Production of anisogametes and gamete motility dimorphism in Monostroma angicava

Abstract

The reproductive strategy of a marine alga with a heteromorphic biphasic life cycle was studied by analyzing various sexual reproductive characters in light of the evolution of anisogamy. Gametophytes of Monostroma angicava were dioecious and their gametes were slightly anisogamous. Volume of gametangium, density of gametangia and area of mature gametangial parts on each gametophyte did not differ from male to female. Therefore, the reproductive biomass investment for gamete production was considered to be the same for each sex. Anisogamy in this alga appeared to be derived from the difference in the number of cell divisions during gametogenesis, because the majority of male gametangia each produced 64 (2 6 ) gametes and the female produced 32 (2 5 ) gametes. This corresponded with measurements of cell size in male and female gametes. Further, the sex ratio was 1:1 for sexually mature plants sampled at Charatsunai. Therefore, it was suggested that in the field twice as many male gametes are released as female gametes. Liberated gametes of both sexes showed positive phototaxis. The swimming velocity of freshly liberated male gametes was a little higher than that of female gametes. Male gametes had the potential to swim for ca. 72 h and female gametes for ca. 84 h. The difference in gamete motility between the two sexes seemed to be related to cell size. Planozygotes were negatively phototactic and swam more rapidly than gametes of either sex.& k w d : Key words Evolution of anisogamy · Gamete motility dimorphism · Monostroma angicava · Reproductive investment& b d y :T. Togashi ( u ) · T. Motomura · T. Ichimura

Evolutionary trajectories explain the diversified evolution of isogamy and anisogamy in marine green algae

“…The evolutionary trajectory might not necessarily go to the direction with the highest fitness. In accordance with observations from nature, phenotypic variance in size of gametes produced within each mating type is constrained (20,21), and loci that determine gamete size are linked to loci that determine mating types (22,23). To keep the model as simple as possible, we use a haploid model that excludes the possibility of simultaneous (i.e., double) mutations in both sexes.…”

Selection for High Gamete Encounter Rates Explains the Success of Male and Female Mating Types

“…Equal total volumes of cytoplasm for each gamete type is reasonable considering arguments that there should be equal investment by both sexes (Smith, 1978, p. 157). Such variations in size and numbers occur among closely related green algae (Cox & Sethian, 1985;Togashi et al, 1997).…”

“…Data on gamete size and speed were found for two species of isogamous green algae (Haxo & Clendenning, 1953;Racey et al, 1981;RuK !er & Nultsch, 1985), one anisogamous green alga (Togashi et al, 1997), one anisogamous aquatic fungus (Pommerville, 1978), sperm of a brown seaweed (Maier & MuK ller, 1986), and sperm of four sea urchin species (Chia et al, 1975;Gray, 1955;Levitan, 1993;Levitan et al, 1991). The rank correlation between gamete speed and volume was not signi"cant by either the Spearman or Kendall methods (StatView).…”

“…Alternatively, a collimating stimulus can be used to maintain a constant orientation (Dusenbery, 1992, p. 397;Pline & Dusenbery, 1987). The gametes of many marine algae are known to be phototactic (Haxo & Clendenning, 1953;Togashi et al, 1997), and they may be using light as a collimating stimulus, which allows them to maintain a straight path over very large distances. So failure to swim straight paths is not a problem for the model, because gametes can avoid the e!ect if necessary.…”

“…Although this may seem improbable, we have little knowledge of what the probabilities actually are. For example, large changes in size can occur by simply changing the number of cell divisions in gametogenesis, as occurs in some algae (Callow et al, 1985;Togashi et al, 1997). Variations in other selective pressures, such as postzygotic selection or predation on gametes, might temporarily change the "tness landscape in ways that eliminate the valley.…”