nipulation and analysis in the former can also be utilized to analyze traits in the latter. Also, it clearly rep-
resents the best model for the examination of the mechanisms involved in that most fundamental aspect
of plant biology, alternation of generations.
II. VOLVOX
A. Volvocales—a Variety of Developmental Potentials
Although probably only distantly related to other plants [1], the order Volvocales contains organisms that
display an interesting variety of developmental potentials. These range from single-celled, free-living or-
ganisms (such as those in the genus Chlamydomonas), through colonial organisms consisting of a single
cell type (genera Gonium,Pandorina,Eudorina) that develops from a somatic into a reproductive cell, to
organisms in which different cells specialize for separate somatic and reproductive functions (genus
Volvox). Thus, within this order are organisms ideal for study of the genetic changes involved in the evo-
lution of multicellularity and of cell specialization. Because the volvox are the most studied in terms of
developmental genetics, this review concentrates on that group and mentions others only for purposes of
comparison. Several reviews [12–14] and the comprehensive book by David Kirk [1] give more detailed
accounts of volvox development than are included here.
B. Asexual Life Cycle and Mutants
The adult of the most studied volvox, V.carteri f.nagariensis, comprises a spheroid of about 2000–4000
terminally differentiated somatic cells that contains, within an extracellular glycoprotein matrix, about 16
developing embryos. Reproduction can be either asexual or sexual. In the asexual cycle (Figure 1), large
reproductive cells (the gonidia), housed within the spheroid, undergo six cycles of mitosis to produce
small embryonic spheres of 32 morphologically indistinguishable cells. At the next division the cells in
the anterior half of each embryo divide asymmetrically to produce 16 larger cells that will give rise to the
next generation of gonidia and 16 smaller cells that will, along with the remainder of the organism, form
the somatic cells. The former divide asymmetrically twice more, releasing more small somatic cell pre-
cursors, and the latter undergo several more rounds of cell division until about 2000 cells are formed, in-
cluding the 16 larger gonidia. Initially, the gonidia are located on the outside of the sphere and the cilia
of the somatic cells point inward, but in the process known as inversion the adult form of internal goni-
dia and external, ciliated somatic cells is produced. After a period of expansion and cytodifferentiation,
the somatic cells of the parental spheroid self-destruct, and the now juvenile Volvoxare dispersed and live
independently.
The haploid nature of all metabolically active cells in Volvoxis a great aid to mutant isolation, and
many have been described that affect the asexual cycle just outlined [15–17]. Their properties suggest
several intriguing possibilities for how the initial split between somatic and germ cell lines is controlled.
Mutants at the pcd(“premature cessation of division,” Ref. 15) and the several mul(“multiple gonidia,”
Ref. 16) loci both display altered cleavage patterns and result in a greater than normal number of gonidia
being formed. This led to the hypothesis that gonidial determination is a direct consequence of the larger
cell size resulting from the asymmetrical cell division [15]. The competing idea [18] is that a cytoplasmic
determinant that is partitioned into the larger cells is responsible for initiating reproductive cell develop-
ment—a phenomenon similar to the pole plasm found to determine germ cell development in Drosophila
[19]. Several lines of evidence favor the former hypothesis [17,20]. For example, if heat shock or micro-
surgery is used to alter the cleavage of early gonidial cells that normally differentiate only into somatic
cells, the larger than normal offspring, which would normally become somatic, differentiate as extra go-
nidia. Detailed studies of normal cleavage patterns and how they give rise to the larger cells that become
gonidia are reviewed in Ref. 1.
In addition to the pcdandmulloci, several other mutant phenotypes have been isolated that alter the
normal developmental sequence. In the lag(“late gonidia”) mutants, which comprise four genetic loci,
the asymmetric cleavages that lead to gonidia occur as usual, but the resultant large cells at first differen-
tiate somatically, producing cilia. Only later do they redifferentiate and follow the germ cell pathway. A
similar phenotype is seen in mutants in the single regAlocus. In this type, gonidial cell development is
normal, but the somatic cells first differentiate normally, then redifferentiate to form small gonidia.
804 WALLACE