Complex Polyploids: Origins, Genomic Composition, and Role of Introgressed Alleles.

Introgression allows polyploid species to acquire new genomic content from diploid progenitors or from other unrelated diploid or polyploid lineages, contributing to genetic diversity and facilitating adaptive allele discovery. In some cases, high levels of introgression elicit the replacement of large numbers of alleles inherited from the polyploid's ancestral species, profoundly reshaping the polyploid's genomic composition. In such complex polyploids, it is often difficult to determine which taxa were the progenitor species and which taxa provided additional introgressive blocks through subsequent hybridization. Here, we use population-level genomic data to reconstruct the phylogenetic history of Betula pubescens (downy birch), a tetraploid species often assumed to be of allopolyploid origin and which is known to hybridize with at least four other birch species. This was achieved by modeling polyploidization and introgression events under the multispecies coalescent and then using an approximate Bayesian computation rejection algorithm to evaluate and compare competing polyploidization models. We provide evidence that B. pubescens is the outcome of an autoploid genome doubling event in the common ancestor of B. pendula and its extant sister species, B. platyphylla, that took place approximately 178,000-188,000 generations ago. Extensive hybridization with B. pendula, B. nana, and B. humilis followed in the aftermath of autopolyploidization, with the relative contribution of each of these species to the B. pubescens genome varying markedly across the species' range. Functional analysis of B. pubescens loci containing alleles introgressed from B. nana identified multiple genes involved in climate adaptation, while loci containing alleles derived from B. humilis revealed several genes involved in the regulation of meiotic stability and pollen viability in plant species.

Distribution of the Bari-I transposable element in stable hybrid strains between Drosophila melanogaster and Drosophila simulans and in Brazilian populations of these species

We analyzed the distribution of the Bari-I transposable element in Drosophila melanogaster (IN(1)AB), its sibling species Drosophila simulans (C167.4) and in eight hybrid strains derived from initial crosses involving D. simulans females and D. melanogaster males of the above cited strains as well as in Brazilian populations of these species. Polymerase chain reaction (PCR) data showed the presence of the Bari-I element among species populations and hybrid strains. Hybridization with a 703 bp probe homologous to the Bari-I sequence showed that the number of Bari-I copies in D. melanogaster IN(1)AB was higher than in D. simulans C167.4 strains. Hybrid strains presented Bari-I sequences related to both parental species. In addition some strains displayed a Bari-I sequence that came from D. melanogaster, suggesting introgression of D. melanogaster genetic material in the background of D. simulans. In contrast, some hybrids showed deletions of D. simulans Bari-I sequences.