Comparing the Human and Sea Urchin Genomes

Abstract

Abstract To compare the genomes of humans and sea urchins seems, at first glance, awkward. The human body is bilaterally symmetrical with a complex head containing many sensory structures, whereas the sea urchin has radial symmetry and a globose form with no suggestion of a head. However, sea urchins belong to a group of animals (the echinoderms) that lies on the same major branch of the tree of life (the deuterostomes) to which humans belong. As echinoderms diverged very early from the major lineage of the deuterostomes, their genomes reflect the basic qualities of this lineage and inform to a deep reach of time the evolutionary changes leading to the human genome. Even though the sea urchin form differs radically from that of vertebrates, they share many of the same gene families. Indeed, the invention of new genes in this evolutionary path is subordinate to the diverse changes in the abundance of genes in existing families. Key Concepts: The relative proximity of sea urchins to humans on the deuterostome branch of the evolutionary tree renders them a good comparison to humans for gene and genome evolution. The sea urchin genome is one‐quarter the size of the human but has about the same number of genes. Gene comparisons between these distantly related species show that differences in body plans result from spatial and temporal differences in gene activity rather than the possession of different genes. The sea urchin HOX gene cluster is modified from the linear arrangement found in humans. Without an adaptive immune system, the sea urchin appears to use a huge expansion in innate immune genes.