Pollen evolution and its taxonomic significance in Cuscuta (dodders, Convolvulaceae)

The pollen morphology of 148 taxa (135 species and 13 varieties) of the parasitic plant genus Cuscuta (dodders, Convolvulaceae) was examined using scanning electron microscopy. Six quantitative characters were coded using the gap-weighting method and optimized onto a consensus tree constructed from three large-scale molecular phylogenies of the genus based on nuclear internal transcribed spacer (ITS) and plastid trn-LF sequences. The results indicate that 3-zonocolpate pollen is ancestral, while grains with more colpi (up to eight) have evolved only in two major lineages of Cuscuta (subg. Monogynella and clade O of subg. Grammica). Complex morphological intergradations occur between species when their tectum is described using the traditional qualitative types—imperforate, perforate, and microreticulate. This continuous variation is better expressed quantitatively as “percent perforation,” namely the proportion of perforated area (puncta or lumina) from the total tectum surface. Tectum imperforatum is likely the ancestral condition, while pollen grains with increasingly larger perforation areas have evolved multiple times. The reticulated tectum, unknown in other Convolvulaceae, has evolved in Cuscuta only in two lineages (subg. Monogynella, and clade O of subg. Grammica). Overall, the morphology of pollen supports Cuscuta as a sister to either the “bifid-style” Convolvulaceae clade (Dicranostyloideae) or to one of the members of this clade. Pollen characters alone are insufficient to reconstruct phylogenetic relationships; however, palynological information is useful for the species-level taxonomy of Cuscuta.