Olfactory mate recognition in a sympatric species pair of three-spined sticklebacks

Mate recognition is critical to the maintenance of reproductiveisolation, and animals use an array of sensory modalities toidentify conspecific mates. In particular, olfactory informationcan be an important component of mate recognition systems. Weinvestigated whether odor is involved in mate recognition ina sympatric benthic and limnetic species pair of three-spinedsticklebacks (Gasterosteus spp.), for which visual cues andsignals are known to play a role in premating isolation. Weallowed gravid females of each species to choose between waterscented by a heterospecific male and water scented by a conspecificmale. Benthic females preferred the conspecific male stimuluswater significantly more often than the heterospecific malestimulus water, whereas limnetic females showed no preference.These species thus differ in their odor and may also differin their use of olfaction to recognize conspecific mates. Thesedifferences are likely a consequence of adaptation to disparateenvironments. Differences in diet, foraging mode, habitat, andparasite exposure may explain our finding that odor might bean asymmetric isolating mechanism in these sympatric sticklebackspecies.     

A test of ecological selection against young-of-the-year hybrids of sympatric sticklebacks

Ecological selection against hybrids, the reduction in hybrid fitness attributed solely to environmental factors, was tested by introducing young-of-the-year benthic, limnetic and F₁ hybrid sticklebacks Gasterosteus aculeatus to divided experimental ponds and lake enclosures. The frequency of hybrids in samples taken at the end was significantly lower than their frequency at introduction. Hybrid survival was significantly lower in pond-sides in which they were initially the most common cross type than in pond-sides in which they were initially rare, suggesting that hybrid survival may be frequency-dependent. Growth rate of F₁ hybrids was marginally lower than benthic growth rates, being significantly lower than in ponds and not different in lake enclosures. The diet of hybrids overlapped with both parent species in ponds and with benthic diets in lake enclosures. The results suggest that ecological selection is acting against young-of-the-year hybrid sticklebacks.     

Ecological selection against hybrids in natural populations of sympatric threespine sticklebacks

Experimental work has provided evidence for extrinsic post-zygotic isolation, a phenomenon unique to ecological speciation. The role that ecological components to reduced hybrid fitness play in promoting speciation and maintaining species integrity in the wild, however, is not as well understood. We addressed this problem by testing for selection against naturally occurring hybrids in two sympatric species pairs of benthic and limnetic threespine sticklebacks (Gasterosteus aculeatus). If post-zygotic isolation is a significant reproductive barrier, the relative frequency of hybrids within a population should decline significantly across the life-cycle. Such a trend in a natural population would give independent support to experimental evidence for extrinsic, rather than intrinsic, post-zygotic isolation in this system. Indeed, tracing mean individual hybridity (genetic intermediateness) across three life-history stages spanning four generations revealed just such a decline. This provides compelling evidence that extrinsic selection plays an important role in maintaining species divergence and supports a role for ecological speciation in sticklebacks.     

Along the speciation continuum in sticklebacks

Speciation can be viewed as a continuum, potentially divisible into several states: (1) continuous variation within panmictic populations, (2) partially discontinuous variation with minor reproductive isolation, (3) strongly discontinuous variation with strong but reversible reproductive isolation and (4) complete and irreversible reproductive isolation. Research on sticklebacks (Gasterosteidae) reveals factors that influence progress back and forth along this continuum, as well as transitions between the states. Most populations exist in state 1, even though some of these show evidence of disruptive selection and positive assortative mating. Transitions to state 2 seem to usually involve strong divergent selection coupled with at least a bit of geographic separation, such as parapatry (e.g. lake and stream pairs and mud and lava pairs) or allopatry (e.g. different lakes). Transitions to state 3 can occur when allopatric or parapatric populations that evolved under strong divergent selection come into secondary contact (most obviously the sympatric benthic and limnetic pairs), but might also occur between populations that remained in parapatry or allopatry. Transitions to state 4 might be decoupled from these selective processes, because the known situations of complete, or nearly complete, reproductive isolation (Japan Sea and Pacific Ocean pair and the recognized gasterosteid species) are always associated with chromosomal rearrangements and environment‐independent genetic incompatibilities. Research on sticklebacks has thus revealed complex and shifting interactions between selection, adaptation, mutation and geography during the course of speciation.     

Genetic variation in MHC class II expression and interactions with MHC sequence polymorphism in three-spined sticklebacks

Genes of the major histocompatibility complex (MHC) have been studied for several decades because of their pronounced allelic polymorphism. Structural allelic polymorphism is, however, not the only source of variability subjected to natural selection. Genetic variation may also exist in gene expression patterns. Here, we show that in a natural population of three-spined sticklebacks (Gasterosteus aculeatus) the expression of MHC class IIB genes was positively correlated with parasite load, which indicates increased immune activation of the MHC when infections are frequent. To experimentally study MHC expression, we used laboratory-bred sticklebacks that were exposed to three naturally occurring species of parasite. We found strong differences in MHC class IIB expression patterns among fish families, which were consistent over two generations, thus demonstrating a genetic component. The average number of MHC class IIB sequence variants within families was negatively correlated to the MHC expression level suggesting compensatory up-regulation in fish with a low (i.e. suboptimal) MHC sequence variability. The observed differences among families and the negative correlation with individual sequence diversity imply that MHC expression is evolutionary relevant for the onset and control of the immune response in natural populations.     

Character shifts in the defensive armor of sympatric sticklebacks

Natural enemies may contribute to the morphological divergence of sympatric species, yet their role has received little attention to date. We tested for character shifts in defensive armor of sympatric threespine sticklebacks (Gasterosteus aculeatus complex) previously shown to exhibit ecological character displacement in traits related to resource use. We scored five defensive armor traits in sympatric benthic and limnetic stickleback species from southwestern British Columbia and compared them with the same traits in nearby allopatric populations in the presence of the same predatory fish (Oncorhynchus sp.). This approach is analogous to tests of ecological character displacement that compare trophic traits of sympatric and allopatric species in the presence of the same community of resource types. Three patterns consistent with character displacement in defensive armor were found. First, limnetics in different lakes had consistently more armor than sympatric benthics. Second, the average amount of armor, averaged over both species, was reduced in sympatry compared to allopatric populations. This reduction was almost entirely the result of shifts by benthic species, whereas armor in limnetics was more similar to that in allopatric populations. Third, differences between sympatric benthics and limnetics in total armor were greater than expected from comparisons with allopatric populations. We interpret these patterns as the result of differences in habitat-specific predation regimes accompanying ecological character displacement and indirect interactions between sympatric stickleback species mediated by their top predators. These results suggest that predation may facilitate, rather than hinder, the process of divergence in sympatry.     

Contrasting patterns of mitochondrial DNA and microsatellite introgressive hybridization between lineages of lake whitefish (Coregonus clupeaformis); relevance for speciation

We performed a combined analysis of mitochondrial DNA (mtDNA) and microsatellite loci among lake whitefish (Coregonus clupeaformis) populations in order to assess the levels of congruence between both types of markers in defining patterns of genetic structuring, introgressive hybridization and inferring population origins in the hybrid zone of the St. John River basin. A second objective was to test the hypothesis that secondary contact between glacial lineages always resulted in the occurrence of sympatric dwarf and normal whitefish ecotypes. Fish were sampled from 35 populations and polymorphism was screened at mtDNA and six microsatellite loci for a total of 688 and 763 whitefish, respectively. Four lakes harbouring a single whitefish population of normal ecotype admixed with mtDNA haplotypes of different lineages were found. This confirmed that secondary contact between whitefish evolutionary lineages did not always result in the persistence of reproductively isolated ecotypes. Microsatellites further supported the definition of distinct glacial lineages by identifying lineage-specific allelic size groups. They also further supported the hypothesis that ecotypes originated from either a single founding lineage (sympatric divergence) or following secondary contacts between lineages (allopatric divergence), depending on the lake. In general, however, the pattern of population differentiation and introgressive hybridization observed at microsatellites was in sharp contrast with that depicted by mtDNA variation. Both factorial correspondence analysis and analysis of admixture proportion revealed a much more pronounced pattern of introgressive hybridization than depicted by mtDNA analyses. Variable levels of introgression indicated that environmental differences may be as important as the historical contingency of secondary contact in explaining the persistence of sympatric ecotypes and the differential pattern of introgressive hybridization among lakes. Whitefish populations from the St. John River basin hybrid zone represent a rare illustration of a continuum of both morphological and genetic differentiation within a given taxon, spanning from complete introgression to possibly complete reproductive isolation, depending on lakes. Thus, each lake may be viewed as a different temporal snapshot taken throughout the gradual process of speciation.     

基于微卫星标记探讨同域分布竹蝗间的基因渐渗

竹蝗是我国竹林的重要害虫。同域分布物种间基因渐渗方面的研究较少,针对竹蝗属的尚未见报道。本文应用5对微卫星标记检测了安徽金寨县青脊竹蝗和黄脊竹蝗同域分布种群的遗传多样性、遗传结构及种群间的渐渗。结果表明,5对微卫星引物检测到的等位基因共60个,每个位点拥有等位基因10～13个,平均为12个;各位点的期望杂合度(He)范围为0.711～0.885,观测杂合度(Ho)均为1。每个位点都具有较高的多态性,平均多态信息含量(PIC)为0.799,说明这些位点都存在着丰富的遗传信息。在整体水平上,所有微卫星位点对之间为显著的连锁不平衡。同域分布的黄脊竹蝗和青脊竹蝗种群个体间发生了基因渐渗,且基因流从黄脊竹蝗转移到青脊竹蝗。最后,讨论并提出了对竹蝗属基因渐渗深入研究的建议。     

Gene exchange between two divergent species of the fungal human pathogen,Coccidioides

The fungal genus Coccidioides is composed of two species, Coccidioides immitis and Coccidioides posadasii. These two species are the causal agents of coccidioidomycosis, a pulmonary disease also known as valley fever. The two species are thought to have shared genetic material due to gene exchange in spite of their long divergence. To quantify the magnitude of shared ancestry between them, we analyzed the genomes of a population sample from each species. Next, we inferred what is the expected size of shared haplotypes that might be inherited from the last common ancestor of the two species and find a cutoff to find what haplotypes have conclusively been exchanged between species. Finally, we precisely identified the breakpoints of the haplotypes that have crossed the species boundary and measure the allele frequency of each introgression in this sample. We find that introgressions are not uniformly distributed across the genome. Most, but not all, of the introgressions segregate at low frequency. Our results show that divergent species can share alleles, that species boundaries can be porous, and highlight the need for a systematic exploration of gene exchange in fungal species.     

A test of ecologically dependent postmating isolation between sympatric sticklebacks

Ecological speciation occurs when reproductive isolation evolves ultimately as a result of divergent natural selection between populations inhabiting different environments or exploiting alternative resources. I tested a prediction of the ecological model concerning the fitness of hybrids between two young, sympatric species of threespine sticklebacks (Benthics and Limnetics). The two species are ecologically and morphologically divergent: the Benthic is adapted to feeding on invertebrates in the littoral zone of the lake whereas the Limnetic is adapted to feeding on zooplankton in the open water. The growth rate of two types of hybrids, the Benthic backcross and the Limnetic backcross, as well as both parent species, was evaluated in enclosures in both parental habitats in the lake. The use of backcrosses is ideal because a comparison of their growth rates in the two habitats estimates an ecologically dependent component of their fitness while controlling for any intrinsic genetic incompatibilities that may exist between the Benthic and Limnetic genomes. The backcross results revealed a striking pattern of ecological dependence: in the littoral zone, Benthic backcrosses grew at approximately twice the rate of Limnetic backcrosses, while in the open water, Limnetic backcrosses grew at approximately twice the rate of Benthic backcrosses. Such a reversal of relative fitness of the two cross-types in the two environments provides strong evidence that divergent natural selection has played a central role in the evolution of postmating isolation between Benthics and Limnetics. Although the rank order of growth rates of all cross-types in the littoral zone was Benthic > Benthic backcross > Limnetic backcross > Limnetic, neither backcross differed significantly from the parent from which it was mainly derived. Implications of this result are discussed in terms of ecological speciation and possible introgressive hybridization between the species. Results in the open water were less clear and were not fully consistent with the ecological model of speciation, mainly as a result of the low growth rate of Limnetics. However, analysis of the diet of the fish in the open water suggests that these enclosures may not have been fully successful at replicating the food regimes characteristic of this habitat.     

Gene flow across species boundaries in sympatric, sexually deceptive Ophrys (Orchidaceae) species

Abstract Orchids of the genus Ophrys (Orchidaceae) are pollinated by male bees and wasps through sexual deception. The Ophrys sphegodes group encompasses several closely related species that differ slightly in floral morphology and are pollinated by different solitary bee species. Populations representing different species of the O. sphegodes group often flower simultaneously in sympatry. To test whether gene flow across the species boundaries occurs in these sympatric populations, or whether they are reproductively isolated, we examined the distribution of genetic variation within and among populations and species of this group. We collected at each of five different localities in southern France and Italy two sympatric, co-flowering Ophrys populations, representing six Ophrys species in total. The six microsatellite loci surveyed were highly variable. Genetic differentiation among geographically distant populations of the same species was lower than differentiation among sympatric populations of different species. However, the strength of genetic differentiation among species was among the lowest reported for orchids. Genotype assignment tests and marker-based estimates of gene flow revealed that gene flow across species boundaries occurred and may account for the low observed differentiation among species. These results suggest that sexual deceit pollination in Ophrys may be less specific than thought, or that rare mistakes occur.     

The rates of introgression and barriers to genetic exchange between hybridizing species: sex chromosomes vs autosomes

Interspecific crossing experiments have shown that sex chromosomes play a major role in reproductive isolation between many pairs of species. However, their ability to act as reproductive barriers, which hamper interspecific genetic exchange, has rarely been evaluated quantitatively compared to Autosomes. This genome-wide limitation of gene flow is essential for understanding the complete separation of species, and thus speciation. Here, we develop a mainland-island model of secondary contact between hybridizing species of an XY (or ZW) sexual system. We obtain theoretical predictions for the frequency of introgressed alleles, and the strength of the barrier to neutral gene flow for the two types of chromosomes carrying multiple interspecific barrier loci. Theoretical predictions are obtained for scenarios where introgressed alleles are rare. We show that the same analytical expressions apply for sex chromosomes and autosomes, but with different sex-averaged effective parameters. The specific features of sex chromosomes (hemizygosity and absence of recombination in the heterogametic sex) lead to reduced levels of introgression on the X (or Z) compared to autosomes. This effect can be enhanced by certain types of sex-biased forces, but it remains overall small (except when alleles causing incompatibilities are recessive). We discuss these predictions in the light of empirical data comprising model-based tests of introgression and cline surveys in various biological systems.     

Nonallopatric and parallel origin of local reproductive barriers between two snail ecotypes

Theory suggests that speciation is possible without physical isolation of populations (hereafter, nonallopatric speciation), but recent nonallopatric models need the support of irrefutable empirical examples. We collected snails (Littorina saxatilis) from three areas on the NW coast of Spain to investigate the population genetic structure of two ecotypes. Earlier studies suggest that these ecotypes may represent incipient species: a large, thick-shelled 'RB' ecotype living among the barnacles in the upper intertidal zone and a small, thin-shelled 'SU' ecotype living among the mussels in the lower intertidal zone only 10-30 m away. The two ecotypes overlap and hybridize in a midshore zone only 1-3 m wide. Three different types of molecular markers [allozymes, mitochondrial DNA (mtDNA) and microsatellites] consistently indicated partial reproductive isolation between the RB and the SU ecotypes at a particular site. However, each ecotype was related more closely to the other ecotype from the same site than to the same ecotype from another site further along the Galician coast (25-77 km away). These findings supported earlier results based solely on allozyme variation and we could now reject the possibility that selection produced these patterns. The patterns of genetic variation supported a nonallopatric model in which the ecotypes are formed independently at each site by parallel evolution and where the reproductive barriers are a byproduct of divergent selection for body size. We argue that neither our laboratory hybridization experiments nor our molecular data are compatible with a model based on allopatric ecotype formation, secondary overlap and introgression.     

The tale of the bad stepfather: male three-spined sticklebacks Gasterosteus aculeatus L. recognize foreign eggs in their manipulated nest by egg cues alone

The ability to discriminate between own and foreign eggs was investigated in brood-caring male three-spined sticklebacks Gasterosteus aculeatus . Males totally cannibalized clutches that contained both foreign and their own eggs significantly more often than sham-manipulated control clutches that only contained their own eggs. These results show that caring males are able to recognize the presence of foreign eggs by egg cues alone.     

Low level infection by eye fluke, Diplostomum spp., affects the vision of three-spined sticklebacks, Gasterosteus aculeatus

Experiments have shown thai infection by low intensities of the eye fluke affects the vision of three-spined stickleback when selecting prey, and also shortens the reactive distance to the prey. The results have consequences for the fitness of the individual, and also for the study of prey selection.     

Ecological divergence associated with mating system causes nearly complete reproductive isolation between sympatric Mimulus species

Speciation often involves the evolution of numerous prezygotic and postzygotic isolating barriers between divergent populations. Detailed knowledge of the strength and nature of those barriers provides insight into ecological and genetic factors that directly or indirectly influenced their origin, and may help predict whether they will be maintained in the face of sympatric hybridization and introgression. We estimated the magnitude of pre- and postzygotic barriers between naturally occurring sympatric populations of Mimulus guttatus and M. nasutus. Prezygotic barriers, including divergent flowering phenologies, differential pollen production, mating system isolation, and conspecific pollen precedence, act asymmetrically to completely prevent the formation of F(1) hybrids among seeds produced by M. guttatus (F(1)g), and reduce F(1) hybrid production among seeds produced by M. nasutus (F(1)n) to only about 1%. Postzygotic isolation is also asymmetric: in field experiments, F(1)g but not F(1)n hybrids had significantly reduced germination rates and survivorship compared to parental species. Both hybrid classes had flower, pollen, and seed production values within the range of parental values. Despite the moderate degree of F(1)g hybrid inviability, postzygotic isolation contributes very little to the total isolation between these species in the wild. We also found that F(1) hybrid flowering phenology overlapped more with M. guttatus than M. nasutus. These results, taken together, suggest greater potential for introgression from M. nasutus to M. guttatus than for the reverse direction. We also address problems with commonly used indices of isolation, discuss difficulties in calculating meaningful measures of reproductive isolation when barriers are asymmetric, and propose novel measures of prezygotic isolation that are consistent with postzygotic measures.     

Phenotypic divergence and reproductive isolation between sympatric forms of Japanese threespine sticklebacks

The threespine stickleback ( Gasterosteus aculeatus ) species complex is well suited for identifying the types of phenotypic divergence and isolating barriers that contribute to reproductive isolation at early stages of speciation. In the present study, we characterize the patterns of genetic and phenotypic divergence as well as the types of isolating barriers that are present between two sympatric pairs of threespine sticklebacks in Hokkaido, Japan. One sympatric pair consists of an anadromous and a resident freshwater form and shows divergence in body size between the forms, despite the lack of genetic differentiation between them. The second sympatric pair consists of two anadromous forms, which originated before the last glacial period and are currently reproductively isolated. These two anadromous forms have diverged in many morphological traits as well as in their reproductive behaviours. Both sexual isolation and hybrid male sterility contribute to reproductive isolation between the anadromous species pair. We discuss the shared and unique aspects of phenotypic divergence and reproductive isolation in the Japanese sympatric pairs compared with postglacial stickleback species pairs. Further studies of these divergent species pairs will provide a deeper understanding of the mechanisms of speciation in sticklebacks.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 671–685.     

Site-specific genetic divergence in parallel hybrid zones suggests nonallopatric evolution of reproductive barriers

The evolution of reproductive isolation in the presence of gene flow is supported by theoretical models but rarely by data. Empirical support might be gained from studies of parallel hybrid zones between interbreeding taxa. We analysed gene flow over two hybrid zones separating ecotypes of Littorina saxatilis to test the expectation that neutral genetic markers will show site-specific differences if barriers have evolved in situ. Distinct ecotypes found in contrasting shore habitats are separated by divergent selection and poor dispersal, but hybrid zones appear between them. Swedish islands formed by postglacial uplift 5000 years ago provide opportunities to assess genetic structure in a recently evolved system. Each island houses a discrete population containing subpopulations of different ecotypes. Hybrid zones between ecotypes may be a product of ecological divergence occurring on each island or a consequence of secondary overlap of ecotypes of allopatric origin that have spread among the islands. We used six microsatellite loci to assess gene flow and genetic profiles of hybrid zones on two islands. We found reduced gene flow over both hybrid zones, indicating the presence of local reproductive barriers between ecotypes. Nevertheless, subpopulations of different ecotypes from the same island were genetically more similar to each other than were subpopulations of the same ecotype from different islands. Moreover, neutral genetic traits separating the two ecotypes across hybrid zones were site-specific. This supports a scenario of in situ origin of ecotypes by ecological divergence and nonallopatric evolution of reproductive barriers.     

Dominance effects strengthen premating hybridization barriers between sympatric species of grasshoppers (Acrididae,Orthoptera)

Sexual selection can lead to the rapid evolution of premating hybridization barriers and allows accelerated diversification and speciation within an evolutionary lineage. Especially during early stages of divergence, hybridization may impede further divergence, which strongly depends on the reproductive success of hybrids. Behavioural sterility of hybrids can limit or even prevent homogenizing gene flow. In this study, we investigated the attractiveness of male courtship songs for females of the grasshopper species Chorthippus biguttulus and C. brunneus and their interspecific F1 and F2 hybrids. Song preferences of females of both species are highly species specific and differ in three parameters: shape of the preference function, preference for syllable pattern and phrase duration. F1 hybrid females of both reciprocal crosses as well as F2 hybrid females resembled closely pure C. biguttulus females in respect of shape of the preference function and preference for syllable pattern, while preference for phrase duration showed an intermediate expression. This resulted in song preferences of hybrid females that closely resembled those of one parental species, that is C. biguttulus females. Such strong dominance effects were rarely reported so far. They represent an effective barrier limiting gene flow between the two species, since hybrid females will backcross to only one parental species and discriminate against hybrid males, which are behaviourally sterile. Such taxon‐specific modes of inheritance may have facilitated the rapid divergence of acoustically communicating grasshoppers of the species group of Chorthippus biguttulus. Our findings have novel implications on the expression of neuronal filters and the evolution of complex courtship signals.     

Ancient hybridization and mitochondrial capture between two species of chipmunks

Models that posit speciation in the face of gene flow are replacing classical views that hybridization is rare between animal species. We use a multilocus approach to examine the history of hybridization and gene flow between two species of chipmunks ( Tamias ruficaudus and T. amoenus ). Previous studies have shown that these species occupy different ecological niches and have distinct genital bone morphologies, yet appear to be incompletely isolated reproductively in multiple areas of sympatry. We compared data from four sequenced nuclear loci and from seven microsatellite loci to published cytochrome b sequences. Interspecific gene flow was primarily restricted to introgression of the T. ruficaudus mitochondrial genome into a sympatric subspecies of T. amoenus , T. a. canicaudus , with the four sequenced nuclear loci showing little to no interspecific allele sharing. Microsatellite data were consistent with high levels of differentiation between the species and also showed no current gene flow between broadly sympatric populations of T. a. canicaudus and T. ruficaudus . Coalescent analyses date the mtDNA introgression event from the mid-Pleistocene to late Pliocene. Overall, these data indicate that introgression has had a minimal impact on the nuclear genomes of T. amoenus and T. ruficaudus despite multiple independent hybridization events. Our findings challenge long-standing assumptions on patterns of reproductive isolation in chipmunks and suggest that there may be other examples of hybridization among the 23 species of Tamias that occur in western North America.