Negative genetic correlation between traits of the Drosophila head, and interspecific divergence in head shape

For morphological traits that are negatively correlated, the genetic correlation (rg) between them might strongly influence patterns of morphological divergence and shape. Here, the pattern of divergence between two sibling species of cactophilic Drosophila, D. buzzatii and D. koepferae, is examined for two traits that are known to be negatively correlated in other Drosophila species: face width (FW) and width of both eyes (EW). Head width (HW, the sum of FW and EW, i. e. the total width of the head capsule) was also examined. Genetic and phenotypic correlations were estimated in the laboratory G2 generation of a sample of wild D. buzzatii derived from a population where D. koepferae is not present. Phenotypic correlations were also estimated in D. buzzatii and D. koepferae from another, very different, population where the species are sympatric. Consistent with studies in other Drosophila species, rg was negative and significant for the correlation between FW and EW, and positive (but nonsignificant at a matrix-wide P-value of 0.05) for the correlations of HW with both FW and EW. This well-defined correlation pattern was also consistent with the phenotypic correlations in both D. buzzatii and D. koepferae. No significant difference in these traits was detected between D. buzzatii populations, but head shape has diverged between D. buzzatii and D. koepferae. Specifically, the two negatively correlated traits, FW and EW, have evolved in opposite directions in these two species, with HW showing no significant interspecific difference. The overall picture of this divergence pattern shows a striking concordance with the present evidence of negative correlations between FW and EW, and is consistent with the notion of rg-related constraints on the pattern of interspecific differentiation.