Rapid and predictable evolution of admixed populations between two Drosophila species pairs

The consequences of hybridization are varied, ranging from the origin of
new lineages, introgression of some genes between species, to the
extinction of one of the hybridizing species. We generated replicate
admixed populations between two pairs of sister species
of Drosophila: D. simulans and D.
mauritiana; and D. yakuba and D. santomea. Each
pair consisted of a continental species and an island endemic. The admixed
populations were maintained by random mating in discrete generations for
over 20 generations. We assessed morphological, behavioral, and
fitness-related traits from each replicate population periodically, and
sequenced genomic DNA from the populations at generation 20. For both
pairs of species, species-specific traits and their genomes regressed to
those of the continental species. A few alleles from the island species
persisted, but they tended to be proportionally rare among all sites in
the genome and were rarely fixed within the populations. This paucity of
alleles from the island species was particularly pronounced on
the X-chromosome. These results indicate that nearly all foreign
genes were quickly eliminated after hybridization and that selection
against the minor species genome might be similar across experimental
replicates.