Homoeosis in Drosophila: The Lethal Syndrome of the Regulator of bithorax (or trithorax) Locus and Its Interaction with Other Homoeotic Loci.

Regulator of bithorax (Rg-bx)(-) [or trithorax (trx)(-)] lethal zygotes show anterior transformations of various cuticular features of the larval thorax and abdomen. The Rg-bx(-) lethal syndrome depends on the dosage of the bithorax gene complex (BX-C), and lack of Rg-bx(+) function is antagonistic to posterior transformations displayed by Polycomb ( Pc)(-) embryos. Significantly, when the BX-C is deleted, the Rg-bx(-) embryos disclose homoeosis of mesothoracic to prothoracic cuticular structures. This homoeotic transformation is due to a reduction in Antennapedia (Antp)(+) gene activity and is consequently dependent on the dosage of the Antennapedia gene complex (ANT-C), suggesting that the Rg-bx(+) activity is necessary for proper expression of the Antp(+) gene. However, the functional relationship between the Rg-bx and Sex combs reduced (Scr) loci in embryogenesis is still to be established.