New insights into pollen morphology and its implications in the phylogeny of Sanguisorba L. (Rosaceae; Sanguisorbeae)

Pollen morphology of 13 species of the genus Sanguisorba (Rosaceae) was examined by light and scanning electron microscopy. The pollen morphology divided the genus into two main groups: (A) tricolporate with tapered colpus tips, and (B) either tricolporate with opened colpus tip or hexacolporate. The former group was further subdivided into those without vestibulum (A1: S. alpina, S. dodecandra, and S. filiformis) and those with vestibulum (A2: S. agrimonoides, S. ancestroides, S. annua, S. cretica, S. minor, and S. verrucosa), whereas the latter group was subdivided into those with colpus narrow and similar to the A1 type (B1: S. canadensis and S. diandra), those with colpus and mesocolpus somewhat equally wide and forming hexacolporate aperture (B3: S. albiflora, S. armena, S. media, S. menziesii, S. parviflora, S. stipulata, and S. tenuifolia), and those with colpus intermediate between the B1 and B3 types (B2: S. hakusanensis, S. microcephala, S. obtusa, S. officinalis, S. polygama, and S. sitchensis). It is suggested that the A1 type aperture would have evolved to the A2 type as a specialized form and the B types (B1 to B3) in a direction in which the hexacolporate aperture was derived. Implications of pollen morphology for infrageneric classification of Sanguisorba are discussed and the results are compared with molecular phylogenetic studies.