A pair of Sox: distinct and overlapping functions of zebrafish sox9 co-orthologs in craniofacial and pectoral fin development

Abstract

Understanding how developmental systems evolve after genome amplification is important for discerning the origins of vertebrate novelties, including neural crest, placodes, cartilage and bone. Sox9 is important for the development of these features, and zebrafish has two co-orthologs of tetrapod SOX9 stemming from an ancient genome duplication event in the lineage of ray-fin fish. We have used a genotype-driven screen to isolate a mutation deleting sox9b function, and investigated its phenotype and genetic interactions with a sox9a null mutation. Analysis of mutant phenotypes strongly supports the interpretation that ancestral gene functions partitioned spatially and temporally between Sox9 co-orthologs. Distinct subsets of the craniofacial skeleton, otic placode and pectoral appendage express each gene, and are defective in each single mutant. The double mutant phenotype is additive or synergistic. Ears are somewhat reduced in each single mutant but are mostly absent in the double mutant. Loss-of-function animals from mutations and morpholino injections, and gain-of-function animals injected with sox9a and sox9b mRNAs showed that sox9 helps regulate other early crest genes, including foxd3, sox10, snai1b and crestin, as well as the cartilage gene col2a1 and the bone gene runx2a;however, tfap2a was nearly unchanged in mutants. Chondrocytes failed to stack in sox9a mutants, failed to attain proper numbers in sox9b mutants and failed in both morphogenetic processes in double mutants. Pleiotropy can cause mutations in single copy tetrapod genes, such as Sox9, to block development early and obscure later gene functions. By contrast, subfunction partitioning between zebrafish co-orthologs of tetrapod genes, such as sox9a and sox9b, can relax pleiotropy and reveal both early and late developmental gene functions.

Osterix‐mCherry transgenic medaka for in vivo imaging of bone formation

“…The corresponding orthologs share significantly similar sequences and expression patterns. These findings indicate that the genetic networks controlling bone formation are highly conserved in all vertebrates (Fisher et al, 2003;Flores et al, 2004;Yan et al, 2005;Renn et al, 2006b;Nemoto et al, 2007).…”

Midkine‐b regulates cell specification at the neural plate border in zebrafish

“…We analyzed expression of foxd3, sox10, sox9b, and snail1b, which label a broad spectrum of ncc subtypes (Yan et al, 2005;Lister et al, 2006). These markers were analyzed at 11-12 hpf (1-3 somite stage) in embryos injected with 150 pg mdkb RNA or 10 ng mdkb MO.…”

PCR survey of hox genes in the goldfish Carassius auratus auratus

“…Extensive comparative genomics studies have corroborated the proposition that gnathostomes have experienced two rounds of genome duplication (2R) (Sidow, '96;Amores et al, '98;Taylor et al, 2003;Vandepoele et al, 2004;Dehal and Boore, 2005). Moreover, another round (3R) genome duplication, the so-called ''fish-specific genome duplication'' (FSGD) took place about 320 mya ago in the ancestral lineage of the teleosts (see e.g., Amores et al, '98;Wittbrodt et al, '98;Gregory and Hebert, '99;Taylor et al, 2003;Hoegg et al, 2004;Vandepoele et al, 2004;Hoegg and Meyer, 2005;Meyer and Van de Peer, 2005;Yan et al, 2005;Crow et al, 2006).…”

“…Teleosts comprise more than 97% of the approximately 25,000 species of actinopterygians (rayfinned fishes) (Nelson,'94) and are the most species-rich and diverse group of vertebrates. Several authors suggested that the FSGD is at least partially responsible for the species diversity of teleosts (e.g., Amores et al, '98;Wittbrodt et al, '98;Meyer and Van de Peer, 2005;Yan et al, 2005; but see Donoghue and Purnell, 2005).…”