Alexey Desnitskiy from Saint Petersburg State University has published a new review of sexual development in the genus Volvox in the International Journal of Plant Reproductive Biology.
The article includes an up-to-date review of Professor Desnitskiy’s own work describing four developmental “programs” in the various species of Volvox:
- The first developmental program is characteristic of V. gigas, V. pocockiae Starr, V. powersii (Shaw) Printz, and V. spermatosphaera Powers. These species have large gonidia, the rate of their division is high, and there is no unequal (asymmetric) division into two cell types.
- The second program is characteristic of V. africanus, V. carteri, V. obversus, and V. reticuliferus and differs from the first program in having an asymmetric division, which forms presumptive reproductive and somatic cells of the next generation.
- The third program is characteristic of V. tertius Meyer, in which gonidia are large, but the rate of division is low, and unequal division is absent.
- The fourth program is characteristic of V. aureus, V. globator, V. rousseletii, V. ferrisii Isaka, Matzuzaki et Nozaki and several other species, in which small gonidia divide slowly and without differentiation of cell lineages.
As I mentioned here, the second through fourth developmental programs were probably derived from the first (it’s important to bear in mind here that since Volvox is polyphyletic, the most recent common ancestor of Volvox is also probably the most recent common ancestor of the family Volvocaceae).
The VcMID gene is present only in the genome of male clone and the knockdown of VcMID leads to the formation of Volvox colonies with fertilizable eggs in the next generation (Geng et al. 2014). Interestingly, partial VcMID knockdown generates self-fertile hermaphrodites. By contrast, expression of this gene after its introduction into the female V. carteri f. nagariensis clone results in the formation of colonies with sperm packets.
That’s right; Volvox males can be turned into females just by preventing the expression of a single regulatory gene (remember, production of eggs and sperm define females and males). That’s not the end of the story, though:
…after such transformations of germ cells (eggs or androgonidia) their number and distribution pattern in colonies remain unchanged. A pseudo-female colony is characterized by the 1:1 ratio of evenly scattered eggs and somatic cells. A pseudo-male colony contains approximately 35 androgonidia, which are located in posterior and equatorial regions of the colony. Therefore, it is clear that sexual reproductive cell patterning (i.e., the number and distribution of sexual reproductive cells and ratio of sexual reproductive cells to somatic cells) is separable from the events of differentiation in two kinds of sexual reproductive cells and that the patterning (unlike the differentiation of sexual cells) is not controlled by the VcMid pathway.
Although changes in VcMid expression are sufficient to change which type of germ cell a colony produces, they don’t change other features that differ between the sexes. Patterns of cell division and differentiation remain unchanged, so the sex-specific differences in these characters must be controlled by something other than VcMid.
Desnitskiy AG (2017) Differentiation of reproductive structures and experimental sex change in Volvox (Chlorophyta, Volvocaceae). International Journal of Plant Reproductive Biology, 9, 65–70.
Geng S, De Hoff P, Umen JG (2014) Evolution of sexes from an ancestral mating-type specification pathway. PLoS Biology, 12, e1001904.