Pollen morphology and its taxonomic utility in the Southern Hemisphere bracteate-prostrate forget-me-nots (Myosotis, Boraginaceae)

The pollen of the informal bracteate-prostrate group of Southern Hemisphere Myosotis sect. Exarrhena (Boraginaceae: Cynogossoideae: Myosotideae) was investigated using scanning electron microscopy (SEM) to obtain taxonomically useful characters for delimiting species and species groups. Pollen grains of one to four individuals of each of 30 different taxonomic entities were imaged using SEM, scored for 16 different morphological characters, described and analysed. Pollen grains are small, heterocolpate, with 8–12 apertures, oblate spheroidal to prolate in equatorial view, with linear pseudocolpi which alternate with the colpori and can be partially to fully fused (‘anastomosing’) to form an apocopial field (‘polar cap’). Colpori ektoapertures are wider and often shorter than pseudocolpi, rhombic or narrowly rhombic with often circular or elliptical endoapertures. The exine is usually verrucate on the endoaperture membranes but densely and evenly granulate elsewhere. Statistical analyses of the pollen morphological dataset recovered three significant clusters, two of which contained the majority of individuals and coincided with the previously categorised Myosotis australis type pollen, while individuals in the third cluster had M. uniflora type pollen. Myosotis brevis (M. discolor type) and M. glabrescens (M. angustata type) represented two additional pollen types and thus had the most distinctive pollen of the whole dataset, whereas M. albiflora and M. tenericaulis showed high intraspecific variability. Important pollen characters separating the clusters included aperture number, polar cap presence, and P:E ratio (shape). Although most species could not be identified based solely on pollen morphology, species with similar habits clustered together, and pollen characters will be useful for delimiting species groups or species when combined with additional morphological, genetic or other datasets. Additional pollen studies of the remaining Myosotis species is warranted, including those from sect. Exarrhena with M. discolor type pollen and the ebracteate-erect species, as well as Northern Hemisphere sect. Myosotis.