An argument for the origins of heterospory in aquatic environments

Abstract

The bifid, grapnel-like processes and apical prominence (acrolamella) found in some heterosporous Middle-Late Devonian spores closely resemble to the bifid processes of acritarchs, dinoflagellates, and some Cretaceous - Recent heterosporous aquatic ferns and the lycopsid Isoetes. The spongy wall ultrastructure of Protobarinophyton pennsylvanicum and Barinophyton citrulliforme shows some similarities to the megaspore wall structure of Azolla, Salvinia, Isoetes and Marsilea. The difference between the microspore and megaspore wall structure seen in B. citrulliforme and P. pennsylvanicum is comparable to the difference found in megaspore and microspore wall structure of Azolla, Salvinia and Isoetes. As the spongy wall structure found in heterosporous aquatic ferns provides buoyancy in an aquatic environment, the same may have been true for Protobarinophyton and Barinophyton and we suggest they probably were aquatic in the dispersal of their spores. These genera are among the oldest heterosporous megaspores known and we suggest that the earliest line of heterospory evolution may be linked to aquatic dispersal of spores and outcrossing in their fertilization during the Middle Devonian. This paper is a review of relevant literature and information. We use these data to support our hypothesis that the reproduction of plants by heterospory has its origins in aquatic environments.