Sexual Dimorphism in Catasetum Orchids: Forcible Pollen Emplacement and Male Flower Competition
GUSTAVO A. ROMERO 1 and
CRAIG E. NELSON 2
1 Department of Biology, Indiana University, Bloomington, IN 47405 and Sub-estación Experimental Amazonas, Fondo Nacional de Investigaciones Agropecuarias, Puerto Ayacucho, Territorio Federal Amazonas, Venezuela 7071.
2 Department of Biology, Indiana University, Bloomington, IN 47405.
Orchids of the Neotropical genus Catasetum have sexually dimorphic flowers; that is, male and female flowers are distinctly different in shape and color. Male flowers forcibly attach a large pollinarium onto euglossine bees. Euglossa bees leave the male flower in response to pollinarium emplacement by Catasetum ochraceum and subsequently avoid male but not female flowers. This evidence suggests that sexual dimorphism promotes pollination. The aversion of the bee to pollinarium attachment and its avoidance of male flowers thereafter apparently reflect competition among male flowers that probably evolved concurrently with sexual dimorphism. Among Catasetum species, the extent of dimorphism is correlated with the degree of apparent aversion caused by the male flower (weight of the pollinarium). Thus, aversive pollinarium emplacement can account for widespread sexual dimorphism in Catasetum and for interspecific variations in its expression.
Submitted on December 24, 1985
Accepted on March 25, 1986
