Reproductive isolation of a new hybrid species, Senecio eboracensis Abbott & Lowe (Asteraceae)

The nature and extent of reproductive isolation was examined between a new self-compatible hybrid species Senecio eboracensis (2n = 40) and its parents, self-incompatible S. squalidus (2n = 20) and self-compatible S. vulgaris (2n = 40). The triploid F-1 of S. eboracensis x S. squalidus exhibited very low seed set ((x) over bar = 0.63%), and F-2 and F-3 progeny were able to recover nominal levels of fertility ((x) over bar = 23.9 and 9.7%), while F-1 and F-2 offspring of S. eboracensis x S. vulgaris showed reduced seed set ((x) over bar = 63.8 and 58.8%). In both cases, evidence from previous work indicates that reduced fertility is associated with meiotic chromosome mispairing, and is a likely consequence of recombining both parental genomes within this new taxon. No hybrid offspring between S. eboracensis and S. squalidus were found in the wild, and only one such hybrid was recorded among 769 progeny produced by S. eboracensis surrounded by S. squalidus on an experimental plot. Natural crossing between S. eboracensis and S. vulgaris was recorded to be very low (between 0 and 1.46%) in the wild, but rose to 18.3% when individuals of S. eboracensis were surrounded by plants of S. vulgaris. It was concluded that strong breeding barriers exist between the new hybrid species and its two parents. Prezygotic isolation between S. eboracensis and S. vulgaris is likely to be largely due to both species reproducing by predominant self-fertilisation. However, differences recorded for germination, seedling survival, time of flowering and characters associated with pollinator attraction, plus significant clumping of juvenile and adult conspecifics in the wild, probably also contribute to reproductive isolation and ecological differentiation.

Divergent environments and population bottlenecks fail to generate premating isolation in Drosophila pseudoobscura

While the feasibility of bottleneck-induced speciation is in doubt, population bottlenecks may still affect the speciation process by interacting with divergent selection. To explore this possibility, I conducted a laboratory speciation experiment using Drosophila pseudoobscura involving 78 replicate populations assigned in a two-way factorial design to both bottleneck (present vs. absent) and environment (ancestral vs. novel) treatments. Populations independently evolved under these treatments and were then tested for assortative mating and male mating success against their common ancestor. Bottlenecks alone did not generate any premating isolation, despite an experimental design that was conducive to bottleneck-induced speciation. Premating isolation also did not evolve in the novel environment treatment, neither in the presence nor absence of bottlenecks. However, male mating success was significantly reduced in the novel environment treatment, both as a plastic response to this environment and as a result of environment-dependent inbreeding effects in the bottlenecked populations. Reduced mating success of derived males will hamper speciation by enhancing the mating success of immigrant, ancestral males. Novel environments are generally thought to promote ecological speciation by generating divergent natural selection. In the current experiment, however, the novel environment did not cause the evolution of any premating isolation and it reduced the likelihood of speciation through its effects on male mating success.

Geographic variation and positive selection on M7 lysin, an acrosomal sperm protein in mussels (Mytilus spp.)

Successful fertilization in free-spawning marine organisms depends on the interactions between genes expressed on the surfaces of eggs and sperm. Positive selection frequently characterizes the molecular evolution of such genes, raising the possibility that some common deterministic process drives the evolution of gamete recognition genes and may even be important for understanding the evolution of prezygotic isolation and speciation in the marine realm. One hypothesis is that gamete recognition genes are subject to selection for prezygotic isolation, namely reinforcement. In a previous study, positive selection on the gene coding for the acrosomal sperm protein M7 lysin was demonstrated among allopatric populations of mussels in the Mytilus edulis species group (M. edulis, M. galloprovincialis, and M. trossulus). Here, we expand sampling to include M7 lysin haplotypes from populations where mussel species are sympatric and hybridize to determine whether there is a pattern of reproductive character displacement, which would be consistent with reinforcement driving selection on this gene. We do not detect a strong pattern of reproductive character displacement; there are no unique haplotypes in sympatry nor is there consistently greater population structure in comparisons involving sympatric populations. One distinct group of haplotypes, however, is strongly affected by natural selection and this group of haplotypes is found within M. galloprovincialis populations throughout the Northern Hemisphere concurrent with haplotypes common to M. galloprovincialis and M. edulis. We suggest that balancing selection, perhaps resulting from sexual conflicts between sperm and eggs, maintains old allelic diversity within M. galloprovincialis.

Bidirectional incompatibility between conspecific populations of Drosophila simulans

Cytoplasmic incompatibility (CI) describes the phenomenon whereby eggs fertilized by sperm from insects infected with a rickettsial endosymbiont fail to hatch. Unidirectional CI between conspecific populations of insects is a well documented phenomenon. Bidirectional CI has, however, only been described in mosquito populations, and recently between closely related species of parasitic wasps, where it is of interest as both an unusual form of reproductive isolation and as a potential means of insect population suppression. Here we report on the first known example of bidirectional CI between conspecific populations of Drosophila simulans. Further, we show that defects as early as the first cleavage division are associated with CI. This observation suggests that the cellular basis of CI involves disruption of processes before or during zygote formation and that CI arises from defects in the structure and/or function of the sperm during fertilization.

Positive selection on an acrosomal sperm protein, M7 lysin, in three species of the mussel genus Mytilus

Marine invertebrate sperm proteins are particularly interesting because they are characterized by positive selection and are likely to be involved in prezyogotic isolation and, thus, speciation. Here, we present the first survey of inter and intraspecific variation of a bivalve sperm protein among a group of species that regularly hybridize in nature. M7 lysin is found in sperm acrosomes of mussels and dissolves the egg vitelline coat, permitting fertilization. We sequenced multiple alleles of the mature protein-coding region of M7 lysin from allopatric populations of mussels in the Mytilus edulis species group (M. edulis, M. galloprovincialis, and M. trossulus). A significant McDonald-Kreitman test showed an excess of fixed amino acid replacing substitutions between species, consistent with positive selection. In addition, Kolmogorov-Smirnov tests showed significant heterogeneity in polymorphism to divergence ratios for both synonymous variation and combined synonymous and non-synonymous variation within M. galloprovincialis. These results indicate that there has been adaptive evolution at M7 lysin and, furthermore, shows that positive selection on sperm proteins can occur even when post-zygotic reproductive isolation is incomplete.

Male-by-female interactions influence fertilization success and mediate the benefits of polyandry in the sea urchin Heliocidaris erythrogramma

Numerous studies have reported that females benefit from mating with multiple males (polyandry) by minimizing the probability of fertilization by genetically incompatible sperm. Few, however, have directly attributed variation in female reproductive success to the fertilizing capacity of sperm. In this study we report on two experiments that investigated the benefits of polyandry and the interacting effects of males and females at fertilization in the free-spawning Australian sea urchin Heliocidaris erythrogramma. In the first experiment we used a paired (split clutch) experimental design and compared fertilization rates within female egg clutches under polyandry (eggs exposed to the sperm from two males simultaneously) and monandry (eggs from the same female exposed to sperm from each of the same two males separately). Our analysis revealed a significant fertilization benefit of polyandry and strong interacting effects of males and females at fertilization. Further analysis of these data strongly suggested that the higher rates of fertilization in the polyandry treatment were due to an overrepresentation of fertilizations due to the most compatible male. To further explore the interacting effects of males and females at fertilization we performed a second factorial experiment in which four mates were crossed with two females (in all eight combinations). In addition to confirming that fertilization success is influenced by male X female interactions, this latter experiment revealed that both sexes contributed significant variance to the observed patterns of fertilization. Taken together, these findings highlight the importance of male X female interactions at fertilization and suggest that polyandry will enable females to reduce the cost of fertilization by incompatible gametes.

The benefits of polyandry in the free-spawning polychaete Galeolaria caespitosa

In many species, females are thought to benefit from polyandry due to the reduced risks of fertilization by genetically incompatible sperm. However, few studies that have reported such benefits have directly attributed variation in female reproductive success to the interacting effects of males and females at fertilization. In this paper, we determine whether male x female interactions influence fertilization in vitro in the free-spawning, sessile polychaete Galeolaria caespitosa. Furthermore, we determined whether polyandry results in direct fertilization benefits for females by experimentally manipulating the number of males contributing towards staged spawning events. To test for male x female interaction effects we performed an initial experiment that crossed seven males with six females (in all 42 combinations), enabling us to assess fertilization rates for each specific male-female pairing and attribute variation in fertilization success to males, females and their interaction. This initial experiment revealed a strong interaction between males and females at fertilization, confirming that certain male-female combinations were more compatible than others. A second experiment tested the hypothesis that polyandry enhances female reproductive success by exposing each female's eggs to either a single male's sperm (monandry) or the sperm from three males simultaneously (polyandry). We performed this second experiment at two ecologically relevant sperm concentrations. This latter experiment revealed a strong fertilization benefit of polyandry, independent of the effects of sperm concentration (which were also significant). We suggest that these direct fertilization gains arising from polyandry will constitute an important source of selection on females to mate multiply in nature.

Interaction between natural and sexual selection during the evolution of mate recognition

The interaction between natural and sexual selection is central to many theories of how mate choice and reproductive isolation evolve, but their joint effect on the evolution of mate recognition has not, to my knowledge, been investigated in an evolutionary experiment. Natural and sexual selection were manipulated in interspecific hybrid populations of Drosophila to determine their effects on the evolution of a mate recognition system comprised of cuticular hydrocarbons (CHCs). The effect of natural selection in isolation indicated that CHCs were costly for males and females to produce. The effect of sexual selection in isolation indicated that females preferred males with a particular CHC composition. However, the interaction between natural and sexual selection had a greater effect on the evolution of the mate recognition system than either process in isolation. When natural and sexual selection were permitted to operate in combination, male CHCs became exaggerated to a greater extent than in the presence of sexual selection alone, and female CHCs evolved against the direction of natural selection. This experiment demonstrated that the interaction between natural and sexual selection is critical in determining the direction and magnitude of the evolutionary response of the mate recognition system.

Evolutionary experiments on mate recognition in the Drosophila serrata species complex

It is becoming increasingly apparent that at least some aspects of the evolution of mate recognition may be amenable to manipulation in evolutionary experiments. Quantitative genetic analyses that focus on the genetic consequences of evolutionary processes that result in mate recognition evolution may eventually provide an understanding of the genetic basis of the process of speciation. We review a series of experiments that have attempted to determine the genetic basis of the response to natural and sexual selection on mate recognition in the Drosophila serrata species complex. The genetic basis of mate recognition has been investigated at three levels: (1) between the species of D. serrata and D. birchii using interspecific hybrids, (2) between populations of D. serrata that are sympatric and allopatric with respect to D. birchii, and (3) within populations of D. serrata. These experiments suggest that it may be possible to use evolutionary experiments to observe important events such as the reinforcement of mate recognition, or the generation of the genetic associations that are central to many sexual selection models.

Combining the analyses of introgressive hybridisation and linkage mapping to investigate the genetic architecture of population divergence in the lake whitefish (Coregonus clupeaformis, Mitchill)

Adaptation and reproductive isolation, the engines of biological diversity, are still elusive when discussing the genetic bases of speciation. Namely, the number of genes and magnitude of selection acting positively or negatively on genomic traits implicated in speciation is contentious. Here, we describe the first steps of an ongoing research program aimed at understanding the genetic bases of population divergence and reproductive isolation in the lake whitefish (Coregonus clupeaformis). A preliminary linkage map originating from a hybrid cross between dwarf and normal ecotypes is presented, whereby some of the segregating AFLP markers were found to be conserved among natural populations. Maximum-likelihood was used to estimate hybrid indices from non-diagnostic markers at 998 AFLP loci. This allowed identification of the most likely candidate loci that have been under the influence of selection during the natural hybridisation of whitefish originating from different glacial races. As some of these loci could be identified on the linkage map, the possibility that selection of traits in natural populations may eventually be correlated to specific chromosomal regions was demonstrated. The future prospects and potential of these approaches to elucidate the genetic bases of adaptation and reproductive isolation among sympatric ecotypes of lake whitefish is discussed.

Evidence for sympatric speciation by host shift in the sea

The genetic divergence and evolution of new species within the geographic range of a single population (sympatric speciation) contrasts with the well-established doctrine that speciation occurs when populations become geographically isolated (allopatric speciation). Although there is considerable theoretical support for sympatric speciation [1, 2], this mode of diversification remains controversial, at least in part because there are few well-supported examples [3]. We use a combination of molecular, ecological, and biogeographical data to build a case for sympatric speciation by host shift in a new species of coral-dwelling fish (genus Gobiodon). We propose that competition for preferred coral habitats drives host shifts in Gobiodon and that the high diversity of corals provides the source of novel, unoccupied habitats. Disruptive selection in conjunction with strong host fidelity could promote rapid reproductive isolation and ultimately lead to species divergence. Our hypothesis is analogous to sympatric speciation by host shift in phytophagous insects [4, 5] except that we propose a primary role for intraspecific competition in the process of speciation. The fundamental similarity between these fishes and insects is a specialized and intimate relationship with their hosts that makes them ideal candidates for speciation by host shift.

Overlapping species boundaries and hybridization within the Monastraea "annularis" reef coral complex in the Pleistocene of the Bahama Islands

Recent molecular analyses indicate that many reef coral species belong to hybridizing species complexes or "syngameons." Such complexes consist of numerous genetically distinct-species or lineages, which periodically split and/or fuse as they extend through time. During splitting and fusion, morphologic intermediates form and species overlap. Here we focus on processes associated with lineage fusion, specifically introgressive hybridization, and the recognition of such hybridization in the fossil record. Our approach involves comparing patterns of ecologic and morphologic overlap in genetically characterized modern species with fossil representatives of the same or closely related species. We similarly consider the long-term consequences of past hybridization on the structure of modern-day species boundaries. Our study involves the species complex Montastraea annularis s.l. and is based in the Bahamas, where, unlike other Caribbean locations, two of the three members of the complex today are not genetically distinct. We measured and collected colonies along linear transects across Pleistocene reef terraces of last interglacial age (approximately 125 Ka) on the islands of San Salvador, Andros, and Great Inagua. We performed quantitative ecologic and morphologic analyses of the fossil data, and compared patterns of overlap among species with data from modern localities where species are and are not genetically distinct. Ecologic and morphologic analyses reveal "moderate" overlap (>10%, but statistically significant differences) and sometimes "high" overlap (no statistically significant differences) among Pleistocene growth forms (= "species"). Ecologic analyses show that three species (massive, column, organ-pipe) co-occurred. Although organ-pipes had higher abundances in patch reef environments, columnar and massive species exhibited broad, completely overlapping distributions and had abundances that were not related to reef environment. For morphometric analyses, we used multivariate discriminant analysis on landmark data and linear measurements. The results show that columnar species overlap "moderately" with organ-pipe and massive species. Comparisons with genetically characterized colonies from Panama show that the Pleistocene Bahamas species have intermediate morphologies, and that the observed "moderate" overlap differs from the morphologic separation among the three modern species. In contrast, massive and columnar species from the Pleistocene of the Dominican Republic comprise distinct morphologic clusters, similar to the modern species; organ-pipe species exhibit "low" overlap (

Ecological speciation

Ecological processes are central to the formation of new species when barriers to gene flow (reproductive isolation) evolve between populations as a result of ecologically-based divergent selection. Although laboratory and field studies provide evidence that 'ecological speciation' can occur, our understanding of the details of the process is incomplete. Here we review ecological speciation by considering its constituent components: an ecological source of divergent selection, a form of reproductive isolation, and a genetic mechanism linking the two. Sources of divergent selection include differences in environment or niche, certain forms of sexual selection, and the ecological interaction of populations. We explore the evidence for the contribution of each to ecological speciation. Forms of reproductive isolation are diverse and we discuss the likelihood that each may be involved in ecological speciation. Divergent selection on genes affecting ecological traits can be transmitted directly (via pleiotropy) or indirectly (via linkage disequilibrium) to genes causing reproductive isolation and we explore the consequences of both. Along with these components, we also discuss the geography and the genetic basis of ecological speciation. Throughout, we provide examples from nature, critically evaluate their quality, and highlight areas where more work is required.

Reinforcement drives rapid allopatric speciation

Allopatric speciation results from geographic isolation between populations. In the absence of gene flow, reproductive isolation arises gradually and incidentally as a result of mutation, genetic drift and the indirect effects of natural selection driving local adaptation(1-3). In contrast, speciation by reinforcement is driven directly by natural selection against maladaptive hybridization(1,4). This gives individuals that choose the traits of their own lineage greater fitness, potentially leading to rapid speciation between the lineages(1,4). Reinforcing natural selection on a population of one of the lineages in a mosaic contact zone could also result in divergence of the population from the allopatric range of its own lineage outside the zone(4-6). Here we test this with molecular data, experimental crosses, field measurements and mate choice experiments in a mosaic contact zone between two lineages of a rainforest frog. We show that reinforcing natural selection has resulted in significant premating isolation of a population in the contact zone not only from the other lineage but also, incidentally, from the closely related main range of its own lineage. Thus we show the potential for reinforcement to drive rapid allopatric speciation.