Molecular heterochrony in the early development of Drosophila

Heterochrony, the relative change of developmental timing, is one of the ma jor modes of macroevolutionary change; it identifies temporally disassociat ed units of developmental evolution. Here, we report the results of a fine- scare temporal study for the expression of the developmental gene hairy and morphological development in three species of Drosophila, D. melanogaster, D. simulans, and D. pseudoobscura, The results suggest that between and am ong closely related species, temporal displacement of ontogenetic trajector y is detected even at the earliest stage of development. Overall, D. simula ns shows the earliest expression, followed by D. melanogaster, and then by D. pseudoobscura. Setting D. melanogaster as the standard, we find the appr oximate time to full expression is accelerated by 13 min, 48 s in D. simula ns and retarded by 24 min in D. pseudoobscura. Morphologically, again with D. melanogaster setting the standard, initiation of cellularization is fast er in D. simulans by 15 min, 42 s; and initiation of morphogenesis is faste r in D. simulans by 18 min, 7 s. These results seem to be consistent with t he finding that the approximate time to full expression of hairy is acceler ated by 13 min, 48 s in D. simulans. On the other hand, the same morphologi cal events are delayed by 5 min, 32 s, and by 11 min, 32 s, respectively, i n D. pseudoobscura. These delays are small, compared with the 24-min delay in full expression. The timing changes, in total, seem consistent with cont inuous phyletic evolution of temporal trajectories. Finally, we speculate t hat epigenetic interactions of hairy expression timing and cell-cycle timin g may have led to morphological differences in the terminal system of the l arvae.