Field-cage experiments on host fidelity in a pair of sympatric phytophagous ladybird beetles

Using field cages, we tested the hypothesis that two phytophagous ladybird beetles, Henosepilachna niponica (Lewis) and Henosepilachna yasutomii Katakura (Coleoptera: Coccinellidae: Epilachninae), are reproductively isolated from each other by their host fidelity alone. We planted host plants of the two species [the thistle Cirsium alpicola Nakai (Asteraceae) for H. niponica and the blue cohosh, Caulophyllum robustum Maxim. (Berberidaceae), for H. yasutomii] in 7 × 7 m cages (BT cages), and planted the two host plants and the beetles’ common food plant, the Japanese nightshade, Solanum japonense Nakai (Solanaceae), in other cages (BJT cages). We then released overwintered reproducing beetles into these cages and recorded their dispersal, the distribution of egg masses, and the incidence of copulations. In BT cages, the beetles moved almost exclusively between their own host plants, and all females laid eggs exclusively on their own host plants. In BJT cages, the beetles moved between their own host plants and the Japanese nightshade, resulting in occasional interspecific mating on the same nightshade plant. Females of both species laid eggs on their own host plants as well as on the Japanese nightshade. These results indicate that host fidelity functions as a strong barrier against gene flow between H. niponica and H. yasutomii. However, this barrier can easily be broken down if a third plant species bridges the beetle populations.     

Genetic divergence with ongoing gene flow is maintained by the use of different hosts in phytophagous ladybird beetles genus Henosepilachna

Adaptation to different environments can promote population divergence via natural selection even in the presence of gene flow – a phenomenon that typically occurs during ecological speciation. To elucidate how natural selection promotes and maintains population divergence during speciation, we investigated the population genetic structure, degree of gene flow and heterogeneous genomic divergence in three closely related Japanese phytophagous ladybird beetles: Henosepilachna pustulosa, H. niponica and H. yasutomii. These species act as a generalist, a wild thistle (Cirsium spp.) specialist and a blue cohosh (Caulophyllum robustum) specialist, respectively, and their ranges differ accordingly. The two specialist species widely co‐occur but are reproductively isolated solely due to their high specialization to a particular host plant. Genomewide amplified fragment‐length polymorphism (AFLP) markers and mitochondrial cytochrome c oxidase subunit I (COI) gene sequences demonstrated obvious genomewide divergence associated with both geographic distance and ecological divergence. However, a hybridization assessment for both AFLP loci and the mitochondrial sequences revealed a certain degree of unidirectional gene flow between the two sympatric specialist species. Principal coordinates analysis (PCoA) based on all of the variable AFLP loci demonstrated that there are genetic similarities between populations from adjacent localities irrespective of the species (i.e. host range). However, a further comparative genome scan identified a few fractions of loci representing approximately 1% of all loci as different host‐associated outliers. These results suggest that these three species had a complex origin, which could be obscured by current gene flow, and that ecological divergence can be maintained with only a small fraction of the genome is related to different host use even when there is a certain degree of gene flow between sympatric species pairs.     

Differential acceptance of and survivorship on a non‐host plant between two geographically separate populations of the phytophagous ladybird beetle Henosepilachna yasutomii

Phytophagous specialists among insects occasionally accept plants they do not or only rarely utilize under natural conditions. This non‐host acceptance could represent an initial stage in host‐range evolution. Here, we examined adult acceptance of and larval survivorship on a non‐host plant – wild thistle, Cirsium kamtschaticum Ledeb. (Asteraceae) – for two geographically separate populations of the phytophagous ladybird beetle Henosepilachna yasutomii Katakura (Coleoptera: Coccinellidae), which normally utilizes blue cohosh, Caulophyllum robustum Maxim. (Berberidaceae), in the wild. The two beetle populations showed lower acceptance of and survivorship on the non‐host plant (thistle) than on the normal host plant (blue cohosh). Furthermore, they differed significantly in their responses to thistle but not to blue cohosh. Even for the beetle population that more readily accepted thistle as adults, acceptance was moderate and the eclosion rate was low, indicating reduced performance on the non‐host plant. Although studies have infrequently focused on non‐host acceptance, this is likely common in phytophagous specialists and thus is a potentially important evolutionary factor, as it may determine the future direction of food‐range evolution.     

Molecular population genetics of a host‐associated sibling species complex of phytophagous ladybird beetles (Coleoptera: Coccinellidae: Epilachninae)

To clarify evolutionary relationships and assess the intensity and persistence of reproductive isolation because of host fidelity in three closely related host‐associated phytophagous ladybird beetles (Henosepilachna pustulosa, Henosepilachna niponica and Henosepilachna yasutomii), we determined sequences of part of the nuclear DNA internal transcribed spacer‐II (ITS‐2) region (388 bp long) for 70 individuals from seven populations, and part of the mitochondrial cytochrome c oxidase subunit I (COI) gene (985 bp long) for 280 individuals from 28 populations across the three species. We failed to detect any polymorphic sites in the ITS‐2 region. We detected 59 COI haplotypes, among which two were shared by different species. While a haplotype network and a phylogenetic gene tree of the COI haplotypes did not support monophyly for any of the three species, the results of AMOVA did not invalidate the classification of these beetles into three species. The isolation‐with‐migration analytic (IMa) test suggested that gene flow between allopatric species (i.e., H. pustulosa versus H. niponica; H. pustulosa versus H. yasutomii) was well prevented, probably because of their limited dispersal power. On the other hand, the IMa test detected a low level of unidirectional gene flow between the sympatric species H. niponica and H. yasutomii, from the former to the latter. This result was consistent with a survival rate of F1 hybrids between the two species that is higher on the host plant of H. yasutomii than on the host plant of H. niponica. High F_ST values among two sympatric and three nearly sympatric population pairs of H. niponica and H. yasutomii, however, indicated that gene flow between the two species has been quite restricted even if it has occurred. Because no reproductive barriers other than the difference in host plants is known for sympatric H. niponica and H. yasutomii, our results suggest that host differentiation and associated ecological divergence function as an effective reproductive barrier between the two species.     

REPRODUCTIVE ISOLATION BY HOST SPECIFICITY IN A PAIR OF PHYTOPHAGOUS LADYBIRD BEETLES

Crossing experiments and food-choice tests show that two sympatric species of phytophagous ladybird beetles, Epilachna niponica and E. yasutomii, are reproductively isolated by host-plant specificity. Adult beetles selected their natural hosts when given choices, though some accepted the host of the other species when no choice was offered. In each species, survival of larvae to the second instar was significantly higher on their own host plant. No evidence for sexual isolation, gametic isolation, hybrid inviability, or reduced hybrid fertility was detected. Reproductive isolation by host specificity is an important prerequisite for certain models of sympatric speciation. Although the present example supports the plausibility of such models, an allopatric origin of host-plant specificity cannot be discounted.     

Local variation in the thistle species Cirsium grayanum affects its utilization by the herbivorous ladybird beetle Henosepilachna niponica

Host plants are the most critical environmental factor for phytophagous arthropods. Adaptation to a novel host will alter the distributional range of an herbivore to include the area the novel host covers, and might promote divergence between populations utilizing the old and new hosts. On the Oshima Peninsula, Hokkaido Island, northern Japan, the ladybird beetle Henosepilachna niponica (Lewis) (Coleoptera: Coccinellidae) usually occurs exclusively on the thistle Cirsium alpicola Nakai (Asteraceae), which is distributed in the southernmost part of the island, although some other Cirsium species often grow in sympatry. At least at one site (Assabu), however, H. niponica depends on another thistle, Cirsium grayanum (Maxim.) Nakai, the most abundant and widely distributed thistle species on the Oshima Peninsula. We examined adult feeding acceptance and preference, and larval performance, in two populations of H. niponica that utilize different species of thistles (the Assabu population occurs on C. grayanum; the Shiriuchi population, on C. alpicola) by testing three kinds of thistles (C. grayanum from Assabu, C. grayanum from Shiriuchi, and C. alpicola from Shiriuchi). Results strongly suggested that among the three plants tested, C. grayanum from Assabu was the most suitable host for H. niponica. We concluded that local differences in C. grayanum mainly explain why it is not used at Shiriuchi, but that there are also local differences in host use among the beetle populations, suggesting some degree of local adaptation. This suggests that different selective regimes associated with different host plants might promote divergence among beetle populations. We conclude that H. niponica is at present unlikely to expand its range to the whole range of C. grayanum.     

Preference-performance linkage in a herbivorous lady beetle: consequences of variability of natural enemies

We investigated the relationship between oviposition preference and offspring performance in a herbivorous lady beetle Epilachna pustulosa on two co-occurring plant species, thistle Cirsium kamtschaticum and blue cohosh Caulophyllum robustum, in 1994 and 1995. The relative importance of bottom-up effects by host plants and top-down effects by natural enemies on offspring performance were determined using field and laboratory experiments. In both years, egg density on blue cohosh was significantly higher than on thistle. A laboratory experiment demonstrated that larval survival from hatching to adult emergence was significantly higher, and developmental period shorter when larvae were reared on blue cohosh compared to thistle. The positive preference-performance linkage varied between years in the field. Top-down effects had a different impact on larval survival on the two host plant species. Arthropod predators, a lady beetle Harmonia axyridis and an earwig Forficula mikado, considerably depressed immature survival on thistle, while they were negligible on blue cohosh. Although the lack of effective predation increased larval survival on blue cohosh, it led to defoliation due to increased larval feeding late in the season. Because of severe intraspecific competition, old larvae had significantly lower survival on blue cohosh than on thistle. In 1994, as larval survival decreased due to defoliation on blue cohosh, the overall survival rate was significantly higher on thistle than on blue cohosh. This survival pattern was opposite to that found in the laboratory experiment. In contrast, in 1995, the increase in predatory lady beetles on thistle caused greater larval mortality. Thus, the overall survival was significantly lower on thistle than on blue cohosh, although severe intraspecific competition occurred on blue cohosh as it had in 1994. Consequently, the offspring performance on the two host plants is largely determined by the relative importance of arthropod predation determining larval survival on thistle and host plant defoliation reducing late larval survival on blue cohosh. These results indicate the important role of spatial and temporal variability of natural enemies on the preference-performance linkage of herbivorous insects. Received: 19 August 1998 / Accepted: 11 January 1999     

Evolution of postzygotic reproductive isolation in galliform birds: analysis of first and second hybrid generations and backcrosses

The process of speciation is a crucial aspect of evolutionary biology. In this study, we analysed the patterns of evolution of postzygotic reproductive isolation in Galliformes using information on hybridization and genetic distance among species. Four main patterns arose: (1) hybrid inviability and sterility in F₁ hybrids increase as species diverge; (2) the presence of geographical overlap does not affect the evolution of postzygotic isolation; (3) the galliforms follow Haldane's rule; (4) hybrid inviability is higher in F₂ than in F₁ hybrids, but does not appear to be increased in the backcrosses. This study contributes to the growing evidence suggesting that the patterns of evolution of postzygotic isolation and the process of speciation are shared among avian groups (and animals in general). In particular, our results support the notion of F₂ hybrid inviability as being key for the maintenance of species genetic integrity when prezygotic isolation barriers are overcome in closely related species, in which postzygotic isolation in the F₁ hybrid might still not be fully developed. To the contrary, hybrids from backcrosses did not show serious inviability problems (at least not more than F₁ hybrids), demonstrating that they could generate gene flow among bird species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 528–542.     

A three-locus system of interspecific incompatibility underlies male inviability in hybrids between Drosophila buzzatii and D. koepferae

In hybrids between the sibling species D. buzzatii and D. koepferae, both sexes are more or less equally viable in the F₁: However, backcross males to D. buzzatii are frequently inviable, apparently because of interspecific genetic incompatibilities that are cryptic in the F₁. We have performed a genetic dissection of the effects of the X chromosome from D. koepferae. We found only two cytological regions, termed hmi-1 and hmi-2, altogether representing 9% of the whole chromosome, which when introgressed into D. buzzatii cause inviability of hybrid males. Observation of the pattern of asynapsis of polytene chromosomes (incomplete pairing, marking introgressed material) in females and segregation analyses were the technique used to infer the X chromosome regions responsible for this hybrid male inviability. The comparison of these results with those previously obtained with the same technique for hybrid male sterility in this same species pair indicate that in the X chromosome of D. koepferae there are at least seven times more regions that produce hybrid male sterility than hybrid male inviability. We have also found that the inviability brought about by the introgression of hmi-1 is suppressed by the cointrogression of two autosomal sections from D. koepferae. Apparently, these three regions conform to a system of species-specific complementary factors involved in an X-autosome interaction that, when disrupted in backcross hybrids by recombination with the genome of its sibling D. buzzatii, brings about hybrid male inviability.     

PATTERNS OF SPECIATION IN DROSOPHILA

To investigate the time course of speciation, we gathered literature data on 119 pairs of closely related Drosophila species with known genetic distances, mating discrimination, strength of hybrid sterility and inviability, and geographic ranges. Because genetic distance is correlated with divergence time, these data provide a cross-section of taxa at different stages of speciation. Mating discrimination and the sterility or inviability of hybrids increase gradually with time. Hybrid sterility and inviability evolve at similar rates. Among allopatric species, mating discrimination and postzygotic isolation evolve at comparable rates, but among sympatric species strong mating discrimination appears well before severe sterility or inviability. This suggests that prezygotic reproductive isolation may be reinforced when allopatric taxa become sympatric. Analysis of the evolution of postzygotic isolation shows that recently diverged taxa usually produce sterile or inviable male but not female hybrids. Moreover, there is a large temporal gap between the evolution of male-limited and female hybrid sterility or inviability. This gap, which is predicted by recent theories about the genetics of speciation, explains the overwhelming preponderance of hybridizations yielding male-limited hybrid sterility or inviability (Haldane's rule).     

Speciation and reduced hybrid female fertility in house mice

In mammals, intrinsic postzygotic isolation has been well studied in males but has been less studied in females, despite the fact that female gametogenesis and pregnancy provide arenas for hybrid sterility or inviability that are absent in males. Here, we asked whether inviability or sterility is observed in female hybrids of Mus musculus domesticus and M. m. musculus, taxa which hybridize in nature and for which male sterility has been well characterized. We looked for parent‐of‐origin growth phenotypes by measuring adult body weights in F1 hybrids. We evaluated hybrid female fertility by crossing F1 females to a tester male and comparing multiple reproductive parameters between intrasubspecific controls and intersubspecific hybrids. Hybrid females showed no evidence of parent‐of‐origin overgrowth or undergrowth, providing no evidence for reduced viability. However, hybrid females had smaller litter sizes, reduced embryo survival, fewer ovulations, and fewer small follicles relative to controls. Significant variation in reproductive parameters was seen among different hybrid genotypes, suggesting that hybrid incompatibilities are polymorphic within subspecies. Differences in reproductive phenotypes in reciprocal genotypes were observed and are consistent with cyto‐nuclear incompatibilities or incompatibilities involving genomic imprinting. These findings highlight the potential importance of reduced hybrid female fertility in the early stages of speciation.     

Larval survival of the phytophagous ladybird,Epilachna yasutomii (Coleoptera,Coccinellidae), on the blue cohosh,Caulophyllum robustum (Ranunculales,Berberidaceae), grown under different environmental conditions

Larval survival of four local populations ofEpilachna yasutomii in the southern part of Nagano Prefecture was evaluated on leaves of blue cohosh,Caulophyllum robustum, growing at two different sites, Takatoh and Nakatachi. Based on larval survival on leaves of TakatohC. robustum, TakatohE. Yasutomii was clearly distinguishable from the three populations in the Minami-Shinano area (Akasawa, Nakatachi and Tenryu). TakatohE. yasutomii developed successfully to emergence onC. robustum from both sites, while Minami-Shinano populations showed a very low survival rate when given TakatohC. robustum. Under controlled plant growth conditions, larval survival of AkasawaE. yasutomii was significantly reduced when givenC. robustum leaves, from either site, exposed to direct solar radiation, which suggested that larval survival was influenced by environmental conditions at the growing sites rather than genetic differences between the plant populations. In contrast, TakatohE. yasutomii developed well onC. robustum leaves from either site exposed to direct solar radiation. TakatohE. yasutomii appears to have some defensive mechanism that allows it to overcome a substance deterring larval development, which increases in leaves exposed to direct solar radiation.     

A host shift from wild blue cohosh to cultivated potato by the phytophagous ladybird beetle, Epilachna yasutomii (Coleoptera, Coccinellidae)

Life history traits of the phytophagous ladybird beetle Epilachna yasutomii were compared between a nonpest population feeding on wild blue cohosh and a pest population feeding on cultivated solanaceous crops, mainly potato. Newly emerged adults of the nonpest population entered diapause early in midsummer when blue cohosh withered, while adults of the pest population were found in tomato and eggplant fields until late autumn. The pest population had larger females, a higher population growth rate, a shorter larval developmental period, and reduced longevity of overwintered females, compared with the nonpest population. ANOVA indicated that all these life history traits were influenced by the food plant, and that the number of eggs laid per female and the longevity of overwintered females were also affected by the population type. These findings suggest that the life history pattern of E. yasutomii changed to high fecundity with a short life span from low fecundity with a long life span as a result of the host shift from wild blue cohosh to cultivated solanaceous crops. Received: May 22, 1998 / Accepted: January 13, 1999     

HYBRIDIZATION STUDIES ON THE HOST RACES OF EUROSTA SOLIDAGINIS: IMPLICATIONS FOR SYMPATRIC SPECIATION

We studied the inheritance of survival ability in host-associated populations of the tephritid fly, Eurosta solidaginis, to test predictions of sympatric speciation models. Eurosta solidaginis induces galls on two species of goldenrod, Solidago altissima and S. gigantea. The host-associated populations have been hypothesized to be host races that originated in sympatry (Craig et al. 1993). We found evidence for disruptive selection for host use, which is a critical assumption of sympatric speciation models. Each host race had higher survival rates on their host plant than on the alternative host. F₁ and backcross hybrids also had lower survival rates than the pure host-race flies on their host plant. Since assortative mating occurs due to host-plant preference (Craig et al. 1993) this would select for divergence in host preference. Low hybrid survival could have been due to strong genetic incompatibilities of the populations or due to host adaptation by each population. Strong genetic incompatibilities would result in poor survival on all host plants, while host adaptation could result in low overall survival with high hybrid survival on some host plants with particularly “benign” environments. High survival of F₁, F₂, and backcross hybrids on some plant genotypes in some years supported the host adaptation hypothesis. F₁ flies mated and oviposited normally and produced viable F₂ and backcross hybrids indicating gene flow is possible between the host races. A few flies developed and emerged on the alternative host plant. This demonstrates that genes necessary to utilize the alternative host exist in both host races. This could have facilitated the origin of one of the populations via a host shift from the ancestral host. The inheritance of survival ability appears to be an autosomal trait. We did not find evidence that survival ability was maternally influenced or sex linked.     

A genomic approach to identify hybrid incompatibility genes

Uncovering the genetic and molecular basis of barriers to gene flow between populations is key to understanding how new species are born. Intrinsic postzygotic reproductive barriers such as hybrid sterility and hybrid inviability are caused by deleterious genetic interactions known as hybrid incompatibilities. The difficulty in identifying these hybrid incompatibility genes remains a rate-limiting step in our understanding of the molecular basis of speciation. We recently described how whole genome sequencing can be applied to identify hybrid incompatibility genes, even from genetically terminal hybrids. Using this approach, we discovered a new hybrid incompatibility gene, gfzf, between Drosophila melanogaster and Drosophila simulans, and found that it plays an essential role in cell cycle regulation. Here, we discuss the history of the hunt for incompatibility genes between these species, discuss the molecular roles of gfzf in cell cycle regulation, and explore how intragenomic conflict drives the evolution of fundamental cellular mechanisms that lead to the developmental arrest of hybrids.     

Parent‐of‐origin growth effects and the evolution of hybrid inviability in dwarf hamsters

Mammalian hybrids often show abnormal growth, indicating that developmental inviability may play an important role in mammalian speciation. Yet, it is unclear if this recurrent phenotype reflects a common genetic basis. Here, we describe extreme parent‐of‐origin‐dependent growth in hybrids from crosses between two species of dwarf hamsters, Phodopus campbelli and Phodopus sungorus. One cross type resulted in massive placental and embryonic overgrowth, severe developmental defects, and maternal death. Embryos from the reciprocal cross were viable and normal sized, but adult hybrid males were relatively small. These effects are strikingly similar to patterns from several other mammalian hybrids. Using comparative sequence data from dwarf hamsters and several other hybridizing mammals, we argue that extreme hybrid growth can contribute to reproductive isolation during the early stages of species divergence. Next, we tested if abnormal growth in hybrid hamsters was associated with disrupted genomic imprinting. We found no association between imprinting status at several candidate genes and hybrid growth, though two interacting genes involved in embryonic growth did show reduced expression in overgrown hybrids. Collectively, our study indicates that growth‐related hybrid inviability may play an important role in mammalian speciation but that the genetic underpinnings of these phenotypes remain unresolved.     

Identification of hemiclonal reproduction in three species of Hexagrammos marine reef fishes

Natural hybrids between the boreal species Hexagrammos octogrammus and two temperate species Hexagrammos agrammus and Hexagrammos otakii were observed frequently in southern Hokkaido, Japan. Previous studies revealed that H. octogrammus is a maternal ancestor of both hybrids; the hybrids are all fertile females and they frequently breed with paternal species. Although such rampant hybridization occurs, species boundaries have been maintained in the hybrid zone. Possible explanations for the absence of introgressions, despite the frequent backcrossing, might include clonal reproduction: parthenogenesis, gynogenesis and hybridogenesis. The natural hybrids produced haploid eggs that contained only the H. octogrammus genome (maternal ancestor) with discarded paternal genome and generated F₁‐hybrid type offspring by fertilization with the haploid sperm of H. agrammus or H. otakii (paternal ancestor). This reproductive mode was found in an artificial backcross hybrid between the natural hybrid and a male of the paternal ancestor. These findings indicate that the natural hybrids adopt hybridogenesis with high possibility and produce successive generations through hybridogenesis by backcrossing with the paternal ancestor. These hybrids of Hexagrammos represent the first hybridogenetic system found from marine fishes that widely inhabit the North Pacific Ocean. In contrast with other hybridogenetic systems, these Hexagrammos hybrids coexist with all three ancestral species in the hybrid zone. The coexistence mechanism is also discussed.     

Gene flow across a hybrid zone maintained by a weak heterogametic incompatibility and positive selection of incompatible alleles

Hybridization between incipient species is more likely to produce sterile or inviable F₁ offspring in the heterogametic (XY or ZW) sex than in the homogametic (XX or ZZ) sex, a phenomenon known as Haldane's rule. Population dynamics associated with Haldane's rule may play an important role in early speciation of sexually reproducing organisms. The dynamics of the hybrid zone maintained by incomplete hybrid inferiority (sterility/inviability) in the heterogametic sex (a ‘weak’ Haldane's rule) caused by a Bateson–Dobzhansky–Muller incompatibility was modelled. The influences and interplays of the strengths of incompatibility, dispersal, density‐dependent regulation (DDR) and local adaptation of incompatible alleles in a scenario of short‐range dispersal (the stepping‐stone model) were examined. It was found that a partial heterogametic hybrid incompatibility could efficiently impede gene flow and maintain characteristic clinal noncoincidence and discordance of alleles. Density‐dependent regulation appears to be an important factor affecting hybrid zone dynamics: it can effectively skew the effects of the partial incompatibility and dispersal as measured by effective dispersal, clinal structures and density depression. Unexpectedly, local adaptation of incompatible alleles in the parental populations, which would be critical for the establishment of the incompatibility, exerts little effect on hybrid zone dynamics. These results strongly support the plausibility of the adaptive origin of hybrid incompatibility and ecological speciation: an adaptive mutation, if it confers a marginal fitness advantage in the local population and happens to cause epistatic inferiority in hybrids, could efficiently drive further genetic divergence that may result in the gene becoming an evolutionary hotspot.     

Distribution of parental DNA markers inEncelia virginensis (Asteraceae: Heliantheae), a diploid species of putative hybrid origin

Morphological, geographical and ecological evidence suggests thatEncelia virginensis is a true-breeding diploid species derived from hybrids ofE. actoni andE. frutescens. To test this hypothesis, we examined the chloroplast and nuclear DNA of severalEncelia species. PCR amplification targeted three separate regions of chloroplast DNA:trnK-2621/trnK-11,rbcL/ORF106, andpsbA3/TrnI-51, which amplify 2600bp, 3300bp and 3200bp fragments respectively. Restriction fragment analysis of chloroplast DNA revealed no variation that could be used to discriminate between the parent species. A RAPD analysis using 109 dekamer primers was used to analyze the nuclear genome.Encelia actoni andE. frutescens were distinguished by several high-frequency RAPD markers. In populations ofE. virginensis, these markers were detected in varying proportions, and no unique markers were found. Evidence from the nuclear genome supports the hypothesis thatE. virginensis is of hybrid origin. ThatE. virginensis may have arisen by normal divergent speciation followed by later introgression remains a possibility, however, and is not formally ruled out here. Diploid hybrid speciation inEncelia differs from other documented cases in that there are no discernible chromosome differences between the species, and all interspecific hybrids are fully fertile. In addition, apparent ecological selection against backcross progeny provides an external barrier to reproduction between F₁ progeny and the parental species. These characteristics suggest that hybrid speciation inEncelia may represent an alternative model for homoploid hybrid speciation involving external reproductive barriers. In particular, this may be the case for other proposed diploid hybrid taxa that also exhibit little chromosomal differentiation and have fertile F₁s.     

Selection on hybrids of ecologically divergent ecotypes of a marine snail: the relative importance of exogenous and endogenous barriers

Unravelling the form of selection acting on hybrids of ecotypes undergoing ecological speciation is essential to understand the mechanisms behind the evolution of reproductive isolation in the face of gene flow. Shell phenotype is known to be affected by natural selection and is involved in the fitness of the marine snail Littorina saxatilis. Here, we studied the association between shell traits and fitness in hybrids in order to determine the relative role of exogenous and endogenous selection in this hybrid zone of L. saxatilis. We show that directional selection is the predominant mode of selection among hybrids. We also show its heterogeneity, affecting different shell traits, within populations at the level of the microhabitat. Therefore, endogenous selection mechanisms are most probably lacking in this hybrid zone and exogenous barriers (pre‐ and post‐zygotic) are possibly one of the main forces behind the evolution of barriers to gene flow between these ecologically divergent ecotypes. This study shows how this barrier might represent an important type of reproductive isolation within ecological speciation, and this should be taken into account in future studies of speciation in hybrid zones. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 391–400.