Dissertation Proposal - The University of Arizona Campus Repository

Table 1. Developmental programs of Volvocales as described by Desnitski (1995).
(e.g., V. tertius), all the divisions are symmetrical as in D1 but the rate of cell division is
slower. D4 (e.g., V. aureus) is considered the most derived developmental program since
palintomy is lost: reproductive cells start as small, flagellated cells, and during embryonic
development cells grow in between cell divisions (binary fission). Although D4
reproductive cells start as flagellated cells, their flagella have essentially no motility
function since they are reabsorbed before the first cell division. Unlike the other Volvox
species, D4 species from the “Euvolvox Section” (e.g., V. rousseletii) retain robust
cytoplasmatic bridges that may be involved in the active transfer of nutrients in the adult
(Kirk 1998).
D1 D2 D3 D4
Size of mature germ cells large large large small
Growth between divisions no no no yes
Rate of divisions fast fast slow slow
Asymmetric division no yes no no
In Volvocales, because of their coherent glycoprotein rigid cell wall, the position of
flagella is fixed. Thus, the basal bodies cannot move laterally and take the position
expected for centrioles during cell division while still remaining attached to the flagella
(as they do in naked green flagellates). Consequently, in undifferentiated colonies with no
somatic cells, motility is inhibited during cell division. This inability to both divide and
maintain flagellar activity is referred to as the “flagellation constraint” (Koufopanou
1994). As the number of cells in the colonies increases, the time spent in the division
phase increases. Therefore, since Volvocales are negatively buoyant, the motility
function so basic to survival is increasingly compromised. Because a flagellum may beat
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