific females. In laboratory experiments P. latipunctata males are capable of triggering embryogenesis in P. formosa females. Field observations support the hypothesis that P. latipunctata is a third host species for P. formosa, indicating that the Amazon molly effectively exploits all available host species for its gynogenetic mode of reproduction. Electronic Publication     

Feeding rates in the sailfin molly Poecilia latipinna and its coexisting sexual parasite,the gynogenetic Amazon molly Poecilia formosa

Feeding rates of the gynogenetic Amazon molly Poecilia formosa and one of its sexual hosts, the sailfin molly Poecilia latipinna, were measured under winter and summer temperature conditions. Food consumption of the unisexual P. formosa in winter conditions was significantly higher than that of P. latipinna, and it is hypothesized that the resulting food stress might have an important influence on the population composition of these closely related fishes via higher winter mortality in P. formosa.     

Effects of Female‐Female Aggression in a Sexual/Unisexual Species Complex

The maintenance of sexual reproduction is still a largely unresolved question in evolutionary biology, and one of the most puzzling aspects of this is the coexistence of sexual and asexual females. This often leads to widespread niche overlap, posing an interesting challenge to competitive exclusion theory. In this study, we investigate how the aggressive behaviors between females in a sexual/unisexual mating complex of mollies (Poecilia latipinna and P. formosa) differ, and the effects these behaviors have on both the performer and receiver of aggressive acts. We exposed females to five treatments: (1) alone (control); (2) a large and small sexual female; (3) a large sexual female and a small unisexual female; (4) a small sexual female and a large unisexual female; and (5) a large and small unisexual female. We found that P. formosa females were on average more aggressive over time when compared to their sexual hosts, P. latipinna. The females of P. formosa, however, incurred a higher cost of being aggressive. Large, aggressor females of this species had a lower body fat content than their conspecific recipients. By contrast, P. latipinna females showed no differentiation between aggressors and recipients. Here, we combine life history methodology and behavioral observations to show that female competition can in fact be quantitatively measured and that these aggressive behaviors are costly to perform. The adaptive value of this interspecies aggression is not yet completely understood, but sexual competition, access to resources, and position within the shoal are all possible functions.     

Are you more than the sum of your parents’ genes? Phenotypic plasticity in a clonal vertebrate and F1 hybrids of its parental species

All known vertebrate clones have originated from hybridization events and some have produced distinct evolutionary lineages via hybrid speciation. Amazon mollies (Poecilia formosa) present an excellent study system to investigate how clonal species have adapted to heterogeneous environments because they are the product of a single hybridization event between male sailfin mollies (Poecilia latipinna) and female Atlantic mollies (Poecilia mexicana). Here, we ask whether the hybrid species differs from the combination of its parental species’ genes in its plastic response to different environments. Using a three-way factorial design, we exposed neonates produced by Amazon mollies and reciprocal F1 hybrid crosses to different thermal (24°C and 29°C) and salinity (0/2, 12, and 20 ppt) regimes. We measured various ontogenetic and life history characteristics across the life span of females. Our major results were as follows: (1) Reaction norms of growth and maturation to temperature and salinity are quite similar between the two hybrid crosses; (2) Amazon molly reaction norms were qualitatively different than the P. latipinna male and P. mexicana female (L×M) hybrids for the ontogenetic variables; (3) Amazon molly reaction norms in reproductive traits were also quite different from L×M hybrids; and (4) The reaction norms of net fertility were very different between Amazon mollies and L×M hybrids. We conclude that best locale for Amazon mollies is not the best locale for hybrids, which suggests that Amazon mollies are not just an unmodified mix of parental genes but instead have adapted to the variable environments in which they are found. Hybridization resulting in asexuality may represent an underappreciated mechanism of speciation because the unlikely events required to produce such hybrids rarely occur and is dependent upon the genetic distance between parental species.     

Polymorphic MHC loci in an asexual fish,the amazon molly (Poecilia formosa; Poeciliidae)

Genes of the major histocompatibility complex (MHC) encode molecules that control immune recognition and are highly polymorphic in most vertebrates. The remarkable polymorphisms at MHC loci may be maintained by selection from parasites, sexual selection, or both. If asexual species show equal (or higher) levels of polymorphisms at MHC loci as sexual ones, this would mean that sexual selection is not necessary to explain the high levels of diversity at MHC loci. In this study, we surveyed the MHC diversity of the asexual amazon molly (Poecilia formosa) and one of its sexual ancestors, the sailfin molly (P. latipinna), which lives in the same habitat. We found that the asexual molly has polymorphic MHC loci despite its clonal reproduction, yet not as polymorphic as the sexual species. Although the nucleotide diversity was similar between the asexual and sexual species, the sexual species exhibited a greater genotypic diversity compared to the asexual one from the same habitats. Within‐genome diversity was similar for MHC class I loci, but for class IIB, the sexual species had higher diversity compared to the asexual — despite the hybrid origins and higher levels of heterozygosity at microsatellite loci in the asexual species. The level of positive selection appears to be similar between the two species, which suggests that these polymorphisms are maintained by selection. Thus, our findings do not allow us to rule out the sexual selection hypothesis for the evolution of MHC diversity, and although the sexual fish has higher levels of MHC‐diversity compared to the asexual species, this may be due to differences in demography, parasites, or other factors, rather than sexual selection.     

How to go extinct by mating too much: population consequences of male mate choice and efficiency in a sexual–asexual species complex

Selection acting on individuals is not predicted to maximize population persistence, yet examples that explicitly quantify conflicts between individual and population level benefits are scarce. One such conflict occurs over sexual reproduction because of the cost of sex: sexual populations that suffer the cost of producing males have only half the growth rate compared to asexuals. Male behaviour can additionally impact population dynamics in a variety of ways, and here we study an example where the impact is unusually clear: the riddle of persistence of sperm‐dependent sexual–asexual species complexes. Here, a sexually reproducing host species coexists with an ameiotically reproducing all‐female sperm parasite. Sexual–asexual coexistence should not be stable because the proportion of asexually reproducing females will rapidly increase and the relative abundance of the sexually reproducing host species will decline. A severe shortage of males will lead to sperm limitation for sexual and asexual females and the system collapses. Male mate choice could reduce the reproductive potential of the asexual species and thus potentially prevent the collapse. In the gynogenetic (sperm‐dependent parthenogenetic) Amazon molly Poecilia formosa and its host (P. latipinna or P. mexicana), males discriminate against asexual females to some extent. Using a population‐dynamical model, we examine the population dynamics of this species complex with varying strengths of male discrimination ability and efficiency with which they locate females and produce sperm. The sexual species would benefit from stronger discrimination, thus preventing being displaced by the asexual females. However, males would be required to evolve preferences that are probably too strong to be purely based upon selection acting on individuals. We conclude that male behaviour does not fully prevent but delays extinction, yet this is highly relevant because low local extinction rates strongly promote coexistence as a metapopulation.     

Size-dependent male mating behaviour and sexual harassment in a population of Atlantic mollies (Poecilia mexicana) from a sulphur creek

The Atlantic molly Poecilia mexicana inhabits a variety of habitats. In previous studies, small males from clear-water populations showed considerably more sexual behaviours than large males. Males from a sulphur creek, the El Azufre, or from a sulphur cave (cave molly) showed comparatively low sexual activity, and typical size-dependent mating behaviour was either absent or large males showed even more sexual behaviour than smaller ones. Sexually deprived cave molly males increased their sexual activity, but small males still did not show more sexual behaviour than large ones, suggesting that the genetic basis for typical size-dependent mating behaviour has been lost. Likely, the extreme abiotic conditions (hypoxia and toxic hydrogen sulphide) have selected against energetically costly behaviours. In this paper, we examined how El Azufre males would respond to sexual deprivation. We found these males to also increase their sexual activity after sexual deprivation but, unlike cave mollies, small males now showed more sexual behaviour than large ones, suggesting that the genetic basis for this behaviour is not lost in this population. Differences between populations may be due to the less harsh characteristics of El Azufre (e.g. less H₂S), resulting in weaker selection against energy-consuming behaviours. Furthermore, we tested for potential costs of male sexual harassment for females in terms of reduced feeding efficiency in the presence of a male, as was shown for P. mexicana from a clear-water population, but was not found in cave mollies or the El Azufre fish. Test females were either starved for 1 week to increase hunger levels or male sexual deprivation and female starvation were combined; however, we found no evidence for male sexual harassment even after these treatments. Our results parallel previous findings in the cave molly and suggest that El Azufre males have lost specific behavioural traits that cause sexual harassment.     

Do I Know You? Species Recognition Operates Within and Between the Sexes in a Unisexual–Bisexual Species Complex of Mollies

Male sailfin mollies (Poecilia latipinna) can be sexually parasitized by closely related, unisexual, gynogenetic Amazon mollies (Poecilia formosa). This study examined possible cues used by male P. latipinna to distinguish between conspecific females and sympatric, heterospecific P. formosa. Digital photos were used to create models to test male P. latipinna preference for model female P. latipinna and P. formosa with a full suite of traits and altered models of P. latipinna and P. formosa. Male P. latipinna significantly preferred models of either species over no stimulus, demonstrating that models elicit a male response. Males also significantly preferred female P. latipinna models over P. formosa models. We also examined species recognition by female sailfin mollies using the same models, and found that female sailfin mollies significantly preferred to associate with female P. latipinna over P. formosa. These results taken together suggest that the use of fish models yield results similar to those studies using live stimuli. Male preference was then tested for unaltered vs. altered models in the following combinations: (i) P. formosa vs. P. formosa with a female P. latipinna fin; (ii) P. formosa vs. P. formosa with a female P. latipinna lateral spot pattern; (iii) P. formosa vs. P. latipinna with a P. formosa fin and their spotless lateral pattern. Males did not significantly prefer models with any isolated traits over the unaltered P. formosa models. Thus, males may be using traits other than the ones isolated for species recognition or males may be using a suite of multiple traits to recognize conspecific females.     

Sexual harassment in a live-bearing fish (<Emphasis Type="Italic">Poecilia mexicana</Emphasis>): influence of population-specific male mating behaviour

We investigated male sexual behaviour and the cost of sexual harassment, as measured by the reduction of female feeding time in the presence of a male, in a cave-dwelling population of Poecilia mexicana, in which sexual harassment does not occur naturally. We asked whether the lack of sexual harassment in this population is due to low sexual activity of the males, or low feeding motivation of the females. We experimentally increased the sexual activity of males or the females feeding motivation, or we used a combination of both treatments. Female feeding time was not lower in the presence of a male than in the presence of a female after sexual deprivation of the males or food deprivation of the females. Only in the combined experiment was female feeding time lower in the presence of a large male than in the presence of a small male, indicating a weak effect of sexual harassment by large males. Virgin females did not suffer a cost of sexual harassment, indicating that sexual experience does not cause the lack of sexual harassment in cave mollies. Males from a surface population, where sexual harassment occurs, significantly reduced the feeding time of cave-dwelling females even though these males exhibited surprisingly little sexual behaviour. The sexual activity of cave mollies did not correlate with male body size in any experiment, indicating that even after sexual deprivation, small cave molly males do not switch to the alternative mating behaviour known in surface-dwelling P. mexicana, where sexual activity is correlated negatively with male body size.     

Lack of species discrimination based on chemical cues by male sailfin mollies, <Emphasis Type="Italic">Poecilia latipinna</Emphasis>

When making mating decisions, individuals may rely on multiple cues from either the same or multiple sensory modalities. Although the use of visual cues in sexual selection is well studied, fewer studies have examined the role of chemical cues in mate choice. In addition, few studies have examined how visual and/or chemical cues affect male mating decisions. Male mate choice is important in systems where males must avoid mating with heterospecific females, as is found in a mating complex of Poecilia. Male sailfin mollies, Poecilia latipinna, are sexually parasitized by gynogenetic Amazon mollies, P. formosa. Little is known about the mechanism by which male sailfin mollies base their mating decisions. Here we tested the hypothesis that male sailfin mollies from an allopatric and a sympatric population with Amazon mollies use multiple cues to distinguish between conspecific and heterospecific females. We found that male sailfin mollies recognized the chemical cues of conspecific females, but we found no support for the hypothesis that chemical cues are by themselves sufficient for species discrimination. Lack of discrimination based on chemical cues alone may be due to the close evolutionary history between P. latipinna and P. formosa. Males from populations sympatric with Amazon mollies did not differentially associate with females of either of the two species when given access to both visual and chemical cues of the females, yet males from the allopatric population did associate more with conspecific females than with heterospecific females in the presence of both chemical and visual cues. The lack of discrimination by males from the sympatric population between conspecific and heterospecific females based on both chemical and visual cues suggests that these males require more complex combinations of cues to distinguish species, possibly due to the close relatedness of these species.     

Divergent female mating preference in a clonal fish

In asexual all-female species, such as the Amazon molly, Poecilia formosa, that depend on sperm from “host males” only to trigger embryogenesis, mate choice does not provide any indirect, genetic benefits to the choosing female, although direct benefits are possible. Asexual species are thought to have a low evolutionary potential or evolvability due to the absence of meiotic recombination. Hence, theory predicts that mating preferences in P. formosa for male ornaments—if existent—should resemble those of females of the two parental species (Poecilia latipinna and Poecilia mexicana) involved in the natural hybridization that gave rise to the asexual P. formosa. When examining the female preference for dummy males with or without black vertical bars in the two parental species and in two lineages of P. formosa, a preference was detected in P. latipinna, but not in P. mexicana females. Interestingly, P. formosa living syntopic with P. latipinna also preferred striped males, while others living syntopic with P. mexicana preferred non-striped males. The evolutionary significance of this phenomenon remains largely unexplained, but it might indicate the evolution of mating preferences in a species with low evolutionary potential. Possible mechanisms include introgression and mitotic gene conversion. Females might use male coloration as indicator mechanisms for male traits that matter in terms of direct benefits.     

A Sexual Preference in the Amazon Molly, Poecilia formosa

The Amazon molly, Poecilia formosa, is an all-female species that reproduces by gynogenesis, i.e., it relies on sperm of males of closely related species to trigger embryogenesis. Sperm is supplied by males of P. latipinna and P. mexicana. Amazon mollies live in sympatry with at least one of these species, a few populations live in sympatry with two sperm-donor species. As P. formosa is sperm dependent, it needs mechanisms for species and mate recognition. To investigate the effect of rearing conditions on sexual preferences of Amazon mollies, we raised Amazon mollies in sympatry with P. latipinna and P. mexicana males. We used simultaneous choice tests to determine the effect of age on female sexual preferences. Immature Amazon mollies do not exhibit a preference if given a choice between a P. latipinna and a P. mexicana male, whereas adult P. formosa do have a preference for the P. latipinna male. We used two different stimuli in this study, live males and videotapes of males.     

Behavioural and life‐history regulation in a unisexual/bisexual mating system: does male mate choice affect female reproductive life histories?

In theory, unisexual taxa have an advantage over ecologically similar bisexual species because unisexuals produce twice as many daughters and, thus, should quickly outcompete coexisting bisexuals in any given population. For sperm‐dependent unisexual (gynogenetic) species, stable coexistence with their bisexual sperm donors can be postulated if male mate choice puts unisexual females at a disadvantage through sperm limitation, thus halving their reproductive output compared to bisexuals (‘behavioural regulation hypothesis’). We tested for a potential life‐history signature of male mate choice in a system of coexisting bisexual sailfin mollies (Poecilia latipinna) and gynogenetic Amazon mollies (Poecilia formosa). Specifically, we gave P. latipinna males an opportunity to freely interact (and mate) with both types of females and, after 25 days, quantified the proportion of (1) females with sperm in their genital tract and (2) pregnant females. A higher proportion of P. latipinna females (53.7%) had sperm in their genital tract (compared to only 25.9% in P. formosa), corroborating a previous study on wild‐caught fish. This translated into a higher frequency (42.6%) of P. latipinna females being pregnant (compared to 29.6% in P. formosa); however, among pregnant females, no significant differences between species in reproductive life‐history traits (such as offspring number or size) were uncovered. Hence, although the findings of the present study confirm that male discrimination against unisexual females leads to reduced reproductive output in unisexuals, the observed magnitude of differences in targeted life histories between the two types of females is unlikely to be the sole factor regulating stable coexistence in this system. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 598–606.     

ORIGIN AND MAINTENANCE OF A FEMALE MATING PREFERENCE

We investigated the role of maintenance and origin of female mating preferences in three species of fish. Poecilia latipinna and P. mexicana are sexual species that hybridized to form the gynogenetic clonal P. formosa, which reproduces asexually but requires sperm to initiate embryogenesis. We demonstrate that all three species display almost identical and statistically indistinguishable preferences for large males. Although processes of good genes, runaway sexual selection, and direct selection could maintain preferences in the sexual species, good genes and runaway sexual selection are unlikely to operate in the asexual species. Furthermore, we found that the most likely direct selection benefit, an increase in fecundity, can also be excluded in the gynogens. We conclude that the most parsimonious explanation for this P. formosa preference is that it was inherited from the parental species and is maintained without forces generated by good genes, runaway selection, or direct selection for increased fecundity. This preference may be maintained because of pleiotropic effects (e.g., sensory bias) or mate searching costs.     

Do ecological niches differ between sexual and asexual lineages of an aphid species?

According to environmental-based theories on the maintenance of sexual reproduction, sexual and asexual populations may coexist if they occupy different ecological niches. The aphid Rhopalosiphum padi offers a good opportunity to test this hypothesis since sexual and asexual lineages show local coexistence during a large part of their respective life-cycles. Because these two reproductive variants are morphologically identical but genetically distinct, we first characterized them using genetic markers in populations of R. padi in areas where sexual and asexual lineages may occur in sympatry. We then inferred the natal host plant of sexual and asexual genotypes by analysing stable isotopic ratios and showed that sexual ones mostly originated from C₃ Poaceae while asexual ones originated from C₃ and C₄ plants, although the majority came from C₄ Poaceae. These findings indicate that ecological niches of sexual and asexual lineages of R. padi differ, offering a plausible explanation for the local coexistence of the two reproductive modes in this species through habitat specialisation.