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.

本研究利用扫描电子显微镜（SEM）对隶属于紫草科（Boraginaceae）牛舌草亚科（Cynogossoideae）勿忘草族（Myosotideae）的南半球勿忘草属（Myosotis）Exarrhena组（sect. Exarrhena）的非正式具苞铺地类群的花粉开展研究，旨在获取可用于界定物种及物种类群的分类学有效性状。研究对30个不同分类单元的1至4个个体的花粉粒进行了扫描电子显微镜成像，针对16项形态性状进行评分，并开展描述与分析。花粉粒体型微小，为异沟型，具8~12个萌发孔；赤道面观呈扁球形至长球形，具与孔沟交替排列的线形假沟，假沟可部分或完全连合("anastomosing")，进而形成极区覆盖域("极帽")。孔沟的外萌发孔较假沟更宽且通常更短，呈菱形或狭菱形，其内萌发孔多为圆形或椭圆形。花粉外壁在内萌发孔膜上通常具疣突，其余区域则密被均匀分布的颗粒。对花粉形态数据集的统计分析得到3个显著聚类群，其中两个聚类群涵盖绝大多数个体，与此前划分的南方勿忘草（Myosotis australis）型花粉一致；第三个聚类群的个体则具有单花勿忘草（M. uniflora）型花粉。短梗勿忘草（Myosotis brevis，M. discolor型）与无毛勿忘草（M. glabrescens，M. angustata型）代表了另外两种花粉类型，因此其花粉是本数据集内最具特异性的；而白花勿忘草（M. albiflora）与细茎勿忘草（M. tenericaulis）则表现出较高的种内变异。区分各聚类群的重要花粉性状包括萌发孔数量、极帽有无以及极轴/赤道轴比率（P:E ratio）。尽管多数物种无法仅通过花粉形态进行鉴定，但具有相似习性的物种会聚类在一起；若结合额外的形态、遗传或其他数据集，花粉性状将有助于界定物种类群或物种。本研究认为有必要对剩余勿忘草属物种开展进一步的花粉研究，包括隶属于Exarrhena组且具有M. discolor型花粉的物种、无苞直立类物种，以及北半球勿忘草属（Myosotis）组物种。