Pollen morphology of Leptodermis (Rubiaceae) and its systematic implications

Leptodermis is a taxonomically problematic genus, and little information about its palynology was known previously. In order to find taxonomic informative characters for species delimitation and their systematic implications, 105 pollen samples, representing 28 species and four varieties, were observed and analyzed. Phylogenetic inference was also conducted based on six chloroplast fragments from 32 taxa to trace the evolutionary trend of pollen characters in Leptodermis. Results showed that two types of pollen can be identified based on the aperture number and position, viz. zonoaperturate pollen with 3 or 4 colpi situated equatorially and pantoaperturate pollen with 6 to 15 apertures evenly spread over the surface forming a polygonal platelike pattern. Pollen polymorphism was rarely observed in a few Leptodermis species. Pollen dimorphism also occurs in Leptodermis, in which the muri are smooth in pollen from pin flowers whereas have minute granules in pollen from thrum flowers. Phylogenetic mapping demonstrated that each of the nine clades matched well with aperture number and position and the 3-colpate condition was plesiomorphic. In Leptodermis, pantoaperturate grains evolved three times independently and were apomorphic. In addition, the frequent occurrence of pollen polymorphism in some species might be explained by shifts among 3-colpate, 4-colpate, and pantoaperturate pollen grains. The pollen-type mapping to the phylogenetic tree revealed that the aperture number and position could well match different phylogenetic clades, suggesting that pollen morphology could provide useful information for taxonomic and systematic studies in Leptodermis.