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PubMed2005

Chromosome speciation: humans, Drosophila, and mosquitoes.

Chromosome rearrangements (such as inversions, fusions, and fissions) may play significant roles in the speciation between parapatric (contiguous) or partly sympatric (geographically overlapping) populations. According to the "hybrid-dysfunction" model, speciation occurs because hybrids with heterozygous chromosome rearrangements produce dysfunctional gametes and thus have low reproductive fitness. Natural selection will, therefore, promote mutations that reduce the probability of intercrossing between populations carrying different rearrangements and thus promote their reproductive isolation. This model encounters a disabling difficulty: namely, how to account for the spread in a population of a chromosome rearrangement after it first arises as a mutation in a single individual. The "suppressed-recombination" model of speciation points out that chromosome rearrangements act as a genetic filter between populations. Mutations associated with the rearranged chromosomes cannot flow from one to another population, whereas genetic exchange will freely occur between colinear chromosomes. Mutations adaptive to local conditions will, therefore, accumulate differentially in the protected chromosome regions so that parapatric or partially sympatric populations will genetically differentiate, eventually evolving into different species. The speciation model of suppressed recombination has recently been tested by gene and DNA sequence comparisons between humans and chimpanzees, between Drosophila species, and between species related to Anopheles gambiae, the vector of malignant malaria in Africa.

Animals↗
PubMed2006

Contrasting genetic structures across two hybrid zones of a tropical reef fish, Acanthochromis polyacanthus (Bleeker 1855).

Hybrid zones are natural laboratories offering insights into speciation processes. Narrow hybrid zones are less common in the sea than on land consistent with higher dispersal among marine populations. Acanthochromis polyacanthus is an unusual bony marine fish with philopatric dispersal that exists as allopatric stocks of white, bicoloured and black fish on the Great Barrier Reef (GBR). At two latitudes, different morphs coexist and hybridize at narrow contact zones. Sequence data from mitochondrial Hypervariable Region 1 revealed contrasting patterns of introgression across these zones. At the northern hybrid zone, a single clade of mitochondrial haplotypes was found in all white fish, hybrids and tens of kilometres into pure bicoloured stock. At the southern hybrid zone, there was no introgression of mitochondrial genes into black fish and hybrids shared the bicoloured haplotypes. Based on this asymmetry, we postulate that black fish from the southern GBR have experienced a selective sweep of their mitochondrial genome, which has resulted in almost total reproductive isolation.

Animals↗
PubMed2006

Receptor expression and sympatric speciation: unique olfactory receptor neuron responses in F1 hybrid Rhagoletis populations.

The Rhagoletis pomonella species complex is one of the foremost examples supporting the occurrence of sympatric speciation. A recent study found that reciprocal F(1) hybrid offspring from different host plant-infesting populations in the complex displayed significantly reduced olfactory host preference in flight-tunnel assays. Behavioral and electrophysiological studies indicate that olfactory cues from host fruit are important chemosensory signals for flies to locate fruit for mating and oviposition. The reduced olfactory abilities of hybrids could therefore constitute a significant post-mating barrier to gene flow among fly populations. The present study investigated the source of changes in the hybrid olfactory system by examining peripheral chemoreception in F(1) hybrid flies, using behaviorally relevant volatiles from the parent host fruit. Single-sensillum electrophysiological analyses revealed significant changes in olfactory receptor neuron (ORN) response specificities in hybrid flies when compared to parent ORN responses. We report that flies from F(1) crosses of apple-, hawthorn- and flowering dogwood-origin populations of R. pomonella exhibited distinct ORN response profiles absent from any parent population. These peripheral alterations in ORN response profiles could result from misexpression of multiple receptors in hybrid neurons as a function of genomic incompatibilities in receptor-gene pathways in parent populations. We conclude that these changes in peripheral chemoreception could impact olfactory host preference and contribute directly to reproductive isolation in the Rhagoletis complex, or could be genetically coupled to other host-associated traits.

Animals↗
PubMed2004

Postzygotic isolating factor in sympatric speciation in Rhagoletis flies: reduced response of hybrids to parental host-fruit odors.

Rhagoletis pomonella is a model for sympatric speciation (divergence without geographic isolation) by means of host-plant shifts. Many Rhagoletis species are known to use fruit odor as a key olfactory cue to distinguish among their respective host plants. Because Rhagoletis rendezvous on or near the unabscised fruit of their hosts to mate, behavioral preferences for fruit odor translate directly into premating reproductive isolation among flies. Here, we report that reciprocal F(1) hybrids between the apple and hawthorn host races of R. pomonella, as well as between the host races and an undescribed sibling species infesting Cornus florida (flowering dogwood) do not respond to host fruit volatiles in wind-tunnel assays at doses that elicit maximal directed flight in parental flies. The reduced ability of hybrids to orient to fruit volatiles could result from a conflict between neural pathways for preference and avoidance behaviors, and it suggests that hybrids might suffer a fitness disadvantage for finding fruit in nature. Therefore, host-specific mating may play a dual role as an important postzygotic as well as a premating reproductive barrier to isolate sympatric Rhagoletis flies.

Animals↗
PubMedDate not supplied

Chromosomal speciation and folliculogenesis in the semisterile interspecific hybrids of the varying lemmings, genus Dicrostonyx.

Karyotypes of Dicrostonyx torquatus torquatus and D. vinogradovi that are very similar phenotypically differ at least by 13-14 chromosomal rearrangements including centric and tandem fusions, fissions and paracentric inversion. In their male hybrid F1 spermatogenesis is arrested on early pre-meiotic stages, the female hybrids are semisterile. Histological investigation of ovaries has demonstrated reduced number of follicles, higher percentage of atretic follicles and an acceleration of folliculogenesis in hybrids as compared with the parental forms. As a result of the acceleration, the number of mature follicles and ovulations is not significantly diminished in the hybrid females, and their semisterility is caused, to a great extent, by high prenatal loss which most likely arises from the chromosomal imbalance in F2 hybrids. Thus, the mechanism suggested by the concept of chromosomal speciation is active, but not alone in forming reproductive isolation between twin species of the varying lemming.

Animals↗
PubMed1995

Ancestry of an isolated subspecies of salamander, Ambystoma tigrinum stebbinsi Lowe: the evolutionary significance of hybridization.

Most phylogenetic systematists assume speciation results in dichotomously branching phylogenies. Hybridization that gives rise to a new lineage can produce character homoplasty that might obscure a species' true history. We report the results of a restriction-enzyme analysis of mitochondrial DNA (mtDNA) variation in three tiger salamander subspecies (Ambystoma trigrinum mavortium, Ambystoma tigrinum nebulosum, and Ambystoma trigrinum stebbinsi) and compare the results to studies of morphological and allozymic variation in these taxa. Allozymically, A. t. mavortium and A. t. nebulosum share most of their genomes (although each has several unique alleles), yet color pattern and mtDNA haplotypes are distinct. Color pattern and allozyme data suggest that A. t. stebbinsi shares a common ancestor with A. t. mavortium, while the A. t. stebbinsi mtDNA haplotype is derived from an A. t. nebulosum haplotype. Thus, our data suggest that A. t. stebbinsi originated through hybridization between A. t. mavortium and A. t. nebulosum. That hybridization can produce recognizably distinct evolutionary entities has long been recognized for plants, but the evolutionary significance of hybridization in animals should be examined more closely. Conservation agencies must recognize that hybrids and hybrid taxa are not necessarily evolutionary "mistakes," and they might have significant importance in the production of natural biodiversity.

Animals↗
PubMed2005

Impact of experimental design on Drosophila sexual isolation studies: direct effects and comparison to field hybridization data.

Many studies of speciation rely critically on estimates of sexual isolation obtained in the laboratory. Here we examine the sensitivity of sexual isolation to alterations in experimental design and mating environment in two sister species of Drosophila, D. santomea and D. yakuba. We use a newly devised measure of mating frequencies that is able to disentangle sexual isolation from species differences in mating propensity. Variation in fly density, presence or absence of a quasi-natural environment, degree of starvation, and relative frequency of species had little or no effect on sexual isolation, but one factor did have a significant effect: the possibility of choice. Designs that allowed flies to choose between conspecific and heterospecific mates showed significantly more sexual isolation than other designs that did not allow choice. These experiments suggest that sexual isolation between these species (whose ranges overlap on the island of São Tomé) is due largely to discrimination against D. yakuba males by D. santomea females. This suggestion was confirmed by direct observations of mating behavior. Drosophila santomea males also court D. yakuba females less ardently than conspecific females, whereas neither males nor females of D. yakuba show strong mate discrimination. Thus, sexual isolation appears to be a result of evolutionary changes in the derived island endemic D. santomea. Surprisingly, as reported in a companion paper (Llopart et al. 2005), the genotypes of hybrids found in nature do not accord with expectations from these laboratory studies: all F1 hybrids in nature come from matings between D. santomea females and D. yakuba males, matings that occur only rarely in the laboratory.

Animals↗
PubMed2006

Hybrid origin of the Pliocene ancestor of wild goats.

Recent theories on speciation suggest that interspecific hybridization is an important mechanism for explaining adaptive radiation. According to this view, hybridization can promote the rapid transfer of adaptations between different species; the hybrid population thus invades new habitats and diversifies into a variety of new species. Although hybridization is well accepted as a fairly common mechanism for diversification in plants, its role in the evolution of animals is more controversial, because reduced fitness would typically condemn animal hybrids to an evolutionary dead-end. Here, we examine DNA sequences of four mitochondrial and four nuclear genes selected for resolving phylogenetic relationships between goats, sheep, and their allies. Our analyses provide evidence of strong discordance for the position of Capra between mitochondrial and nuclear phylogenies. We suggest that the common ancestor of wild goats arose from interspecific hybridization, and that the mitochondrial genome of a species better adapted to life at high altitudes was transferred via this route into the common ancestor of Capra. We propose that the acquisition of more efficient mitochondria has conferred a selective advantage on goats, allowing their rapid adaptive radiation during the Plio-Pleistocene epoch. Our study therefore agrees with theories that predict an important role for interspecific hybridization in the evolution and diversification of animal species.

Altitude↗
PubMed2004

Reticulate sympatric speciation in Cameroonian crater lake cichlids.

BACKGROUND: Traditionally the rapid origin of megadiverse species flocks of extremely closely related species is explained by the combinatory action of three factors: Disruptive natural selection, disruptive sexual selection and partial isolation by distance. However, recent empirical data and theoretical advances suggest that the diversity of complex species assemblages is based at least partially on the hybridization of numerous ancestral allopatric lineages that formed hybrids upon invasion of new environments. That reticulate speciation within species flocks may occur under sympatric conditions after the primary formation of species has been proposed but not been tested critically. RESULTS: We reconstructed the phylogeny of a complex cichlid species flock confined to the tiny Cameroonian crater lake Barombi Mbo using both mitochondrial and nuclear (AFLP) data. The nuclear phylogeny confirms previous findings which suggested the monophyly and sympatric origin of the flock. However, discordant intra-flock phylogenies reconstructed from mitochondrial and nuclear data suggest strongly that secondary hybridization among lineages that primarily diverged under sympatric conditions had occurred. Using canonical phylogenetic ordination and tree-based tests we infer that hybridization of two ancient lineages resulted in the formation of a new and ecologically highly distinct species, Pungu maclareni. CONCLUSIONS: Our findings show that sympatric hybrid speciation is able to contribute significantly to the evolution of complex species assemblages even without the prior formation of hybrids derived from allopatrically differentiated lineages.

Journal Article↗
PubMed2003

Reduced meiotic fitness in hybrids with heterozygosity for heterochromatin in the speciating Mus terricolor complex.

Mus terricolor I, II and III are the three chromosomal species which differ in stable autosomal short-arm heterochromatin variations established in homozygous condition. Analysis of meiosis in the laboratory-generated F1 male hybrids from crosses (both ways) between M. terricolor I and II and between M. terricolor I and III shows high frequencies of pairing abnormalities at pachytene. The backcross (N3 generation) male hybrids between M. terricolor I and II have meiotic abnormalities as in the F1 male hybrids, though to a lesser extent. They show difference in pairing abnormalities in the different karyotypic forms; the backcross hybrids heterozygous for the heterochromatic short arms have more anomalies compared to the homokaryotypic hybrids. This suggests a negative influence of the heterochromatin heterozygosity in meiotic pairing. The results indicate a role for heterochromatin variations in the development of a reproductive barrier in the speciating M. terricolor complex.

Animals↗
PubMed2005

On-line hyphenation of capillary electrophoresis with flame-heated furnace atomic absorption spectrometry for trace mercury speciation.

Capillary electrophoresis (CE) was directly interfaced to flame-heated furnace atomic absorption spectrometry (FHF-AAS) via a laboratory-made thermospray interface for nanoliter trace element speciation. The CE-FHF-AAS interface integrated the superiorities of stable CE separation, complete sample introduction, and continuous vaporization for AAS detection without the need of extra external heat sources and any post-column derivation steps. To demonstrate the usefulness of the developed hybrid technique for speciation analysis, three environmentally significant and toxic forms of methylmercury (MeHg), phenylmercury (PhHg), and inorganic mercury (Hg(II)) were taken as model analytes. Baseline separation of the three mercury species was achieved by CE in a 60 cm long x 75 microm inner diameter fused-silica capillary at 20 kV and using a mixture of 100 mM boric acid and 10% v/v methanol (pH 8.30) as running electrolyte. The precision (relative standard deviation, RSD, n = 7) of migration time, peak area and peak height for the mercury species at 500 microg x L(-1) (as Hg) level were in the range of 0.9-1.2%, 1.5-1.9%, and 1.4-2.0%, respectively. The detection limit (S/N = 3) of three mercury species was 3.0 +/- 0.15 pg (as Hg), corresponding to 50.8 +/- 2.4 microg x L(-1) (as Hg) for 60 nL sample injection, which was almost independent on specific mercury species. The developed hybrid technique was successfully applied to the speciation analysis of mercury in a certified reference material (DORM-2, dogfish muscle).

Electrophoresis, Capillary↗
PubMed2005

Barrier to gene flow between two ecologically divergent Populus species, P. alba (white poplar) and P. tremula (European aspen): the role of ecology and life history in gene introgression.

The renewed interest in the use of hybrid zones for studying speciation calls for the identification and study of hybrid zones across a wide range of organisms, especially in long-lived taxa for which it is often difficult to generate interpopulation variation through controlled crosses. Here, we report on the extent and direction of introgression between two members of the "model tree" genus Populus: Populus alba (white poplar) and Populus tremula (European aspen), across a large zone of sympatry located in the Danube valley. We genotyped 93 hybrid morphotypes and samples from four parental reference populations from within and outside the zone of sympatry for a genome-wide set of 20 nuclear microsatellites and eight plastid DNA restriction site polymorphisms. Our results indicate that introgression occurs preferentially from P. tremula to P. alba via P. tremula pollen. This unidirectional pattern is facilitated by high levels of pollen vs. seed dispersal in P. tremula (pollen/seed flow = 23.9) and by great ecological opportunity in the lowland floodplain forest in proximity to P. alba seed parents, which maintains gene flow in the direction of P. alba despite smaller effective population sizes (N(e)) in this species (P. alba N(e)c. 500-550; P. tremula N(e)c. 550-700). Our results indicate that hybrid zones will be valuable tools for studying the genetic architecture of the barrier to gene flow between these two ecologically divergent Populus species.

Austria↗
PubMed2025

Patterns of Genomic Divergence and Introgression in Two Primulina Hybrid Zones.

Hybrid zones have long been promoted as natural laboratories for understanding the mechanisms of speciation. Multiple or replicated hybrid zones are particularly informative, as they allow for assessing the consistency of genomic divergence and introgression across different environmental contexts and demographic histories, thereby improving our understanding of the factors that drive or hinder speciation on a broader scale. Here, using whole-genome resequencing data, we compare the patterns of genomic divergence and introgression in two Primulina hybrid zones. We found that genomic divergence in both hybrid zones is largely shaped by neutral processes, with only a few genomic regions showing signatures of balancing or lineage-specific selection. Genomic cline analyses identified numerous SNPs that showed significantly steeper clines and biased centres than the genome-wide expectation in both hybrid zones, consistent with the existence of reproductive barriers. Within regions of restricted gene flow, we identified 21 genes shared between the two hybrid zones. Annotation of gene function revealed that several genes are involved in reproductive processes. In addition, many zone-specific outlier loci were linked to genes associated with pollen and flower development, suggesting that these barriers may contribute to reproductive isolation under localised ecological conditions. Overall, these findings suggest that while certain reproductive barriers remain consistent across independent hybrid zones, others may be contingent on local environmental contexts. Our results demonstrate that both general and zone-specific mechanisms contribute to reproductive isolation in Primulina, providing empirical evidence that some genomic barriers recur across independent hybrid zones while others arise through localised adaptation.

Lamiales↗
PubMed1982

Immunological factors in Peromyscus speciation.

Reciprocal interspecific F1 hybrids of deermice (Peromyscus maniculatus) and oldfield mice (P. polionotus) differ significantly and substantially in fetal and placental, as well as adult, size and weight. Hybrid fetal mortality is associated with large conceptus size. Skin grafts were exchanged between and within the two species to ascertain whether any relationship exists between mean graft retention time and body size of fetuses and adults. P. maniculatus skin grafted to P. polionotus rejected significantly earlier than the reciprocal xenograft. All interspecific graft combinations rejected significantly earlier than intraspecific grafts. Pre-immunization of female P. maniculatus with con- and trans-specific paternal spleen cell antigens reduced fetal, placental, neonatal, and ten-day size compared with controls. Size, weight, fertility, and graft rejection data were compared with several theoretical models. The data were consistent with the hypothesis that immunological disparity between the species could produce marked size variations in reciprocal hybrids. Multiple minor histocompatibility factors can account for large placental size and fetal mortality in Peromyscus hybrids. Physiological reproductive isolation may result from immunological differences between closely allied species.

Animals↗
PubMed2003

Gene flow between cassava, Manihot esculenta Crantz, and wild relatives.

Controlled and natural hybridization between cassava and wild relatives does occur. Barriers within the genus appear to be weak due to recent evolution of the group. All Manihot species examined cytogenetically have a chromosome number of 2n = 36. However, they behave meiotically as diploids. The weak interspecific barriers have led to an extremely heterozygous gene pool that may begin a sequence of hybridization followed by speciation. Introgression from cassava into a number of wild species (M. neusana, M. alutacea, M. reptans and M. anomala) has been detected by both morphological marker genes and molecular techniques. Winged fruit, setaceous bracteoles, and wide leaf sinus were dominant genes that came from cassava and appeared in the hybrids. The characteristic protein bands of cassava were recognized in the hybrid seed protein electrophoresis.

Crosses, Genetic↗
PubMedDate not supplied

Artificial life and speciation, a case study: heterochromatin and speciation in the Microtus savii Group (Rodentia-Arvicolinae).

Artificial life is a tool which is used for simulation of peculiar cases of evolutionary events. The main characteristic of artificial life is that with this technique it is possible to simulate for a high number of generations the evolution of a population of individuals. Each individual is characterised by a small number of parameters, but each individual has its own evolutive story. So far it is possible to simulate the evolution of a population of some thousands specimens, for a high number of generations. The realistic aspect of the simulation is that each specimen is taken individually. In our opinion this instrument is very useful to simulate the evolution of the hybrids barrier during speciation. For this reason it is applied to a peculiar case of speciation, that of the Savi pine vole (Microtus savii) whose experimental data were recently investigated.

Animals↗
PubMed1995

Application of DNA fingerprinting to the taxonomy of European blackberry species.

Thousands of blackberry species (Rubus subgenus Rubus) have been described from Central and Northern Europe. These species are usually polyploid and pseudogamous. Since apomixis is only facultative, the species have retained the possibility to produce some offspring by sexual recombination. Hybridization of restriction-enzyme-digested DNA samples to the M13, (AC)/(TG) and 33.15 probes has yield valuable information on the occurrence and distribution of genetic variation in Rubus. Intraspecific variation proved to be very restricted, with identical DNA fingerprints sometimes being exhibited by Swedish, Danish and German populations of the same species. By comparison, interspecific variation was usually pronounced, suggesting that most species have been derived through genetic recombination. Speciation through interspecific hybridization was demonstrated in one case. Several critical taxa were investigated; some of these proved to have unique DNA fingerprints, whereas others appear to be identical with previously described species. Evaluation of raspberry cultivars, obtained through several generations of vegetative propagation or with various in vitro techniques, failed to detect any DNA fingerprint variation, suggesting that the fingerprints are somatically stable. Blackberry seedlings, obtained by pollinating various species as well as interspecific hybrids with pollen from yet other species, were successfully evaluated with DNA fingerprinting to determine the relative proportions of the apomictic and sexual seed set.

DNA Fingerprinting↗