Phylogenomics, biogeography, and evolution in the American palm genus Brahea

Abstract

Background and AimsSlow rates of molecular evolution at low taxonomic levels hamper studies of relationships among species, and subsequent biogeographic and evolutionary analyses. An example is the genus Brahea, which is among the most poorly understood lineages of American palms and is characterized by a wide variety of growth forms and intermediate morphological features.\n\nMethodsWe generated approximately 400 kb of genome-scale data from all three genomes for the 11 currently described species of Brahea to infer phylogenetic relationships, reconstruct ancestral growth form, estimate ancestral geographic ranges, and test for niche equivalency among closely related species with geographic overlap.\n\nKey ResultsRelationships receive strong support, and conform to previous subgeneric assignments, except for placement of the dwarf species B. moorei within subgenus Erythea. Our robust phylogenetic hypothesis reveals trends in growth form including an overall increase in height in the B. armata clade, and independent evolution of dwarf forms from taller ancestors in the B. pimo and B. dulcis clades. Ancestral range estimation reveals roles of dispersal (e.g. B. edulis on Guadalupe Island) and sympatric speciation in some cases (e.g. in the B. armata clade), but is equivocal in others (e.g. in the B. pimo clade). We find evidence of niche non-equivalency among species within the B. armata clade in northwestern Mexico, and some evidence of niche non-equivalency between B. berlandieri and B. dulcis, the former of which is synonymized under B. dulcis.\n\nConclusionsOur findings have implications for the complex biogeographic history in Central America and Mexico, suggesting that sympatric speciation and dispersal are the predominant processes of species diversification. Future studies should include population-level sampling across the genus, along with morphological and ecological information, to assess distinctness among species and, particularly, levels of gene flow, in an integrative fashion.