Adaptive evolution of sperm bindin tracks egg incompatibility in neotropical sea urchins of the genus Echinometra

Bindin is a gamete recognition protein known to control species-specificsperm-egg adhesion and membrane fusion in sea urchins. Previousanalyses have shown that diversifying selection on bindin aminoacid sequence is found when gametically incompatible speciesare compared, but not when species are compatible. The presentstudy analyzes bindin polymorphism and divergence in the threeclosely related species of Echinometra in Central America: E.lucunter and E. viridis from the Caribbean, and E. vanbruntifrom the eastern Pacific. The eggs of E. lucunter have evolveda strong block to fertilization by sperm of its neotropicalcongeners, whereas those of the other two species have not.As in the Indo-West Pacific (IWP) Echinometra, the neotropicalspecies show high intraspecific bindin polymorphism in the samegene regions as in the IWP species. Maximum likelihood analysisshows that many of the polymorphic codon sites are under mildpositive selection. Of the fixed amino acid replacements, mosthave accumulated along the bindin lineage of E. lucunter. Weanalyzed the data with maximum likelihood models of variationin positive selection across lineages and codon sites, and withmodels that consider sites and lineages simultaneously. Ourresults show that positive selection is concentrated along theE. lucunter bindin lineage, and that codon sites with aminoacid replacements fixed in this species show by far the highestsignal of positive selection. Lineage-specific positive selectionparalleling egg incompatibility provides support that adaptiveevolution of sperm proteins acts to maintain recognition ofbindin by changing egg receptors. Because both egg incompatibilityand bindin divergence are greater between allopatric speciesthan between sympatric species, the hypothesis of selectionagainst hybridization (reinforcement) cannot explain why adaptiveevolution has been confined to a single lineage in the AmericanEchinometra. Instead, processes acting to varying degrees withinspecies (e.g., sperm competition, sexual selection, and sexualconflict) are more promising explanations for lineage-specificpositive selection on bindin.     

Physical forces experienced by echinoid eggs in the oviduct during spawning: comparison of the geminate pair Echinometra vanbrunti and Echinometra lucunter

During spawning, eggs of echinoids are released from the gonad and traverse a narrow oviduct-gonopore complex before being released into the water surrounding the spawning female. As the eggs traverse the oviduct-gonopore complex, a velocity gradient develops within the fluid formed by the eggs. This velocity gradient imposes a shear stress on the eggs that ranges from a maximal value at the walls of the oviduct-gonopore complex and declines linearly to zero at the center of the complex. In addition to shear stress, the eggs of some echinoids also experience strain. Strain is imposed on the eggs in situations where the diameter of the egg is greater than the diameter of the oviduct-gonopore complex. Both shear stress and strain have the potential to damage echinoid eggs during spawning and are likely selective pressures acting on the physical properties of protective extracellular layers, which surround the eggs of these organisms. The amounts of shear stress and strain experienced by eggs of a geminate pair of echinoids, Echinometra vanbrunti and E. lucunter, are calculated. Calculations of shear stress based on the mean spawning rates and dimensions of the oviduct-gonopore complexes of specimens collected during this study ranged up to 30 Pa for E. vanbrunti and up to 17 Pa for E. lucunter. To obtain conservative estimates of shear stress, these calculations were repeated incorporating a spawning rate that was reduced by one order of magnitude below the mean rate measured for both species. These calculations yielded shear stresses that ranged up to 13 and 6 Pa for E. vanbrunti and E. lucunter, respectively. The diameter of the oviduct-gonopore complex of echinoids increases linearly with the diameter of the tests of the adults. Because the diameter of the oviduct-gonopore complex influences the amount of shear stress experienced by eggs of echinoids, shear stress calculations were repeated incorporating the full range of potential diameters of the oviduct-gonopore complexes from data in the literature. Using the mean spawning rates obtained from each species, the amount of shear stress experienced by eggs ranged up to 32 Pa for E. vanbrunti and up to 398 Pa for E. lucunter. When the mean spawning rates were reduced by one order of magnitude, the amount of shear stress experienced by E. vanbrunti eggs ranged up to 14 Pa and up to 140 Pa for the eggs of E. lucunter. The eggs of E. vanbrunti do not experience strain during spawning because the diameter of the its eggs is smaller than the diameter of its oviduct-gonopore complex at all stages of adult growth. In contrast, the diameter of the eggs of E. lucunter is larger than the diameter of the oviduct-gonopore complex at small adult sizes and the eggs of this species experience true strains ranging up to 1.2. These results indicate that there are substantial differences in the amounts of physical forces that the eggs of echinoids experience during spawning both within species at different stages of adult growth and between species. Different amounts of shear stress and strain experienced by eggs among echinoid species may select for differences in the mechanical properties of protective extracellular layers that surround them.     

Reproductive periodicity of the echinoids Diadema and Echinometra on the two coasts of Panama

Patterns of reproductive periodicity in the regular echinoid species Diadema mexicanum A. Agassiz and Echinometra vanbrunti A. Agassiz from the Bay of Panama, and Diadema antillarum Phillipi, Echinometra lucunter (Linnaeus) and E. viridis A. Agassiz from the Caribbean coast of the Isthmus of Panama exhibit dissimilarities that reflect the differences of the environments they inhabit. Populations of the two species from the seasonal Bay of Panama display synchronous, well-defined, reproductive cycles. Spawning appears to be timed so that newly metamorphosed sea urchins, rather than larvae, can benefit from the increased food production concomitant with dry season upwelling. On the less seasonal Caribbean shore reproductive periodicity is less defined. Populations of Diadema antillarum and Echinometra lucunter from the vicinity of the Panama Canal show indications of periodicity, while those from a locality 20 km to the east display little tendency for synchrony between individuals. The much rarer E. viridis, on the other hand, maintains well-defined, population-wide cycles in both localities. It is suggested that in a constant environment the intensity of selection for synchrony between individual gametogenic cycles may be inversely proportional to population density.     

Multiple gene genealogies reveal asymmetrical hybridization and introgression among strongylocentrotid sea urchins

The evolution of incompatibilities between eggs and sperm is thought to play important roles in establishing and maintaining reproductive isolation among species of broadcast-spawning marine invertebrates. However, the effectiveness of gametic isolation in initiating the speciation process and/or in limiting the introgression of genes among species at later stages of divergence remains largely unknown. In the present study, we collected DNA sequence data from five loci in four species of Strongylocentrotus sea urchins ( S. droebachiensis , S. pallidus , S. purpuratus , and S. franciscanus ) to test whether the susceptibility of S. droebachiensis eggs to fertilization by heterospecific sperm results in gene flow between species. Despite the potential for introgression, a small but statistically significant signal of introgression was observed only between the youngest pair of sister taxa ( S. pallidus and S. droebachiensis ) that was strongly asymmetrical (from the former into the latter). No significant gene flow was observed for either S. purpuratus or S. franciscanus despite the ability of their sperm to readily fertilize the eggs of S. droebachiensis . Our results demonstrate that asymmetrical gamete compatibilities in strongylocentrotids can give rise to asymmetrical patterns of introgression but suggest that gamete traits alone cannot be responsible for maintaining species integrities. The genetic boundaries between strongylocentrotid urchin species in the northeast Pacific appear to be related to postzygotic isolating mechanisms that scale with divergence times and not intrinsic gametic incompatibilities per se .     

FUNCTIONAL INCOMPATIBILITY BETWEEN THE FERTILIZATION SYSTEMS OF TWO ALLOPATRIC POPULATIONS OF CALLOSOBRUCHUS MACULATUS (COLEOPTERA: BRUCHIDAE)

Abstract Recent studies indicate that postcopulatory sexual selection may represent an important component of the speciation process by initiating reproductive isolation via the evolutionary divergence of fertilization systems. Using two geographically isolated populations of the polyandrous beetle Callosobruchus maculatus, we investigated divergence in fertilization systems by determining the extent of postcopulatory functional incompatibility. Through reciprocal, cross‐population matings we were able to separately estimate the effects of male and female population origin and their interaction on the extent of last‐male sperm precedence, female receptivity to further copulation and female oviposition. Our results indicate partial incompatibility between the fertilization systems of the two populations at all three functional levels. Males derived from the same population as females outcompete rival, allopatric males with respect to sperm preemption, sperm protection, and ability to stimulate female oviposition. This pattern is reciprocated in both populations indicating that postcopulatory, prezygotic events represent important mechanisms by which between‐population gene flow is reduced. We suggest the partial gametic isolation observed is a by‐product of the coevolution of male and female fertilization systems by a process of cryptic female choice. Our results are consistent with a mechanism akin to conventional mate choice models although they do not allow us to reject antagonistic sexual coevolution as the mechanism of cryptic female choice.     

CRYPTIC CHOICE OF CONSPECIFIC SPERM CONTROLLED BY THE IMPACT OF OVARIAN FLUID ON SPERM SWIMMING BEHAVIOR

Despite evidence that variation in male–female reproductive compatibility exists in many fertilization systems, identifying mechanisms of cryptic female choice at the gamete level has been a challenge. Here, under risks of genetic incompatibility through hybridization, we show how salmon and trout eggs promote fertilization by conspecific sperm. Using in vitro fertilization experiments that replicate the gametic microenvironment, we find complete interfertility between both species. However, if either species’ ova were presented with equivalent numbers of both sperm types, conspecific sperm gained fertilization precedence. Surprisingly, the species’ identity of the eggs did not explain this cryptic female choice, which instead was primarily controlled by conspecific ovarian fluid, a semiviscous, protein‐rich solution that bathes the eggs and is released at spawning. Video analyses revealed that ovarian fluid doubled sperm motile life span and straightened swimming trajectory, behaviors allowing chemoattraction up a concentration gradient. To confirm chemoattraction, cell migration tests through membranes containing pores that approximated to the egg micropyle showed that conspecific ovarian fluid attracted many more spermatozoa through the membrane, compared with heterospecific fluid or water. These combined findings together identify how cryptic female choice can evolve at the gamete level and promote reproductive isolation, mediated by a specific chemoattractive influence of ovarian fluid on sperm swimming behavior.     

Conspecific Sperm Precedence Is a Reproductive Barrier between Free-Spawning Marine Mussels in the Northwest Atlantic Mytilus Hybrid Zone

Reproductive isolation at the gamete stage has become a focus of speciation research because of its potential to evolve rapidly between closely related species. Conspecific sperm precedence (CSP), a type of gametic isolation, has been demonstrated in a number of taxa, both marine and terrestrial, with the potential to play an important role in speciation. Free-spawning marine invertebrates are ideal subjects for the study of CSP because of a likely central role for gametic barriers in reproductive isolation. The western Atlantic Mytilus blue mussel hybrid zone, ranging from the Atlantic Canada to eastern Maine, exhibits characteristics conducive to the study of CSP. Previous studies have shown that gametic incompatibility is incomplete, variable in strength and the genotype distribution is bimodal—dominated by the parental species, with a low frequency of hybrids. We conducted gamete crossing experiments using M. trossulus and M. edulis individuals collected from natural populations during the spring spawning season in order to detect the presence or absence of CSP within this hybrid zone. We detected CSP, defined here as a reduction in heterospecific offspring from competitive fertilizations in vitro compared to that seen in non-competitive fertilizations, in five of the twelve crosses in which conspecific crosses were detectable. This is the first finding of CSP in a naturally hybridizing population of a free-spawning marine invertebrate. Our findings support earlier predictions that CSP can promote assortative fertilization in bimodal hybrid zones, further advancing their hypothesized progression towards full speciation. Despite strong CSP numerous heterospecific fertilizations remain, reinforcing the hypothesis that compatible females are a source of hybrid offspring in mixed natural spawns.     

Strong reproductive isolation between closely related tropical sea urchins (genus Echinometra)

Morphological, mitochondrial DNA, and single-copy nuclear DNA differences show that the tropical sea urchin Echinometra mathaei is composed of at least four independent gene pools. Evolutionary distance between species measured with restriction-site changes (for mitochondrial DNA) and thermal renaturation (for single-copy nuclear DNA) is 1%-3% nucleotide divergence. Thus these are the most closely related sea urchin species known. Despite this genetic similarity, strong blocks to interspecific fertilization exist in this genus. Between two Hawaiian species, few eggs are fertilized in hybrid crosses, even in the presence of excess sperm. Microscopic examination of such crosses shows that sperm attachment to heterologous eggs is inhibited. Measures of genetic distance between species can help reveal the tempo of speciation and allow comparisons of morphological, biochemical, and ecological characteristics to be made in an evolutionary framework. Our results show that strong reproductive isolation can evolve by changes in egg-sperm recognition without extensive genetic divergence between species. Such mechanisms are most easily studied in free-spawning animals such as sea urchins but as well may represent an important aspect of speciation in species with internal fertilization.     

Evolutionary patterns of two major reproduction candidate genes (Zp2 and Zp3) reveal no contribution to reproductive isolation between bovine species

Background  

It has been established that mammalian egg zona pellucida (ZP) glycoproteins are responsible for species-restricted binding of sperm to unfertilized eggs, inducing the sperm acrosome reaction, and preventing polyspermy. In mammals, ZP apparently represents a barrier to heterospecific fertilization and thus probably contributes to reproductive isolation between species. The evolutionary relationships between some members of the tribe Bovini are complex and highly debatable, particularly, those involving Bos and Bison species for which interspecific hybridization is extensively documented. Because reproductive isolation is known to be a major precursor of species divergence, testing evolutionary patterns of ZP glycoproteins may shed some light into the speciation process of these species. To this end, we have examined intraspecific and interspecific genetic variation of two ZP genes (Zp2 and Zp3) for seven representative species (111 individuals) from the Bovini tribe, including five species from Bos and Bison, and two species each from genera Bubalus and Syncerus.     

HYBRIDIZATION OF SYMPATRIC PATIRIELLA SPECIES (ECHINODERMATA: ASTEROIDEA) IN NEW SOUTH WALES

Three species of the asteroid genus Patiriella occur sympatrically in New South Wales and the possibility for hybridization among them was examined through a series of cross-fertilization experiments. Patiriella calcar and P. gunnii are morphologically distinct as adults but indistinguishable as larvae. Patiriella exigua is morphologically distinct in both its adult and larval morphologies. The gametes of P. calcar and P. gunnii were reciprocally compatible: laboratory crosses between these species produced viable hybrid juveniles. In crosses between female P. calcar and male P. gunnii, most of the juveniles metamorphosed with an arm number intermediate between that of the parents, whereas crosses between female P. gunnii and male P. calcar produced juveniles with an arm number more similar to the maternal phenotype. Heterospecific crosses with P. exigua resulted in low fertilization rates, and viable hybrids were not produced. This species appears capable of self-fertilization. Because hybrids between P. calcar and P. gunnii were viable, neither gametic incompatibility nor hybrid inviability appears to ensure reproductive isolation between these species. Ecological or habitat segregation and temporal separation in breeding may isolate these species in the field. The results demonstrate that if gamete surface recognition molecules are involved in fertilization of P. calcar and P. gunnii, then they are not strongly species specific, at least at the sperm concentrations used in this study. Reproductive isolation between these species has evolved despite their gametic compatibility. In contrast, P. exigua is isolated from its congeners because of gametic incompatibility and several features characteristic of its reproduction and development. The implications of these findings for reproductive isolation and speciation of Patiriella and for the evolution of reproductive isolation in free-spawning marine organisms are discussed.     

Analysis of Early Embryogenesis in Rainbow and Banded Darters (Percidae: Etheostoma) Reveals Asymmetric Postmating Barrier

This study examines early embryogenesis in two species of darters, Etheostoma caeruleum and E. zonale (Teleostei: Percidae), and their hybrids. Results document the course of ontogeny from fertilization until the onset of pigmented eyes. Comparing fertilization and developmental success of conspecific versus heterospecific crosses revealed an asymmetric postmating reproductive barrier. E. caeruleum eggs treated with E. zonale sperm exhibited fertilization and developmental success similar to that of conspecific crosses. In contrast, E. zonale eggs treated with E. caeruleum sperm exhibited reduced fertilization relative to conspecific crosses and abnormal development. Development in this latter cross was compromised at all stages, but appeared to be concentrated around epiboly, or cell migration. As epiboly represents the stage of ontogeny when zygotic genes of both species jointly contribute to embryogenesis, results provide insight into the genetic mechanisms underlying postmating barriers in Etheostoma. Finally, the observed asymmetry in fertilization success is consistent with predictions based on the behavioral ecology of these species. Etheostoma zonale spawn in the open water column, whereas E.␣caeruleum bury their eggs under the substrate. The observed fertilization barrier may have therefore resulted from selection favoring increased fertilization specificity in E. zonale.     

A MULTILOCUS TEST OF SIMULTANEOUS DIVERGENCE ACROSS THE ISTHMUS OF PANAMA USING SNAPPING SHRIMP IN THE GENUS ALPHEUS

The completion of the Panamanian Isthmus is one of the greatest natural experiments in evolution, sending multiple species pairs from a broad range of taxonomic groups on independent evolutionary trajectories. The resulting transisthmian sister species have been used as model systems for examining consequences that accompany cessation of gene flow in formerly panmictic populations. However, variance in pairwise genetic distances of these "geminates" often exceeds expectations, seemingly conflicting with the assumption that separation of populations was contemporaneous with the final closure of the Isthmus. Multilocus datasets and coalescent-based analytical methods can be used to estimate divergence times while accounting for variance in gene divergence that predates isolation, thus removing the need to invoke unequal divergence times. Here we present results from Bayesian analyses of sequence data from seven nuclear and one mitochondrial marker in eight transisthmian species pairs in the snapping shrimp genus Alpheus . Divergence times in two species pairs were shown to occur much earlier than the Isthmus final closure, but much of the variance in pairwise genetic distances from cytochrome oxidase I (COI) was explained when ancestral polymorphisms were accounted for. Results illustrate how coalescent approaches may be more appropriate for dating recent divergences than for estimating ancient speciation events.     

SPERM COMPETITION PROMOTES ASYMMETRIES IN REPRODUCTIVE BARRIERS BETWEEN CLOSELY RELATED SPECIES

Reproductive barriers between closely related species are often incomplete and asymmetric, but the evolutionary significance of these well-known phenomena remains unsolved. We test the hypothesis that the degree of gametic incompatibility in reciprocal crosses is associated to levels of sperm competition because this selective force favors both increased sperm competitiveness and ovum defensiveness. Using three species of Mus with high, intermediate, and low levels of sperm competition, we examined fertilization rates in competitive and noncompetitive contexts. We found that the influence of sperm competition upon sperm competitiveness is as strong as it is upon ovum defensiveness, revealing an effect upon female gametes so far overlooked. As a result, fertilization success was strongly related to differences in sperm competition levels between species providing sperm and ova, thus generating major asymmetries in reciprocal crosses. When placed in competition, conspecific sperm maintained levels of fertilization success similar to those found in noncompetitive contexts, at the expense of the success of heterospecific sperm. When only heterospecific sperm competed, species with highest levels of sperm competition outcompeted others and asymmetries were exacerbated. We conclude that sperm competition explains both the degree of gametic isolation and the degree of asymmetries between closely related species.     

Sperm divergence in a passerine contact zone: Indication of reinforcement at the gametic level

Postcopulatory sexual selection may promote evolutionary diversification in sperm form, but the contribution of between‐species divergence in sperm morphology to the origin of reproductive isolation and speciation remains little understood. To assess the possible role of sperm diversification in reproductive isolation, we studied sperm morphology in two closely related bird species, the common nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia), that hybridize in a secondary contact zone spanning Central and Eastern Europe. We found: (1) striking divergence between the species in total sperm length, accompanied by a difference in the length of the mitochondrial sperm component; (2) greater divergence between species in sperm morphology in sympatry than in allopatry, with evidence for character displacement in sperm head length detected in L. megarhynchos; (3) interspecific hybrids showing sperm with a length intermediate between the parental species, but no evidence for decreased sperm quality (the proportion of abnormal spermatozoa in ejaculates). Our results demonstrate that divergence in sperm morphology between the two nightingale species does not result in intrinsic postzygotic isolation, but may contribute to postcopulatory prezygotic isolation. This isolation could be strengthened in sympatry by reinforcement.     

TROPICS ACCELERATE THE EVOLUTION OF HYBRID MALE STERILITY IN DROSOPHILA

Understanding the evolutionary mechanisms that facilitate speciation and explain global patterns of species diversity has remained a challenge for decades. The most general pattern of species biodiversity is the latitudinal gradient, whereby species richness increases toward the tropics. Although such a global pattern probably has a multitude of causes, recent attention has focused on the hypothesis that speciation and the evolution of reproductive isolation occur faster in the tropics. Here, I tested this prediction using a dataset on premating and postzygotic isolation between recently diverged Drosophila species. Results showed that while the evolution of premating isolation was not greater between tropical Drosophila relative to nontropical species, postzygotic isolation evolved faster in the tropics. In particular, hybrid male sterility was much greater among tropical Drosophila compared to nontropical species pairs of similar genetic age. Several testable explanations for the novel pattern are discussed, including greater role for sterility‐inducing bacterial endosymbionts in the tropics and more intense sperm–sperm competition or sperm–egg sexual conflict in the tropics. The results imply that processes of speciation in the tropics may evolve at different rates or may even be somewhat different from those at higher latitudes.     

Quantitative analysis of gametic incompatibility between closely related species of neotropical sea urchins

Species of the sea urchin genus Echinometra found on the two coasts of Panamá are recently diverged and only partially isolated by incomplete barriers to interspecific fertilization. This study confirms previous work that revealed incompatibility between the eggs of the Atlantic E. lucunter and the sperm of the other two neotropical species, whereas eggs of its sympatric congener E. viridis and allopatric E. vanbrunti are largely compatible with heterospecific sperm. Here we quantify fertilization using a range of sperm dilutions. We demonstrate a much stronger block to cross-species fertilization of E. lucunter eggs than was previously shown at fixed sperm concentrations, and mild incompatibility of the other two species' eggs where previous crosses between species were not distinguishable from within-species controls. Additionally, we present evidence for intraspecific variation in egg receptivity towards heterospecific sperm. Our findings here again discount the "reinforcement model" as a viable explanation for the pattern of prezygotic isolation. Gamete incompatibility in these Echinometra has appeared recently-within the last 1.5 million years-but is weaker in sympatry than in allopatry. Accidents of history may help explain why incompatibility of eggs emerged in one species and not in others. Compensatory sexual selection on sperm in this species could follow, and promote divergence of proteins mediating sperm-egg recognition.     

PHYLOGENETIC ANALYSIS OF THE GENUS EUGLENA (EUGLENOPHYCEAE) WITH PARTICULAR REFERENCE TO THE TYPE SPECIES EUGLENA VIRIDIS1

Euglena viridis (subgenus Euglena) serves as the type species for the genus Euglena. In this study, molecular phylogenetic analyses using a small subunit (SSU) and a combined SSU–partial large subunit rDNA data set for members of the genus Euglena showed that strains identified as E. viridis on the basis of morphology are distributed between two separate nonsister clades. Although all the E. viridis strains examined were morphologically indistinguishable and possessed spherical mucocysts and stellate chloroplasts with one paramylon center, there was a high degree of sequence divergence between the E. viridis strains in different clades, making this a cryptic species. Like E. viridis, all taxa from the subgenus Euglena are characterized by having one or more stellate chloroplasts with paramylon grains clustered around the center of the chloroplast. These additional taxa were divided into four clades in all the molecular analyses. Strains of Euglena stellata formed two nonsister clades whose members had a single aggregate chloroplast with paramylon center and spindle‐shaped mucocysts. A geniculata clade included species with one or two stellate chloroplasts with paramylon centers and spherical mucocysts, and the cantabrica clade had members with one stellate chloroplast with paramylon center and spherical mucocysts often arranged in spiral rows. Interspersed among these were three additional clades bearing taxa from the subgenus Calliglena that contains members with discoid plastids and pyrenoids that may or may not be capped with paramylon. These taxa formed a laciniata clade, mutabilis clade, and gracilis clade. This study demonstrates that E. viridis and E. stellata are cryptic species that can only be distinguished at the molecular level. Because E. viridis is the designated type species for the genus Euglena, we designated an epitype for E. viridis.     

GENETIC AND MORPHOLOGICAL DIVERGENCE AMONG MORPHOTYPES OF THE ISOPOD EXCIROLANA ON THE TWO SIDES OF THE ISTHMUS OF PANAMA

Excirolana braziliensis is a dioecious marine isopod that lives in the high intertidal zone of sandy beaches on both sides of Central and South America. It possesses no larval stage and has only limited means of adult dispersal. Indirect estimates of gene flow have indicated that populations from each beach exchange less than one propagule per generation. Multivariate morphometrics have discovered three morphs of this species in Panama, two of them closely related and found on opposite sides of Central America (“C morph” in the Caribbean and “C′ morph” in the eastern Pacific), the third found predominantly in the eastern Pacific (“P morph”). Though the P and C′ morphs are seldom found on the same beach, they have overlapping latitudinal ranges in the eastern Pacific. A related species, Excirolana chamensis, has been described from the Pacific coast of Panama. Each beach contains populations that remain morphologically and genetically stable, but a single drastic change in both isozymes and morphology has been documented. We studied isozymes and multivariate morphology of 10 populations of E. braziliensis and of one population of E. chamensis. Our objective was to assess the degree of genetic and morphological variation, the correlation of divergence on these two levels of integration, the phylogenetic relationships between morphs, and the possible contributions of low vagility, low gene flow, and occasional extinction and recolonization to the genetic structuring of populations. Genetic distance between the P morph, on one hand, and the other two morphotypes of E. braziliensis, on the other, was as high as the distance between E. braziliensis and E. chamensis. Several lines of evidence agree that E. chamensis and the P morph had diverged from other morphs of E. braziliensis before the rise of the Panama Isthmus separated the C and C′ forms, and that the P morph constitutes a different species. A high degree of genetic differentiation also exists between populations of the same morph. On the isozyme level, every population can be differentiated from every other on the basis of at least one diagnostically different locus, regardless of geographical distance or morphological affiliation. Morphological and genetic distances between populations are highly correlated. However, despite the high degree of local variation, evolution of E. braziliensis as a whole has not been particularly rapid; divergence between the C and C′ morphs isolated for 3 million yr by the Isthmus of Panama is not high by the standard of within-morph differentiation or by comparison with other organisms similarly separated. Alleles that are common in one population may be absent from another of the same morph, yet they appear in a different morph in a separate ocean. The high degree of local differentiation, the exclusive occupation of a beach by one genotype with rare arrival of foreign individuals that cannot interbreed freely with the residents, the genetic stability of populations with infrequent complete replacement by another genetic population, and the sharing by morphs of polymorphisms that are not shared by local populations, all suggest a mode of evolution concentrated in rare episodes of extinction and recolonization, possibly coupled with exceptional events of gene flow that help preserve ancestral variability in both oceans.     

Speciation with gene flow: Evidence from a complex of alpine butterflies (Coenonympha,Satyridae)

Until complete reproductive isolation is achieved, the extent of differentiation between two diverging lineages is the result of a dynamic equilibrium between genetic isolation and mixing. This is especially true for hybrid taxa, for which the degree of isolation in regard to their parental species is decisive in their capacity to rise as a new and stable entity. In this work, we explored the past and current patterns of hybridization and divergence within a complex of closely related butterflies in the genus Coenonympha in which two alpine species, C. darwiniana and C. macromma, have been shown to result from hybridization between the also alpine C. gardetta and the lowland C. arcania. By testing alternative scenarios of divergence among species, we show that gene flow has been uninterrupted throughout the speciation process, although leading to different degrees of current genetic isolation between species in contact zones depending on the pair considered. Nonetheless, at broader geographic scale, analyses reveal a clear genetic differentiation between hybrid lineages and their parental species, pointing out to an advanced stage of the hybrid speciation process. Finally, the positive correlation observed between ecological divergence and genetic isolation among these butterflies suggests a potential role for ecological drivers during their speciation processes.     

Asymmetric conspecific sperm precedence in relation to spawning times in the Montastraea annularis species complex (Cnidaria: Scleractinia)

In broadcast spawners, prezygotic reproductive isolation depends on differences in the spatial and temporal patterns of gamete release and gametic incompatibility. Typically, gametic incompatibility is measured in no‐choice crosses, but conspecific sperm precedence (CSP) can prevent hybridization in gametes that are compatible in the absence of sperm competition. Broadcast spawning corals in the Montastraea annularis species complex spawn annually on the same few evenings. Montastraea franksi spawns an average of 110 min before M. annularis, with a minimum gap of approximately 40 min. Gametes are compatible in no‐choice heterospecific assays, but it is unknown whether eggs exhibit choice when in competition. Hybridization depends on either M. franksi eggs remaining unfertilized and in proximity to M. annularis when the latter species spawns or M. franksi sperm remaining in sufficient viable concentrations when M. annularis spawns. We found that the eggs of the early spawning M. franksi demonstrate strong CSP, whereas CSP appears to be lacking for M. annularis eggs. This study provides evidence of diverging gamete affinities between these recently separated species and suggests for the first time that selection may favour CSP in earlier spawning species when conspecific sperm is diluted and aged and is otherwise at a numeric and viability disadvantage with heterospecific sperm.