Apertures with lids: distribution and significance of operculate pollen in monocotyledons

In pollen terminology, an operculum is an area of exine covering a pollen aperture like a lid. Monosulcate‐operculate, pontoperculate, disulculate, disulcate, zonasulculate, and zonasulcate aperture types are often confused in the monocot pollen literature. Various factors contribute to this confusion, including the existence of intermediate forms. Other factors include the presence of artifacts (such as the collapse of thin‐walled pollen examined using SEM) and insufficient data, either on mature pollen (including absence of ultrastructural data using TEM) or on developmental stages, especially the critical tetrad stage. In this article, we review records of monosulcate‐operculate pollen in monocots in relation to recent phylogenetic concepts, using data both from the literature and our own observations, including new records of operculate pollen inDoryanthesandChamaerops. The exine that forms the operculum often has a simpler structure than that of the rest of the grain and is underlain by thick, often channeled, apertural intine. With the exception of monocots, opercula are rare or absent in basal angiosperms with monosulcate pollen. Within monocots, monosulcate‐operculate pollen is absent from basal monocots (Acorusand Alismatales) and is relatively infrequent among commelinids, except for some Dasypogonaceae, Arecaceae, and Poales, in which the aperture is reduced to an ulcus. In contrast, it occurs frequently in the lilioid orders Liliales, particularly Liliaceae, Melanthiaceae, and Uvulariaceae; and Asparagales, particularly Doryanthaceae, Iridaceae, Tecophilaeaceae, and Agavaceae. Species with operculate pollen often have relatives with insulae or granules on the aperture membrane or with diaperturate pollen. In some taxa, monosulcate‐operculate pollen may represent a route in the evolution of diaperturate pollen from monoaperturate pollen, although this requires further testing. The operculum has evolved several times independently in monocots and may have a variety of related functions, such as protection of the delicate apertural area from pathogens and/or dehydration, particularly in taxa from dry habitats, and a role in harmomegathy.