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PubMed2002

SRY evolution in Cebidae (Platyrrhini: Primates).

Sex determination in mammals is dependent on the presence of SRY, which codes for a protein with a DNA binding motif (the HMG-box domain). Here we analyze the evolution of SRY among seven genera of New World monkeys belonging to the family Cebidae. Estimates of the number of synonymous and nonsynonymous substitutions indicated the absence of positive selection acting on SRY evolution. The presence of indels at the C-terminus coding region in different genera and species maintained an open reading frame, indicating a selective pressure constraining the evolution of this coding region. Available data on the fertility of natural and captive interspecific hybrids failed to show any relationship between SRY evolution and speciation for the genera herein studied. Our phylogenetic arrangement for Cebidae genera was similar to previous topologies based on mitochondrial and autosomal DNA sequences. This arrangement also corroborated the division of Cebus into two species groups. However, for Callithrix the differences among SRY topology and those derived from autosomal and mitochondrial genes suggested a Y-chromosome ancestral polymorphism.

Amino Acid Sequence↗
PubMed2006

Evolutionary biology: how did the human species form?

A recent analysis has shown that divergence between human and chimpanzee varies greatly across the genome. Although this is consistent with "hybridisation" between the diverging human and chimp lineages, such observations can be explained more simply by the null model of allopatric speciation.

Animals↗
PubMed1992

Mapping and characterization of a 'speciation gene' in Drosophila.

Almost nothing is known about the identity of the genes causing reproductive isolation between species. As a first step towards molecular isolation of a 'speciation gene', I mapped and partly characterized a gene causing hybrid male sterility in Drosophila. This analysis shows that sterility of D. melanogaster males who carry the 'dot' fourth chromosome from D. simulans is due entirely to a very small region of the D. simulans chromosome (including only about 5 salivary gland bands or approximately 250 kb of DNA). Thus the hybrid sterility effect of the D. simulans fourth chromosome is almost surely due to a single gene of very large effect (here named hms, hybrid male sterile). Hms is zygotically acting, and the D. simulans allele of hms is completely recessive. Furthermore, complementation tests suggest that hms is not an allele of any known locus in D. melanogaster.

Alleles↗
PubMed1993

Haldane's rule has multiple genetic causes.

Haldane's rule states that "When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous [heterogametic or XY] sex". This rule represents one of the few patterns characterizing animal speciation. Traditional explanations of Haldane's rule claim that heterogametic hybrids are unfit because they lack an X chromosome that is 'compatible' with the autosomes of one species. Recent work shows that this explanation is incorrect for hybrid sterility: contrary to prediction, homogametic hybrids carrying both X chromosomes from the same species remain fertile. Until now, similar tests have not been performed for hybrid inviability. Here I show that homogametic hybrids who carry both X chromosomes from the same species are inviable. These results show tht the genetic causes of Haldane's rule differ for hybrid sterility versus inviability. Haldane's rule does not, therefore, have a single genetic basis.

Animals↗
PubMed2002

Assessment of rDNA IGS as a molecular marker in the Simulium damnosum complex.

For five cytospecies of the Simulium damnosum Theobald complex of blackflies (Diptera: Simuliidae) from West Africa, both ends of the intergenic spacer region (IGS) of the rDNA have been sequenced with the aim of developing specific molecular markers. No specific differences in these two regions were detected between Simulium sanctipauli V. & D., Simulium sirbanum V. & D., Simulium soubrense V. & D., Simulium squamosum Enderlein and Simulium yahense V. & D., except in the number of A subrepeats at the 5' end of the IGS (two in S. squamosum and four or five in the others) and in position 310 of the 3' end (a C in S. squamosum and a G in the others). However, genetic distances within and between species overlapped. These DNA sequences had no strong phylogenetic signal, and the trees obtained were mostly unresolved. Although most sequences from S. squamosum clustered together, a few of them were more similar to those in other cytospecies. These results could be explained either by hybridization with genetic introgression or by ancestral polymorphism and recent speciation.

Africa, Western↗
PubMed1993

Mitochondrial DNA polymorphisms in subterranean mole-rats of the Spalax ehrenbergi superspecies in Israel, and its peripheral isolates.

Patterns of mitochondrial DNA (mtDNA) variation were examined in 133 mole-rats constituting all four chromosomal species (2n = 52, 2n = 54, 2n = 58, and 2n = 60) of the Spalax ehrenbergi superspecies in Israel, as well as the peripheral isolates of 2n = 60. In the main range of the complex, a total of 28 mtDNA haplotypes were found in 64 mole-rats, with most haplotypes being unique to either a single chromosomal species or population. mtDNA divergence increased from low to high diploid number in a north-to-south direction in Israel. Overall levels of mtDNA diversity were unexpectedly the highest in the 2n = 60, the youngest species of the complex. The mtDNA haplotypes can be separated into two major groups, 2n = 52-54 and 2n = 58-60, and a phylogenetic analysis for each group revealed evidence of a few haplotypes not sorted by diploid number. The overall patterns of mtDNA divergence seen within and among the four chromosomal species are consistent with the parapatric mode of speciation as suggested from previous studies of allozyme and DNA hybridization. In a separate data set the patterns of mtDNA variation were examined across the main geographic range and across peripheral semi-isolates and isolates of the 2n = 60 chromosomal species. Fifteen haplotypes were found in 69 mole-rats. High levels of mtDNA diversity characterized the main range, semi-isolated, and even some desert isolated populations. The peripheral isolates contain much mtDNA diversity, including novel haplotypes.

Animals↗
PubMed2005

Postmating-prezygotic isolation is not an important source of selection for reinforcement within and between species in Drosophila pseudoobscura and D. persimilis.

Most work on adaptive speciation to date has focused on the role of low hybrid fitness as the force driving reinforcement (the evolution of premating isolation after secondary contact that reduces the likelihood of matings between populations). However, recent theoretical work has shown that postmating, prezygotic incompatibilities may also be important in driving premating isolation. We quantified premating, postmating-prezygotic, and early postzygotic fitness effects in crosses among three populations: Drosophila persimilis, D. pseudoobscura USA (sympatric to D. persimilis), and D. pseudoobscura Bogotá (allopatric to D. persimilis). Interspecific matings were more likely to fail when they involved the sympatric populations than when they involved the allopatric populations, consistent with reinforcement. We also found that failure rate in sympatric mating trials depended on whether D. persimilis females were paired with D. pseudoobscura males or the reverse. This asymmetry most likely indicates differences in discrimination against heterospecific males by females. By measuring egg laying rate, fertilization success and hatching success, we also compared components of postmating-prezygotic and early postzygotic isolation. Postmating-prezygotic fitness costs were small and not distinguishable between hetero- and conspecific crosses. Early postzygotic fitness effects due to hatching success differences were also small in between-population crosses. There was, however, a postzygotic fitness effect that may have resulted from an X-linked allele found in one of the two strains of D. pseudoobscura USA. We conclude that the postmating-prezygotic fitness costs we measured probably did not drive premating isolation in these species. Premating isolation is most likely driven in sympatric populations by previously known hybrid male sterility.

Adaptation, Physiological↗
PubMed2007

Hybridization among cryptic species of the cellar fungus Coniophora puteana (Basidiomycota).

In this study we have analysed the genetic variation and phylogeography in a global sample of the cellar fungus Coniophora puteana, which is an important destroyer of wooden materials indoor. Multilocus genealogies of three DNA regions (beta tubulin, nrDNA ITS and translation elongation factor 1alpha) revealed the occurrence of three cryptic species (PS1-3) in the morphotaxon C. puteana. One of the lineages (PS3) is apparently restricted to North America while the other two (PS1-2) have wider distributions on multiple continents. Interspecific hybridization has happened between two of the lineages (PS1 and PS3) in North America. In three dikaryotic isolates, two highly divergent beta tubulin alleles coexisted, one derived from PS1 and one from PS3. Furthermore, one isolate included a recombinant ITS sequence, where ITS1 resembled the ITS1 version of PS3 while ITS2 was identical to a frequent PS1 ITS2 version. This pattern must be due to hybridization succeeded by intralocus recombination in ITS. The results further indicated that introgression has happened between subgroups appearing in PS1. We hypothesize that the observed reticulate evolution is due to previous allopatric separation followed by more recent reoccurrence in sympatry, where barriers to gene flow have not yet evolved. A complex phylogeographical structure is observed in the morphotaxon C. puteana caused by (i) cryptic speciation; (ii) the interplay between natural migration and distribution patterns and probably more recent human mediated dispersal events; and (iii) hybridization and introgression.

Base Sequence↗
PubMed2004

Prevalence of interspecific hybrids amongst asexual fungal endophytes of grasses.

Epichloë endophytes are fungal symbionts of grasses that span a continuum including asexual mutualists that are vertically transmitted, obligately sexual pathogens that are horizontally transmitted, and mixed-strategy symbionts with both mutualistic and pathogenic capabilities. Here we show that processes of genome evolution differ markedly for the different symbiont types. Genetic and phylogenetic analysis was conducted of a broad taxonomic, ecological and geographical sample of sexual and asexual isolates, in which were identified and sequenced alleles of genes for beta-tubulin (tub2) and translation elongation factor 1-alpha (tef1), and microsatellite alleles were identified by length polymorphisms. The majority of asexual isolates had two or three alleles of most loci, but every sexual isolate had only single alleles for each locus. Phylogenetic analysis of tub2 and tef1 indicated that in all instances of multiple alleles in an isolate, the alleles were derived from different sexual species. It is concluded that, whereas horizontally transmissible species had haploid genomes and speciation occurred cladistically, most of the strictly seedborne mutualists were interspecific hybrids with heteroploid (aneuploid or polyploid) genomes. Furthermore, the phylogenetic evidence indicated that, in at least some instances, hybridization followed rather than caused evolution of the strictly seedborne habit. Therefore, the abundance of hybrid species among grass endophytes, and their prevalence in many host populations suggests a selective advantage of hybridization for the mutualistic endophytes.

Base Sequence↗
PubMed2004

Molecular phylogeny of the plant pathogenic genus Botrytis and the evolution of host specificity.

The cosmopolitan genus Botrytis contains 22 recognized species and one hybrid. The current classification is largely based on morphological characters and, to a minor extent, on physiology and host range. In this study, a classification of the genus was constructed based on DNA sequence data of three nuclear protein-coding genes (RPB2, G3PDH, and HSP60) and compared with the traditional classification. Sexual reproduction and the host range, important fitness traits, were traced in the tree and used for the identification of major evolutionary events during speciation. The phylogenetic analysis corroborated the classical species delineation. In addition, the hybrid status of B. allii (B. byssoidea x B. aclada) was confirmed. Both individual gene trees and combined trees show that the genus Botrytis can be divided into two clades, radiating after the separation of Botrytis from other Sclerotiniaceae genera. Clade 1 contains four species that all colonize exclusively eudicot hosts, whereas clade 2 contains 18 species that are pathogenic on either eudicot (3) or monocot (15) hosts. A comparison of Botrytis and angiosperm phylogenies shows that cospeciation of pathogens and their hosts have not occurred during their respective evolution. Rather, we propose that host shifts have occurred during Botrytis speciation, possibly by the acquisition of novel pathogenicity factors. Loss of sexual reproduction has occurred at least three times and is supposed to be a consequence of negative selection.

Base Sequence↗
PubMed2003

Reproductive isolation driven by the combined effects of ecological adaptation and reinforcement.

Recent years have seen a resurgence of interest in the process of speciation but few studies have elucidated the mechanisms either driving or constraining the evolution of reproductive isolation. In theory, the direct effects of reinforcing selection for increased mating discrimination where interbreeding produces hybrid offspring with low fitness and the indirect effects of adaptation to different environments can both promote speciation. Conversely, high levels of homogenizing gene flow can counteract the forces of selection. We demonstrate the opposing effects of reinforcing selection and gene flow in Timema cristinae walking-stick insects. The magnitude of female mating discrimination against males from other populations is greatest when migration rates between populations adapted to alternate host plants are high enough to allow the evolution of reinforcement, but low enough to prevent gene flow from eroding adaptive divergence in mate choice. Moreover, reproductive isolation is strongest under the combined effects of reinforcement and adaptation to alternate host plants. Our findings demonstrate the joint effects of reinforcement, ecological adaptation and gene flow on progress towards speciation in the wild.

Adaptation, Biological↗
PubMed2001

Allozyme analysis of the hybrid origin of Arisaema ehimense (Araceae).

Allozyme diversity was examined in the Japanese jack-in-the pulpit species, Arisaema serratum and A. tosaense, and their putative hybrid species, A. ehimense (all diploid). Arisaeme ehimense contains mostly alleles found in one or both of the putative parent species but few unique alleles, partially supporting the hypothesis that A. ehimense is of hybrid origin from the two species, although the possibility that A. ehimense arose via divergent speciation cannot be excluded. Because only limited information was gained from the survey of allozyme variation, additional taxon-specific markers from A. serratum and A. tosaense are required to test rigorously the hybrid origin of A. ehimense. A phenogram based on allele frequencies suggested that introgression occurs between A. serratum and A. tosaense where these species are sympatric.

Alleles↗
PubMed2003

Engineering evolution to study speciation in yeasts.

The Saccharomyces 'sensu stricto' yeasts are a group of species that will mate with one another, but interspecific pairings produce sterile hybrids. A retrospective analysis of their genomes revealed that translocations between the chromosomes of these species do not correlate with the group's sequence-based phylogeny (that is, translocations do not drive the process of speciation). However, that analysis was unable to infer what contribution such rearrangements make to reproductive isolation between these organisms. Here, we report experiments that take an interventionist, rather than a retrospective approach to studying speciation, by reconfiguring the Saccharomyces cerevisiae genome so that it is collinear with that of Saccharomyces mikatae. We demonstrate that this imposed genomic collinearity allows the generation of interspecific hybrids that produce a large proportion of spores that are viable, but extensively aneuploid. We obtained similar results in crosses between wild-type S. cerevisiae and the naturally collinear species Saccharomyces paradoxus, but not with non-collinear crosses. This controlled comparison of the effect of chromosomal translocation on species barriers suggests a mechanism for the generation of redundancy in the S. cerevisiae genome.

Aneuploidy↗
PubMed2001

A genetic interpretation of ecologically dependent isolation.

Hybrids may suffer a reduced fitness both because they fall between ecological niches (ecologically dependent isolation) and as a result of intrinsic genetic incompatibilities between the parental genomes (ecologically independent isolation). Whereas genetic incompatibilities are common to all theories of speciation, ecologically dependent isolation is a unique prediction of the ecological model of speciation. This prediction can be tested using reciprocal transplants in which the fitness of various genotypes is evaluated in both parental habitats. Here we expand a quantitative genetic model of Lynch (1991) to include two parental environments. We ask whether a sufficient experimental design exists for detecting ecologically dependent isolation. Analysis of the model reveals that by using both backcrosses in both parental environments, environment-specific additive genetic effects can be estimated while correcting for any intrinsic genetic isolation. Environment-specific dominance effects can also be estimated by including the F1 and F2 in the reciprocal transplant. In contrast, a reciprocal transplant comparing only F1s or F2s to the parental species cannot separate ecologically dependent from intrinsic genetic isolation. Thus, a reduced fitness of F1 or F2 hybrids relative to the parental species is not sufficient to demonstrate ecological speciation. The model highlights the importance of determining the contribution of genetic and ecological mechanisms to hybrid fitness if inferences concerning speciation mechanisms are to be made.

Adaptation, Physiological↗
PubMed1998

Spermatogenesis in common shrews, Sorex araneus, from a hybrid zone with extensive Robertsonian polymorphism.

The analysis of the fertility of hybrid and nonhybrid individuals from a chromosomal contact zone gives us the possibility of studying the role of chromosomes in speciation processes. In this study, homozygous, "simple" and "complex" Robertsonian heterozygous male common shrews (Sorex araneus) from the Abisko-Sidensjö chromosomal hybrid zone in Sweden were analysed. The degree of germ cell death was estimated, sperm counts were performed and the testis and seminal vesicles weighed in each individual. Chromosome interactions and synapsis at pachytene were examined under the EM. The weight of the testis was significantly different in the three karyotypic groups and "complex" heterozygotes suffered higher germ cell death than homozygotes and "simple" heterozygotes. Interactions between chromosomes at pachytene were rare. Nonhomologous pairing in the centromeric regions of autosomal trivalents (side arms) was common while asynapsed segments were seldom found. Thus, the difference in the reproductive characteristics of the common shrew might be due to genetic factors rather than Robertsonian translocations.

Animals↗
PubMed2024

The centromere landscapes of four karyotypically diverse Papaver species provide insights into chromosome evolution and speciation.

Understanding the roles played by centromeres in chromosome evolution and speciation is complicated by the fact that centromeres comprise large arrays of tandemly repeated satellite DNA, which hinders high-quality assembly. Here, we used long-read sequencing to generate nearly complete genome assemblies for four karyotypically diverse Papaver species, P. setigerum (2n = 44), P. somniferum (2n = 22), P. rhoeas (2n = 14), and P. bracteatum (2n = 14), collectively representing 45 gapless centromeres. We identified four centromere satellite (cenSat) families and experimentally validated two representatives. For the two allopolyploid genomes (P. somniferum and P. setigerum), we characterized the subgenomic distribution of each satellite and identified a "homogenizing" phase of centromere evolution in the aftermath of hybridization. An interspecies comparison of the peri-centromeric regions further revealed extensive centromere-mediated chromosome rearrangements. Taking these results together, we propose a model for studying cenSat competition after hybridization and shed further light on the complex role of the centromere in speciation.

Centromere↗
PubMed2004

Contrasting genetic structure of adults and progeny in a Louisiana iris hybrid population.

Studies of natural hybridization have suggested that it may be a creative stimulus for adaptive evolution and speciation. An important step in this process is the establishment of fit recombinant genotypes that are buffered from subsequent recombination with unlike genotypes. We used molecular markers and a two-generation sampling strategy to infer the extent of recombination in a Louisiana iris hybrid zone consisting predominantly of Iris fulva-type floral phenotypes. Genotypic diversity was fairly high, indicating that sexual reproduction is frequent relative to clonal reproduction. However, we observed strong spatial genetic structure even after controlling for clonality, which implies a low level of pollen and seed dispersal. We therefore used cluster analysis to explore the hypothesis that the fulva-type hybrids are an admixture of groups between which there has been limited recombination. Our results indicate that several such groups are present in the population and are strongly localized spatially. This spatial pattern is not attributable strictly to a lack of mating opportunities between dissimilar genotypes for two reasons: (1) relatedness of flowering pairs was uncorrelated with the degree of overlap in flowering, and (2) paternity analysis shows that pollen movement among the outcross fraction occurred over large distances, with roughly half of all paternity attributed to pollen flow from outside the population. We also found evidence of strong inbreeding depression, indicated by contrasting estimates of the rate of self-fertilization and the average inbreeding coefficient of fulva-type hybrids. We conclude that groups of similar hybrid genotypes can be buffered from recombination at small spatial scales relative to pollen flow, and selection against certain recombinant genotypes may be as important as or more important than clonal reproduction and inbreeding.

Cluster Analysis↗
PubMed2000

Sexual selection and speciation in field crickets.

Recent theoretical work has shown that sexual selection may cause speciation under a much wider range of conditions than previously supposed. There are, however, no empirical studies capable of simultaneously evaluating several key predictions that contrast this with other speciation models. We present data on male pulse rates and female phonotactic responses to pulse rates for the field cricket Gryllus texensis; pulse rate is the key feature distinguishing G. texensis from its cryptic sister species G. rubens. We show (i) genetic variation in male song and in female preference for song, (ii) a genetic correlation between the male trait and the female preference, and (iii) no character displacement in male song, female song recognition, female species-level song discrimination, or female song preference. Combined with previous work demonstrating a lack of hybrid inviability, these results suggest that divergent sexual selection may have caused speciation between these taxa.

Animals↗