Evol Ecol Res 10: 281-293 (2008)     Full PDF if your library subscribes.

The genetic consequences of evolving two sexes: the genetic structure of distylous and dioecious species of Erythroxylum

César A. Abarca,1 Angélica Martínez-Bauer,1 Francisco Molina-Freaner2 and César A. Domínguez1*

1Departamento de Ecología Evolutiva, Instituto de Ecología, Universidad Nacional Autónoma de México; AP 70-275, México DF 04510, México and  2Departamento de Ecología de la Biodiversidad, Instituto de Ecología, Universidad Nacional Autónoma de México; AP 1354, Hermosillo, Sonora 83250, México

Author to whom all correspondence should be addressed.
e-mail: tejada@servidor.unam.mx


Question: Is the evolution of dioecy associated with increasing levels of genetic variation?

Background and hypothesis: Dioecy and self-incompatibility are seen as alternative mechanisms promoting outcrossing and avoiding the deleterious effects of inbreeding depression. Under this formulation, we would expect similar genetic variation between dioecious and self-incompatible species. In contrast, differences in genetic variation between these two breeding systems would indicate that sexual specialization has genetic effects that go beyond the avoidance of inbreeding.

Organisms and methods: We studied the genetic structure of one distylous and one dioecious species of Erythroxylum by means of ISSR markers. We measured the level and distribution of genetic diversity in five populations of the dioecious E. rotundifolium and in six populations of the self-incompatible distylous E. havanense.

Results: Expected heterozygosity and Shannon genetic diversity in populations of the distylous E. havanense were significantly greater than in populations of dioecious E. rotundifolium. Differences between species were small but differences among populations accounted for a large fraction of the genetic variation. Levels of differentiation were slightly greater among populations of the dioecious species. Overall, our results indicate that the evolution of dioecy in Erythroxylum is associated with a significant reduction in genetic diversity and increasing population differentiation.

Keywords: Baker’s hypothesis, dioecy, distyly, Erythroxylum, genetic differentiation, genetic diversity, sexual dimorphism.

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