A Developmental Genetic Analysis of the C2H2 Zinc Finger Encoding Gene, Disconnected, in Drosophila Melanogaster and Tribolium Castaneum

Abstract

The Hox proteins are highly conserved homeodomain transcription factors that play a crucial role in the embryonic development of all animals, and the Hox genes encoding these proteins are conserved in a clustered genomic arrangement that is usually co-linear with their anterior/posterior domains of expression. Also called 'selector' genes, the Hox genes are presumed encode factors regulating specific genetic cascades that confer precise and unique regional identities along the anterior/posterior body axis. Because different Hox proteins are able to bind very similar, and sometimes identical, target DNA sequences, it is likely that co-factors play a role in guiding Hox protein functional specificity during animal development. However, few such co-factors have been identified.

The objective of this research is to address several questions regarding a previously identified genetic Hox co-factor encoded by the disconnected (disco) gene during the embryonic patterning of the Drosophila melanogaster larval head. First, what is the function of disco during embryonic development, and how does the encoded Disco protein function with the Hox proteins to direct patterning? Second, what upstream factors regulate the spatial and temporal expression of disco during embryonic development? Finally, given that the disco gene is conserved across animal phyla, as is the case with the Hox genes, is the Hox co-factor function also conserved? These questions are examined using the model organisms Drosophila melanogaster (the fruit fly) and Tribolium castaneum (the red flour beetle).

In Drosophila, developmental and genetic analysis techniques are utilized to examine the role of disco as a genetic co-factor for two HOM-C/HOX genes, Deformed (Dfd) and Sex combs reduced (Scr), during embryonic development. disco expression defines domains in which these Hox proteins can function, as evidenced by target gene activation and homeotic transformations. disco is further shown to have a regional embryonic patterning role as part of an interactive network of C2H2 zinc finger proteins that includes the trunk patterning gene and Hox co-factor, teashirt (tsh). The results of this work leads to the proposal of a model for Hox protein function in Drosophila in which different combinations of regionally expressed C2H2 zinc finger proteins and Hox proteins confer specific segment identities along the embryonic body axis. This work goes on to demonstrate that the Gap proteins Giant, Hunchback, and Krüppel regulate the spatial and temporal expression of disco and tsh. In particular, Giant is shown to be a key regulator of a repressive cascade that positions the embryonic gnathal/trunk boundary through its regulation of disco and tsh spatial expression. Giant's regulation of disco and tsh is part of a previously unrecognized segment identity role for this protein in the anterior portion of the Drosophila embryo. Finally, study of disco function is presented in an organism diverged by 300 million years from Drosophila, Tribolium castaneum. The embryonic expression patterns of the Tribolium disco are examined and compared to Drosophila disco expression. Embryonic and adult developmental functions of the Tribolium disco gene are investigated utilizing parental and larval RNA interference. The results reveal a significant role for the Tribolium gene in appendage patterning and elongation.