A combination of molecular markers identifies evolutionarily significant units in Cedrela odorata L. (Meliaceae) in Costa Rica

The necessity for conservation of the genetic component of biodiversity is now widely recognised. A broad genetic base is required to maintain evolutionary potential and the population erosion occurring in much of the world's forests threatens the genetic integrity of many tree species. Spanish Cedar (Cedrela odorata L.) has been under severe pressure for generations and is now the focus of a study aimed at assessing the levels and distribution of genetic diversity in remaining populations. Ten Costa Rican populations were analysed using chloroplast and AFLP markers. The overall level of diversity was as expected for an outcrossing, long-lived, woody species (HT = 0.27). However, this concealed a deep divergence within the species, for chloroplast and AFLP (phiCT = 0.83) markers. Populations were differentiated in two groups that exhibited contrasting habitat preferences and two ecotypes, wet and dry, were identified. Within the ecotypes, all but one population were fixed for a single chloroplast haplotype and within populations, total genomic diversity levels were low (HS = 0.03-0.13). Populations possessing the dry ecotype maintained significantly more diversity than those from wet regions. Within the wet ecotype group, pairwise genetic distance between populations fitted an isolation by distance model. The group was strongly subdivided and showed isolation by distance around the southern edge of the central mountain ranges. The genetic divergence of the two ecotypes, observed at both organellar and nuclear loci, identifies evolutionarily significant units that, taken together with previous studies of the species, provide a rational basis on which to build a conservation policy for the species.