Molecular and genetic analysis of the Tribolium Ultrabithorax ortholog, Ultrathorax

Ultrabithorax (Ubx) is essential for the proper patterning of the posterior thorax and anterior abdomen in Drosophila. The Coleoptera and Diptera diff er in the organization and structure of their thorax and anterior abdomen. Changes in the regulation of Ubx and/or its downstream target genes are pre dicted to underlie these altered morphologies, We exploited the feasibility of genetic analysis in the red flour beetle, Tribolium castaneum, to exami ne the role of its Ubx ortholog in development. We analyzed genomic and cDN A clones that predict a polypeptide with nearly 100% identity with the Dros ophila Ubx gene in the homeodomain and nanking sequences. Southern blot: an alysis indicates that these clones represent DNA sequences within the Homeo tic complex (HOM-C) of Tribolium. Phenotypic analysis of mutant variants of the Ultrathorax (Utx) gene, and its location within the beetle HOM-C, stro ngly supports Utx being the Tribolium ortholog of Ubx. The embryonic expres sion-pattern of Ubx-homologous transcripts coincides with the phenotypes as sociated with Utx mutations, providing support that the Ubx-homologous clon ed DNA represents the Utx locus. By mid-germband extension Utx transcripts are expressed in a pattern similar to Ubx in Drosophila. However, during ea rly germband formation Utx transcripts differ in both spatial and temporal progression. Utx expression is initially detected in parasegments 4 and 5 ( T1p-T3a) as they are established during early germband formation. This is t he first report of the wild-type parasegmental expression of an insect Ubx ortholog extending through parasegment 4. The earlier and more anterior exp ression in the thorax may underlie the modification of the Coleopteran thor ax.