Clearly the lagandregAgenes are critically important to regulating the germ-soma dichotomy. Finally,
mutants of a third type, gls(“gonidialess”), undergo no asymmetric cleavages and all cells develop so-
matically; clearly, this mutant cannot reproduce and to be maintained must be carried in a regAback-
ground. Interestingly, regA/glsdouble mutants resemble more primitive colonial volvocaleans, such as
those in the genus Eudorina.
Based on the nature of the mutant phenotypes described here, a model for how these genes control
Volvoxdevelopment has been proposed by Kirk and colleagues (Figure 2). In the model the glsgene is
directly responsible for the asymmetric cleavages; the mulgenes specify the exact times and places for
these divisions to occur. In the larger cells the laggenes become active, and their products keep those cells
from undergoing somatic differentiation. In the smaller cells the regAgene product prevents any devel-
opment as a reproductive cell, and in the absence of laggene products the pathway of somatic develop-
ment is followed. The complementary nature of the lagandregAgenes is particularly apparent here: the
former prevent expression of somatic cell genes, and the latter prevent expression of germ cell genes. The
cloning of some of these genes (see later) is allowing the model to be examined and tested at an entirely
new level of detail.
An especially noteworthy feature of the regAlocus is its hypermutability [21]. It was found that regA
mutants appeared at an exceptionally high frequency after treatment with agents that interfere with DNA
DEVELOPMENTAL GENETICS IN LOWER PLANTS 805
Figure 1 The asexual life cycle of Volvox carterias synchronized by a light-dark cycle. Mature gonidia
(asexual reproductive cells) undergo a rapid series of cleavage divisions, certain of which are asymmetric. The
larger cells resulting from these unequal divisions will become the gonidia of the next generation, while the
small cells will become part of the somatic cell population. At the end of cleavage, all cells that will be present
in the adult are present in undifferentiated form, but the embryo is inside-out with respect to the adult configu-
ration. The adult orientation is achieved through the process called inversion. Following inversion, both the
parental spheroid and the juveniles contained within it expand by deposition of extracellular matrix. Midway
through expansion, the juveniles hatch and swim away, leaving a “hulk” of parental somatic cells that will un-
dergo programmed cell death. The juvenile spheroids continue to expand while their gonidia mature, preparing
for a new round of embryogenesis. Under the synchronizing influence of the light-dark cycle, one asexual life
cycle is completed every 48 hr, cleavage (which takes about 7 hr) begins near the end of a light period, and in-
version (which takes less than an hour) occurs in the dark period. (From Ref. 13.)