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 differ in the organization and structure of their thorax and anterior abdomen. Changes in the regulation of Ubx and/or its downstream target genes are predicted to underlie these altered morphologies. We exploited the feasibility of genetic analysis in the red flour beetle, Tribolium castaneum, to examine the role of its Ubx ortholog in development. We analyzed genomic and cDNA clones that predict a polypeptide with nearly 100% identity with the Drosophila Ubx gene in the homeodomain and flanking sequences. Southern blot analysis indicates that these clones represent DNA sequences within the Homeotic complex (HOM-C) of Tribolium. Phenotypic analysis of mutant variants of the Ultrathorax (Utx) gene, and its location within the beetle HOM-C, strongly supports Utx being the Tribolium ortholog of Ubx. The embryonic expression pattern of Ubx-homologous transcripts coincides with the phenotypes associated with Utx mutations, providing support that the Ubx-homologous cloned DNA represents the Utx locus. By mid-germband extension Utx transcripts are expressed in a pattern similar to Ubx in Drosophila. However, during early 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 the first report of the wild-type parasegmental expression of an insect Ubx ortholog extending through parasegment 4. The earlier and more anterior expression in the thorax may underlie the modification of the Coleopteran thorax.