The evolution of Hox paralog group 2 developmental gene function in the osteichthyes.

摘要

Clustered Hox genes constitute an interesting case of evolutionarily conserved transcription factors. While their genomic organization, embryonic expression and ability to result in homeotic transformations when mutated are generally conserved between arthropods and vertebrates, changes in their expression or function have been hypothesized to be generators of morphological evolution in the animal kingdom. Given their demonstrated function in specifying morphology along the anterior-posterior axis of vertebrate embryos and the fact that several rounds of Hox gene duplication occurred during incipient stages of vertebrate evolution, the amplification of Hox gene number was hypothesized to have allowed the evolution of greater organismic complexity in the vertebrate lineage. Accordingly, appropriate testing of this theory should include studies of Hox gene function in lineages representative of pre- and post-duplication states. Herein we investigated the post-duplication function of Hox genes from paralog group 2 (PG2) in the Nile tilapia (Oreochromis niloticus ) to test hypotheses related to post-duplication divergence of gene function. Our findings are compared to those from previous studies conducted in zebrafish and several tetrapod models to infer a more precise picture of Hox PG2 gene function evolution. Our functional study shows that Nile tilapia Hox PG2 genes have partially redundant roles in second pharyngeal arch specification, which appears somewhat similar to the redundancy of function observed in zebrafish, but also reveals previously undescribed roles of PG2 genes in posterior arch specification and segmentation. While this latter role was not previously observed in zebrafish, it parallels that of mouse Hoxa2, suggesting that PG2 gene-dependent specification of posterior arch morphology is an ancestral osteichthyan trait. Our data demonstrate that Hox PG2 gene functions have diverged between the lineages leading to zebrafish---the Ostaryophysans---and to Nile tilapia---the Acanthopterygians---possibly as a result of differential post-duplication retention of PG2 duplicates in these lineages.