Extra DNA replication and cell proliferation in wing imaginal discs of aDrosophila species hybrid.

Species hybrids obtained from corsses betweenD. azteca andD. athabasca display characteristic growth anomalies. These are restricted to hybrid males while hybrid females resemble the pure species. The ratio of the wing length to body length is 1 in pure species, 1.3 in hybrid males from the crossD. azteca x D. athabasca and 0.65 in hybrid males from the reciprocal cross. This indicates a disproportionate enlargement and diminuation, respectively, restricted to some tissues. Whereas the number of ommatidia in the compound eye is normal, the number of wing cells is approximately twice as high in hybrid giant males as in hybrid females and pure species. Microdeterminations of the DNA content of wing discs show that an extra DNA replication step occurs during the early pupal stage of hybrid giant males; the pupal stage of these males lasts about 58 h longer than that of the pure species. In eye-antenna discs no extra DNA synthesis has been found. Incorporation experiments reveal a delayed uptake of radioactively labeled thymidine into the DNA of wing discs in late third instar larvae and early pupae of hybrid giant males. The results of this study are interpreted to indicate that a disturbed balance between sex-linked and autosomal genes in hybrid males is responsible for the modified rhythm of DNA synthesis and of cell proliferation in certain cell types.