Introgressive hybridization betweenBrahea dulcisandBrahea nitida(Arecaceae) in Mexico: evidence from morphological and PCR–RAPD patterns

Abstract

Gene introgression usually results from spontaneous hybridization among closely related species in sympatric populations. This phenomenon has recently become a frequent topic of interest in evolutionary and conservation biology. A palm species complex occurring in Mexico formed by two native species, Brahea dulcis (Kunth) C. Martius and Brahea nitida André, and their putative hybrids, was analyzed with morphological and molecular markers in four hybrid zones and two pure sites for each parental species. Our results showed that the closeness of the hybrids to an allopatric site of either parental species (B. nitida, mesic habitat, and B. dulcis, xeric habitat) is directly related to their morphological and genetic similarity. Also, B. dulcis differed significantly from B. nitida in all the examined characters. Morphological and molecular data were highly coincident and supported the hypothesized hybridization. Hybrid phenotypes were morphologically parent-like for 46.4% of traits, intermediate for 33.2% of traits, and transgressive for 10.0% of traits. Thus, individual hybrids appear to be a mosaic of parent-like, intermediate, and transgressive phenotypes. Our results suggest that B. nitida has been genetically assimilated by B. dulcis in Xochiapala, and in the remaining hybrid zones a unidirectional pattern of gene flow towards B. nitida exists. Therefore, hybridization may threaten the conservation status of species through genetic assimilation and may confound the ability to distinguish among taxa.