The coexistence of closely related plant species is both widespread and common, but presents a puzzle: it is unclear how, despite their ecological and genetic similarities, congeners persist as recognizably different taxa without fusing through hybridization. Reproductive isolating barriers are known to preserve species boundaries by limiting gene flow between species, but overall their relative importance in isolating sympatric closely related species is unknown, and few studies have addressed this question in a phylogenetic framework. The objective of this study is to understand the prepollination barriers that limit the gene flow among 6 sympatric species of Bursera (Burseraceae) and how these barriers vary according to the phylogenetic distance of the species. The study was conducted in a tropical dry forest in the pacific slope of Mexico, at Chamela Biosphere Reserve where B. heteresthes, B. excelsa, B. arborea, B. fagaroides, B. instabilis, and B. sp (fagaroides clade co –occur under the same rainfall regime and soil type and present differing phylogenetic relationships within the genus. During a preliminary study, we examined the flowering phenology, breeding system, and pollination biology of the species.
Bursera starts flowering in May, and during the first month of flowering when the study was performed, only two species (B. instabilis and B. sp) where found flowering. This indicates that asynchronous/non-overlapping flowering period is the first acting barrier between the most closely related pairs of Bursera species in the study site (1st pair: B. instabilis and B. arborea, which may be sister species within the simaruba clade; and 2nd pair B. fagaroides and B.sp, both in the fagaroides clade). The species do not exhibit pollinator isolation, sharing the most common pollinators. Two bee species were observed visiting males and females of both plant species, collecting pollen and having contact with stigmas. The populations of B. instabilis and B. sp are dioecious, and apomixis was not found. Both staminate and pistillate flowers contain nectar. The flowers of both species last about one day, but open at different times of the day, suggesting that asynchronous floral phenology could be an important barrier limiting the gene flow between the plant species.