Spore ultrastructural features and significance of their diverse ornamental elements in the evolutionary history of the order Trichiales (Myxomycetes, Amoebozoa)

Spores are the dispersal and reproductive units in Myxomycetes, and their ornamentation, usually at the light microscopy resolution limit, is taxonomically meaningful. Here, we analyze with scanning and transmission electron microscopy the spore ultrastructural features in Trichiales, one of the most morphologically diverse orders. In Trichiales, the spore wall consists of two layers, an outer one, including the ornamentation, and an inner layer subdivided into two sections. The diversity of ornamental elements includes verrucae, bacula, pila, muri, and cristae, the two latter creating reticulate patterns. Each of these elements defines a broadly recognized ornamentation type except cristae, from which we differentiate for the first time the cristate reticulate and patched subtypes. Besides, our results point out a previous incorrect classification of the spores of the species Trichia decipiens and T. scabra. Advances in Trichiales phylogeny showed that the capillitium ornamental elements, such as the spirals, do not reflect phylogenetic relationships among the species. The same seems to apply to some of spore ornamentation types, e.g., baculate. Consequently, we may consider the need to combine multiple characters, like the spore and capillitium ornamentation, so that Trichiales systematics better reflect the still cryptic phylogenetic affinities.