Evol Ecol Res 8: 617-628 (2006)     Full PDF if your library subscribes.

Gamete behaviours and the evolution of ‘marked anisogamy’: reproductive strategies and sexual dimorphism in Bryopsidales marine green algae

Tatsuya Togashi,1,4* Masaru Nagisa,2 Tatsuo Miyazaki,1 Jin Yoshimura,1,3 John L. Bartelt4 and Paul Alan Cox4

1Marine Biosystems Research Center, Chiba University, Chiba, Japan,  2Department of Mathematics and Informatics, Faculty of Science, Chiba University, Chiba, Japan,  3Department of Systems Engineering, Shizuoka University, Hamamatsu, Japan and  4Institute for Ethnobotany, National Tropical Botanical Garden, Kalaheo, HI, USA

Address all correspondence to T. Togashi, Marine Biosystems Research Center, Chiba University, Amatsu-Kominato 299-5502, Japan.
e-mail: togashi@faculty.chiba-u.jp


Questions: In some species of markedly anisogamous marine green algae, why do male gametes alone have no phototactic device? Why do some of their partners (female gametes) have pheromonal attraction systems?

Simulation methods: Three-dimensional swimming and fertilization model with parameters based on experimental data of gametic traits (gamete sizes, swimming speeds and trajectories) and pseudo-parallel algorithms compiled for rapid calculations with large numbers of gametes.

Key assumptions: The biomass allocated to produce gametes is similar between mating types. Each spherical gamete swims three-dimensionally at a speed determined by its diameter. Swimming speed is inversely proportional to gamete size according to Stoke’s law.

Results: Under conditions in which disturbance under water may be a consideration, ‘markedly anisogamous species’ have a greater chance of successful fertilization than species with phototactic gametes in both sexes. This is more conspicuous in species with larger female gametes or pheromonal attraction systems. However, in deeper water, species with non-phototactic gametes in both sexes generally have a numerical advantage in mating over species with phototactic gametes. Field observations and existing data on gamete size tend to support these predictions.

Keywords: anisogamy, gamete behaviour, marine green algae, pheromonal attraction, phototaxis.

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