Genetic analysis of extra sex combs in the hybrids between Drosophila subobscura and D. madeirensis

Drosophila subobscura and D. madeirensis are closely related species, the first distributed over a large area and the latter restricted to the island of Madeira. These species can hybridize in laboratory conditions, yielding fertile females and sterile males. Hybrid offspring show several phenotypic anomalies, including sex combs on the second and third pairs of legs in males. The extra sex comb trait is a homeotic transformation of the mesothoracic and metathoracic legs into prothoracic legs. This anomaly is observed almost exclusively in F₁ males with D. madeirensis mothers. Analysis of backcross males shows that D. subobscura and D. madeirensis have diverged at a minimum of four autosomal loci affecting the extra sex comb anomaly. In addition, some incompatibility involving the X chromosome and/or a maternal effect is also implicated.