Local hibridization in subtropical mountain habitats: Can Cedrela (Meliaceae) maintain species’ identity in sympatry?

Abstract

Congener species with incomplete reproductive barriers that coexist along environmental gradients may be prone to ecological divergence, in spite of the potential for hybridization in sympatry. We analyzed distribution patterns of isozymes, plastid, and nuclear DNA sequences at regional and local scales in three timber Cedrela species of the subtropics in the northern Argentina (Cedrela angustifolia, C. balansae, and C. saltensis), to test whether populations of distinct species have diverged in montane habitats in relation to their ecological characteristics. Cedrela balansae and C. angustifolia can be identified by diagnostic isozyme alleles. Nuclear ITS sequences yielded intraindividual polymorphism; ambiguous bases were shared between C. balansae and C. saltensis while those of C. angustifolia were part of its intraspecific polymorphism. Chloroplast DNA consisted mainly of a low-elevation haplotype present in C. balansae and C. saltensis and other found in C. angustifolia which in turn was shared locally by all species in sympatry. Multivariate UPGMA analysis of isozymes and Bayesian phylogeny of haplotype ITS yielded concordant patterns. Populations of C. angustifolia clustered in one group and were separated from the rest whereas those of C. saltensis and C. balansae were grouped together in one cluster. This indicates that stronger reproductive barriers exist between C. angustifolia and the low-elevation taxa C. saltensis and C. balansae which seem to maintain continuous gene flow. Nonetheless, under particular environmental settings, i.e., the three species hybridized sometime in the past and later became differentiated through ecological divergence.