ALLOMETRIC RELATIONSHIPS BETTER EXPLAIN SEED COAT MICROSCULPTURE TRAITS IN MENTZELIA SECTION BARTONIA (LOASACEAE) THAN ECOLOGY OR DISPERSAL

Premise of research. Variation in seed coat microsculpture traits provides reliable characters for species delimitation as well as synapomorphies that support phylogenetic hypotheses; however, botanists have not yet identified the ecological or evolutionary reasons why closely related species differ in microsculpture traits. Methodology. A statistical phylogenetic approach was applied to seed variation in Mentzelia section Bartonia (Loasaceae) to determine whether seed traits are correlated with ecological variation, dispersal strategies, and/or each other. Fifty-six taxa were measured for capsule, whole seed, and seed coat microsculpture traits (cell area, cell shape, sinus depth, papillae number, and papillae length) and compared to each other and to bioclimate variation. Pivotal results. We determined that seed coat traits covary with each other; specifically, seeds with larger cells have more sinuate walls and more numerous small papillae. The hypothesis that seed coat microsculpture traits directly covary with climate or dispersal strategy was unsupported. Conclusions. We conclude that bioclimate variation does not directly select for seed microsculpture traits in Mentzelia section Bartonia but, rather, indirectly selects for larger seeds that, in turn, are correlated with larger cells. Cell sizes are ultimately responsible for the variation in microsculpture traits such as sinus depth, papillae number, and papillae length.