Transcriptional readthrough of Hox genes Ubx and Antp and their divergent post‐transcriptional control during crustacean evolution

Abstract

Hox genes are in principle tandemly arranged in an order colinear with their order of expression along the anterior-posterior axis. Combinations of Hox proteins encode information that specifies the unique characteristics of axial regions in the metazoan body plan. The independent regulation of Hox genes achieved by differential promoter activity is essential for the expression of Hox proteins in distinct territories and thereby creating a full repertoire of Hox codes. Here we report the abundant expression of transcriptional readthrough products of two adjacent Hox genes, Ubx, and Antp, in five crustacean species of Branchiopoda and Malacostraca. Bicistronic mRNA places Antp under the control of the Ubx promoter, which is active in the posterior segments of two branchiopodans Daphnia and Artemia, and would normally reduce the complexity of Hox codes if translated. This does not occur, however, as the translational capability of the bicistronic mRNA is limited. In Daphnia, bicistronic Ubx/Antp mRNA produced no significant level of either UBX or ANTP. In Artemia, on the other hand, the bicistronic mRNA produced only UBX, and replaced the role of monocistronic Ubx mRNA. In this way, multiple post-transcriptional control mechanisms in two extant branchiopodans can be seen as preventing the potentially deleterious consequences of Hox gene fusion.

The significance of Hox gene collinearity

“…This is reported in crustaceans, but posttranscriptional mechanisms prevent the potentially adverse effects of shared expression domains (Shiga et al, 2006). It has been suggested that human Hox genes Hoxc6, Hoxc5 and Hoxc4 may be transcribed in placenta as a single primary transcript, and that this may account for how the same 5′ non-coding exon can become spliced on to both Hoxc6 and Hoxc4 mature mRNAs .…”

Development of an RNA interference method in the cladoceran crustacean Daphnia magna

“…In turn, this will lead to a better understanding of the ecology, evolutionary biology, and molecular biology of D. magna. In particular, this technique will increase suitability of D. magna for developmental studies (Shiga et al 2002(Shiga et al , 2006 and establish this species as a new model organism for evolutionary developmental biology.…”

“Crustacea”: Comparative Aspects of Larval Development

“…Also the expression of other Hox genes has been studied for Artemia franciscana (Averof and Akam 1995 ;Shiga et al 2006 ) at an ontogenetic stage in which the fi rst thoracopods just start to develop. Antennapedia is expressed from the ( 2008 )) posterior part of the maxillulary segment throughout the entire trunk and Ultrabithorax from the fi rst trunk segment onwards, and Abdominal -A expression starts about one segment more posteriorly (discussion on a linkage of the Ubx and Abd-A expression in Shiga et al 2006 ).…”

“…Antennapedia is expressed from the ( 2008 )) posterior part of the maxillulary segment throughout the entire trunk and Ultrabithorax from the fi rst trunk segment onwards, and Abdominal -A expression starts about one segment more posteriorly (discussion on a linkage of the Ubx and Abd-A expression in Shiga et al 2006 ). Abdominal -B is expressed far more posteriorly, in the genital segments and gonads.…”