Morphological Character Mapping on a Molecular Phylogeny Using Pollen Variation in the Cryptanthinae (Boraginaceae)

Phylogenetic classification at the species level in the Boraginaceae is notoriously difficult when relying solely on morphological data. Studies are currently in progress to generate a well-supported phylogenetic tree of this family using molecular data. A molecular based phylogeny may reveal the characters that evolved slowly enough to have the same state in closely related taxa found in some key palynological traits used in previous classifications. Pollen attributes were collected on seventy four species across six genera in the subtribe Cryptanthinae using Scanning Electron Microscopy (SEM). The pollen data exhibit features which are taxonomically informative including shape, aperture type, sculpturing, and size. Cryptanthinae pollen encompasses three of the nine Erdtman (1966) shape categories, and seven of the eleven Faegri and Iversen (1975) sub-shape categories. Their aperture types include heterocolpate, zonoporate, and zonocolpate forms. They are sculpted with fossulate, foveolate, echinate, reticulate, and gemmate clavate surfaces. They range in size from 4.85μm long and 1.92μm wide to 40.85μm long and 25.60μm wide. Some of them have a transverse groove and others do not. The same is true for the presence or absence of polar apertures. These characteristics were mapped on a molecular phylogeny to observe evolutionary trends. Biogeographic data such as habitat moisture, range of distribution, flowering period, and style type were also mapped on the molecular phylogeny to uncover selection pressures responsible for the high level of morphological diversity in this subfamily. This analysis revealed habitat moisture level as one of the driving forces behind pollen subshape diversity in the Cryptanthinae.