Multilamellated zones in the sporoderm of heterosporous lycopsids: adaptive trait or evolutionary spandrel?

Background Multilamellated zones (MZs) are small areas in the innermost portion of sporopollenin-containing walls of spores of heterosporous lycopsids, where this layer of the sporopollenin wall becomes thicker by splitting into interconnected lamellae. The detection of MZs is difficult, but accumulating information on their occurrence suggests possible evolutionary and taxonomic importance, requiring updating and reevaluation. Scope The paper reviews the occurrences of MZs in spores of fossil and extant lycopsids (since the Devonian to modern time). The majority of MZ records are ascribed with different degrees of certainty to the isoetalean lineage, but they are also observed in some members of Selaginellales. The only known homosporous plant with MZs is the Early Devonian Leclercqia. As to modern Isoetes and Selaginella, some species of these genera possess them, but only in microspores. It is still unclear when megaspores lost MZs and whether it was a single event. The available information is incomplete: ultrastructural characteristics are available for microspores of several taxa of fossil heterosporous lycopsids, but there is a lack of data on their megaspores. Conclusions The occurrence of MZs almost exclusively in heterosporous lycopsids and their consistent presence in isoetaleans are begging for phylogenetic interpretation, but their occurrence in some species and absence in other species of the same genera and especially the dearth of relevant information on many heterosporous lycopsids warrant cautious interpretation. The location of MZs only in the vicinity of the haptotypic mark and the fact that they appeared as early as in Devonian and are still present in extant species suggest that they have some function. Although the most widely circulated hypothesis relates them to spore germination, no confirmation has been obtained so far from extant species.