IN SEARCH OF MOLECULAR-ORIGINS OF CELLULAR-DIFFERENTIATION IN VOLVOX AND ITS RELATIVES

This chapter presents a summary of the morphological and biochemical evidence that supports the volvocine-lineage hypothesis. It describes attempts to evaluate the validity of this hypothesis objectively with molecular phylogenetic methods. The chapter also reviews advances that have occurred in three areas of Volvox research:(1) the genetic and molecular control of germ or soma differentiation, (2) the mechanism of action of the sexual pheromone, and (3) the comparative anatomy and evolution of selected structural genes of Volvox and Chlamydomonas. The volvocine algae (Chlamydomonas and the members of the family Volvocaceae) all have a fundamentally similar and distinctive pattern of subcellular organization. However, the volvocine algae can be conceptually lined up in a series (Chlamydomonas, Gonium, Pandorina, Eudorina, Pleodorina, and Volvox), in which there is a progressive increase in each of the parameters—cell number, organismic size, tendency for some cells to differentiate terminally as somatic cells, and ratio of somatic-cell number to reproductive-cell number. The volvocine lineage hypothesis postulates that this conceptual series may actually reflect the evolutionary history of the group. The aspect of Volvox carteri development that is most interesting from an evolutionary genetic standpoint is the aspect that most clearly differentiates it from other genera of volvocine algae, and from most other species of Volvox, i.e., the early and complete segregation of separate germ and somatic-cell lineages. © 1992, Elsevier Science Publishers, B.V.