Population genomics of an emergent tri-species hybrid zone

Isolating barriers that drive speciation are commonly studied in the
context of two-species hybrid zones. There is however evidence that more
complex introgressive relationships are common in nature. Here, we use
field observations and genomic analysis, including the sequencing and
assembly of a novel reference genome, to study an emergent hybrid zone
involving two colliding hybrid zones of three woodpecker species:
red-breasted, red-naped, and yellow-bellied sapsuckers (Sphyrapicus ruber,
S. nuchalis, and S. varius). Surveys of the area surrounding Prince
George, British Columbia, Canada, show that all three species are
sympatric, and Genotyping-by-Sequencing identifies hybrids from each
species pair and birds with ancestry from all three species. Observations
of phenotypes and genotypes of mated pairs provide evidence for
assortative mating, though there is some heterospecific pairing.
Hybridization is more extensive in this tri-species hybrid zone than in
two di-species hybrid zones. However, there is no evidence of a hybrid
swarm, and admixture is constrained to contact zones, so we classify this
region as a tension zone and invoke selection against hybrids as a likely
mechanism maintaining species boundaries. Analysis of sapsucker age
classes does not show disadvantages in hybrid survival to adulthood, so we
speculate the selection upholding the tension zone may involve hybrid
fecundity. Gene flow among all sapsuckers in di-species hybrid zones
suggests introgression likely occurred before the formation of this
tri-species hybrid zone and might result from bridge hybridization,
vagrancies, or other three-species interactions.