The ornate surfaces of male and female reproductive cells in flowering plants have long attracted attention for their variety and evolutionary significance. These structures, and the molecules involved in sexual interactions, remain among the most rapidly evolving and diverse characteristics known.

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Pollen and Stigma Structure and Function: The Role of Diversity in Pollination

We have amassed structural data for 108 characters and 52 taxa of magnoliids and basal monocots and eudicots, including observations by P. K. Endress and A. Igersheim on flowers. These data were analyzed separately and in combination with rbcL, 18S, and atpB sequences. Besides confirming agreements between previous analyses of both kinds of data (reduction of Magnoliales to six families; relation of Piperaceae, Saururaceae, Lactoris, and Aristolochiaceae), trees based on this data set show shifts toward molecular results (separation of Illiciaceae and Schisandraceae from Winteraceae and of Amborella, Austrobaileya, Trimeniaceae, and Chloranthaceae from Laurales; relation of Winteraceae and Canellaceae) plus continued conflicts (association of Chloranthaceae with Amborella and Trimeniaceae and of Nymphaeales with monocots). In cases where molecular and morphological trees conflict, combined analyses of morphological and molecular data generally yield the same topologies as molecular analyses, but morpholog...

https://www.jstor.org/stable/10.1086/317578

Morphological Phylogenetic Analysis of Basal Angiosperms: Comparison and Combination with Molecular Data

The outer pollen wall, or exine, is more structurally complex than any other plant cell wall, comprising several distinct layers, each with its own organizational pattern. Since elucidation of the basic events of pollen wall ontogeny using electron microscopy in the 1970s, knowledge of their developmental genetics has increased enormously. However, self-assembly processes that are not under direct genetic control also play an important role in pollen wall patterning. This review integrates ultrastructural and developmental findings with recent models for self-assembly in an attempt to understand the origins of the morphological complexity and diversity that underpin the science of palynology.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2007.02060.x

Pollen wall development in flowering plants

Pollen Morphology and Plant Taxonomy: Angiosperms

Increasingly robust understanding of angiosperm phylogeny allows more secure reconstruction of the flower in the most recent common ancestor of extant angiosperms and its early evolution. The surprising emergence of several extant and fossil taxa with simple flowers near the base of the angiosperms-Chloranthaceae, Ceratophyllum, Hydatellaceae, and the Early Cretaceous fossil Archaefructus (the last three are water plants)-has brought a new twist to this problem. We evaluate early floral evolution in angiosperms by parsimony optimization of morphological characters on phylogenetic trees derived from morphological and molecular data. Our analyses imply that Ceratophyllum may be related to Chloranthaceae, and Archaefructus to either Hydatellaceae or Ceratophyllum. Inferred ancestral features include more than two whorls (or series) of tepals and stamens, stamens with protruding adaxial or lateral pollen sacs, several free, ascidiate carpels closed by secretion, extended stigma, extragynoecial compitum, and one or several ventral pendent ovule(s). The ancestral state in other characters is equivocal: e.g., bisexual vs. unisexual flowers, whorled vs. spiral floral phyllotaxis, presence vs. absence of tepal differentiation, anatropous vs. orthotropous ovules. Our results indicate that the simple flowers of the newly recognized basal groups are reduced rather than primitively simple.

https://bsapubs.onlinelibrary.wiley.com/doi/pdf/10.3732/ajb.0800047

Reconstructing the ancestral angiosperm flower and its initial specializations.

Microsporogenesis is highly labile in early‐branching angiosperms, i.e., those with mostly sulcate pollen, compared with the tricolpate and tricolpate‐derived eudicots. New records of microsporogenesis in basal angiosperms (19 taxa were examined), together with a review of the literature, demonstrate that the existing typology has been too strictly applied; several basal angiosperms have apparently intermediate forms and therefore do not fit easily into simultaneous or successive categories. Intermediate forms include the “modified simultaneous” type, where ephemeral cell plates are formed after the first meiotic division but then disperse, and simultaneous cleavage follows the second meiotic division. This relative diversity reflects a range of variation in number and position of pollen apertures in basal angiosperms, although both monosulcate and inaperturate pollen may occur in conjunction with either simultaneous or successive microsporogenesis. However, many taxa with inaperturate pollen have success...

Evolution of Microsporogenesis in Angiosperms

Pollen morphology is summarized for most families of magnoliids, including unpublished observations. This survey explores the extent to which pollen types and possible evolutionary trends in these basal angiosperms can be linked to the results of recent molecular and nonmolecular cladistic analyses. Particular consideration is given to comparing pollen of the “ANITA” clades—similar to the “magnoliid I” group—with the “eumagnoliid” clade (“magnoliid II” group) within which the monocots are grouped and which is more closely related to higher dicots (eudicots). I conclude that there are no fundamental differences between pollen of the ANITA and eumagnoliid groups. The ancestral angiosperm pollen grain was probably monosulcate with a distal aperture and a homogeneous or partly granular ectexine and possessed an endexine and a two‐layered intine with a channeled outer layer, with the latter probably initially restricted to apertural regions.

https://www.jstor.org/stable/10.1086/317573

F. Bruce Sampson

Papers

Evolution of Microsporogenesis in Angiosperms

Pollen Diversity in Some Modern Magnoliids