Zebrafish dentition in comparative context

Abstract

Studies of the zebrafish (Danio rerio) promise to contribute much to an understanding of the developmental genetic mechanisms underlying diversification of the vertebrate dentition. Tooth development, structure, and replacement in the zebrafish largely reflect the primitive condition of jawed vertebrates, providing a basis for comparison with features of the more extensively studied mammalian dentition. A distinctive derived feature of the zebrafish dentition is restriction of teeth to a single pair of pharyngeal bones. Such reduction of the dentition, characteristic of the order Cypriniformes, has never been reversed, despite subsequent and extensive diversification of the group in numbers of species and variety of feeding modes. Studies of the developmental genetic mechanism of dentition reduction in the zebrafish suggest a potential explanation for irreversibility in that tooth loss seems to be associated with loss of developmental activators rather than gain of repressors. The zebrafish and other members of the family Cyprinidae exhibit species-specific numbers and arrangements of pharyngeal teeth, and extensive variation in tooth shape also occurs within the family. Mutant screens and experimental alteration of gene expression in the zebrafish are likely to yield variant tooth number and shape phenotypes that can be compared with those occurring naturally within the Cyprinidae. Such studies may reveal the relative contribution to trends in dental evolution of biases in the generation of variation and sorting of this variation by selection or drift.

Selection and constraint underlie irreversibility of tooth loss in cypriniform fishes

“…The oral region of zebrafish larvae expresses the transcription factor pitx2, a marker of tooth-competent epithelium, but lacks the expression of markers of the dental placode, the earliest morphologically visible sign of tooth development (12,21). We found that arrest of pharyngeal tooth development in nkt mutant homozygotes resembles that found in the wild-type oral region.…”

“…Teeth in this group were lost from the entire mouth cavity and upper pharynx at least 50 million y ago and remain only on the fifth ceratobranchial bones of the lower posterior pharynx (Fig. 1A), where they serve the function of mechanical processing of food, especially by chewing (11,12). Such reduction of dentition is thought to have evolved as an adaptation to suction feeding in bottom deposits (11,13,14).…”

“…Identifying the genetic pathways altered in association with cypriniform dentition reduction is expected to provide insight into potential constraints leading to irreversibility, and is facilitated by the membership of the zebrafish (Danio rerio) model species in the group (12). One candidate gene for tooth loss encodes the TNF family ligand Ectodysplasin (Eda) (19), as zebrafish homozygotes for the nkt loss-of-function mutation in this gene completely lack adult dentition (20).…”

Evolution and developmental diversity of tooth regeneration

“…The zebrafish (Danio rerio) is a highly derived Teleost species of the order Cypriniformes and over the past 30 years has become the top fish developmental model owing to its transparency during embryogenesis, the ease of vast embryo production and the development of genomic tools. The zebrafish has become a useful model for tooth development and manipulation [8] however the tiny teeth of zebrafish and teeth in all Cypriniformes are confined to the back of the gill arches, embedded deep within the pharyngeal cavity, with oral teeth in this order having been lost approximately 50 million years ago [9].…”

Expression of protocadherin 18 in the CNS and pharyngeal arches of zebrafish embryos

“…Teleost fish including zebrafish often have diversified alleles homologous to a mammalian counterpart supposedly due to ancient whole gene duplications (Stock, 2007, Wittbrodt et al, 1998. Blast searches indicate that there are two alleles (EU267178 and our AB297803) in the zebrafish genome homologous to mammalian pcdh18.…”