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Evol Ecol Res 11: 713-729 (2009)     Full PDF if your library subscribes.

Evolution of dwarf males and a variety of sexual modes in barnacles: an ESS approach

Satoru Urano1, Sachi Yamaguchi2, Shigeyuki Yamato3, Satoshi Takahashi2 and Yoichi Yusa2

1Center for Ecological Research, Kyoto University, Otsu, Shiga,  2Faculty of Science, Nara Women’s University, Nara and  3Seto Marine Biological Laboratory, Kyoto University, Shirahama, Wakayama, Japan

Correspondence: S. Urano, Center for Ecological Research, Kyoto University, Otsu, Shiga 520-2113, Japan.


Questions: Why do barnacles have many modes of sexuality, including hermaphroditism, androdioecy (large hermaphrodites with dwarf males), and dioecy (large females with dwarf males)? Can mating group size, relative body size, competitive advantage or survival rate of dwarf male individuals explain which type of sexuality is favoured by natural selection?

Mathematical methods: We developed an ESS model to investigate factors affecting the optimal proportion of larvae that become dwarf males (q*). Allocation to male function of large hermaphrodites is calculated according to Charnov’s sex allocation theory, although sperm competition with dwarf males is taken into account. Our model is based on a life history of androdioecious barnacles, which includes hermaphroditism (q* = 0) and dioecy (q* > 0 and the male allocation of large hermaphrodites = 0) as special cases. We incorporate average mating group size (m) into the model, together with body size, competitive advantage, and survival rate of dwarf males relative to large hermaphrodites.

Results: The proportion of dwarf males, q*, increases from 0 (hermaphroditism) as mating group size decreases, and approaches 0.5 when group size, m, nears 0. At the latter extreme, large individuals should become females instead of hermaphrodites. Thus mating group size can explain the major trend of sexuality in barnacles: hermaphroditism in relatively large mating groups, androdioecy in smaller groups, and dioecy in even smaller groups. Relative body size, competitive advantage, and survival rate of dwarf males all have positive effects on the evolutionarily stable proportion of males. If there is a simple trade-off between body size and survival rate, survival rate will have the greater influence on sexuality.

Keywords: androdioecy, hermaphrodite, mating system, sex ratio, sex allocation, sperm competition.

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