Organization and evolution of animal colonies

Abstract

Based on the theory of organization and evolution of colonies in an extinct group of hemichordates graptolites (Urbanek, 1960, 1973) the relationship between the events in the late astogeny of bryozoan colonies and their somatic and reproductive cycle. The bryozoan colonies with simple organization are compared with graptoloid colonies and their structure is interpreted within the framework of the morphogen gradient theory. A morphogen produced by the founder-zooid (oozooid) diffuses along the long axis of the colonies and regulates the phenotypic expression of the size and structure of zooids. Evolutionary changes in the graptoloid colonies involve introduction of new characters and their spreading is also accompanied by gradient changes of their manifestation. Evolutionary mechanisms in bryozoan colonies are considered in terms of penetrance and expressivity of genes. In contrast to graptolites, many bryozoan colonies display multiple zones of astogeny changes and repetitions.