HOW DO SINGLE HOMEOTIC GENES CONTROL MULTIPLE SEGMENT IDENTITIES

Each of the homeotic genes of the bithorax complex of Drosophila defin es the identities of more than one body segment. The mechanisms by whi ch this occurs have been elusive. In a recent report, Castelli-Gair an d Akam((1)) analyze in detail the control of parasegment 5 and paraseg ment 6 identities by the bithorax complex gene Ubx. Their results indi cate that differences in the spatial and temporal expression patterns of Ubx are critical in determining differences between these parasegme nts. However, dose effects observed by others indicate that parasegmen t-specific differences in the level of Ubx expression are also importa nt. For the other BX-C genes, parasegment-specific expression of prote in isoforms, or combinatorial control dependent on the expression patt erns of other spatially restricted regulators, may also play a role.