Gametic behavior in a marine green alga, Monostroma angicava: an effect of phototaxis on mating efficiency |
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Authors: | T Togashi Taizo Motomura Terunobu Ichimura Paul Alan Cox |
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Institution: | (1) Institute of Algological Research, Hokkaido University, Muroran 051-0003, Japan, e-mail: togashi@bio.sci.hokudai.ac.jp, Tel.: +81-143-22 2846; Fax: +81-143-22 4135, JP;(2) National Tropical Botanical Garden, Lawai, Kauai, Hawaii 96765, USA, US |
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Abstract: | The role of phototactic behavior of gametes was tested experimentally in the 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 source. In contrast, they did not respond to a red light source. Their swimming velocity
did not differ between these two illuminating light sources. It was, therefore, suggested that the search ability of the gamete
itself might not vary between phototactic and non-phototactic conditions. The number of zygotes formed during the mating process
may be expressed as the product of the number of encounters between male and female gametes and the fraction of encounters
that result in sexual fusion. In this study, with high densities of male and female gametes mixed in test tubes, almost all
minor (fewer in number) gametes fused sexually within 10 min. After dilution of the gamete suspensions by half, mating efficiency
in test tubes illuminated by white light from above was higher than that in dark controls. This suggests that male and female
gametes gathered at the water surface through their positive phototaxis, thus increasing the rate of encounters. Mating efficiency
also decreased if the test tubes were illuminated from above by white light and also shaken. Since negative phototaxis is
clearly shown in planozygotes, we suggest that positive phototaxis of male and female gametes in M. angicava is an adaptive trait for increasing the rate of gametic encounters rather than for the dispersal of zygotes as previously
reported for zoospores of some marine algae.
Received: 12 February 1999 / Revision accepted: 24 May 1999 |
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Keywords: | Evolution of anisogamy Mating behavior Mating efficiency Monostroma angicava Phototaxis |
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