Molecular evidence of hybridization in two native invasive species: Tithonia tubaeformis and T. rotundifolia (Asteraceae) in Mexico
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The evolutionary genetics of invasive species has been relatively unexplored. Hybridization of invasive populations can generate novel genotypes, stimulating the colonization of new environments. A sunflower complex occurring in Mexico formed by two native invasive species, Tithonia tubaeformis and T. rotundifolia was analyzed with molecular markers (RAPDs) in five hybrid zones and two pure sites for each parental species. We tested if morphological atypical individuals between T. tubaeformis and T. rotundifolia that occur in sympatry are the result of hybridization between these two species, in which case genetic diversity in mixed stands would be higher in comparison with pure parental stands. Total DNA of 230 individuals was analyzed with 17 diagnostic markers and six species-specific markers. Molecular data support the hypothesis of hybridization and a bidirectional pattern of gene flow in this complex. Cluster analysis suggests that individuals from the same parental species were more similar among themselves than to putative hybrids, indicating occasional hybridization with segregation in hybrid types or backcrossing to parents. Hybrid populations had the highest levels of genetic diversity in comparison with nonmixed/allopatric populations of their putative parentals. We suggest that hybridization between invasive species may result in the creation of genotypes with an increased capacity for colonization of new habitats. Moreover, invasive species with incipient reproductive barriers may overlap with species of narrow distribution range and increase their possible hybridization rates.