PATTERNS OF POSTZYGOTIC ISOLATION IN FROGS

Hybrid incompatibility is consistent with a hybrid origin of Heliconius heurippa Hewitson from its close relatives, Heliconius cydno Doubleday and Heliconius melpomene Linnaeus

Abstract Shared ancestral variation and introgression complicates the reconstruction of phylogenetic relationships among closely related taxa. Here we use overall genomic compatibility as an alternative estimate of species relationships in a group where divergence is rapid and genetic exchange is common. Heliconius heurippa, a butterfly species endemic to Colombia, has a colour pattern genetically intermediate between H. cydno and H. melpomene: its hindwing is nearly indistinguishable from that of H. melpomene and its forewing band is an intermediate phenotype between both species. This observation has lead to the suggestion that the pattern of H. heurippa arose through hybridization. We present a genetic analysis of hybrid compatibility in crosses between the three taxa. Heliconius heurippa x H. cydno and female H. melpomene x male H. heurippa yield fertile and viable F1 hybrids, but male H. melpomene x female H. heurippa crosses yield sterile F1 females. In contrast, Haldane's rule has previously been detected between H. melpomene and H cydno in both directions. Therefore, H. heurippa is most closely related to H. cydno, with some evidence for introgression of genes from H. melpomene. The results are compatible with the hypothesis of a hybrid origin for H. heurippa. In addition, backcrosses using F1 hybrid males provide evidence for a large Z(X)-chromosome effect on sterility and for recessive autosomal sterility factors as predicted by Dominance Theory.

On the theoretical and empirical framework for studying genetic interactions within and among species

We present a quantitative genetic (QG) interpretation of the Bateson-Dobzhansky-Muller (BDM) genetic model of speciation in order to unify the theoretical framework for understanding how the genetic differentiation of populations is associated with the process of speciation. Specifically, we compare the QG theory of joint scaling with the Turelli-Orr mathematical formulation of the BDM model. By formally linking the two models, we show that a wealth of empirical methods from QG can be brought to bear on the study of the genetic architecture of hybrid phenotypes to better understand the connections, if any, between microevolution within populations and macroevolution in the origin of species. By integrating the two theories, we make additional novel predictions that enrich the opportunities for empirically testing speciation genetic theory or facets of it, such as Haldane's rule. We show that the connection between the two theories is simple and straightforward for autosomal genes but not for sex-linked genes. Differences between the two approaches highlight key conceptual issues concerning the relevance of epistasis to evolution within and among lineages and to differences in the process of speciation in hermaphrodites and in organisms with separate sexes.

Identifying spiders through DNA barcodes

With almost 40 000 species, the spiders provide important model systems for studies of sociality, mating systems, and sexual dimorphism. However, work on this group is regularly constrained by difficulties in species identification. DNA-based identification systems represent a promising approach to resolve this taxonomic impediment, but their efficacy has only been tested in a few groups. In this study, we demonstrate that sequence diversity in a standard segment of the mitochondrial gene coding for cytochrome c oxidase I (COI) is highly effective in discriminating spider species. A COI profile containing 168 spider species and 35 other arachnid species correctly assigned 100% of subsequently analyzed specimens to the appropriate species. In addition, we found no overlap between mean nucleotide divergences at the intra- and inter-specific levels. Our results establish the potential of COI as a rapid and accurate identification tool for biodiversity surveys of spiders.

Genetics of hybrid incompatibility between Lycopersicon esculentum and L. hirsutum

We examined the genetics of hybrid incompatibility between two closely related diploid hermaphroditic plant species. Using a set of near-isogenic lines (NILs) representing 85% of the genome of the wild species Lycopersicon hirsutum (Solanum habrochaites) in the genetic background of the cultivated tomato L. esculentum (S. lycopersicum), we found that hybrid pollen and seed infertility are each based on 5-11 QTL that individually reduce hybrid fitness by 36-90%. Seed infertility QTL act additively or recessively, consistent with findings in other systems where incompatibility loci have largely been recessive. Genetic lengths of introgressed chromosomal segments explain little of the variation for hybrid incompatibility among NILs, arguing against an infinitesimal model of hybrid incompatibility and reinforcing our inference of a limited number of discrete incompatibility factors between these species. In addition, male (pollen) and other (seed) incompatibility factors are roughly comparable in number. The latter two findings contrast strongly with data from Drosophila where hybrid incompatibility can be highly polygenic and complex, and male sterility evolves substantially faster than female sterility or hybrid inviability. The observed differences between Lycopersicon and Drosophila might be due to differences in sex determination system, reproductive and mating biology, and/or the prevalence of sexual interactions such as sexual selection.

A fine-scale genetic analysis of hybrid incompatibilities in Drosophila

The sterility and inviability of species hybrids is thought to evolve by the accumulation of genes that cause generally recessive, incompatible epistatic interactions between species. Most analyses of the loci involved in such hybrid incompatibilities have suffered from low genetic resolution. Here I present a fine-resolution genetic screen that allows systematic counting, mapping, and characterizing of a large number of hybrid incompatibility loci in a model genetic system. Using small autosomal deletions from D. melanogaster and a hybrid rescue mutation from D. simulans, I measured the viability of hybrid males that are simultaneously hemizygous for a small region of the D. simulans autosomal genome and hemizygous for the D. melanogaster X chromosome. These hybrid males are exposed to the full effects of any recessive-recessive epistatic incompatibilities present in these regions. A screen of approximately 70% of the D. simulans autosomal genome reveals 20 hybrid-lethal and 20 hybrid-semilethal regions that are incompatible with the D. melanogaster X. In further crosses, I confirm the epistatic nature of hybrid lethality by showing that all of the incompatibilities are rescued when the D. melanogaster X is replaced with a D. simulans X. Combined with information from previous studies, these results show that the number of recessive incompatibilities is approximately eightfold larger than the number of dominant ones. Finally, I estimate that a total of approximately 191 hybrid-lethal incompatibilities separate D. melanogaster and D. simulans, indicating extensive functional divergence between these species' genomes.

Asymmetrical crossing barriers in angiosperms

Patterns of reproductive isolation between species may provide insight into the mechanisms and evolution of barriers to interspecific gene exchange. We used data from published interspecific hybridization experiments from 14 genera of angiosperms in order to test for the presence of asymmetrical barriers to gene exchange. Reproductive isolation was examined at three life-history stages: the ability of interspecific crosses to produce seeds, the viability of F1 hybrids, and the fertility of F1 hybrids. Statistically significant asymmetries in the strength of reproductive isolation between species were detected in all genera and at each of the three life-history stages. Asymmetries in seed production may be caused by a variety of mechanisms including differences in stigma/style lengths, self compatibility and differential fruit abortion. Asymmetries in post-zygotic isolation are probably caused by nuclear-cytoplasmic interactions. Asymmetrical reproductive isolation between plant taxa may have important implications for the dynamics of hybrid zones, the direction of genetic introgression and the probability of reinforcement.