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PubMed1989

Mutation rates in hybrids between sibling species of Drosophila.

It has recently been suggested that sterility or inviability in species hybrids might result from the movement of transposable elements. Because such movement is often detectable by an increased mutation rate, I studied the effect of interspecific hybridization in the Drosophila melanogaster group on the mutation rate of X-linked visibles. This rate did not differ between hybrids and intraspecific controls. This was also true for the germ-line excision rate of a transposable element, although the rate of somatic excision was two to sixfold higher in hybrids than in pure species. Combined with previous work, these results do not support a role for transposable elements in speciation.

Animals↗
PubMed1999

Hybrid zones and the genetic architecture of a barrier to gene flow between two sunflower species.

Genetic analyses of reproductive barriers represent one of the few methods by which theories of speciation can be tested. However, genetic study is often restricted to model organisms that have short generation times and are easily propagated in the laboratory. Replicate hybrid zones with a diversity of recombinant genotypes of varying age offer increased resolution for genetic mapping experiments and expand the pool of organisms amenable to genetic study. Using 88 markers distributed across 17 chromosomes, we analyze the introgression of chromosomal segments of Helianthus petiolaris into H. annuus in three natural hybrid zones. Introgression was significantly reduced relative to neutral expectations for 26 chromosomal segments, suggesting that each segment contains one or more factors that contribute to isolation. Pollen sterility is significantly associated with 16 of these 26 segments, providing a straightforward explanation of why this subset of blocks is disadvantageous in hybrids. In addition, comparison of rates of introgression across colinear vs. rearranged chromosomes indicates that close to 50% of the barrier to introgression is due to chromosomal rearrangements. These results demonstrate the utility of hybrid zones for identifying factors contributing to isolation and verify the prediction of increased resolution relative to controlled crosses.

Alleles↗
PubMed1997

The role of Wolbachia bacteria in reproductive incompatibilities and hybrid zones of Diabrotica beetles and Gryllus crickets.

A rickettsial bacterium in the genus Wolbachia is the cause of a unidirectional reproductive incompatibility observed between two major beetle pests of maize, the western corn rootworm, Diabrotica virgifera virgifera, and the Mexican corn rootworm, D. v. zeae. These subspecies are allopatric except for two known regions of sympatry in Texas and Mexico. We demonstrate that populations of D. v. virgifera, with the exception of two populations in southern Arizona, are infected with a strain of Wolbachia. Populations of D. v. zeae are not infected. Treatment of D. v. virgifera with tetracycline eliminated the Wolbachia and removed the reproductive incompatibility. Similar patterns of reproductive incompatibility exist among taxa of the cricket genus Gryllus. Gryllus assimilis, G. integer, G. ovisopis, G. pennsylvanicus, and G. rubens are infected with Wolbachia whereas G. firmus is usually not. Populations of G. rubens and G. ovisopis carry the same Wolbachia strain, which is distinct from that of G. integer. G. pennsylvanicus is infected with two Wolbachia strains, that found in G. rubens and one unique to G. pennsylvanicus. Moreover, a proportion of G. pennsylvanicus individuals harbors both strains. Wolbachia may have influenced speciation in some members of the genus Gryllus by affecting the degree of hybridization between species. Given that Wolbachia infections are relatively common in insects, it is likely that other insect hybrid zones may be influenced by infections with Wolbachia.

Animals↗
PubMed1991

Sex chromosomes and speciation.

Studies of reproductive isolation between animal species have shown (i) that if one sex of the hybrids between two species is sterile or inviable, it is usually the heterogametic sex (Haldane's rule), and (ii) the genes on the sex chromosomes play a particularly large role in hybrid sterility and inviability. We propose an explanation for these two observations which is based on the changes in chromosome conformation which take place during gametogenesis. These changes are far greater in sex chromosomes than in autosomes. They are also greater in the heterogametic than in the homogametic sex. We suggest that the sensitivity of hybrids of the heterogametic sex to the genetic divergence that occurs during periods of population isolation is partly the result of the failure of their sex chromosomes to undergo appropriate conformational changes. This hypothesis explains why the sex chromosomes play a disproportionate role in post-zygotic, but not in pre-zygotic, isolation, and why often only the germ line is sensitive to hybridization.

Animals↗
PubMed1977

Diplotene chromosomes of Xenopus hybrid oocytes.

Observation on meiotic diplotene chromosomes from oocytes of Xenopus species and subspecies hybrids are reported. Species interrelationships are established on the basis of the number of bivalent in the respective hybrids. Polyploid oocytes were found and their origin by supplementary endoreduplication placed at the differentiation stage of gametogenesis when oogonia become oocytes. The significance of polyploid oocytes for speciation is discussed.

Animals↗
PubMed2006

Phylogenetic networks from multi-labelled trees.

It is now quite well accepted that the evolutionary past of certain species is better represented by phylogenetic networks as opposed to trees. For example, polyploids are typically thought to have resulted through hybridization and duplication, processes that are probably not best represented as bifurcating speciation events. Based on the knowledge of a multi-labelled tree relating collection of polyploids, we present a canonical construction of a phylogenetic network that exhibits the tree. In addition, we prove that the resulting network is in some well-defined sense a minimal network having this property.

Algorithms↗
PubMed1997

Comparative mapping of human alphoid satellite DNA repeat sequences in the great apes.

Heterochromatic regions of chromosomes contain highly repetitive, tandemly arranged DNA sequences that undergo very rapid variation compared to unique DNA sequences that are predominantly conserved. In this study the chromosomal basis of speciation has been looked at in terms of repeat sequences. We have hybridized twenty-one chromosome-specific human alphoid satellite DNA probes to metaphase spreads of the chimpanzee (Pan troglodytes), gorilla (Gorilla gorilla), and orangutan (Pongo pygmaeus) to investigate the evolutionary relationship of heterochromatic regions among such hominoid species. The majority of the probes did not hybridize to their corresponding equivalent chromosome but presented hybridization signals on non-corresponding chromosomes. Such observations suggest that rapid changes may have occurred in the ancestral alphoid satellite DNA sequence, resulting in divergence among the great ape species.

Animals↗
PubMed2000

Genomic imprinting and environmental disease susceptibility.

Genomic imprinting is one of the most intriguing subtleties of modern genetics. The term "imprinting" refers to parent-of-origin-dependent gene expression. The presence of imprinted genes can cause cells with a full parental complement of functional autosomal genes to specifically express one allele but not the other, resulting in monoallelic expression of the imprinted loci. Genomic imprinting plays a critical role in fetal growth and behavioral development, and it is regulated by DNA methylation and chromatin structure. This paper summarizes the Genomic Imprinting and Environmental Disease Susceptibility Conference held 8-10 October 1998 at Duke University, Durham, North Carolina. The conference focused on the importance of genomic imprinting in determining susceptibility to environmentally induced diseases. Conference topics included rationales for imprinting: parental antagonism and speciation; methods for imprinted gene identification: allelic message display and monochromosomal mouse/human hybrids; properties of the imprinted gene cluster human 11p15.5 and mouse distal 7; the epigenetics of X-chromosome inactivation; variability in imprinting: imprint erasure, non-Mendelian inheritance ratios, and polymorphic imprinting; imprinting and behavior: genetics of bipolar disorder, imprinting in Turner syndrome, and imprinting in brain development and social behavior; and aberrant methylation: methylation and chromatin structure, methylation and estrogen exposure, methylation of tumor-suppressor genes, and cancer susceptibility. Environmental factors are capable of causing epigenetic changes in DNA that can potentially alter imprint gene expression and that can result in genetic diseases including cancer and behavioral disorders. Understanding the contribution of imprinting to the regulation of gene expression will be an important step in evaluating environmental influences on human health and disease.

Animals↗
PubMed2006

Asymmetrical reinforcement and Wolbachia infection in Drosophila.

Reinforcement refers to the evolution of increased mating discrimination against heterospecific individuals in zones of geographic overlap and can be considered a final stage in the speciation process. One the factors that may affect reinforcement is the degree to which hybrid matings result in the permanent loss of genes from a species' gene pool. Matings between females of Drosophila subquinaria and males of D. recens result in high levels of offspring mortality, due to interspecific cytoplasmic incompatibility caused by Wolbachia infection of D. recens. Such hybrid inviability is not manifested in matings between D. recens females and D. subquinaria males. Here we ask whether the asymmetrical hybrid inviability is associated with a corresponding asymmetry in the level of reinforcement. The geographic ranges of D. recens and D. subquinaria were found to overlap across a broad belt of boreal forest in central Canada. Females of D. subquinaria from the zone of sympatry exhibit much stronger levels of discrimination against males of D. recens than do females from allopatric populations. In contrast, such reproductive character displacement is not evident in D. recens, consistent with the expected effects of unidirectional cytoplasmic incompatibility. Furthermore, there is substantial behavioral isolation within D. subquinaria, because females from populations sympatric with D. recens discriminate against allopatric conspecific males, whereas females from populations allopatric with D. recens show no discrimination against any conspecific males. Patterns of general genetic differentiation among populations are not consistent with patterns of behavioral discrimination, which suggests that the behavioral isolation within D. subquinaria results from selection against mating with Wolbachia-infected D. recens. Interspecific cytoplasmic incompatibility may contribute not only to post-mating isolation, an effect already widely recognized, but also to reinforcement, particularly in the uninfected species. The resulting reproductive character displacement not only increases behavioral isolation from the Wolbachia-infected species, but may also lead to behavioral isolation between populations of the uninfected species. Given the widespread occurrence of Wolbachia among insects, it thus appears that there are multiple ways by which these endosymbionts may directly and indirectly contribute to reproductive isolation and speciation.

Animals↗
PubMed2002

Molecular phylogeny of hybridizing species from the genus Spartina Schreb. (Poaceae).

Interspecific hybridization events have been reported in the genus Spartina Schreb. (Poaceae), involving the east American species Spartina alterniflora, and including either introgression (e.g., with the western American Spartina foliosa) or allopolyploid speciation (e.g., with the Euro-African Spartina maritima). Molecular phylogenetic analysis of the genus has been undertaken in order to understand phylogenetic relationships and genetic divergence among these hybridizing species. Twelve Spartina species have been sequenced for two nuclear DNA regions (ITS of ribosomal DNA, and part of the Waxy gene) and one chloroplast DNA spacer (trnT-trnL). Separate and conditional combined phylogenetic analyses using Cynodon dactylon as the outgroup have been conducted. Spartina is composed of two lineages. The first clade includes all hexaploid species: the Euro-African S. maritima (2n = 60), the East-American S. alterniflora (2n = 62) and the West-American S. foliosa (2n = 60). Spartina alterniflora appears as a closely related sister species to S. foliosa. Although belonging to the same lineage, Spartina maritima appears consistently more genetically differentiated from S. alterniflora than S. foliosa. The tetraploid species S. argentinensis (2n = 40) is placed at the base of this first clade according to the Waxy data, but its position is not well resolved by the other sequences. The second well-supported main lineage within genus Spartina includes the other tetraploid American species. Significant incongruence has been encountered between the waxy based tree and both the ITS and trnT-trnL trees concerning the position of S. densiflora, suggesting a possible reticulate evolution for this species. The results agree with hybridization patterns occurring in Spartina: introgression involving closely related species (S. alterniflora and S. foliosa) on one hand, and alloploid speciation involving more differentiated species (S. alterniflora and S. maritima) on the other hand.

Cell Nucleus↗
PubMed2002

Chromosomal map of the model legume Lotus japonicus.

Lotus japonicus is a model plant for the legume family. To facilitate map-based cloning approaches and genome analysis, we performed an extensive characterization of the chromosome complement of the species. A detailed karyotype of L. japonicus Gifu was built and plasmid and BAC clones, corresponding to genetically mapped markers (see the accompanying article by Sandal et al. 2002, this issue), were used for FISH to correlate genetic and chromosomal maps. Hybridization of DNA clones from 32 different genomic regions enabled the assignment of linkage groups to chromosomes, the comparison between genetic and physical distances throughout the genome, and the partial characterization of different repetitive sequences, including telomeric and centromeric repeats. Additional analysis of L. filicaulis and its F(1) hybrid with L. japonicus demonstrated the occurrence of inversions between these closely related species, suggesting that these chromosome rearrangements are early events in speciation of this group.

Centromere↗
PubMed1997

Genetic basis of sperm and testis length differences and epistatic effect on hybrid inviability and sperm motility between Drosophila simulans and D. sechellia.

Results are reported from a genetic study of hybrid inviability and three 'fertilization traits' (sperm motility and length, and testis size) that affect hybrid sterility between the sibling species Drosophila simulans and D. sechellia. The main findings are as follows. (i) For sperm length there was a dominant effect of the D. simulans genome over that of D. sechellia, and the Y chromosome of D. sechellia in the background of D. simulans reduced the sperm length. (ii) In contrast, testis length, in spite of its generally high correlation with sperm length, showed an additive effect. (iii) We found a strong asymmetric incompatibility between the D. sechellia X chromosome and D. simulans autosomes: D. sechellia X chromosome with D. simulans autosomes, but not the reverse, showed a significant reduction in testis length as well as in hybrid inviability compared to the parental species. (iv) Between the two autosomes, chromosome 3 had a greater effect on these traits than chromosome 2, and there was additionally an epistatic effect between these chromosomes with respect to their parental vs. recombinant status: recombinant chromosomes 2 and 3, together, had lower viability than any other combination. (v) The testis size in the backcross generation was greater than the parental species, suggesting that some modifier genes are being released from their species-specific genetic control. (vi) The species-specific homogeneity of the genome was important for all three traits--offspring viability, hybrid male fertility and testis length. These results are discussed with respect to the role of sexual selection and genetic divergence during speciation.

Animals↗
PubMedDate not supplied

Genetic rescue of inviable hybrids between Drosophila melanogaster and its sibling species.

Post-mating mechanisms are central to the establishment of reproductive isolation between different, but closely related, species. Post-mating isolation mechanisms include hybrid breakdown, hybrid sterility and hybrid lethality and may, in some cases, be reinforced by pre-mating mechanisms such as ethological differentiation. In the Drosophila melanogaster species sub-group post-mating reproductive isolation is ensured by both the inviability and the sterility of hybrids. For example when D. melanogaster females are crossed to D. simulans males the hybrid progeny are normally all female; the hybrid males die as third instar larvae. The viable hybrid females are totally sterile. Little is known of the genetic basis for either hybrid sterility or hybrid inviability, although Coyne and others have begun a genetic analysis of the sterility of hybrids within this species sub-group. We have discovered a single gene difference that rescues the otherwise inviable male hybrids from the cross between D. melanogaster females and males of its three closest relatives. The study of this locus may shed light on the genetic control of both speciation and development.

Animals↗
PubMed1997

Genetic variation segregating in natural populations of Tribolium castaneum affecting traits observed in hybrids with T. freemani.

We investigated patterns of within-species genetic variation for traits observed in hybrids (hybrid numbers, hybrid sex ratios, and hybrid male deformities) between two species of flour beetles, Tribolium castaneum and T. freemani. We found genetic variation segregating among four natural populations of T. castaneum as well as within these populations. For some hybrid traits, we observed as much variation among populations 750 km apart as between populations on different continents, suggesting genetic differentiation at a local scale. Within natural populations, the variation segregating among sires is greater than that found in an earlier study for an outbred laboratory population and comparable to that observed between inbred lines derived from the outbred stock by eight generations of brother-sister mating. When sires from T. castaneum are mated to conspecific and heterospecific females, we do not observe a significant correlation at the level of the family mean between the intraspecific and interspecific phenotypes, suggesting the independence of the hybrid traits from comparable traits within species. We discuss our findings in relation to the evolutionary genetics of speciation and the expression of epistatic genetic variance in interspecific crosses.

Animals↗
PubMed2006

Timing and tempo of primate speciation.

Published molecular clocks for primates are used to estimate typical divergence times for phylogroups (1.6 Ma), species (3.3 Ma), sister species (2.7 Ma), genera (8.9 Ma) and sister genera (8.6 Ma). Significant median differences exist between major groups (infraorders and superfamilies) for various divergence times. These data are employed to estimate typical maximum duration of speciation. Typical primate values (1.1 Ma) suggest this process to be faster than is characteristic of many vertebrates. However, after considering divergence times for hybridizing congeneric and confamilial primates, this value is likely only to estimate the commencement of prezygotic isolating mechanisms, rather than the completion of reproductive isolation. Thus, speciation typically takes around 1.0 Ma to more than 4.0 Ma to occur, depending on whether prezygotic or post-zygotic isolating mechanisms are emphasized. Typical primate genus age is around 5.3 Ma, but we note differences among major groups. In light of these estimates, the classification of humans and chimpanzees is reconsidered using a molecular yardstick approach. Three taxonomic frameworks may flow from molecular analyses, all of them having major implications for understanding the evolution of humans and chimpanzees.

Animals↗
PubMed2001

Reinforcement during ecological speciation.

Reinforcement of pre-zygotic isolation can result when any of several kinds of selection act against hybrids. This paper investigates the situation where hybrids are selected against for ecological reasons, for example when there is no niche for individuals that are phenotypically intermediate between the parental species. The calculations here show how much ecological selection can lead to the reinforcement of a female mating preference or an assortative mating trait that is expressed in both sexes. The model allows for the ecological trait to be affected by any number of loci, but assumes that selection is weak and the introgression rate small. The effect of selection against hybrids increases rapidly as the difference between the mean phenotypes of the two populations increases. When genetic variation in the ecological trait is caused by many loci, stabilizing selection on it further contributes to reinforcement.

Animals↗
PubMed2006

Reproductive relationships and degree of synapsis in the polytene chromosomes of the Drosophila buzzatii species cluster.

The process of speciation occurs through the evolution of any of several forms of reproductive isolation between taxa, including inviability of hybrids. In this work, strains derived from allopatric populations of Drosophila buzzatii cluster species were experimentally crossed in order to evaluate their reproductive and cytogenetic relationships, and to contribute toward understanding the reproductive isolation in this group of sibling species. Although intrastrain crosses were highly fertile, we consider it relevant to discuss the differences in intra- and interspecific fertility and fecundity here. Among 30 interspecific crosses, about 63% were partially or completely sterile. Fifty three percent of interspecific F1 crosses (female and male F1 crossed) were also partially or completely sterile, in contrast to only one out of 24 intraspecific F1 crosses that was partially sterile. An analysis of hybrid polytene chromosomes revealed complete synapsis, except in the microchromosomes (VI) and in the proximal region of the X chromosome. The intraspecific divergence observed in this study and the variable degree of chromosome pairing shown here reveal part of the complexity of the speciation process pertinent to Drosophila buzzatii cluster, which is consistent with different traits studied in this cluster.

Animals↗
PubMed1998

Sex-related genes, directional sexual selection, and speciation.

Reproductive isolation and speciation can result from the establishment of either premating or postmating barriers that restrict gene flow between populations. Recent studies of speciation have been dominated by a molecular approach to dissect the genetic basis of hybrid male sterility, a specific form of postmating reproductive isolation. However, relatively little attention has been paid to the evolution of genes involved in premating isolation and genes generally involved in other sex-related functions (e.g., mating behavior, fertilization, spermatogenesis, sex determination). We have assembled DNA sequences from 51 nuclear genes and classified them based on their functional characteristics. The proportion of nonsynonymous to synonymous nucleotide substitutions were compared between Drosophila melanogaster, Drosophila simulans, and Drosophila pseudoobscura, as well as between Caenorhabditis elegans and Caenorhabditis briggsae. We found a high ratio of nonsynonymous to synonymous substitutions for sex-related genes (i.e., genes involved in mating behavior, fertilization, spermatogenesis, or sex determination). The results suggest that directional sexual selection has shaped the evolution of sex-related genes and that these changes have more likely occurred during the early stages of speciation. It is possible that directional selection becomes relaxed after reproductive isolation has been completed between more distantly related species (e.g., D. melanogaster and D. pseudoobscura). However, a saturation in the number of nucleotide substitutions since the time of species separation may mask any sign of directional selection between more distantly related species.

Animals↗