Does a predatory insect contribute to the divergence between cave- and surface-adapted fish populations?

Sex-specific life-history trait expression in hybrids of a cave- and surface-dwelling fish (Poecilia mexicana, Poeciliidae)

Evaluating the fitness of hybrids can provide important insights into genetic differences between species or diverging populations. We focused on surface- and cave-ecotypes of the widespread Atlantic molly Poecilia mexicana and raised F1 hybrids of reciprocal crosses to sexual maturity in a common-garden experiment. Hybrids were reared in a fully factorial 2 × 2 design consisting of lighting (light vs. darkness) and resource availability (high vs. low food). We quantified survival, ability to realize their full reproductive potential (i.e., completed maturation for males and 3 consecutive births for females) and essential life-history traits. Compared to the performance of pure cave and surface fish from a previous experiment, F1s had the highest death rate and the lowest proportion of fish that reached their full reproductive potential. We also uncovered an intriguing pattern of sex-specific phenotype expression, because male hybrids expressed cave molly life histories, while female hybrids expressed surface molly life histories. Our results provide evidence for strong selection against hybrids in the cave molly system, but also suggest a complex pattern of sex-specific (opposing) dominance, with certain surface molly genes being dominant in female hybrids and certain cave molly genes being dominant in male hybrids.

Wall-following - Phylogenetic context of an enhanced behaviour in stygomorphic Sinocyclocheilus (Cypriniformes: Cyprinidae) cavefishes

With 75 known species, the freshwater fish genus Sinocyclocheilus is the largest cavefish radiation in the world and shows multiple adaptations for cave-dwelling (stygomorphic adaptations), which include a range of traits such as eye degeneration (normal-eyed, micro-eyed and eyeless), depigmentation of skin, and in some species, the presence of "horns". Their behavioural adaptations to subterranean environments, however, are poorly understood. Wall-following (WF) behaviour, where an organism remains in close contact with the boundary demarcating its habitat when in the dark, is a peculiar behaviour observed in a wide range of animals and is enhanced in cave dwellers. Hence, we hypothesise that wall-following is also present in Sinocyclocheilus, possibly enhanced in eyeless species compared to eye bearing (normal-/micro-eyed species). Using 13 species representative of Sinocyclocheilus radiation and eye morphs, we designed a series of assays, based on pre-existing methods for Astyanax mexicanus behavioural experiments, to examine wall-following behaviour under three conditions. Our results indicate that eyeless species exhibit significantly enhanced intensities of WF compared to normal-eyed species, with micro-eyed forms demonstrating intermediate intensities in the WF distance. Using a mtDNA based dated phylogeny (chronogram with four clades A-D), we traced the degree of WF of these forms to outline common patterns. We show that the intensity of WF behaviour is higher in the subterranean clades compared to clades dominated by normal-eyed free-living species. We also found that eyeless species are highly sensitive to vibrations, whereas normal-eyed species are the least sensitive. Since WF behaviour is presented to some degree in all Sinocyclocheilus species, and given that these fishes evolved in the late Miocene, we identify this behaviour as being ancestral with WF enhancement related to cave occupation. Results from this diversification-scale study of cavefish behaviour suggest that enhanced wall-following behaviour may be a convergent trait across all stygomorphic lineages.

Sex-dependent increase of movement activity in the freshwater isopod Asellus aquaticus following adaptation to a predator-free cave habitat

Populations experiencing negligible predation pressure are expected to evolve higher behavioral activity. However, when sexes have different expected benefits from high activity, the adaptive shift is expected to be sex-specific. Here, we compared movement activity of one cave (lack of predation) and three adjacent surface (high and diverse predation) populations of Asellus aquaticus, a freshwater isopod known for its independent colonization of several caves across Europe. We predicted 1) higher activity in cave than in surface populations, with 2) the difference being more pronounced in males as they are known for active mate searching behavior, while females are not. Activity was assessed both in the presence and absence of light. Our results supported both predictions: movement activity was higher in the cave than in the surface populations, particularly in males. Relaxed predation pressure in the cave-adapted population is most likely the main selective factor behind increased behavioral activity, but we also showed that the extent of increase is sex-specific.

Exploratory behaviour divergence between surface populations, cave colonists and a cave population in the water louse, Asellus aquaticus

Abstract

Behaviour is considered among the most important factors in colonising new habitats. While population divergence in behaviour is well-documented, intraspecific variation in exploratory behaviour in species with populations successfully colonising and adapting to extreme (compared to the ‘typical’) habitats is less understood. Here, by studying surface- vs. cave-adapted populations of water louse (Asellus aquaticus), we tested whether (i) adaptation to the special, ecologically isolated cave habitat includes a decrease in explorativeness and (ii) recent, surface-type cave colonists are more explorative than their surface conspecifics from the source population. We repeatedly tested dispersal related novel area exploration and dispersal speed in both the presence and absence of light. We found that surface populations showed higher behavioural activity in dark than in light, and they were more explorative and dispersed faster than their cave conspecifics. Recent colonists showed a trend of higher dispersal speed compared to their source surface population. We suggest that extreme and isolated habitats like caves might work as ‘dispersal traps’ following successful colonisation, because adaptation to these habitats includes the reduction of explorativeness. Furthermore, we suggest that individuals with higher explorativeness are likely to colonise markedly new environments. Finally, we provide experimental evidence about surface A. aquaticus moving more in dark than in light.

Significance statement

Environmental conditions in caves are differing drastically from those of the surface. Consequently, animals colonising subterranean habitats are subject to different selective forces than those experienced by the ancestral surface-living population. Behaviour is believed to be a key factor in successful colonisation to novel habitats; however, intraspecific behavioural variation in species with both surface- and cave-adapted populations is less known. Here, we compared dispersal related novel area exploration and dispersal speed across surface and cave-adapted populations of the freshwater crustacean Asellus aquaticus. Our results show that cave-adapted A. aquaticus are significantly less explorative and disperse slower than surface-type populations, indicating that caves may act as ‘dispersal traps’, where adaptation includes the loss of explorativeness. Also, recent cave colonists show a trend to be faster dispersers than peers from the surface source population, suggesting that individuals with higher explorativeness are likely to colonise markedly different new environments.

An interaction mechanism for the maintenance of fission-fusion dynamics under different individual densities

Animals often show high consistency in their social organisation despite facing changing environmental conditions. Especially in shoaling fish, fission-fusion dynamics that describe for which periods individuals are solitary or social have been found to remain unaltered even when density changed. This compensatory ability is assumed to be an adaptation towards constant predation pressure, but the mechanism through which individuals can actively compensate for density changes is yet unknown. The aim of the current study is to identify behavioural patterns that enable this active compensation. We compared the fission-fusion dynamics of two populations of the live-bearing Atlantic molly (Poecilia mexicana) that live in adjacent habitats with very different predator regimes: cave mollies that inhabit a low-predation environment inside a sulfidic cave with a low density of predatory water bugs (Belostoma sp.), and mollies that live directly outside the cave (henceforth called "surface" mollies) in a high-predation environment. We analysed their fission-fusion dynamics under two different fish densities of 12 and 6 fish per 0.36 m2. As expected, surface mollies spent more time being social than cave mollies, and this difference in social time was a result of surface mollies being less likely to discontinue social contact (once they had a social partner) and being more likely to resume social contact (once alone) than cave mollies. Interestingly, surface mollies were also less likely to switch among social partners than cave mollies. A random walk simulation predicted each population to show reduced social encounters in the low density treatment. While cave mollies largely followed this prediction, surface mollies maintained their interaction probabilities even at low density. Surface mollies achieved this by a reduction in the size of a convex polygon formed by the group as density decreased. This may allow them to largely maintain their fission-fusion dynamics while still being able to visit large parts of the available area as a group. A slight reduction (21%) in the area visited at low densities was also observed but insufficient to explain how the fish maintained their fission-fusion dynamics. Finally, we discuss potential movement rules that could account for the reduction of polygon size and test their performance.

Female Choice Undermines the Emergence of Strong Sexual Isolation between Locally Adapted Populations of Atlantic Mollies (Poecilia mexicana)

Divergent selection between ecologically dissimilar habitats promotes local adaptation, which can lead to reproductive isolation (RI). Populations in the Poecilia mexicana species complex have independently adapted to toxic hydrogen sulfide and show varying degrees of RI. Here, we examined the variation in the mate choice component of prezygotic RI. Mate choice tests across drainages (with stimulus males from another drainage) suggest that specific features of the males coupled with a general female preference for yellow color patterns explain the observed variation. Analyses of male body coloration identified the intensity of yellow fin coloration as a strong candidate to explain this pattern, and common-garden rearing suggested heritable population differences. Male sexual ornamentation apparently evolved differently across sulfide-adapted populations, for example because of differences in natural counterselection via predation. The ubiquitous preference for yellow color ornaments in poeciliid females likely undermines the emergence of strong RI, as female discrimination in favor of own males becomes weaker when yellow fin coloration in the respective sulfide ecotype increases. Our study illustrates the complexity of the (partly non-parallel) pathways to divergence among replicated ecological gradients. We suggest that future work should identify the genomic loci involved in the pattern reported here, making use of the increasing genomic and transcriptomic datasets available for our study system.

Intrasexual competition enhances reproductive isolation between locally adapted populations

During adaptation to different habitat types, both morphological and behavioral traits can undergo divergent selection. Males often fight for status in dominance hierarchies and rank positions predict reproductive success. Ecotypes with reduced fighting abilities should have low reproductive success when migrating into habitats that harbor ecotypes with superior fighting abilities. Livebearing fishes in the Poecilia mexicana-species complex inhabit not only regular freshwater environments, but also independently colonized sulfidic (H₂S-containing) habitats in three river drainages. In the current study, we found fighting intensities in staged contests to be considerably lower in some but not all sulfidic surface ecotypes and the sulfidic cave ecotype compared with populations from non-sulfidic surface sites. This is perhaps due to selection imposed by H₂S, which hampers oxygen uptake and transport, as well as cellular respiration. Furthermore, migrants from sulfidic habitats may lose fights even if they do not show overall reduced aggressiveness, as physiological performance is likely to be challenged in the non-sulfidic environment to which they are not adapted. To test this hypothesis, we simulated migration of H₂S-adapted males into H₂S-free waters, as well as H₂S-adapted cave-dwelling males into sulfidic surface waters. We found that intruders established dominance less often than resident males, independent of whether or not they showed reduced aggressiveness overall. Our study shows that divergent evolution of male aggressive behavior may also contribute to the maintenance of genetic differentiation in this system and we call for more careful evaluation of male fighting abilities in studies on ecological speciation.

Complexities of gene expression patterns in natural populations of an extremophile fish (Poecilia mexicana, Poeciliidae)

Variation in gene expression can provide insights into organismal responses to environmental stress and physiological mechanisms mediating adaptation to habitats with contrasting environmental conditions. We performed an RNA-sequencing experiment to quantify gene expression patterns in fish adapted to habitats with different combinations of environmental stressors, including the presence of toxic hydrogen sulphide (H2 S) and the absence of light in caves. We specifically asked how gene expression varies among populations living in different habitats, whether population differences were consistent among organs, and whether there is evidence for shared expression responses in populations exposed to the same stressors. We analysed organ-specific transcriptome-wide data from four ecotypes of Poecilia mexicana (nonsulphidic surface, sulphidic surface, nonsulphidic cave and sulphidic cave). The majority of variation in gene expression was correlated with organ type, and the presence of specific environmental stressors elicited unique expression differences among organs. Shared patterns of gene expression between populations exposed to the same environmental stressors increased with levels of organismal organization (from transcript to gene to physiological pathway). In addition, shared patterns of gene expression were more common between populations from sulphidic than populations from cave habitats, potentially indicating that physiochemical stressors with clear biochemical consequences can constrain the diversity of adaptive solutions that mitigate their adverse effects. Overall, our analyses provided insights into transcriptional variation in a unique system, in which adaptation to H2 S and darkness coincide. Functional annotations of differentially expressed genes provide a springboard for investigating physiological mechanisms putatively underlying adaptation to extreme environments.

Sex-specific local life-history adaptation in surface- and cave-dwelling Atlantic mollies (Poecilia mexicana)

Cavefishes have long been used as model organisms showcasing adaptive diversification, but does adaptation to caves also facilitate the evolution of reproductive isolation from surface ancestors? We raised offspring of wild-caught surface- and cave-dwelling ecotypes of the neotropical fish Poecilia mexicana to sexual maturity in a 12-month common garden experiment. Fish were raised under one of two food regimes (high vs. low), and this was crossed with differences in lighting conditions (permanent darkness vs. 12:12 h light:dark cycle) in a 2 × 2 factorial design, allowing us to elucidate potential patterns of local adaptation in life histories. Our results reveal a pattern of sex-specific local life-history adaptation: Surface molly females had the highest fitness in the treatment best resembling their habitat of origin (high food and a light:dark cycle), and suffered from almost complete reproductive failure in darkness, while cave molly females were not similarly affected in any treatment. Males of both ecotypes, on the other hand, showed only weak evidence for local adaptation. Nonetheless, local life-history adaptation in females likely contributes to ecological diversification in this system and other cave animals, further supporting the role of local adaptation due to strong divergent selection as a major force in ecological speciation.

Light triggers habitat choice of eyeless subterranean but not of eyed surface amphipods

Boundaries of species distributions are the result of colonization-extinction processes. Survival on the boundary depends on how well individuals discriminate optimal from suboptimal habitat patches. Such behaviour is called habitat choice and was only rarely applied to macroecology, although it links species ecological niche and species distribution. Surface and subterranean aquatic species are spatially strongly segregated, even in the absence of physical barriers. We explored whether a behavioural response to light functions as a habitat choice mechanism that could explain species turnover between surface and subterranean aquatic ecosystems. In a controlled laboratory experiment, we studied the behavioural response to light of ten pairs of surface and subterranean amphipods that permanently co-occur in springs. Surface species showed a weak photophobic, photoneutral, and in one case, photophilic response, whereas all subterranean species showed a strong photophobic response. Eyeless subterranean but not eyed surface amphipods appear to orient themselves with light cues. On a local scale, this difference possibly diminishes harmful interactions between the co-occurring amphipods, whereas on a regional scale, photophobia could explain limited dispersal and a high degree of endemism observed among subterranean species.

Reduction of Energetic Demands through Modification of Body Size and Routine Metabolic Rates in Extremophile Fish

Variation in energy availability or maintenance costs in extreme environments can exert selection for efficient energy use, and reductions in organismal energy demand can be achieved in two ways: reducing body mass or metabolic suppression. Whether long-term exposure to extreme environmental conditions drives adaptive shifts in body mass or metabolic rates remains an open question. We studied body size variation and variation in routine metabolic rates in locally adapted populations of extremophile fish (Poecilia mexicana) living in toxic, hydrogen sulfide-rich springs and caves. We quantified size distributions and routine metabolic rates in wild-caught individuals from four habitat types. Compared with ancestral populations in nonsulfidic surface habitats, extremophile populations were characterized by significant reductions in body size. Despite elevated metabolic rates in cave fish, the body size reduction precipitated in significantly reduced energy demands in all extremophile populations. Laboratory experiments on common garden-raised fish indicated that elevated routine metabolic rates in cave fish likely have a genetic basis. The results of this study indicate that adaptation to extreme environments directly impacts energy metabolism, with fish living in cave and sulfide spring environments expending less energy overall during routine metabolism.

Patterns of Macroinvertebrate and Fish Diversity in Freshwater Sulphide Springs

Extreme environments are characterised by the presence of physicochemical stressors and provide unique study systems to address problems in evolutionary ecology research. Sulphide springs provide an example of extreme freshwater environments; because hydrogen sulphide’s adverse physiological effects induce mortality in metazoans even at micromolar concentrations. Sulphide springs occur worldwide, but while microbial communities in sulphide springs have received broad attention, little is known about macroinvertebrates and fish inhabiting these toxic environments. We reviewed qualitative occurrence records of sulphide spring faunas on a global scale and present a quantitative case study comparing diversity patterns in sulphidic and adjacent non-sulphidic habitats across replicated river drainages in Southern Mexico. While detailed studies in most regions of the world remain scarce, available data suggests that sulphide spring faunas are characterised by low species richness. Dipterans (among macroinvertebrates) and cyprinodontiforms (among fishes) appear to dominate the communities in these habitats. At least in fish, there is evidence for the presence of highly endemic species and populations exclusively inhabiting sulphide springs. We provide a detailed discussion of traits that might predispose certain taxonomic groups to colonize sulphide springs, how colonizers subsequently adapt to cope with sulphide toxicity, and how adaptation may be linked to speciation processes.

Selection from parasites favours immunogenetic diversity but not divergence among locally adapted host populations

The unprecedented polymorphism in the major histocompatibility complex (MHC) genes is thought to be maintained by balancing selection from parasites. However, do parasites also drive divergence at MHC loci between host populations, or do the effects of balancing selection maintain similarities among populations? We examined MHC variation in populations of the livebearing fish Poecilia mexicana and characterized their parasite communities. Poecilia mexicana populations in the Cueva del Azufre system are locally adapted to darkness and the presence of toxic hydrogen sulphide, representing highly divergent ecotypes or incipient species. Parasite communities differed significantly across populations, and populations with higher parasite loads had higher levels of diversity at class II MHC genes. However, despite different parasite communities, marked divergence in adaptive traits and in neutral genetic markers, we found MHC alleles to be remarkably similar among host populations. Our findings indicate that balancing selection from parasites maintains immunogenetic diversity of hosts, but this process does not promote MHC divergence in this system. On the contrary, we suggest that balancing selection on immunogenetic loci may outweigh divergent selection causing divergence, thereby hindering host divergence and speciation. Our findings support the hypothesis that balancing selection maintains MHC similarities among lineages during and after speciation (trans-species evolution).

Gradient evolution of body colouration in surface- and cave-dwelling Poecilia mexicana and the role of phenotype-assortative female mate choice

Ecological speciation assumes reproductive isolation to be the product of ecologically based divergent selection. Beside natural selection, sexual selection via phenotype-assortative mating is thought to promote reproductive isolation. Using the neotropical fish Poecilia mexicana from a system that has been described to undergo incipient ecological speciation in adjacent, but ecologically divergent habitats characterized by the presence or absence of toxic H2S and darkness in cave habitats, we demonstrate a gradual change in male body colouration along the gradient of light/darkness, including a reduction of ornaments that are under both inter- and intrasexual selection in surface populations. In dichotomous choice tests using video-animated stimuli, we found surface females to prefer males from their own population over the cave phenotype. However, female cave fish, observed on site via infrared techniques, preferred to associate with surface males rather than size-matched cave males, likely reflecting the female preference for better-nourished (in this case: surface) males. Hence, divergent selection on body colouration indeed translates into phenotype-assortative mating in the surface ecotype, by selecting against potential migrant males. Female cave fish, by contrast, do not have a preference for the resident male phenotype, identifying natural selection against migrants imposed by the cave environment as the major driver of the observed reproductive isolation.

Genetic differentiation and selection against migrants in evolutionarily replicated extreme environments

We investigated mechanisms of reproductive isolation in livebearing fishes (genus Poecilia) inhabiting sulfidic and nonsulfidic habitats in three replicate river drainages. Although sulfide spring fish convergently evolved divergent phenotypes, it was unclear if mechanisms of reproductive isolation also evolved convergently. Using microsatellites, we found strongly reduced gene flow between adjacent populations from different habitat types, suggesting that local adaptation to sulfidic habitats repeatedly caused the emergence of reproductive isolation. Reciprocal translocation experiments indicate strong selection against immigrants into sulfidic waters, but also variation among drainages in the strength of selection against immigrants into nonsulfidic waters. Mate choice experiments revealed the evolution of assortative mating preferences in females from nonsulfidic but not from sulfidic habitats. The inferred strength of sexual selection against immigrants (RI(s)) was negatively correlated with the strength of natural selection (RI(m)), a pattern that could be attributed to reinforcement, whereby natural selection strengthens behavioral isolation due to reduced hybrid fitness. Overall, reproductive isolation and genetic differentiation appear to be replicated and direct consequences of local adaptation to sulfide spring environments, but the relative contributions of different mechanisms of reproductive isolation vary across these evolutionarily independent replicates, highlighting both convergent and nonconvergent evolutionary trajectories of populations in each drainage.

Genomic resources for a model in adaptation and speciation research: characterization of the Poecilia mexicana transcriptome

Background

Elucidating the genomic basis of adaptation and speciation is a major challenge in natural systems with large quantities of environmental and phenotypic data, mostly because of the scarcity of genomic resources for non-model organisms. The Atlantic molly (Poecilia mexicana, Poeciliidae) is a small livebearing fish that has been extensively studied for evolutionary ecology research, particularly because this species has repeatedly colonized extreme environments in the form of caves and toxic hydrogen sulfide containing springs. In such extreme environments, populations show strong patterns of adaptive trait divergence and the emergence of reproductive isolation. Here, we used RNA-sequencing to assemble and annotate the first transcriptome of P. mexicana to facilitate ecological genomics studies in the future and aid the identification of genes underlying adaptation and speciation in the system.

Description

We provide the first annotated reference transcriptome of P. mexicana. Our transcriptome shows high congruence with other published fish transcriptomes, including that of the guppy, medaka, zebrafish, and stickleback. Transcriptome annotation uncovered the presence of candidate genes relevant in the study of adaptation to extreme environments. We describe general and oxidative stress response genes as well as genes involved in pathways induced by hypoxia or involved in sulfide metabolism. To facilitate future comparative analyses, we also conducted quantitative comparisons between P. mexicana from different river drainages. 106,524 single nucleotide polymorphisms were detected in our dataset, including potential markers that are putatively fixed across drainages. Furthermore, specimens from different drainages exhibited some consistent differences in gene regulation.

Conclusions

Our study provides a valuable genomic resource to study the molecular underpinnings of adaptation to extreme environments in replicated sulfide spring and cave environments. In addition, this study adds to the increasing number of genomic resources in the family Poeciliidae, which are widely used in comparative analyses of behavior, ecology, evolution, and medical genetics.

Divergent evolution of male aggressive behaviour: another reproductive isolation barrier in extremophile poeciliid fishes?

Reproductive isolation among locally adapted populations may arise when immigrants from foreign habitats are selected against via natural or (inter-)sexual selection (female mate choice). We asked whether also intrasexual selection through male-male competition could promote reproductive isolation among populations of poeciliid fishes that are locally adapted to extreme environmental conditions [i.e., darkness in caves and/or toxic hydrogen sulphide (H(2)S)]. We found strongly reduced aggressiveness in extremophile P. oecilia mexicana, and darkness was the best predictor for the evolutionary reduction of aggressiveness, especially when combined with presence of H(2)S. We demonstrate that reduced aggression directly translates into migrant males being inferior when paired with males from non-sulphidic surface habitats. By contrast, the phylogenetically old sulphur endemic P. sulphuraria from another sulphide spring area showed no overall reduced aggressiveness, possibly indicating evolved mechanisms to better cope with H(2)S.

Speciation in caves: experimental evidence that permanent darkness promotes reproductive isolation

Divergent selection through biotic factors like predation or parasitism can promote reproductive isolation even in the absence of geographical barriers. On the other hand, evidence for a role of adaptation to abiotic factors during ecological speciation in animals is scant. In particular, the role played by perpetual darkness in establishing reproductive isolation in cave animals (troglobites) remains elusive. We focused on two reproductively isolated ecotypes (surface- and cave-dwelling) of the widespread livebearer Poecilia mexicana, and raised offspring of wild-caught females to sexual maturity in a 12-month common-garden experiment. Fish were reared in light or darkness combined with high- or low-food conditions. Females, but not males, of the surface ecotype suffered from almost complete reproductive failure in darkness, especially in the low-food treatment. Furthermore, surface fish suffered from a significantly higher rate of spontaneous, stress-related infection with bacterial columnaris disease. This experimental evidence for strong selection by permanent darkness on non-adapted surface-dwelling animals adds depth to our understanding of the selective forces establishing and maintaining reproductive isolation in cave faunas.

An indigenous religious ritual selects for resistance to a toxicant in a livebearing fish

Human-induced environmental change can affect the evolutionary trajectory of populations. In Mexico, indigenous Zoque people annually introduce barbasco, a fish toxicant, into the Cueva del Azufre to harvest fish during a religious ceremony. Here, we investigated tolerance to barbasco in fish from sites exposed and unexposed to the ritual. We found that barbasco tolerance increases with body size and differs between the sexes. Furthermore, fish from sites exposed to the ceremony had a significantly higher tolerance. Consequently, the annual ceremony may not only affect population structure and gene flow among habitat types, but the increased tolerance in exposed fish may indicate adaptation to human cultural practices in a natural population on a very small spatial scale.

Toxic hydrogen sulphide and dark caves: pronounced male life-history divergence among locally adapted Poecilia mexicana (Poeciliidae)

Chronic environmental stress is known to induce evolutionary change. Here, we assessed male life-history trait divergence in the neotropical fish Poecilia mexicana from a system that has been described to undergo incipient ecological speciation in adjacent, but reproductively isolated toxic/nontoxic and surface/cave habitats. Examining both field-caught and common garden-reared specimens, we investigated the extent of differentiation and plasticity of life-history strategies employed by male P. mexicana. We found strong site-specific life-history divergence in traits such as fat content, standard length and gonadosomatic index. The majority of site-specific life-history differences were also expressed under common garden-rearing conditions. We propose that apparent conservatism of male life histories is the result of other (genetically based) changes in physiology and behaviour between populations. Together with the results from previous studies, this is strong evidence for local adaptation as a result of ecologically based divergent selection.

Locally adapted fish populations maintain small-scale genetic differentiation despite perturbation by a catastrophic flood event

Background

Local adaptation to divergent environmental conditions can promote population genetic differentiation even in the absence of geographic barriers and hence, lead to speciation. Perturbations by catastrophic events, however, can distort such parapatric ecological speciation processes. Here, we asked whether an exceptionally strong flood led to homogenization of gene pools among locally adapted populations of the Atlantic molly (Poecilia mexicana, Poeciliidae) in the Cueva del Azufre system in southern Mexico, where two strong environmental selection factors (darkness within caves and/or presence of toxic H₂S in sulfidic springs) drive the diversification of P. mexicana. Nine nuclear microsatellites as well as heritable female life history traits (both as a proxy for quantitative genetics and for trait divergence) were used as markers to compare genetic differentiation, genetic diversity, and especially population mixing (immigration and emigration) before and after the flood.

Results

Habitat type (i.e., non-sulfidic surface, sulfidic surface, or sulfidic cave), but not geographic distance was the major predictor of genetic differentiation. Before and after the flood, each habitat type harbored a genetically distinct population. Only a weak signal of individual dislocation among ecologically divergent habitat types was uncovered (with the exception of slightly increased dislocation from the Cueva del Azufre into the sulfidic creek, El Azufre). By contrast, several lines of evidence are indicative of increased flood-induced dislocation within the same habitat type, e.g., between different cave chambers of the Cueva del Azufre.

Conclusions

The virtual absence of individual dislocation among ecologically different habitat types indicates strong natural selection against migrants. Thus, our current study exemplifies that ecological speciation in this and other systems, in which extreme environmental factors drive speciation, may be little affected by temporary perturbations, as adaptations to physico-chemical stressors may directly affect the survival probability in divergent habitat types.

Complementary effect of natural and sexual selection against immigrants maintains differentiation between locally adapted fish

Adaptation to ecologically heterogeneous environments can drive speciation. But what mechanisms maintain reproductive isolation among locally adapted populations? Using poeciliid fishes in a system with naturally occurring toxic hydrogen sulfide, we show that (a) fish from non-sulfidic sites (Poecilia mexicana) show high mortality (95 %) after 24 h when exposed to the toxicant, while locally adapted fish from sulfidic sites (Poecilia sulphuraria) experience low mortality (13 %) when transferred to non-sulfidic water. (b) Mate choice tests revealed that P. mexicana females exhibit a preference for conspecific males in non-sulfidic water, but not in sulfidic water, whereas P. sulphuraria females never showed a preference. Increased costs of mate choice in sulfidic, hypoxic water, and the lack of selection for reinforcement due to the low survival of P. mexicana may explain the absence of a preference in P. sulphuraria females. Taken together, our study may be the first to demonstrate independent-but complementary-effects of natural and sexual selection against immigrants maintaining differentiation between locally adapted fish populations.

Natural and sexual selection against immigrants maintains differentiation among micro-allopatric populations

Local adaptation to divergent environmental conditions can promote population genetic differentiation even in the absence of geographic barriers and hence lead to speciation. But what mechanisms contribute to reproductive isolation among diverging populations? We tested for natural and sexual selection against immigrants in a fish species inhabiting (and adapting to) nonsulphidic surface habitats, sulphidic surface habitats and a sulphidic cave. Gene flow is strong among sample sites situated within the same habitat type, but low among divergent habitat types. Our results indicate that females of both sulphidic populations discriminate against immigrant males during mate choice. Furthermore, using reciprocal translocation experiments, we document natural selection against migrants between nonsulphidic and sulphidic habitats, whereas migrants between sulphidic cave and surface habitats did not exhibit increased mortality within the same time period. Consequently, both natural and sexual selection may contribute to isolation among parapatric populations, and selection against immigrants may be a powerful mechanism facilitating speciation among locally adapted populations even over very small spatial distances.