Figure 2. Genetic incompatibilities between the hybrid Italian sparrow and its parent species.

(A) Representative geographic cline (ND2) for the mitochondrion and Z-linked genes shifting significantly between the Italian and Spanish sparrows of mainland Italy/Sicily and Sardinia. Colors refer to posterior likelihood of belonging to group corresponding to the Italian sparrow (>0.9, no color) relative to the Spanish sparrow (<0.1, red). The numbers refer to three transects through the Italian-house sparrow hybrid zone in the Alps. Black dots denote sampling locations. (B) Genomic location (in zebra finch) of genes inferred to be involved in hybrid-parent reproductive isolation. Blue outlines denote genes shifting significantly between the Italian and house sparrow, and red outlines denote genes shifting significantly between the Italian and Spanish sparrow. Markers highlighted in yellow have significantly steeper clines (significant β) than the neutral expectation according to a BGC-analysis (see main text) in addition to being significantly skewed towards either hybrid-parent species boundary (significant α), and hence represent the strongest candidate RI genes. Markers in white have significant α only. Chromosomal location for the Z-linked and Chr. 4A genes are indicated. (C) Geographic clines along transect 2 for the three genes shifting significantly in the Italian-house sparrow hybrid zone in the Alps. Upper panel shows results from the Z-linked genes CHD1Z/CETN3, lower panel shows results for the autosomal gene RPS4. Colors refer to posterior likelihood of belonging to group corresponding to the house sparrow (>0.9, white) relative to the Italian sparrow (<0.1, red). Black dots denote sampling locations.