Selection and isolation define a heterogeneous divergence landscape between hybridizing Heliconius butterflies.

Hybridizing species provide a powerful system to identify the processes that shape genomic variation and maintain species boundaries. However, complex histories of isolation, gene flow, and selection often generate heterogeneous genomic landscapes of divergence that complicate reconstruction of the speciation history. Here, we explore patterns of divergence to reconstruct recent speciation in the erato clade of Heliconius butterflies. We focus on the genomic landscape of divergence across three contact zones of the species H. erato and H. himera. We show that these hybridizing species have an intermediate level of divergence in the erato clade, which fits with their incomplete levels of reproductive isolation. Using demographic modeling and the relationship between admixture and divergence with recombination rate variation, we reconstruct histories of gene flow, selection, and demographic change that explain the observed patterns of genomic divergence. We find that periods of isolation and selection within populations, followed by secondary contact with asymmetrical gene flow are key factors in shaping the heterogeneous genomic landscapes. Collectively, these results highlight the effectiveness of demographic modeling and recombination rate estimates to disentangling the distinct contributions of gene flow and selection to patterns of genomic divergence.

Epigenetic processes in a tetraploid mammal.

Polyploidy has played a most important role in speciation and evolution of plants and animals. It is thought that low frequency of polyploidy in mammals is due to a dosage imbalance that would interfere with proper development in mammalian polyploids. The first tetraploid mammal, Tympanoctomys barrerae (Octodontidae), appears to be an exception to this rule. In this study we investigated X chromosome inactivation (XCI) and genomic imprinting in T. barrerae, two epigenetic processes usually involved in dosage control in mammalian genomes. The imprinting status of the Peg1 gene was determined by Peg1 allelic expression studies. The inactive X chromosome was identified on interphase nuclei by immunofluorescence using specific antisera raised against Met3H3K27 and macroH2A1. Quantitative PCR was used to compare the Peg1/Dmd ratio in T. barrerae and in its most closely related diploid species, Octomys mimax. Our data demonstrate that parental-specific silencing of at least one gene and normal X chromosomal dosage mechanism are conserved in the tetraploid genome. We hypothesize a concerted action of genetic and epigenetic mechanisms during the process of functional diploidization of this tetraploid genome.