REPRODUCTIVE CHARACTER DISPLACEMENT IN THE OSTRACOD GENUS CYTHERE

Abstract

ABSTRACT The male copulatory organs of 10 Recent species of Cythere were compared and the relationship between the copulatory organ and geographic distribution was examined. For 9 pairs of species whose geographic distributions overlap each other, the copulatory organs are morphologically different. Conversely, the pairs of species whose copulatory organs are similar show either of the following characteristics: (1) their distribution areas are completely isolated, or (2) their distribution areas are very close or more or less overlap each other, but their reproductive seasons or their habitats are different. The significant morphologic differences between the copulatory organs of such sympatric pairs are thus interpreted as responsible for mechanical reproductive isolation. Examination of copulatory organs is very useful in ostracod systematics, especially for discrimination of sympatric species. However, the results do not always coincide with their phylogenetic relationship which is well reconstructed from the ontogenetic development of the distribution pattern of normal pores. In the case of Cythere, the diversification of copulatory organs does not seem to reflect taxonomic distance between species because of their function. Finally, geographic variation of the male copulatory organ of Cythere omotenipponicais shown.