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

The necessity for conservation of the geneticcomponent of biodiversity is now widelyrecognised. A broad genetic base is required tomaintain evolutionary potential and thepopulation erosion occurring in much of theworld's forests threatens the genetic integrityof many tree species. Spanish Cedar (Cedrela odorata L.) has been under severepressure for generations and is now the focusof a study aimed at assessing the levels anddistribution of genetic diversity in remainingpopulations. Ten Costa Rican populations wereanalysed using chloroplast and AFLP markers.The overall level of diversity was as expectedfor an outcrossing, long-lived, woody species(HT = 0.27). However, this concealeda deep divergence within the species, forchloroplast and AFLP (ΦCT = 0.83)markers. Populations were differentiated in twogroups that exhibited contrasting habitatpreferences and two ecotypes, wet and dry, wereidentified. Within the ecotypes, all but onepopulation were fixed for a single chloroplasthaplotype and within populations, total genomicdiversity levels were low (HS= 0.03–0.13). Populations possessing the dryecotype maintained significantly more diversitythan those from wet regions. Within the wetecotype group, pairwise genetic distancebetween populations fitted an isolation bydistance model. The group was stronglysubdivided and showed isolation by distancearound the southern edge of the centralmountain ranges. The genetic divergence of thetwo ecotypes, observed at both organellar andnuclear loci, identifies evolutionarilysignificant units that, taken together withprevious studies of the species, provide arational basis on which to build a conservationpolicy for the species.