Molecular basis of urostyle development in frogs: genes and gene regulation underlying an evolutionary novelty.

Evolutionary novelties entail the origin of morphologies that enable new functions. These features can arise through changes to gene function and regulation. One key novelty is the fused rod at the end of the vertebral column in anurans, the urostyle. This feature is composed of a coccyx and a hypochord, both of which ossify during metamorphosis. To elucidate the genetic basis of these features, we used laser capture microdissection of these tissues and did RNA-seq and ATAC-seq at three developmental stages in tadpoles of Xenopus tropicalis. RNA-seq reveals that the coccyx and hypochord have two different molecular signatures. Neuronal (TUBB3) and muscle markers (MYH3) are upregulated in coccygeal tissues, whereas T-box genes (TBXT, TBXT.2), corticosteroid stress hormones (CRCH.1) and matrix metallopeptidases (MMP1, MMP8 and MMP13) are upregulated in the hypochord. ATAC-seq reveals potential regulatory regions that are observed in proximity to candidate genes that regulate ossification identified from RNA-seq. Even though an ossifying hypochord is only present in anurans, this ossification between the vertebral column and the notochord resembles a congenital vertebral anomaly seen prenatally in humans caused by an ectopic expression of the TBXT/TBXT.2 gene. This work opens the way to functional studies that can elucidate anuran bauplan evolution.