Low divergence in Dlx gene expression between dentitions of the medaka (Oryzias latipes) versus high level of expression shuffling in osteichtyans.

Serially homologous structures are believed to originate from the redeployment of a genetic cascade in different locations of the body. Serial homologs may diverge at the genetic and morphological level and acquire developmental independency (individualization). Teeth are repeated units that form dentitions found on different bones of the oral-pharyngeal cavity in gnathostomes and provide a good model to study such processes. Previous comparisons of dlx gene expression patterns between mouse oral teeth and zebrafish pharyngeal teeth showed a high level of divergence. Furthermore, these genes are differentially expressed in different teeth of the zebrafish, and in the mouse they are responsible for tooth identity (incisors vs. molars). We examined the potential divergence of dlx gene expression between oral and pharyngeal teeth by examining the expression pattern in the development of the first generation teeth of the medaka and comparing it with data from the zebrafish and the mouse. Out of the seven medaka dlx genes, five are expressed during odontogenesis compared with six in both the zebrafish and the mouse. The only difference observed between oral and pharyngeal teeth in the medaka is an earlier expression of dlx5a in the oral dental epithelium. The subset of dlx genes expressed in the medaka, zebrafish, and mouse is slightly different but their detailed expression patterns are highly divergent. Our results demonstrate a low constraint on dlx gene expression shuffling in the odontogenic cascade within osteichtyans but the non-individualization of oral and pharyngeal dentitions in the medaka.