Meeting Abstract
Jaws are an evolutionary innovation found in the majority of vertebrates (gnathostomes) which facilitate the capture of prey. One of its key features is a central jaw joint that allows the articulation of its rigid skeleton, yet we are uncertain as to the origins of this joint. To address jaw joint evolution, we investigated the skeletal development of an early diverging jawless vertebrate, the sea lamprey Petromyzon marinus. During their larval stage, their skeleton contains tissue known as mucocartilage that shares morphological and genetic features with the jaw joint. We asked whether mucocartilage may be homologous with joint tissue in gnathostomes by investigating genes in sea lamprey that are related to joint development. To do this, we conducted in situ hybridizations on embryos for three genes of interest (BarxA, TrpsA, and GDF5/6/7b). We also performed alcian stains to visualize skeletal development. To assess the function of these genes, we used CRISPR-cas9 mutagenesis to assess differences in gene expression and cartilage development in mutants. Our in situ hybridizations show expression of BarxA and TrpsA in both mucocartilage and rigid skeleton, contrasting their more exclusive gnathostome orthologues. These results were reinforced in TrpsA and BarX mutants, which show global loss of cartilage. GDF5/6/7b is expressed early in the anterior head and ventral pharynx, coinciding with future sites of mucocartilage. However, GDF5/6/7b mutants show only a slight loss of these tissues and loss of expression of TrpsA. Our results suggest that genes involved in the vertebrate jaw joint were co-opted from more generalized roles in skeletal development. We discuss how mucocartilage may not be as homogenous as previously thought and how this may shape our understanding of the jaw joint.
