Comparative exine development from the post-tetrad stage in the early-divergent lineages of Ranunculales: the genera Euptelea and Pteridophyllum

Abstract

Studies of pollen wall development produce a great deal of morphological data that supplies useful information regarding taxonomy and systematics. We present the exine development of Euptelea and Pteridophyllum, two taxa whose pollen wall development has never previously been studied using transmission electron microscopy. Both genera are representatives of the two earliest-diverging families of the order Ranunculales and their pollen data are important for the diagnosis of the ancestral pollen features in eudicots. Our observations show these genera are defined by having microechinate microreticulate exine ornamentation, perforate tectum, columellate morphology of the infratectum and the existence of a foot layer and endexine. The presence of lamellations is detected during the early stages of development in the nexine of both genera, especially in the apertures. Euptelea presents remains of the primexine layer during the whole maturation process, a very thin foot layer, and a laminate exinous oncus in the apertural region formed by ectexine and endexine elements. Pteridophyllum has a thicker tectum than Euptelea, a continuous foot layer and a thicker endexine. In the apertures, the exinous oncus is formed by islets and granules of endexine, in contrast to the Euptelea apertures. The secretory tapetum produces orbicules in both genera, but they have different morphology and electron-density. Comparisons with pollen data from related orders and families confirm the ancestral states for the pollen of eudicots proposed in previous studies: reticulate and echinate surfaces, columellate infractectum and a thin foot layer relative to the thickness of the ectexine. According to our observations, we propose considering the possibility of a polymorphic state for the aperture number in the ancestor of Ranunculales, and suggest the development of orbicules as the ancestral state in this order.

Nectar bacteria stimulate pollen germination and bursting to enhance their fitness

“…We utilized pollen from the California native poppy Eschscholzia californica (Papaveraceae) as it produces a large amount of pollen, is easily obtainable near our laboratory to allow for minimal effects of storage, and because flowers are covered by a cap that is pushed off just before its first opening with the sun, so flowers can be collected before insect or wind activity introduces contaminating microbes. Pollen of Papaveraceae has ectoapertures (54,55) meaning that the outermost layer of the exine shell is missing from the aperture region but there are still other sporopollenin layers surrounding the intine (56).…”

The Role of INAPERTURATE POLLEN1 as a Pollen Aperture Factor Is Conserved in the Basal Eudicot Eschscholzia californica (Papaveraceae)

“…At the base of the eudicot clade there was a transition in the pollen grain aperture system, from pollen with one polar aperture –typical of basal angiosperms and monocots– to pollen with three apertures in equatorial positions, which may have been a key innovation involved in the success and diversification of eudicots ( Furness and Rudall, 2004 ). Because of the phylogenetic position of Ranunculales, between monocots and the core eudicots, and its high pollen aperture system diversity ( Blackmore et al, 1995 ; Pérez-Gutiérrez et al, 2016 ; Zhang et al, 2017 ), the Ranunculales species provide the opportunity to study the conservation of genetic mechanisms involved in the aperture system and its evolution. Unlike pollen from the species in which INP1 was previously studied, pollen of E. californica develops between five and seven colpate apertures.…”