Figure 1

A schematic representation of asymmetric mitochondrial DNA introgression via hybridization and species displacement. Figure 1a depicts the transfer of mitochondrial DNA across the species boundary via introgressive hybridization. Large circles reflect the nuclear DNA composition of individuals, small ones their mitochondrial DNA type. There is an initial hybridization event between the members of two species, a red female and a green male. The F1 offspring contain a mix of red and green nuclear DNA, but only red mitochondrial DNA (due to mitochondrial DNA’s matrilineal transmission). Subsequent backcrossing of admixed females with green males over the generations dilutes the red nuclear DNA out, in effect reconstituting the green species’ nuclear genome, but with red mitochondrial DNA (ensured by mitochondrial DNA’s clonal transmission). Figure 1b shows how the outcompeting of one species by another can result in asymmetric mitochondrial DNA introgression. Small circles now reflect the spatial distribution of mitochondrial DNA type and the background the geographical nuclear DNA composition of the population. At the top, the ranges of a red and a green species are in allopatry, but green expands its range towards red. In the middle, green and red have become parapatric. The species hybridize at the contact zone, where red mitochondrial DNA introgresses into the green species (as in Figure 1a). At the bottom, green shifts its distribution further to the right, at the expense of red. As the members of the green species leading the expansion contain red instead of green mitochondrial DNA, only red mitochondrial DNA spreads in the region where the green species displaces the red species. This figure is based on [3, 5].