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PubMed2001

Incipient speciation by sexual isolation in Drosophila: concurrent evolution at multiple loci.

Drosophila melanogaster from Zimbabwe and nearby regions shows strong but asymmetric sexual isolation from its cosmopolitan counterparts. By creating stable chromosome-substitution lines, earlier studies were able to show that the two major autosomes have very large effects on both male mating success and female mating preference. In this study, we genetically dissect this sexual isolation by recombination analysis between a whole-chromosome substitution line (which carries a Zimbabwe-derived third chromosome) and a strain with seven visible markers on that chromosome. Four loci are responsible for male mating success and three others are found to control female mating preference. Because male and female traits are not closely linked, their strong association among isofemale lines is most likely a reflection of sexual selection in nature. The results suggest that a large number of behavioral loci may evolve concurrently in the incipient stage of speciation before other aspects of reproductive isolation (such as hybrid sterility) have become evident. The results shed light on the population genetic processes underlying the formation of nascent species, as well as modes of speciation.

Animals↗
PubMed2004

The Allonemobius-Wolbachia host-endosymbiont system: evidence for rapid speciation and against reproductive isolation driven by cytoplasmic incompatibility.

Evidence for the evolution of fertilization incompatibilities and rapid speciation can be biased by the occurrence of hybridization and reproductive endosymbionts such as Wolbachia. For example, patterns of mitochondrial DNA (mtDNA) variation can be obscured by mitotypes hitchhiking on extrachromosomal elements like Wolbachia, while such endosymbionts can also induce phenotypes that mirror the operation of intrinsic fertilization incompatibilities between species. Therefore, before strong inferences can be drawn concerning the rates and processes of speciation in arthropod systems, we must first assess whether extrinsic endosymbionts obscure patterns of speciation. Here, I use the Allonemobius fasciatus-socius species complex to determine what role Wolbachia has played in the presumed rapid divergence of this complex by analyzing patterns of mtDNA and nuclear DNA variation in conjunction with sequence and cytoplasmic incompatibility data on Wolbachia. Data on molecular variation suggest that Wolbachia has not induced a strong selective sweep of the mitochondrial genome; nor does Wolbachia appear to induce cytoplasmic incompatibility. Preliminary evidence indicates that a third species identified within this complex, A. sp. nov. Tex, is partially reproductively isolated from A. socius, its closest relative, via conspecific sperm precedence or some form of postzygotic isolation. Moreover, shared mitotypes between A. sp. nov. Tex and A. socius may indicate the occurrence of a hybrid zone between these species near the border of Texas and Louisiana, although they may also represent shared ancestral polymorphisms. Molecular data also indicate that all three species in this complex diverged from a common ancestor as recently as 3000-30,000 years ago. Finally, the radiation of this complex from its ancestral population likely occurred in the presence of one strain of Wolbachia, thus suggesting a minimal role for Wolbachia during this burst of speciation. In total, barriers to gene flow do appear to have evolved very rapidly in this group of crickets.

Animals↗
PubMed2003

Evidence of a hybrid-zone in Atlantic cod (Gadus morhua) in the Baltic and the Danish Belt Sea revealed by individual admixture analysis.

The study of hybrid zones is central to our understanding of the genetic basis of reproductive isolation and speciation, yet very little is known about the extent and significance of hybrid zones in marine fishes. We examined the population structure of cod in the transition area between the North Sea and the Baltic Sea employing nine microsatellite loci. Genetic differentiation between the North Sea sample and the rest increased along a transect to the Baltic proper, with a large increase in level of differentiation occurring in the Western Baltic area. Our objective was to determine whether this pattern was caused purely by varying degrees of mechanical mixing of North Sea and Baltic Sea cod or by interbreeding and formation of a hybrid swarm. Simulation studies revealed that traditional Hardy-Weinberg analysis did not have sufficient power for detection of a Wahlund effect. However, using a model-based clustering method for individual admixture analysis, we were able to demonstrate the existence of intermediate genotypes in all samples from the transition area. Accordingly, our data were explained best by a model of a hybrid swarm flanked by pure nonadmixed populations in the North Sea and the Baltic Sea proper. Significant correlation of gene identities across loci (gametic phase disequilibrium) was found only in a sample from the Western Baltic, suggesting this area as the centre of the apparent hybrid zone. A hybrid zone for cod in the ecotone between the high-saline North Sea and the low-saline Baltic Sea is discussed in relation to its possible origin and maintenance, and in relation to a classical study of haemoglobin variation in cod from the Baltic Sea/Danish Belt Sea, suggesting mixing of two divergent populations without interbreeding.

Animals↗
PubMed2004

Partial island submergence and speciation in an adaptive radiation: a multilocus analysis of the Cuban green anoles.

Sympatric speciation is often proposed to account for species-rich adaptive radiations within lakes or islands, where barriers to gene flow or dispersal may be lacking. However, allopatric speciation may also occur in such situations, especially when ranges are fragmented by fluctuating water levels. We test the hypothesis that Miocene fragmentation of Cuba into three palaeo-archipelagos accompanied species-level divergence in the adaptive radiation of West Indian Anolis lizards. Analysis of morphology, mitochondrial DNA (mt DNA) and nuclear DNA in the Cuban green anoles (carolinensis subgroup) strongly supports three pre dictions made by this hypothesis. First, three geographical sets of populations, whose ranges correspond with palaeo-archipelago boundaries, are distinct and warrant recognition as independent evolutionary lineages or species. Coalescence of nuclear sequence fragments sampled from these species and the large divergences observed between their mtDNA haplotypes suggest separation prior to the subsequent unification of Cuba ca. 5 Myr ago. Second, molecular phylogenetic relationships among these species reflect historical geographical relationships rather than morphological similarity. Third, all three species remain distinct despite extensive geographical contact subsequent to island unification, occasional hybridization and introgression of mtDNA haplotypes. Allopatric speciation initiated during partial island submergence may play an important role in speciation during the adaptive radiation of Anolis lizards.

Animals↗
PubMed2006

Wax plants disentangled: a phylogeny of Hoya (Marsdenieae, Apocynaceae) inferred from nuclear and chloroplast DNA sequences.

Hoya (Marsdenieae, Apocynaceae) includes at least 200 species distributed from India to the Pacific Islands. We here infer major species groups in the genus based on combined sequences from the chloroplast atpB-rbcL spacer, the trnL region, and nuclear ribosomal DNA ITS region for 42 taxa of Hoya and close relatives. To assess levels of ITS polymorphism, ITS sequences for a third of the accessions were obtained by cloning. Most ITS clones grouped by species, indicating that speciation in Hoya usually predates ITS duplication. One ITS sequence of H. carnosa, however, grouped with a sequence of the morphologically similar H. pubicalyx, pointing to recent hybridization or the persistence of paralogous copies through a speciation event. The topology resulting from the combined chloroplast and nuclear data recovers some morphology-based sections, such as Acanthostemma and Eriostemma, as well as a well-supported Australian/New Guinean clade. The combined data also suggest that morphological adaptations for ant-symbiosis evolved at least three times within Hoya.

Apocynaceae↗
PubMed2003

Phylogeography and introgressive hybridization: chipmunks (genus Tamias) in the northern Rocky Mountains.

If phylogeographic studies are to be broadly used for assessing population-level processes relevant to speciation and systematics, the ability to identify and incorporate instances of hybridization into the analytical framework is essential. Here, we examine the evolutionary history of two chipmunk species, Tamias ruficaudus and Tamias amoenus, in the northern Rocky Mountains by integrating multivariate morphometrics of bacular (os penis) variation, phylogenetic estimation, and nested clade analysis with regional biogeography. Our results indicate multiple examples of mitochondrial DNA introgression layered within the evolutionary history of these nonsister species. Three of these events are most consistent with recent and/or ongoing asymmetric introgression of mitochondrial DNA across morphologically defined secondary contact zones. In addition, we find preliminary evidence where a fourth instance of nonconcordant characters may represent complete fixation of introgressed mitochondrial DNA via a more ancient hybridization event, although alternative explanations of convergence or incomplete sorting of ancestral polymorphisms cannot be dismissed with these data. The demonstration of hybridization among chipmunks with strongly differentiated bacular morphology contradicts long-standing assumptions that variation within this character is diagnostic of complete reproductive isolation within Tamias. Our results illustrate the utility of phylogeographic analyses for detecting instances of reticulate evolution and for incorporating this and other information in the inference of the evolutionary history of species.

Animals↗
PubMed2005

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. In contrast, speciation by reinforcement is driven directly by natural selection against maladaptive hybridization. This gives individuals that choose the traits of their own lineage greater fitness, potentially leading to rapid speciation between the lineages. 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. 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.

Animals↗
PubMed1998

A rapidly evolving homeobox at the site of a hybrid sterility gene.

The homeodomain is a DNA binding motif that is usually conserved among diverse taxa. Rapidly evolving homeodomains are thus of interest because their divergence may be associated with speciation. The exact site of the Odysseus (Ods) locus of hybrid male sterility in Drosophila contains such a homeobox gene. In the past half million years, this homeodomain has experienced more amino acid substitutions than it did in the preceding 700 million years; during this period, it has also evolved faster than other parts of the protein or even the introns. Such rapid sequence divergence is driven by positive selection and may contribute to reproductive isolation.

Amino Acid Sequence↗
PubMed2006

Testing introgressive hybridization hypotheses using statistical network analysis of nuclear and cytoplasmic haplotypes in the leaf beetle Timarcha goettingensis species complex.

Previous studies of leaf beetles (Chrysomelidae) in the Timarcha goettingensis species complex using mitochondrial (cox2) and nuclear (ITS-2 rRNA) markers revealed two main clades confined to the Iberian Peninsula and the rest of Europe but showing incongruent distributions indicative of gene exchange between both groups. Because of the anastomosing nature of hybridization, which disrupts the cladistic structure of character variation, phylogenetic trees might be inappropriate to represent and study this process. Here we test for evidence of hybridization in the T. goettingensis complex by analyzing the extra homoplasy arising in hybrid genomes from the simultaneous analysis of genetically independent markers. Haplotype networks obtained by Templeton's statistical parsimony analysis were generated for combined (concatenated) cox2 and ITS-2 sequences from 167 individuals of the T. goettingensis complex. Networks were used to detect runs of homoplasious characters physically clustered along a nucleotide sequence, as evidence for recombination between both gene partitions. A hypergeometric tail probability for the chance occurrence of physically clustered character changes on the connections linking networks of genotypes was applied. The test recognized two instances of statistically significant clustering, indicating the presence of cox2-ITS-2 mosaic genotypes and reticulation of both main T. goettingensis clades, supporting the reticulate origin of samples of T. maritima in southwestern France and T. sinuatocollis/T. monserratensis in the eastern Pyrenees. Although the assessment of reticulation in DNA sequences does not provide direct proof for hybridization, the geographical distribution of mosaic genotypes in the vicinity of "pure" genotypes supports the effect of gene flow between the two divergent lineages. The study demonstrates the utility of statistical parsimony networks for the detection of hybrids in the growing number of phylogeographic studies based on multiple gene markers.

Adaptor Proteins, Vesicular Transport↗
PubMed2001

Hybridation of different chiral separation techniques with ICP-MS detection for the separation and determination of selenomethionine enantiomers: chiral speciation of selenized yeast.

Enantioseparation and determination of selenomethionine enantiomers in selenized yeast was investigated using chiral separation techniques based on different principles, coupled on-line to inductively coupled plasma mass spectrometry (ICP-MS) for selenium-specific detection. High performance liquid chromatography (HPLC) on a beta-cyclodestrin (beta-CD) column, cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC), gas chromatography (GC) on a Chirasil-L-Val column, and HPLC on a Chirobiotic T column have been investigated as the chiral separation techniques. For HPLC separation on the beta-CD column, and also for CD-MEKC, selenomethionine enantiomers were derivatized with NDA/CN(-). For chiral separation by GC, selenomethionine enantiomers were converted into their N-trifluoroacetyl (TFA)-O-alkyl esters. The developed hybridation methodologies are compared with respect to enantioselectivity, sensitivity and analysis time. The usefulness of the best-suited method [HPLC (Chirobiotic T)-ICP-MS] was demonstrated by its application to the successful chiral speciation of selenium and D-and L-selenomethionine content determination in selenized yeast.

Chromatography, Gas↗
PubMed1999

Transgressive segregation, adaptation and speciation.

The production of extreme or 'transgressive' phenotypes in segregating hybrid populations has been speculated to contribute to niche divergence of hybrid lineages. Here, we assess the frequency of transgressive segregation in hybrid populations, describe its genetic basis and discuss the factors that best predict its occurrence. From a survey of 171 studies that report phenotypic variation in segregating hybrid populations, we show that transgression is the rule rather than the exception. In fact, 155 of the 171 studies (91%) report at least one transgressive trait, and 44% of 1229 traits examined were transgressive. Transgression occurred most frequently in intraspecific crosses involving inbred, domesticated plant populations, and least frequently in interspecific crosses between outbred, wild animal species. Quantitative genetic studies of plant hybrids consistently point to the action of complementary genes as the primary cause of transgression, although overdominance and epistasis also contribute. Complementary genes appear to be common for most traits, with the possible exception of those with a history of disruptive selection. These results lend credence to the view that hybridization may provide the raw material for rapid adaptation and provide a simple explanation for niche divergence and phenotypic novelty often associated with hybrid lineages.

Adaptation, Biological↗
PubMed1970

Chromosome number variation in a stick insect Didymuria violescens (Leach).

Seven major races, with diploid numbers ranging from 26 to 40, and three types of sex-chromosome mechanism were found in the Australian phasmatid Didymuria violescens (Leach). The differences between chromosome complements are mainly due to translocations between autosomes, and to translocations between autosomes and sex chromosomes. The geographic pattern of chromosome variation and the characteristics of hybrids implicate chromosomal rearrangements in mechanisms of speciation.

Australia↗
PubMed1999

Incongruence between chloroplast and species phylogenies in Eucalyptus subgenus Monocalyptus (Myrtaceae).

Seventy-eight polymorphic cpDNA (chloroplast DNA) characters were found in 13 closely related taxa from Eucalyptus series Amygdalinae (subgenus Monocalyptus) and seven potential outgroup taxa. The strict consensus of six cladograms generated from cpDNA data confirmed monophyly of Monocalyptus. However, cpDNA phylogeny within Monocalyptus was incongruent with taxonomic classification, being more related to geography, even when accessions were from divergent series. Monocalyptus cpDNA formed two major clades. On the island of Tasmania cpDNA was restricted to a single clade, exhibited very little variation, and was phylogenetically related to cpDNA found in central and western Victoria. In contrast, cpDNA of mainland monocalypt taxa was more variable, even within the Amygdalinae. Four out of six Tasmanian Amygdalinae species were polymorphic. The difference between cpDNA of replicates was often greater than differences between species from different series. The low level of cpDNA variation and extensive morphological intergradation between the Tasmanian endemics suggest recent speciation. However, the transfer of cpDNA through hybridization between lineages is the most likely explanation for the observed sharing of cpDNA across series. This study highlights that the geographical pattern to cpDNA variation in Eucalyptus may be an important source of information on past plant distributions in Australia.

Journal Article↗
PubMed2005

Chromosome inversions, local adaptation and speciation.

We study the evolution of inversions that capture locally adapted alleles when two populations are exchanging migrants or hybridizing. By suppressing recombination between the loci, a new inversion can spread. Neither drift nor coadaptation between the alleles (epistasis) is needed, so this local adaptation mechanism may apply to a broader range of genetic and demographic situations than alternative hypotheses that have been widely discussed. The mechanism can explain many features observed in inversion systems. It will drive an inversion to high frequency if there is no countervailing force, which could explain fixed differences observed between populations and species. An inversion can be stabilized at an intermediate frequency if it also happens to capture one or more deleterious recessive mutations, which could explain polymorphisms that are common in some species. This polymorphism can cycle in frequency with the changing selective advantage of the locally favored alleles. The mechanism can establish underdominant inversions that decrease heterokaryotype fitness by several percent if the cause of fitness loss is structural, while if the cause is genic there is no limit to the strength of underdominance that can result. The mechanism is expected to cause loci responsible for adaptive species-specific differences to map to inversions, as seen in recent QTL studies. We discuss data that support the hypothesis, review other mechanisms for inversion evolution, and suggest possible tests.

Adaptation, Physiological↗
PubMed2003

A rapidly evolving MYB-related protein causes species isolation in Drosophila.

Matings among different species of animals or plants often result in sterile or lethal hybrids. Identifying the evolutionary forces that create hybrid incompatibility alleles is fundamental to understanding the process of speciation, but very few such alleles have been identified, particularly in model organisms that are amenable to experimental manipulation. We report here the cloning of the first, to our knowledge, Drosophila melanogaster gene involved in hybrid incompatibilities, Hybrid male rescue (Hmr). Hmr causes lethality and female sterility in hybrids among D. melanogaster and its sibling species. We have found that Hmr encodes a protein with homology to a family of MYB-related DNA-binding transcriptional regulators. The HMR protein has evolved both amino acid substitutions and insertions and deletions at an extraordinarily high rate between D. melanogaster and its sibling species, including in its predicted DNA-binding domain. Our results suggest that hybrid lethality may result from disruptions in gene regulation, and we also propose that rapid evolution may be a hallmark of speciation genes in general.

Amino Acid Sequence↗
PubMed2006

Speciation in Drosophila: from phenotypes to molecules.

Study of the genetics of speciation--and especially of the genetics of intrinsic postzygotic isolation-has enjoyed remarkable progress over the last 2 decades. Indeed progress has been so rapid that one might be tempted to ask if the genetics of postzygotic isolation is now wrapped up. Here we argue that the genetics of speciation is far from complete. In particular, we review 2 topics where recent work has revealed major surprises: 1) the role of meiotic drive in hybrid sterility and 2) the role of gene transposition in speciation. These surprises, and others like them, suggest that evolutionary biologists may understand less about the genetic basis of speciation than seemed likely a few years ago.

Animals↗
PubMed2007

Searching the genomes of inbred mouse strains for incompatibilities that reproductively isolate their wild relatives.

Identification of the genes that underlie reproductive isolation provides important insights into the process of speciation. According to the Dobzhansky-Muller model, these genes suffer disrupted interactions in hybrids due to independent divergence in separate populations. In hybrid populations, natural selection acts to remove the deleterious heterospecific combinations that cause these functional disruptions. When selection is strong, this process can maintain multilocus associations, primarily between conspecific alleles, providing a signature that can be used to locate incompatibilities. We applied this logic to populations of house mice that were formed by hybridization involving two species that show partial reproductive isolation, Mus domesticus and Mus musculus. Using molecular markers likely to be informative about species ancestry, we scanned the genomes of 1) classical inbred strains and 2) recombinant inbred lines for pairs of loci that showed extreme linkage disequilibria. By using the same set of markers, we identified a list of locus pairs that displayed similar patterns in both scans. These genomic regions may contain genes that contribute to reproductive isolation between M. domesticus and M. musculus. This hypothesis can now be tested using laboratory crosses and surveys of introgression in the wild.

Animals↗