EMBRYOS AND ANCESTORS: RECONSTRUCTING GENE REGULATORY NETWORKS AND EMBRYONIC DEVELOPMENT IN ANCESTRAL ECHINOIDS (Invited Presentation)

A critical process in early embryonic development is partitioning fields of cells in the embryo into distinct regions of gene expression that will later give rise to the specialized cells and tissues of the adult/larva. Orchestrating this process are suites of regulatory genes that control the expression of other regulatory genes. These assemblages of regulatory genes are recursively wired in the genome as genetic circuits, the architecture of which can be represented as a gene regulatory network (GRN). As development unfolds, each cell begins to express a cell-type specific set of regulatory genes, or regulatory state, that is the outcome of GRN process. These molecular mechanisms form a powerful conceptual toolkit and are opening new avenues of research for studying the evolution of early embryonic development. Here, we detail how these concepts are applied in echinoderms and how they afford reconstruction of ancestral GRNs and regulatory states that likely existed in the ancestral embryos of modern echinoids. The backbone of this effort is the extensively-detailed GRN driving early development of the camarodont sea urchin Strongylocentrotus purpuratus. Comparative analyses of this circuitry in modern representatives of distantly- and closely-related echinoid clades afford triangulation of molecular and developmental evolutionary events. Integrating comparative data of embryonic development with paleontological data and molecular-dating informs assumptions regarding the genomic and morphological transformations that must have occurred in the ancestral lineages leading to modern echinoid clades. Extending these methods to all echinoderm classes and continuing integration of new fossil data will allow us to elaborate in striking molecular detail the evolutionary events that led to the modern diversity of developmental systems in echinoderms.