Mussaenda Pubescens (Rubiaceae): A Case on the Evolution of Dioecy from Stigma-height Dimorphism

Herkogamy is a common strategy employed by hermaphroditic angiosperms to reduce self interference within flowers. Stigma-height dimorphism is a less well-known reciprocal herkogamy. In species with stigma-height dimorphism, individuals within a population have two floral-morphs and flowers of different morphs differ significantly in style length but present similar anther position. This polymorphism has been reported sporadically among a few angiosperm families. Compared to heterostyly, stigma-height polymorphism receives less attention and is so far poorly understood.

Recently, the Plant Phylogenetics and Reproductive Biology Group of South China Botanical Garden conducted a series of studies on Mussaenda pubescens, a species with morphological stigma-height dimorphism. Experimental pollinations demonstrated that the species is functionally dioecious despite morphological hermaphroditism. The long-styled (L) morph possesses sterile pollen and functions as a female, whereas the short-styled (S) morph is female sterile and functions as a male. Male sterility in the L-morph results from earlier degradation of the tapetum, while female sterility of the S-morph appears to be mainly due to the failure of functional megaspore establishment. Self- and intra-morph pollinations of the S-morph were consistent with those expected from dimorphic incompatibility.

Therefore, in M. pubescens, cryptic dioecy has evolved from stigma-height dimorphism as a result of morph-specific sterility mutations. This is the first case that dioecy evolves from stigma-height dimorphism, which is otherwise always associated with the evolutionary build-up of distyly.

Li, A. M., X. Q. Wu, D. X. Zhang & S. H. Barrett. 2010. Cryptic dioecy inMussaenda pubescens (Rubiaceae): a species with stigma-height dimorphism. Annals of Botany doi:10.1093/aob/mcq146, available online at www.aob.oxfordjournals.org