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Results for “Reproductive Isolation”

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PubMed2006

Gene transposition as a cause of hybrid sterility in Drosophila.

We describe reproductive isolation caused by a gene transposition. In certain Drosophila melanogaster-D. simulans hybrids, hybrid male sterility is caused by the lack of a single-copy gene essential for male fertility, JYAlpha. This gene is located on the fourth chromosome of D. melanogaster but on the third chromosome of D. simulans. Genomic and molecular analyses show that JYAlpha transposed to the third chromosome during the evolutionary history of the D. simulans lineage. Because of this transposition, a fraction of hybrids completely lack JYAlpha and are sterile, representing reproductive isolation without sequence evolution.

Animals↗

PubMed1998

Sexual conflict and speciation.

We review the significance of two forms of sexual conflict (different evolutionary interests of the two sexes) for genetic differentiation of populations and the evolution of reproductive isolation. Conflicting selection on the alleles at a single locus can occur in males and females if the sexes have different optima for a trait, and there are pleiotropic genetic correlations between the sexes for it. There will then be selection for sex limitation and hence sexual dimorphism. This sex limitation could break down in hybrids and reduce their fitness. Pleiotropic genetic correlations between the sexes could also affect the likelihood of mating in interpopulation encounters. Conflict can also occur between (sex-limited) loci that determine behaviour in males and those that determine behaviour in females. Reproductive isolation may occur by rapid coevolution of male trait and female mating preference. This would tend to generate assortative mating on secondary contact, hence promoting speciation. Sexual conflict resulting from sensory exploitation, polyspermy and the cost of mating could result in high levels of interpopulation mating. If females evolve resistance to make pre- and postmating manipulation, males from one population could be more successful with females from the other, because females would have evolved resistance to their own (but not to the allopatric) males. Between-locus sexual conflict could also occur as a result of conflict between males and females of different populations over the production of unfit hybrids. We develop models which show that females are in general selected to resist such matings and males to persist, and this could have a bearing on both the initial level of interpopulation matings and the likelihood that reinforcement will occur. In effect, selection on males usually acts to promote gene flow and to restrict premating isolation, whereas selection on females usually acts in the reverse direction. We review theoretical models relevant to resolution of this conflict. The winning role depends on a balance between the 'value of winning' and 'power' (relating to contest or armament costs): the winning role is likely to correlate with high value of winning and low costs. Sperm-ovum (or sperm-female tract) conflicts (and their plant parallels) are likely to obey the same principles. Males may typically have higher values of winning, but it is difficult to quantify 'power', and females may often be able to resist mating more cheaply than males can force it. We tentatively predict that sexual conflict will typically result in a higher rate of speciation in 'female-win' clades, that females will be responsible for premating isolation through reinforcement, and that 'female-win' populations will be less genetically diverse.

Alleles↗

PubMed2003

Evaluation of the courtship and of the hybrid male sterility among Drosophila buzzatii cluster species (Diptera, Drosophilidae).

In the Drosophila repleta group the establishment of subgroups and complexes made on the basis of morphological and cytological evidences is supported by tests of reproductive isolation. Among species in the repleta group, the buzzatii cluster, due to its polymorphism and polytipism, is an excellent material for ecological and speciation studies. Some interspecific crosses involving Drosophila seriema, Drosophila sp. B, D. koepferae and D. buzzatii strains were completely sterile while others involving strains from these species produced F1 hybrids that did not yield F2. In the present work, data on courtship duration and copula occurrence obtained in the analysis of flies from parental sterile crosses and on spermatozoon mobility observed in F1 hybrids that did not yield F2 are presented. Copula did not occur during one hour of observation and the spermatozoon also did not show mobility at any of the analyzed stages (3, 7, 9 and 10 days old). There was a high variation in courtship average duration and in the percentage of males that courted the females. The reproductive isolation mechanisms indicated by these observations were pre and post-zygotic, as supported by the absence of copula and male sterility. Data obtained also showed the occurrence of different degrees of reproductive compatibility among the strains classified as the same species but from distinct geographic localities.

Animals↗

PubMed2005

Morphometric discrimination of two allozymically diagnosed sibling species of the Echinorhynchus gadi Zoega in Muller complex (Acanthocephala) in the North Sea.

Allozyme electrophoresis was used to detect biological species of the E. gadi complex from gadids from the northern North Sea. A fixed difference at one of nine enzyme loci surveyed confirmed the existence of two reproductively isolated, sympatric species. Mixed infections of two E. gadi spp. (termed A and B) were observed in Gadus morhua and Pollachius virens. E. gadi sp. B was also found in Melanogrammus aeglefinus and Merlangius merlangus. The presence of gravid females of E. gadi spp. A and B in the same host species, P. virens, and sometimes in the same host individual, indicates that neither differential host-specificity nor seasonal differences in mating time are responsible for their reproductive isolation. Morphological study of probosces from electrophoretically identified specimens demonstrated that the two species can be discriminated in graphical and cluster analyses of hook morphometrics. E. gadi sp. I (of Vainola etal., 1994) and E. gadi sp. A are probably conspecific.

Acanthocephala↗

PubMed2005

Adaptive speciation when assortative mating is based on female preference for male marker traits.

Adaptive speciation occurs when frequency-dependent ecological interactions generate conditions of disruptive selection to which lineage splitting is an adaptive response. Under such selective conditions, evolution of assortative mating mechanisms enables the break-up of the ancestral lineage into diverging and reproductively isolated descendent species. Extending previous studies, I investigate models of adaptive speciation due to the evolution of indirect assortative mating that is based on three different mating traits: the degree of assortativity, a female preference trait and a male marker trait. For speciation to occur, linkage disequilibria between different mating traits, e.g. between female preference and male marker traits, as well as between mating traits and the ecological trait, must evolve. This can lead to novel speciation scenarios, e.g. when reproductive isolation is generated by a splitting in the degree of assortativeness, with one of the emerging lineages mating assortatively, and the other one disassortatively. I investigate the effects of variation in various model parameters on the likelihood of speciation, as well as robustness of speciation to introducing costs of assortative mating. Even though in the models presented speciation requires the genetic potential for strong assortment as well as rather restrictive ecological conditions, the results show that adaptive speciation due to the evolution of assortative mating when mate choice is based on separate female preference and male marker traits is a theoretically plausible evolutionary scenario.

Adaptation, Biological↗

PubMed2005

Counteracting selective regimes and host preference evolution in ecotypes of two species of walking-sticks.

The evolution of ecological specialization has been a central topic in ecology because specialized adaptations to divergent environments can result in reproductive isolation and facilitate speciation. However, the order in which different aspects of habitat adaptation and habitat preference evolve is unclear. Timema walking-stick insects feed and mate on the host plants on which they rest. Previous studies of T. cristinae ecotypes have documented divergent, host-specific selection from visual predators and the evolution of divergent host and mate preferences between populations using different host-plant species (Ceanothus or Adenostoma). Here we present new data that show that T. podura, a nonsister species of T. cristinae, has also formed ecotypes on these host genera and that in both species these ecotypes exhibit adaptive divergence in color-pattern and host preference. Color-pattern morphs exhibit survival trade-offs on different hosts due to differential predation. In contrast, fecundity trade-offs on different hosts do not occur in either species. Thus, host preference in both species has evolved before divergent physiological adaptation but in concert with morphological adaptations. Our results shed light onto which traits are involved in the initial stages of ecological specialization and ecologically based reproductive isolation.

Adaptation, Biological↗

PubMed2002

Allozyme analysis of cryptic species in the Asplenium niduscomplex from West Java, Indonesia.

In various fern species, a large amount of rbcL sequence variation has been reported, and it is possible that these species contain several reproductively isolated cryptic species. In our previous study on Asplenium nidus L., it was suggested that the plants growing in Mt. Halimun National Park, West Java, Indonesia, consist of several cryptic species based on the results of crossing experiments among rbcL sequence types. In this study, we examined allozyme polymorphisms of five rbcL sequence types found in West Java in order to test the hypothesis that the assemblages of A. nidus delimited based on the rbcL sequences are separate Mendelian populations and gene flow is disrupted by reproductive isolation from one another. The calculated fixation indices suggested that the individuals in each rbcL type are randomly crossing at least in the investigated localities. Nevertheless, these rbcL-based assemblages were genetically differentiated in allozymes that are encoded in their nuclear genomes, and it is also suggested that gene flow is disrupted even between sympatrically distributed pairs of rbcL sequence types. Therefore, our findings support the view that the five rbcL sequence types in West Java are potential cryptic species.

Journal Article↗

PubMed2002

Population structure, history and gene flow in a group of closely related land snails: genetic variation in Partula from the Society Islands of the Pacific.

Previous studies of Partula land snails from the Society Islands, French Polynesia, have shown that populations within species are highly differentiated in terms of their morphology, behaviour, ecology and molecular genetic variation. Despite this level of variability, differences between species are sometimes small, possibly reflecting the fact that reproductive isolation is not always complete and there exists the opportunity for genetic exchange between taxa through hybridization. The present study uses sequence data from a mitochondrial gene to further investigate genetic variation in Society Island Partula. Most populations are found in this study to be highly differentiated, but within individual species there seems to be no simple relationship either between genetic distance and geographical proximity, or between variation in mitochondria and that in allozymes or morphological characteristics. Among species there appears to be no simple correlation between degrees of reproductive isolation and genetic relatedness according to mitochondrial DNA. The results suggest that past events as well as ongoing drift and selection may have been important in affecting patterns of variation. Similarities among species at specific localities suggest that there must have been some genetic exchange in the past, although this may not necessarily reflect ongoing rates of hybridization. The discrepancy between results for different markers probably reflects the differential effects of drift and selection on mitochondrial and nuclear genes.

Animals↗

PubMed1996

Rescue of hybrid sterility in crosses between D. melanogaster and D. simulans.

The genetic analysis of reproductive isolation between species of Drosophila has now reached the resolution necessary to start answering one of the fundamental questions of evolution: what is the genetic basis of species differences? A.H. Sturtevant, one of the founders of Drosophila genetics, was fascinated by this question and thought he had found a way to analyse it when he realized that 'Drosophila melanogaster' was actually two species: D. melanogaster and D. simulans. By passing genes between these two species he hoped to investigate their genetic differences directly. No doubt he was disappointed to find that the D. melanogaster/D. simulans hybridization resulted only in unisexual sterile hybrids, a disappointment appreciated all the more by modern evolutionary biologists. Seventy-five years after Sturtevant's description of D. melanogaster/D. simulans hybrid sterility, we have discovered a strain of D. simulans that produces fertile female hybrids in crosses with D. melanogaster. Our discovery promises to bring the enormous resolution of D. melanogaster genetics to the study of reproductive isolation and species differences.

Animals↗

PubMed2000

Species concepts and malaria parasites: detecting a cryptic species of Plasmodium.

Species of malaria parasite (phylum Apicomplexa: genus Plasmodium) have traditionally been described using the similarity species concept (based primarily on differences in morphological or life-history characteristics). The biological species concept (reproductive isolation) and phylogenetic species concept (based on monophyly) have not been used before in defining species of Plasmodium. Plasmodium azurophilum, described from Anolis lizards in the eastern Caribbean, is actually a two-species cryptic complex. The parasites were studied from eight islands, from Puerto Rico in the north to Grenada in the south. Morphology of the two species is very similar (differences are indistinguishable to the eye), but one infects only erythrocytes and the other only white blood cells. Molecular data for the cytochrome b gene reveal that the two forms are reproductively isolated; distinct haplotypes are present on each island and are never shared between the erythrocyte-infecting and leucocyte-infecting species. Each forms a monophyletic lineage indicating that they diverged before becoming established in the anoles of the eastern Caribbean. This comparison of the similarity, biological and phylogenetic species concepts for malaria parasites reveals the limited value of using only similarity measures in defining protozoan species.

Animals↗

PubMed2006

Apparent 'sympatric' speciation in ecologically similar herbivorous beetles facilitated by multiple colonizations of an island.

Coexistence of recently diverged and ecologically similar sister species in complete sympatry represents a particularly compelling case for sympatric speciation. This study investigates the possible sympatric origin of two coexisting bark beetle taxa that utilize the same host plant on the island of La Palma in the Canary Islands. Aphanarthrum subglabrum and Aphanarthrum glabrum ssp. nudum breed inside dead twigs of Euphorbia lamarckii plants and are closely related to the allopatric A. glabrum ssp. glabrum in Tenerife, El Hierro and La Gomera. We tested the various speciation hypotheses in a genealogical context, using mitochondrial gene fragments from Cytochrome Oxidase I and 16S, and nuclear gene fragments from Enolase, Elongation Factor 1alpha and Histone H3. Phylogenetic analyses of the combined nuclear DNA data strongly supported a sister relationship between two sympatric and reproductively isolated taxa in La Palma. However, network analyses of subdivided nonrecombinant segments of the Enolase locus indicated a closer relationship between the two allopatric A. glabrum subspecies, suggesting multiple colonizations of this island. A bimodal distribution of mtDNA haplotypes in La Palma further documented the independent colonization of this island, with asymmetric introgression of mtDNA between two lineages. Consequently, the sympatric origin of the La Palma species is concluded to have involved allopatric phases before the parallel colonization of this island and subsequent introgression at some loci. The clear genetic and morphological evidence for reproductive isolation between these species suggests that the sympatric completion of divergence was either due to initial genetic incompatibility, morphological character displacement in male genitalia, or a combination of these factors.

Animals↗

PubMed2001

Gene flow in wild chimpanzee populations: what genetic data tell us about chimpanzee movement over space and time.

The isolation of phylogenetically distinct primate immunodeficiency viruses from at least seven wild-born, captive chimpanzees indicates that viruses closely related to HIV-1 may be endemic in some wild chimpanzee populations. The search for the chimpanzee population or populations harbouring these viruses is therefore on. This paper attempts to answer the question of whether or not such populations of chimpanzees are likely to exist at all, and, if so, where they are likely to be found. We summarize what is known about gene flow in wild populations of chimpanzees, both between major phylogeographical subdivisions of the species, and within these subdivisions. Our analysis indicates that hitherto undocumented reproductively isolated chimpanzee populations may in fact exist. This conclusion is based on the observation that, despite limited geographical sampling and limited numbers of genetic loci, conventional notions of the nature and extent of chimpanzee gene flow have recently been substantially revised. Molecular genetic studies using mitochondrial DNA sequences and hypervariable nuclear microsatellite markers have indicated the existence of heretofore undocumented barriers to chimpanzee gene flow. These studies have identified at least one population of chimpanzees genetically distinct enough to be classified into a new subspecies (Pan troglodytes vellerosus). At the same time, they have called into question the long-accepted genetic distinction between eastern chimpanzees (Pan troglodytes schweinfurthii) and western equatorial chimpanzees (Pan troglodytes troglodytes). The same studies have further indicated that gene flow between local populations is more extensive than was previously thought, and follows patterns sometimes inconsistent with those documented through direct behavioural observation. Given the apparently incomplete nature of the current understanding of chimpanzee gene flow in equatorial Africa, it seems reasonable to speculate that a chimpanzee population or populations may exist which both harbour the putative HIV-1 ancestor, and which have remained reproductively isolated from other chimpanzee populations over the time-scale relevant to the evolution of the SIVcpz-HIV-1 complex of viruses. Continued extensive sampling of wild chimpanzee populations, both for their genes and their viruses, should be performed quickly considering the high probability of extinction that many wild chimpanzee populations face today. The history of human-chimpanzee contacts is discussed.

Africa↗

PubMed2001

The role of hybridization in evolution.

Hybridization may influence evolution in a variety of ways. If hybrids are less fit, the geographical range of ecologically divergent populations may be limited, and prezygotic reproductive isolation may be reinforced. If some hybrid genotypes are fitter than one or both parents, at least in some environments, then hybridization could make a positive contribution. Single alleles that are at an advantage in the alternative environment and genetic background will introgress readily, although such introgression may be hard to detect. 'Hybrid speciation', in which fit combinations of alleles are established, is more problematic; its likelihood depends on how divergent populations meet, and on the structure of epistasis. These issues are illustrated using Fisher's model of stabilizing selection on multiple traits, under which reproductive isolation evolves as a side-effect of adaptation in allopatry. This confirms a priori arguments that while recombinant hybrids are less fit on average, some gene combinations may be fitter than the parents, even in the parental environment. Fisher's model does predict heterosis in diploid F1s, asymmetric incompatibility in reciprocal backcrosses, and (when dominance is included) Haldane's Rule. However, heterosis arises only when traits are additive, whereas the latter two patterns require dominance. Moreover, because adaptation is via substitutions of small effect, Fisher's model does not generate the strong effects of single chromosome regions often observed in species crosses.

Alleles↗

PubMed2004

Sympatric speciation by sexual selection alone is unlikely.

According to Darwin, sympatric speciation is driven by disruptive, frequency-dependent natural selection caused by competition for diverse resources. Recently, several authors have argued that disruptive sexual selection can also cause sympatric speciation. Here, we use hypergeometric phenotypic and individual-based genotypic models to explore sympatric speciation by sexual selection under a broad range of conditions. If variabilities of preference and display traits are each caused by more than one or two polymorphic loci, sympatric speciation requires rather strong sexual selection when females exert preferences for extreme male phenotypes. Under this kind of mate choice, speciation can occur only if initial distributions of preference and display are close to symmetric. Otherwise, the population rapidly loses variability. Thus, unless allele replacements at very few loci are enough for reproductive isolation, female preferences for extreme male displays are unlikely to drive sympatric speciation. By contrast, similarity-based female preferences that do not cause sexual selection are less destabilizing to the maintenance of genetic variability and may result in sympatric speciation across a broader range of initial conditions. Certain groups of African cichlids have served as the exclusive motivation for the hypothesis of sympatric speciation by sexual selection. Mate choice in these fishes appears to be driven by female preferences for extreme male phenotypes rather than similarity-based preferences, and the evolution of premating reproductive isolation commonly involves at least several genes. Therefore, differences in female preferences and male display in cichlids and other species of sympatric origin are more likely to have evolved as isolating mechanisms under disruptive natural selection.

Animals↗

PubMed1999

Apoptosis detected in hybrids between nicotiana glutinosa and N. repanda expressing lethality

Hybrid lethality is one of the mechanisms for reproductive isolation. Apoptotic features were detected in the cells of hybrid seedlings of Nicotiana glutinosa L. x N. repanda Willd. Condensation of chromatin and fragmentation of nuclei were observed in the leaf protoplasts isolated from hybrid seedlings expressing this lethality. Fragmentation of nuclei was correlated with the progression of lethal symptoms, as confirmed by fluorimetry of the nuclear DNA using laser scanning cytometry. Agarose gel analysis of DNA extracted from hybrid leaves showing lethality revealed a specific ladder pattern suggesting nucleosomal fragmentation associated with nuclear fragmentation. In-situ detection of DNA fragmentation using terminal deoxyribonucleotidyl transferase-mediated dUTP-fluorescein nick end labeling (TUNEL) showed that this process occurred in all leaf cells. This is the first evidence that apoptosis can induce suicide of hybrid plants, thus leading to reproductive isolation.

Journal Article↗

PubMed1999

Spatial genetic structure of delphinium nuttallianum populations: inferences about gene flow

The spatial genetic structure of a plant population provides a potential record of past gene flow and mating. We used hierarchical F-statistics and spatial autocorrelation to characterize spatial genetic differentiation of allozymes in adult Delphinium nuttallianum plants within and among six natural populations separated from one another by up to 3 km. Previous direct estimates suggested that gene flow is highly localized, averaging << 10 m. Earlier studies of seed-set, pollen-tube growth and progeny fitness suggested that partial reproductive isolation exists between plants growing too close together (<3 m) and too far apart (>100 m). Thus we anticipated substantial genetic differentiation on scales of a few to hundreds of metres. However, we detected little differentiation among the six populations, among replicate study plots within populations, or among subsections of study plots, except at the smallest scale of cm to m. These results suggest that relatively rare long-distance pollen movement has gone undetected and that postpollination selection may further modify genetic structure during the life cycle. Lack of differentiation is not at odds with the observation of partial reproductive isolation, because some loci may respond to spatial variation in selection without this response being evident at marker loci.

Journal Article↗

PubMed1998

Effects of A and B Wolbachia and host genotype on interspecies cytoplasmic incompatibility in Nasonia.

Wolbachia endosymbionts cause postmating reproductive isolation between the sibling species Nasonia vitripennis and N. giraulti. Most Nasonia are doubly infected with a representative from each of the two major Wolbachia groups (A and B). This study investigates the role of single (A or B) and double (A and B) Wolbachia infections in interspecies cytoplasmic incompatibility (CI) and host genomic influences on the incompatibility phenotype. Results show that the single A Wolbachia harbored in N. vitripennis (wAv) is bidirectionally incompatible with the single A Wolbachia harbored in N. giraulti (wAg). Results also indirectly show that the N. vitripennis wBv is bidirectionally incompatible with the N. giraulti wBg. The findings support current phylogenetic evidence that suggests these single infections have independent origins and were acquired via horizontal transfer. The wAv Wolbachia expresses partial CI in the N. vitripennis nuclear background. However, following genomic replacement by introgression, wAv expresses complete CI in the N. giraulti background and remains bidirectionally incompatible with wAg. Results show that double infections can reinforce interspecies reproductive isolation through the addition of incompatibility types and indicate that the host genome can influence incompatibility levels. This study has implications for host-symbiont coevolution and the role of Wolbachia in speciation.

Animals↗

PubMed2005

Interspecific phylogeography of the Stator limbatus species complex: the geographic context of speciation and specialization.

Diversification in phytophagous insects is often attributed to a propensity toward specialization and to a tendency for speciation to be associated with host-shifts. Phylogenetic analysis revealed a sister relationship between the generalist Stator limbatus and the specialist host-shifted Stator beali, providing a system to examine the genealogical and geographic origins of the main processes involved in this diversification: host-shifts, specialization, and reproductive isolation. We examine the interspecific phylogeographic relationships between these species using mitochondrial DNA sequence data. S. beali is derived within S. limbatus, rendering the latter paraphyletic and suggesting a budding process of speciation. The inherent polarity in this genealogical pattern indicates that the specialist habit, clumping oviposition behavior, and distinct genitalia of S. beali are all derived from the ancestral S. limbatus. The phylogeography of S. limbatus also shows strong geographic structure with divergences corresponding to known biogeographic boundaries, indicating that this evolutionary signal has not been erased by the vagaries of history. However, the derivation of S. beali and the evolution of reproductive isolation between the two species does not correspond to these known biogeographic boundaries, as S. beali and its sister clade of S. limbatus are restricted to the same geographic province. The geographic proximity of diversification combined with a divergence time estimated at the beginning of the Pleistocene indicates that speciation likely occurred very rapidly, although further genetic and ecological work is necessary to examine the mode of speciation. This study provides the historical context for ongoing evolutionary, ecological, and quantitative genetic research on the divergence in diet breadth between these species.

Animals↗