Isolation and re-connection: the formation of a ring-shaped speciation
continuum in an odorous frog (Odorrana margaretae)
Abstract
The Green Odorous Frog (Odorrana margaretae) around the Sichuan Basin of
western China displays a ring-shaped distributional pattern and
possesses multiple replicate contact zones between lineages at various
levels of differentiation. To understand its unique speciation history
and mechanisms, we obtained 1,540 SNPs from 29 populations and 227
individuals using ddRAD sequencing. Population structure analysis
revealed three groups within the species: The West, the North & South,
and the East groups. These groups were initially isolated at
~2.03 million years ago, and subsequent post-glacial
expansion produced the current ring-shaped distribution around Sichuan
Basin with three contact zones. Hybridization in those zones involved
lineages with different levels of divergence and produced greatly
different outcomes. Both the hybrid zones at southwest (S-W) and
southeast (E-NS) of the Basin have extensive admixture and less barrier
effect. Consequently, the southern region has the highest genetic
diversity and becomes an ‘evolutionary melting pot’. In contrast, the
hybrid zone at northwestern corner (N-W), which resembles the overlap
zone between two expansion terminals of a ring species, has limited
admixture with a narrow geographic cline, suggesting partial
reproductive isolation between the northern and western populations. The
three hybrid zones likely resemble three time points along a speciation
continuum; while both E-NS and S-W hybrid zones are merging, the N-W
zone may have passed the ‘tipping point’ and is destined for a complete
reproductive isolation over time.