Gametic behavior in a marine green alga, Monostroma angicava: an effect ofphototaxis on mating efficiency

The role of phototactic behavior of gametes was tested experimentally in th e slightly anisogamous marine green alga Monostroma angicava Kjellman, and the effect of phototaxis on mating efficiency was discovered. Both male and female gametes showed positive phototaxis in response to a white light sou rce. In contrast, they did not respond to a red light source. Their swimmin g velocity did not differ between these two illuminating light sources. it was, therefore, suggested that the search ability of the gamete itself migh t not vary between phototactic and non-phototactic conditions. The number o f zygotes formed during the mating process may be expressed as the product of the number of encounters between male and female gametes and the fractio n of encounters that result in sexual fusion. In this study? with high dens ities of male and female gametes mixed in test tubes, almost all minor (few er in number) gametes fused sexually within 10 min. After dilution of the g amete suspensions by half mating efficiency in test tubes illuminated by wh ite light from above was higher than that in dark controls. This suggests t hat male and female gametes gathered at the water surface through their pos itive phototaxis, thus increasing the rate of encounters. Mating efficiency also decreased if the test tubes were illuminated from above by white ligh t and also shaken. Since negative phototaxis is clearly shown in planozygot es, we suggest that positive phototaxis of male and female gametes in M. an gicava is an adaptive trait for increasing the rate of gametic encounters r ather than for the dispersal of zygotes as previously reported for zoospore s of some marine algae.