Molecular evidence and ecological niche modeling reveal an extensive hybrid zone among three Bursera species (section Bullockia)

Abstract

The genus Bursera, includes ~100 shrub and trees species in tropical dry forests with its center of diversification and endemism in Mexico. Morphologically intermediate individuals have commonly been observed in Mexican Bursera in areas where closely related species coexist. These individuals are assumed to result from interspecific hybridization, but no molecular evidence has supported their hybrid origins. This study aimed to investigate the existence of interspecific hybridization among three Mexican Bursera species (Bullockia section: B. cuneata, B. palmeri and B. bipinnata) from nine populations based on DNA sequences (three nuclear and four chloroplast regions) and ecological niche modeling for three past and two future scenario projections. Results from the only two polymorphic nuclear regions (PEPC, ETS) supported the hybrid origin of morphologically intermediate individuals and revealed that B. cuneata and B. bipinnata are the parental species that are genetically closer to the putative hybrids. Ecological niche modeling accurately predicted the occurrence of putative hybrid populations and showed a potential hybrid zone extending in a larger area (74,000 km2) than previously thought. Paleo-reconstructions showed a potential hybrid zone existing from the Last Glacial Maximum (~ 21 kya) that has increased since the late Holocene to the present. Future ecological niche projections show an increment of suitability of the potential hybrid zone for 2050 and 2070 relative to the present. Hybrid zone changes responded mostly to an increase in elevational ranges. Our study provides the first insight of an extensive hybrid zone among three Mexican Bursera species based on molecular data and ecological niche modeling.

Natural Hybridization between Bursera bicolor × B. glabrifolia (Burseraceae) Complex: Molecular and Chemical Evidence

“…Regardless of the variations noted in how species partition cpDNA diversity, they widely share the same haplotypes, even at a fine spatial scale. These last observations are supported by previous studies in Mexico between B. schlechtendalli × B. morelensis [55], B. bipinnata × B. gracilis [18], B. bipinnata × B. cuneata [56], and B. cuneata, B. bipinnata, and B. palmeri [21]. They support the view that introgressive hybridization was not limited to sporadic events during long periods of sympatry.…”

“…The Bayesian analysis documented the occurrence of nine hybrid individuals that showed admixture between B. bicolor and B. glabrifolia involved in this complex in two sympatric populations. These results are congruent with a significant increase in genetic diversity levels in hybrid zones (number of haplotypes; gene diversity; Shannon diversity index) in relation to the parental species, indicating that there is introgressive hybridization in the sympatric zones [5,21,53,54]. Our results suggest that hybrid zones favor an increase in genetic diversity levels.…”

“…Mexico is considered one of the main centers of diversification of this genus [18][19][20], containing 100 species, out of which 85% are considered endemic. Despite the recent identification of hybrid zones and of hybridization events between Bursera species [21], there are no studies addressing the analysis of these hybrid zones with an integrative approach using molecular and chemical markers. In the present study, two Bursera species were selected, B. bicolor (Will.…”

Complete chloroplast genomes of three copal trees (Bursera: Bullockia): comparative analysis and phylogenetic relationships

“…The four divergent genes observed in Bursera can be informative for further taxonomic studies aiming at resolving phylogenetic relationships within closely related species. For instance, the psbA-trnH and rps16 genes used in previous phylogenetic analysis did not provide enough discriminating power between closely related species (e.g., [13,40]) or for distinguishing the origin of putative Bursera hybrids [41].…”