HOMEOTIC COMPLEX AND TEASHIRT GENES COOPERATE TO ESTABLISH TRUNK SEGMENTAL IDENTITIES IN DROSOPHILA

Homeotic genes determine the identities of metameres in Drosophila. We have examined functional aspects of the homeotic gene teashirt by ect opically expressing its product under the control of a heat-shock prom oter during embryogenesis. Our results confirm that the gene is critic al for segmental identity of the larva. Under mild heat-shock conditio ns, the Teashirt protein induces an almost complete transformation of the labial to prothoracic segmental identity, when expressed before 8 hours of development. Positive autoregulation of the endogenous teashi rt gene and the presence of Sex combs reduced protein in the labium ex plain this homeosis. Patterns in the maxillary and a more anterior hea d segment are partly replaced with trunk ones, Additional Teashirt pro tein has no effect on the identity of the trunk segments where the gen e is normally expressed; teashirt function is overriden by some homeot ic complex acting in the posterior trunk. Strong heat-shock regimes pr ovoke novel defects: ectopic sense organs differentiate in posterior a bdominal segments and trunk pattern elements differentiate in the nint h abdominal segment. Teashirt acts in a partially redundant way with c ertain homeotic complex proteins but co-operates with them for the est ablishment of specific segment types, We suggest that Teashirt and HOM -C proteins regulate common sets of downstream target genes.