GENOME EVOLUTION IN NEOPROTEROZOIC BILATERIANS

Judging from trace fossil evidence, the earliest bilaterian radiations occurred among small-bodied, vermiform, benthic organisms similar to living acoelomorphs in morphological grade (i.e., paracoelomates). The pattern of occurrence of regulatory genes in living phyla indicates that those early bilaterians already possessed a nearly or quite complete array of representatives of the important regulatory gene families, some organized into clusters. The Hox gene cluster, which mediates the identity of anteroposterior modules, is the best known of those. In living bilaterian phyla in which it is has been studied, the Hox cluster tends to be stable within phyla but is composed of unique combinations of genes in each phylum (with a single exception, Arthropoda and Oncychophora). Thus, Hox cluster evolution has changed since before the rise of crown groups. In Nematoda, the living phylum of acoelomorph grade in which Hox gene function is known, Hox genes mediate anteroposterior positioning of individual cell types. Therefore, Hox clusters presumably responded directly to the morphological evolution among radiating paracoelomates as they became adapted to the challenges associated with the disparate habitat conditions within the mosaic of benthic environments, suffering losses or acquiring additions as appropriate. In genomes within more complex body plans, Hox genes came to control the expression of downstream cascades of the genes that underlay the anteroposterior differentiation of complex structures. Responses to adaptive challenges tended to be met by changing patterns of downstream gene expression or in some cases by changes in Hox gene expression domains, while Hox clusters themselves were chiefly conserved. Thus, bilaterian Hox cluster evolution was chiefly a Neoproterozoic phenomenon.