Nuclear ribosomal DNA phylogeny and its implications for evolutionary trends in Mexican Bursera (Burseraceae)

The genus Bursera (Burseraceae) is one of the most diversified and abundant groups of plants of the tropical dry forests of Mexico. In order to provide a basis for better understanding of its evolutionary biology, we reconstructed a phylogeny of 57 species and varieties using the nucleotide sequences of the internal transcribed spacer regions (ITS1 and ITS2) of 18S-26S and the 5.8S coding region of nuclear ribosomal DNA. We used four species of the allied genera Commiphora and Boswellia and one species of Spondias (Anacardiaceae) as outgroups. Our results support the views that Bursera is monophyletic and more closely related to Commiphora than to Boswellia. The division of Bursera into sections Bullockia and Bursera is also strongly supported by our phylogeny. Several other subclades also had high bootstrap values, especially within section Bursera. We use the phylogeny as a basis for discussing evolutionary tendencies in bark, leaves, breeding systems, and fruits.