Extensive chromosomal repatterning in two congeneric species: Pytilia melba, L. and Pytilia phoenicoptera Swainson (Estrildidae; Aves)

Chromosomal analysis of two species of African finches of the genus Pytilia has been carried out using both G- and C-banding. The karyotypes of these two species were found to differ radically, not only from each other, but also from those of other species in the family Estrildidae. The differences are due to paracentric and pericentric inversions and to tandem fusions. However, not all of the karyotypic differences can be explained by conventional mechanisms. These results are discussed in relation to the role of karyotypic rearrangement in avian evolution and the conversion of microchromosomes to macrochromosomes.     

Genetics and evolution of hybrid male sterility in house mice

Comparative genetic mapping provides insights into the evolution of the reproductive barriers that separate closely related species. This approach has been used to document the accumulation of reproductive incompatibilities over time, but has only been applied to a few taxa. House mice offer a powerful system to reconstruct the evolution of reproductive isolation between multiple subspecies pairs. However, studies of the primary reproductive barrier in house mice-hybrid male sterility-have been restricted to a single subspecies pair: Mus musculus musculus and Mus musculus domesticus. To provide a more complete characterization of reproductive isolation in house mice, we conducted an F(2) intercross between wild-derived inbred strains from Mus musculus castaneus and M. m. domesticus. We identified autosomal and X-linked QTL associated with a range of hybrid male sterility phenotypes, including testis weight, sperm density, and sperm morphology. The pseudoautosomal region (PAR) was strongly associated with hybrid sterility phenotypes when heterozygous. We compared QTL found in this cross with QTL identified in a previous F(2) intercross between M. m. musculus and M. m. domesticus and found three shared autosomal QTL. Most QTL were not shared, demonstrating that the genetic basis of hybrid male sterility largely differs between these closely related subspecies pairs. These results lay the groundwork for identifying genes responsible for the early stages of speciation in house mice.     

Transgressive character expression in a hybrid sunflower species

Diploid hybrid lineages often are ecologically distinct from their parental species. However, it is unclear whether this niche divergence is typically achieved via hybrid intermediacy, a mixture of parental traits, and/or the evolution of extreme (transgressive) morphological and ecophysiological features. Here we compare an extensively studied hybrid sunflower species, Helianthus anomalus, with its putative parents, H. annuus and H. petiolaris, for 41 morphological and 12 ecophysiological traits. Helianthus anomalus was morphologically intermediate for one trait (2.4%), parental-like for 23 traits (56.1%), and transgressive for 17 traits (41.5%). For ecophysiological traits, H. anomalus was not significantly different from one or both parents for nine traits (75%), and was transgressive for the remaining three (25%). Thus, H. anomalus appears to be a mosaic of parental-like and transgressive phenotypes. Although the fitness effects of the transgressive characters are not yet known, many of these characters are consistent with adaptations reported for other sand dune plants. Genetic studies are currently underway to ascertain whether these extreme characters arose as a direct byproduct of hybridization or whether they evolved via mutational divergence.     

An axonemal dynein at the Hybrid Sterility 6 locus: implications for t haplotype-specific male sterility and the evolution of species barriers

Poor sperm motility characterized by a distinct aberration in flagellar waveform known as ``curlicue' is a hallmark of t haplotype (t) homozygous male sterility. Previous studies have localized ``curlicue' and a flagellar developmental defect, ``whipless', to the Hybrid Sterility 6 locus (Hst6), between the markers Pim1 and Crya1. More recent heterospecific breeding experiments between Mus spretus (Spretus) and Mus musculus domesticus (Domesticus) have mapped the primary source(s) of both ``curlicue' and ``whipless' to a small sub-locus of Hst6, Curlicue a (Ccua). Here we report the complete physical isolation of the Ccua locus and the identification of a candidate gene for expression of both ``whipless' and ``curlicue' at its proximal end, an axonemal dynein heavy chain gene, Dnahc8, formerly mapped by interspecific backcross analysis near Pim1. Dnahc8 mRNA expression commences in the Domesticus wild-type testis just prior to flagellar assembly and is testis-specific in the adult male. However, expression of Dnahc8 is not readily evident in the testis of either Spretus or ``whipless' animals (Domesticus males homozygous for the Spretus allele of Dnahc8). Our results argue that Dnahc8 is fundamental to flagellar organization and function in Domesticus, but not Spretus, and suggest that Dnahc8 is integral to both Hst6- and t-specific male infertility. Received: 13 July 1999 / Accepted: 25 August 1999     

Origin and evolution of animal hybrid species

The increasing number of hybrid species, discovered in both vertebrates and invertebrates by the combined use of chromosome, allozyme and molecular markers, calls for a reevaluation of hybrid speciation and reticulate evolution In animals. The array of reproductive strategies recently detected In phylogenetically related stick Insects allows us to Investigate, using a comparative approach, questions such as the relationship between hybridization and unisexuality, and short- versus long-term evolutionary success of hybrid species. Unexpected similarities are now apparent in hybrid evolution of animals as varied as insects, snails, fish, frogs and lizards. Hybrid species may combine, to some extent, the main advantage of sex (genetic diversity) with those of clonal reproduction. This explains why these species are often so successful, and indicates a potential use of some hybrid species in experimental biology and resource management (e.g. mass production of animal proteins).     

Genetic interactions underlying hybrid male sterility in the Drosophila bipectinata species complex

Understanding genetic mechanisms underlying hybrid male sterility is one of the most challenging problems in evolutionary biology especially speciation. By using the interspecific hybridization method roles of Y chromosome, Major Hybrid Sterility (MHS) genes and cytoplasm in sterility of hybrid males have been investigated in a promising group, the Drosophila bipectinata species complex that consists of four closely related species: D. pseudoananassae, D. bipectinata, D. parabipectinata and D. malerkotliana. The interspecific introgression analyses show that neither cytoplasm nor MHS genes are involved but X-Y interactions may be playing major role in hybrid male sterility between D. pseudoananassae and the other three species. The results of interspecific introgression analyses also show considerable decrease in the number of males in the backcross offspring and all males have atrophied testes. There is a significant positive correlation between sex - ratio distortion and severity of sterility in backcross males. These findings provide evidence that D. pseudoananassae is remotely related with other three species of the D. bipectinata species complex.     

A genealogical view of chromosomal evolution and species delimitation in the Drosophila virilis species subgroup

Chromosomal arrangement was a historically important character used for defining taxonomic boundaries. The Drosophila virilis species group exhibits a series of chromosomal rearrangements, and the resulting differences among karyotypes were primary characters originally used to define taxa within the group. However, some chromosomally divergent forms have not been sufficiently resolved in phylogenetic reconstructions of DNA sequences from several nuclear genes. Sequences of mitochondrial regions have the potential for finer-scale resolution of closely related taxa; therefore, sequences of two mitochondrial genes were used to examine phylogenetic relationships within the chromosomally variable virilis subgroup. Sequences were obtained from multiple strains of the Palearctic species, D. virilis and D. lummei, and the Nearctic species, D. novamexicana and two chromosomal forms of D. americana. Analyses support the recent emergence of the different chromosomal forms in North America. However, none of these chromosomally divergent forms exhibit reciprocal monophyly of their mtDNA sequences, which is the requirement for attaining genealogical species status.     

Comparative genetics of hybrid incompatibility: sterility in two Solanum species crosses

The genetic basis of hybrid sterility can provide insight into the genetic and evolutionary origins of species barriers. We examine the genetics of hybrid incompatibility between two diploid plant species in the plant clade Solanum sect. Lycopersicon. Using a set of near-isogenic lines (NILs) representing the wild species Solanum pennellii (formerly Lycopersicon pennellii) in the genetic background of the cultivated tomato S. lycopersicum (formerly L. esculentum), we found that hybrid pollen and seed infertility are each based on a modest number of loci, male (pollen) and other (seed) incompatibility factors are roughly comparable in number, and seed-infertility QTL act additively or recessively. These findings are remarkably consistent with our previous analysis in a different species pair, S. lycopersicum x S. habrochaites. Data from both studies contrast strongly with data from Drosophila. Finally, QTL for pollen and seed sterility from the two Solanum studies were chromosomally colocalized, indicating a shared evolutionary history for these QTL, a nonrandom genomic distribution of loci causing sterility, and/or a proclivity of certain genes to be involved in hybrid sterility. We show that comparative mapping data can delimit the probable timing of evolution of detected QTL and discern which sterility loci likely evolved earliest among species.     

Epistasis and hybrid sterility in Saccharomyces

Hybrid sterility is thought to be due to deleterious epistatic interactions between genes from different species. Here we demonstrate that dominant genic incompatibility does not contribute to sterility in hybrids between Saccharomyces cerevisiae and five closely related species. Sterile diploids were made fertile by genome doubling to produce hybrid tetraploids. Based on these and previous results, we conclude that neither genic incompatibility nor classical chromosomal speciation models apply.     

Extensive chromosomal polymorphism revealed by ribosomal DNA and satellite DNA loci in 13 Citrus species

Little is known about the chromosomal variability and polymorphism existing in mitotic chromosomes of Citrus, mainly due to lack of reliable chromosomal markers and small chromosome size. To test the hypothesis of chromosomal polymorphism and provide the foundation of the genome organization in the Citrus cultivars, we have developed molecular cytogenetic markers for 13 Citrus species collected from Jeju island, Korea. In this study, we demonstrated that the chromosomal locations of cytogenetic markers are quite variable and extremely polymorphic, in contrast to the previous studies. The data obtained in this study will be of utmost importance in cytological systematics and karyotyping of the Citrus species.     

Site-specific genetic divergence in parallel hybrid zones suggests nonallopatric evolution of reproductive barriers

The evolution of reproductive isolation in the presence of gene flow is supported by theoretical models but rarely by data. Empirical support might be gained from studies of parallel hybrid zones between interbreeding taxa. We analysed gene flow over two hybrid zones separating ecotypes of Littorina saxatilis to test the expectation that neutral genetic markers will show site-specific differences if barriers have evolved in situ. Distinct ecotypes found in contrasting shore habitats are separated by divergent selection and poor dispersal, but hybrid zones appear between them. Swedish islands formed by postglacial uplift 5000 years ago provide opportunities to assess genetic structure in a recently evolved system. Each island houses a discrete population containing subpopulations of different ecotypes. Hybrid zones between ecotypes may be a product of ecological divergence occurring on each island or a consequence of secondary overlap of ecotypes of allopatric origin that have spread among the islands. We used six microsatellite loci to assess gene flow and genetic profiles of hybrid zones on two islands. We found reduced gene flow over both hybrid zones, indicating the presence of local reproductive barriers between ecotypes. Nevertheless, subpopulations of different ecotypes from the same island were genetically more similar to each other than were subpopulations of the same ecotype from different islands. Moreover, neutral genetic traits separating the two ecotypes across hybrid zones were site-specific. This supports a scenario of in situ origin of ecotypes by ecological divergence and nonallopatric evolution of reproductive barriers.     

Genetic basis of hybrid male sterility among three closely related species of Drosophila

The genetic basis of hybrid male sterility among three closely related species, Drosophila bipectinata, D. parabipectinata and D. malerkotliana has been investigated by using backcross analysis methods. The role of Y chromosome, major hybrid sterility (MHS) genes (genetic factors) and cytoplasm (non-genetic factor) have been studied in the hybrids of these three species. In the species pair, bipectinata--parabipectinata, Y chromosome introgression of parabipectinata in the genomic background of bipectinata and the reciprocal Y chromosome introgression were unsuccessful as all males in second backcross generation were sterile. Neither MHS genes nor cytoplasm was found important for sterility. This suggests the involvement of X-Y, X-autosomes or polygenic interactions in hybrid male sterility. In bipectinata--malerkotliana and parabipectinata--malerkotliana species pairs, Y chromosome substitution in reciprocal crosses did not affect male fertility. Backcross analyses also show no involvement of MHS genes or cytoplasm in hybrid male sterility in these two species pairs. Therefore, X- autosome interaction or polygenic interaction is supposed to be involved in hybrid male sterility in these two species pairs. These findings also provide evidence that even in closely related species, genetic interactions underlying hybrid male sterility may vary.     

Molecular mechanisms of hybrid sterility in rice

Hybrid sterility presents a major bottleneck in hybrid crop breeding and causes postzygotic reproductive isolation in speciation.Here, we summarize the current understanding of the genetics of rice hybrid sterility and highlight new advances in deciphering the molecular basis of the major genetic loci for hybrid sterility in rice. We also discuss practical strategies for overcoming reproductive barriers to utilize hybrid vigor in inter-specific and inter-subspecific hybrid rice breeding.     

Extensive parallelism in protein evolution

Background  

Independently evolving lineages mostly accumulate different changes, which leads to their gradual divergence. However, parallel accumulation of identical changes is also common, especially in traits with only a small number of possible states.