Characterization of hybrids and contact zones
Based on our test using only informative markers, we found that
individuals of hybrid ancestry had generally low levels of
heterozygosity across the three contact zones (Figure 4). Moreover, we
also found a continuous distribution of hybrid indices between the two
parental subspecies in the corvina-hicksii andhoffmanni-hicksii contact zones (Figure 4A,B). Interestingly, we
found virtually no corvina-hoffmanni hybrids (Figure 4C), in this
contact zone; all individuals with mixed ancestry can be confidently
assigned to one of the parental subspecies based on the genetic makeup,
contrary to the other two contact zones.
We fitted geographic clines along both contact zones in which subspecies
hybridize, we fitted clines for the hybrid index, plumage traits, and
significantly different morphometric traits. First, we identified
morphometric differences between distant populations, testing for the
effect of subspecies and sex, and found corvina andhicksii differed in beak size, wing length, and tail length,
while hoffmanni and hicksii differed in beak size, tail
length, and tarsus length (Table 1). For color patches, we focused on
those patches that differed between subspecies. In both contact zones,
the best fit model for the hybrid index had a fixed scaling from 0 to 1
and no tails fitted (Table 2). Plumage brightness of the different
patches modeled showed a clinal distribution in which the brightness of
the rump and belly in males for the corvina-hicksii contact zone,
and the brightness of the throat for the hoffmanni-hicksiicontact zone had cline centers that were coincident with their
respective hybrid index clines. However, the center of the throat’s
brightness for the corvina-hicksii contact zone is displaced
~60 km to the East, with no overlap of the 95% CI
(Figure 5A,C; Table 2). Plumage brightness and genetic clines differ in
cline widths, with plumage clines showing narrower widths than the
hybrid index clines (Figure 5A,C; Table 2).
When the most distant populations differed in morphometric traits, we
found a general pattern in which hicksii is smaller than the
other two subspecies, showing on average smaller beaks and shorter tails
than corvina and hoffmanni , and shorter wings thancorvina . However, hoffmanni had, on average, shorter tarsi
than hicksii (Table 1). Morphometric traits along thecorvina-hicksii contact zones (tail length and beak size) were
not coincident with respect to the hybrid index, with tail length and
beak size having displaced centers (Figure 5B; Table 2). Along thehoffmanni-hicksii contact zone, cline centers of beak size and
tail length are coincident with the correspondent hybrid index based on
the overlapping of the 95% CI (Figure 5D; Table 2). However, we found
important intra-population variation, with none of the morphometric
traits exhibiting a clear clinal distribution when modeling average
trait values per population and, instead, showed a more linear, smoother
transition from one parental population to the other, which result in
low confidence of cline parameters (Figure 5B,D; Table 2). For example,
the CI of the cline width of the tarsus length betweenhoffmanni-hicksii and wing length between corvina-hicksiiwere estimated to be between ~20 km to 1200 km wide (not
graphed, Table 2).