Seed viability in Bursera: The relative contribution of environmental variation and phylogenetic relatedness

Seed viability is a crucial factor affecting the regeneration potential of seeds, but there is little information on how this trait varies across habitats, populations, species, and higher taxonomic units, as well as on the relative contributions of evolutionary history and environmental factors to this trait. Here we evaluate the relative contributions of climatic variability and phylogenetic history to seed viability in a group of species of Bursera that belong to two distinct lineages (sections Bursera and Bullockia). We analyzed 39 seed lots from 11 species, comprising several populations and 2 years, and estimated embryo presence and viability in each lot. We examined spatial and temporal variation in viability and analyzed the relationship between this trait and water availability using regression models; the amount of phylogenetic signal in seed viability was also estimated. Mean seed viability was consistently low in species of section Bursera (mean 29.7%) and substantially higher in species of section Bullockia (mean 79.6%). A high proportion of unviable seeds were embryoless. Spatial and temporal variation in seed viability was significant but usually low among populations and species. The relationship between water availability and seed viability was not significant, but the amount of phylogenetic signal in seed viability was high and significant (λ = 0.74). There is a clear difference in seed viability among the two sections of Bursera. During the split of the two sections in the middle Eocene, the reorganization of the genome may have involved changes in fruit morphology associated with differences in parthenocarpy among them.