CLONAL ANALYSIS IN HYBRIDS BETWEEN DROSOPHILA-MELANOGASTER AND DROSAPHILA SIMULANS

We have analysed the viability of cellular clones induced by mitotic r ecombination in Drosophila melanogaster/D. simulans hybrid females dur ing larval growth. These clones contain a portion of either melanogast er or simulans genomes in homozygosity. Analysis has been carried out for the X and the second chromosomes, as well as for the 3L chromosome ann. Clones were not found in certain structures, and in others they appeared in a very low frequency. Only in abdominal tergites was a sig nificant number of clones observed, although their frequency was lower than in melanogaster abdomens. The bigger the portion of the genome t hat is homozygous, the less viable is the recombinant melanogaster/sim ulans hybrid clone. The few clones that appeared may represent cases i n which mitotic recombination took place in distal chromosome interval s, so that the clones contained a small portion of either melanogaster or simulans chromosomes in homozygosity. Moreover, Lhr, a gene of D. simulans that suppresses the lethality of male and female melanogaster /simulans hybrids, does not suppress the lethality of the recombinant melanogaster/simulans clones. Thus, it appears that there is not just a single gene, but at least one per tested chromosome arm (and maybe m ore) that cause hybrid lethality. Therefore, the two species, D. melan ogaster and D. simulans, have diverged to such a degree that the absen ce of part of the genome of one species cannot be substituted by the c orresponding part of the genome of the other, probably due to the form ation of co-adapted gene complexes in both species following their div ergent evolution after speciation. The disruption of those co-adapted gene complexes would cause the lethality of the recombinant hybrid clo nes.