Pollen Evolution in the Early‐Divergent Monocot Order Alismatales

Abstract

Alismatales are a key clade in monocot evolution, as they are sister to all other monocots, excluding Acorales. However, phylogenetic relationships within the order are poorly resolved, prompting the need for a reevaluation of pollen and tapetum characters, which have proved useful in other monocot groups, such as lilioids. Development of monosulcate pollen in Aponogeton (Aponogetonaceae) and Butomus (Butomaceae) and of pantoporate pollen in Echinodorus (Alismataceae) is examined here. Phylogenetic mapping of pollen and tapetum characters, based on both original observations and an extensive literature search, demonstrates that Alismatales are characterized by a potential synapomorphy: a plasmodial tapetum. Microsporogenesis is successive, a character shared with Acorus and probably other potential outgroups. Linked character syndromes, involving thin‐walled or exineless, inaperturate (omniaperturate) pollen with a smooth or reticulate surface, have evolved iteratively within the order, probably related to the aquatic or semiaquatic habit of many alismatids. Extremely long (up to 1500 μm) filamentous pollen, associated with hydrophilous pollination, occurs in three closely related families (Cymodoceaceae, Posidoniaceae, and Zosteraceae). Pantoporate spiny pollen is a potential synapomorphy for Alismataceae (including Limnocharitaceae). Alismatales share some characters (plasmodial tapetum and reniform pollen) with Pandanales, although further work is required to investigate whether this apparent homoplasy is of systematic significance. Tofieldiaceae possess two interesting apomorphies (simultaneous microsporogenesis and a secretory tapetum) that are also found in Petrosaviales and some Dioscoreales.