Evolution of segmentation in chordates: dissecting the genetic mechanism of somitogenesis in the basal chordate, amphioxus

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Non-technical. Understanding how the thousands of genes that make up an organism’s genome (the genotype) interact to create the tissues and organs that together make up that organism (the phenotype) is one of the five ‘grand challenges of 21st century biology’ identified by the National Research Council. Dissecting this genotype-phenotype relationship is of great practical importance and is the focus of enormous research efforts. Importantly, it is key to understanding how organisms evolve and has wide-ranging implications for the health sciences, agriculture, biotechnology, and ecology. To address the genotype-phenotype relationship, this research uses amphioxus, a simple fish-like animal whose ancestors separated from the evolutionary line leading to vertebrates half a billion years ago. The specific question asked is how the amphioxus genome produces the segmented body muscles (similar to the repeating muscles along each side of a vertebrate such as a fish that allow side to side movement). Technical. The research will elucidate the gene network mediating muscle segmentation, and show at the single-cell level how this network controls the partitioning of cells into discrete segments (somites). Comparisons with vertebrates will show how this genetic mechanism has changed as vertebrates became larger and more complex. In amphioxus, muscle segments extend the full length of the body, but in vertebrates, there are none in the head. It is controversial whether vertebrates lost head segments or they are an amphioxus novelty. Much of the gene network for muscle segmentation appears to be conserved between amphioxus and vertebrates—in both, a “clock” specifies when segments will form, and Notch/Delta signaling mediates segmentation. However, in vertebrates, gradients of secreted proteins, chiefly Fgfs, specify where segment boundaries will form, but in amphioxus only the head segments, not those in the trunk or tail, require Fgfs. To test the theory that as vertebrates grew larger and lost head segments, the Fgf gradient was co-opted from the head to the posterior segments, key signaling pathways (Wnt/β-catenin; Fgf, Notch) in developing amphioxus will be perturbed with chemical inhibitors and activators at specific developmental times. Gene knockdown and overexpression will show gene hierarchy. Effects on morphology of segmentation will be reconstructed in 3-D by serial blockface scanning electron microscopy (SBSEM), which when done at intervals can show the steps in morphogenesis at the single cell level. Researchers will involve undergraduates and high school students directly in the project and also partner with the Birch Aquarium at Scripps Institution of Oceanography and the San Diego County Office of Education to lead workshops for high school teachers. These workshops will use locally-available marine invertebrates to show teachers how to instruct students in experimental design and data analysis—skills that are fundamental to training the next generation of scientists. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.