Morphological and phylogenetic evidence that the novel leaf structures of multivein Selaginella schaffneri are derived traits

Abstract

Microphylls, simple leaves with a single vein and no leaf gap, are the typical lycophyte leaves. However, Selaginella schaffneri has complex veins. Structural features and phylogeny associated with this unusual venation have remained unknown. We studied the leaf, venation, spore structures, and phylogeny of S. schaffneri, with S. erythropus as a typical Selaginella for comparison. Leaf veins of both S. schaffneri and S. erythropus originate from a single vascular strand in the stem and have no leaf gaps. In S. schaffneri, this single vascular strand prominently enlarges as a hub-like vein node at the leaf base and then divides multiply in the leaf blade. Unusual structures, more commonly found in angiosperms, are revealed, including vessels, bundle sheath cells, three stomatal types, and differentiated mesophyll tissue. Other unusual structures include transparent zones on the leaf margin and a complex open hexagonal three-dimensional structure on the megaspore walls. Fifty one concatenated protein-coding genes from plastomes were used to construct the phylogeny of S. schaffneri within Selaginellaceae, which shows that S. schaffneri, together with the sanguinolenta group, is the earliest-diverging lineage of subgenus Stachygynandrum. The unusual structures of S. schaffneri are consistent with drought resistance. However, these structures are not known in more basal members of Selaginella and appear to be derived in S. schaffneri. The leaf veins of S. schaffneri, originating from the branching of a single vein, imply a variation on a microphyll. Despite the general simplicity of structure in Selaginella, S. schaffneri shows unusual structural homoplasy with angiosperms in these traits.