It's all about genetic (in)compatability!

In a recent article published in the Proceedings of the Royal Society B (non-potluck), Pulido-Santacruz et al. describes speciation via post-zygotic isolation in a region of the Amazon that could be considered a hybrid swarm. Seven species, or sub-species, come into geographical contact in the headwater region of the Teles Pieres River, and all are known to hybridize. However, little is known about the extent of hybridization in this region or in any species-rich region in the world.

The authors studied hybridization between two groups: Willsornis (scale-backed antbirds and Xingu antbirds;Figure 1a), and Xiphorhynchus (elegant and Spinx's woodcreepers; Figure 1b). All four populations, although they are known to hybridize, are known as 'morphologically cryptic species'. This means that the two parental taxa look almost exactly alike (in this case the plumage and song patterns are indistinguishable), but their genomes are actually separated sufficiently to be warranted as a distinct group.

They sampled two sub-species of each taxon and generated genotype data via SNP's of 144 antbirds, 128 woodcreepers, and 43 individuals from the contact zone. They used these SNP's to determine a hybrid index (HI) and based on the results of the proportion of hybrid-like individuals, they used modeling to determine that post-zygotic isolation (i.e. genetic incompatibility) is likely maintaining distinct species in this region.

Figure 1. From Paulid-Santacruz et al. 2018; represents geographical sampling (a & b) and population STRUCTURE (c & d) for the distinct groups sampled in this study.

After running a STRUCTURE analysis (Figure 1, c and d), the authors also found clear differentiation between sub-species further away from the hybrid zone (in pink and yellow). The sub-species closer to the hybrid zone were also differentiated (green and blue), but contact zone individuals contained a mixed-genetic composition (purple).

The authors conclude from these analysis that strong post-zygotic isolation drove the accumulation of genetic incompatibilities over long periods of time, likely due to the absence of strong divergent selection. However this points to more evidence that it is genetic compatibility, driven by both selection and time since divergence, that is the central facet to speciation via introgression.

- Elizabeth (Liz) Tapanes
2nd non-potluck