Cephalopod Hox genes and the origin of morphological novelties

Abstract

Cephalopods are a diverse group of highly derived molluscs, including nautiluses, squids, octopuses and cuttlefish. Evolution of the cephalopod body plan from a monoplacophoran-like ancestor entailed the origin of several key morphological innovations contributing to their impressive evolutionary success. Recruitment of regulatory genes, or even pre-existing regulatory networks, may be a common genetic mechanism for generating new structures. Hox genes encode a family of transcriptional regulatory proteins with a highly conserved role in axial patterning in bilaterians; however, examples highlighting the importance of Hox gene recruitment for new developmental functions are also known. Here we examined developmental expression patterns for eight out of nine Hox genes in the Hawaiian bobtail squid Euprymna scolopes, by whole-mount in situ hybridization. Our data show that Hox orthologues have been recruited multiple times and in many ways in the origin of new cephalopod structures. The manner in which these genes have been co-opted during cephalopod evolution provides insight to the nature of the molecular mechanisms driving morphological change in the Lophotrochozoa, a clade exhibiting the greatest diversity of body plans in the Metazoa.

Heterotopic expression of MPF2 is the key to the evolution of the Chinese lantern of Physalis , a morphological novelty in Solanaceae

“…This assumption needs to be further substantiated. Regardless of the precise reason in this particular case, recruitment of an existing transcription factor into a different functional context might be a common phenomenon in the evolution of morphological novelties (3,50).…”

Terminal addition, the Cambrian radiation and the Phanerozoic evolution of bilaterian form

“…In each case the loss of serial musculature is evident in the muscle scars found in an evolutionary succession (e.g., Wingstrand 1985;Peel 1991). In cephalopods, modification of Hox gene specification of axial structures is evident during this process and the exact nature of the axial organization becomes difficult to compare with more elongate bilaterians (Lee et al 2003).…”

Clustered brachiopod Hox genes are not expressed collinearly and are associated with lophotrochozoan novelties

“…Hox genes are prone to gains (15-17) and losses (18)(19)(20)(21), and their arrangement in a cluster can be interrupted, or even completely disintegrated (22)(23)(24)(25). Furthermore, the collinear character of the Hox gene expression can fade temporally (24,26,27) and/or spatially (28). In addition, Hox genes have diversified their roles during development, extending beyond providing spatial information (29).…”

“…In addition, Hox genes have diversified their roles during development, extending beyond providing spatial information (29). They also are involved in patterning different tissues (30), and often have been recruited for the evolution and development of novel morphological traits, such as vertebrate limbs (31,32), cephalopod funnels and arms (28), and beetle horns (33).…”