ABR: Evolution of Left-Right Asymmetry in Chordates using Cephalochordates as a Proxy for the Ancestral Chordate

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

Investigators

Abstract

Linda Z. Holland, P.I. PROPOSAL NUMBER: 1353688 PROPOSAL TITLE: ABR: Evolution of Left-Right Asymmetry in Chordates using Cephalochordates as a Proxy for the Ancestral Chordate Nontechnical description/broader impacts Cephalochordates (amphioxus and lancelets) are closely related to vertebrates but simpler both structurally and at the gene level as their genomes have not undergone the two rounds of whole-genome duplication that occurred in ancestral vertebrates. While these duplications are useful for humans as a defect in one gene can often be compensated for by a duplicate, the lack of gene duplication in amphioxus makes it ideal for determining how individual genes work together to create an embryo and adult. Because amphioxus is so vertebrate-like, the genetic mechanisms that pattern, for example, the kidney or brain are fundamentally the same for both groups. The difference is that vertebrates have created additional complexity by elaborating upon these basic mechanisms. A major outstanding question is how left/right (LR) asymmetry evolved. Vertebrates are superficially symmetric, but internal organs are asymmetrically arranged across the left/right (LR) axis. This study aims to elucidate the genetic mechanisms of LR asymmetry using two distant amphioxus genera (Branchiostoma and Asymmetron). Importantly, hybrids between the two genera yield larvae intermediate in asymmetry. This study has far reaching implications for understanding gene networks and how they evolve. A collaboration with bioinformaticians will help identify genes that are active at particular developmental stages. It will give undergraduates research experience. During 2013-2014, 13 undergraduates including 6 under-represented minorities participated in research in the Holland laboratory. One postdoctoral fellow will be trained. Results will be presented at meetings and published in scientific journals. Outreach with the Birch Aquarium at Scripps and the San Diego Unified Schools' Enhancing Science Education through Technology program will give middle school students virtual tours of the Holland laboratory with direct interactions via SKYPE. The work will foster cross-disciplinary education via exchanges of students/postdocs between the PI and collaborators, allowing bioinformatics students to learn biology and biology students to learn bioinformatics. Technical description The basal chordate amphioxus has long been thought to hold clues to vertebrate origins. This proposal addresses a major unresolved question: What is the genetic basis of LR asymmetry in chordates and how has it changed during evolution? The specific aim is to elucidate the evolution of LR asymmetry in chordates using two distant amphioxus genera (Branchiostoma and Asymmetron) as proxies for ancestral chordates. Except for left-sided expression of the conserved triad of Nodal, Lefty and Pitx in embryos of both vertebrates and amphioxus, the fundamental genetic basis of LR patterning in chordates is elusive. It is proposed that specification of LR asymmetry begins with skewed maternal Nodal and is refined by BMP suppressing Nodal on the right. The simplicity of amphioxus embryos allows dissection of gene networks starting with the egg and ending with a vertebrate-like embryo. Branchiostoma and Asymmetron offer the "Goldilocks Principle"- alike enough to be sure that two structures are homologous and unlike enough to elucidate genetic differences in LR asymmetry. Hybrids between the two genera have intermediate asymmetry. The Holland lab pioneered research on B. floridae, was the first to study Asymmetron development and establish year-round breeding of both species in the laboratory. sequencing the A. lucayanum genome and is currently sequencing the A. lucayanum genome. The two genera are similar, but LR asymmetries differ in several respects. Importantly, Branchiostoma has gonads on both sides, but Asymmetron has them only on the right. The approach combines in situ hybridization, antibody labeling, manipulating gene function, RNA-Seq and bioinformatics. Comparisons of the molecular basis of LR patterning and germ cell migration in both species will elucidate the fundamental basis of LR asymmetry in chordates and test the overall hypothesis that vertebrates evolved from an amphioxus-like ancestor. The results promise to be applicable not just to cephalochordates but also to vertebrates and may transform ideas on evolution of the myriad variations vertebrates added to the basic theme of a small non-yolky embryo gastrulating by invagination.