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Pollen Evolution

Scanning electron micrograph of pollen of Meliosma pinnata ssp. macrophylla (Sabiaceae). Polar view showing three colporate apertures. Scale bar = 5µm.

Considerable depth of expertise in comparative pollen morphology at Kew includes current staff members Carol Furness (Acanthaceae, monocots, eudicots, general and developmental), Hannah Banks (Fabales, Nelumbonaceae), and Paula Rudall (monocots and general), former staff members Keith Ferguson (Fabaceae and general) and Madeline Harley (Arecaceae, Burseraceae, Lamiaceae, Liliaceae, Iridaceae and general), plus numerous research visitors.

This expertise, in parallel with improved hypotheses of angiosperm relationships based on molecular sequence data, permit the re-assessment of key pollen and anther characters in angiosperms. Work on this began with the monocots (see Monocot pollen evolution) and has been expanded since 2000 to include basal angiosperms and eudicots. This long-term project is based in the Jodrell Laboratory and involves collaborations with colleagues from Kew and other institutions. The aims are (1) to review pollen and anther characters of systematic significance throughout the angiosperms and their seed-plant outgroups (e.g. tapetum type, microsporogenesis, pollen apertures, microgametophyte), and (2) to target specific groups for more detailed study, particularly Fabales (legumes and relatives). Research techniques include both scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Our results indicate that a secretory tapetum is plesiomorphic in angiosperms although both plasmodial and invasive tapeta occur in basal lineages. Simultaneous microsporogenesis is plesiomorphic in angiosperms, which can be divided into two groups: basal angiosperms and monocots with monosulcate (or monosulcate-derived) pollen apertures which arise by simultaneous or successive microsporogenesis, and eudicots, with tricolpate (or tricolpate-derived apertures) which arise only by simultaneous microsporogenesis. Endoapertures (the inner openings of compound apertures) are common in eudicots but infrequent in early-divergent eudicot lineages. All these pollen and anther characters demonstrate evidence of a high degree of experimentation prior to evolutionary canalisation of some key morphological features.

In Fabales, there is a long history of research on legume (Fabaceae) pollen at RBG Kew, begun by Keith Ferguson in the late 1970’s. Recently a thesis on pollen structure in caesalpinioid legumes was successfully presented by Hannah Banks for her PhD. This included work on development and evolution of ‘Zwischenkörper’ (unusual structures associated with pollen apertures), polyads in mimosoid legumes, and the enigmatic apertures of Duparquetia (Fabaceae) pollen. The latter were investigated using confocal microscopy techniques. Two overviews of caesalpinioid pollen were published in Advances in Legume Systematics: (1) on the detarioid legumes (Caesalpinioideae), and (2) on tribes Caesalpinieae and Cassieae (Caesalpinioidae).  Surveys of pollen of legume relatives Surianaceae, Quillajaceae and Polygalaceae have been undertaken involving Frances Claxton, a Kew sandwich student. That on Surianaceae and Quillajaceae has been published and Polygalaceae is nearing completion.

Other families for which detailed studies of pollen morphology have been carried out include Burseraceae and Lamiaceae, in conjunction with molecular phylogenetic analyses. The Miocene fossil pollen Fupingopollenites has been compared with modern eudicot pollen in a collaboration with Wei-Ming Wang (Chinese Academy of Sciences, Nanjing).

A total of 36 papers on pollen structure have been produced in peer-reviewed publications since 2000, including six in higher impact journals. Ongoing projected research output is at least two publications per year.

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