Seed coat sculpture in Comanthera (Eriocaulaceae) and its implications on taxonomy and phylogenetics

Abstract

Eriocaulaceae occurs primarily in the Neotropics, diversifying mostly in mountainous areas in southeastern Brazil. They are popularly known as everlasting plants, because once dried, they retain the appearance of living structures, being widely used as ornamentals. Most of the commercialized species belong to the genus Comanthera L.B.Sm., which was reestablished from Syngonanthus Ruhland. Comanthera groups 41 species, 26 in Comanthera subg. Comanthera and 15 in C. subg. Thysanocephalus, all restricted to South America. Phylogenetic analyses show Syngonanthus and Comanthera, as well as both Comanthera subgenera, as monophyletic and sister groups. Seed coat ornamentation proved to be useful and informative to help in determining taxonomic relationships in many plant groups. This study aims to characterize the external morphology of the seed coat of Comanthera and its subgenera using scanning electron microscopy, and discuss its taxonomic and phylogenetic implications. Seeds from nine species of Comanthera were analyzed, seven of which belong to C. subg. Comanthera and two to C. subg. Thysanocephalus. The results were compared with four species of Syngonanthus and one of Leiothrix. In Comanthera and Leiothrix the external periclinal walls remain intact, whereas they degenerate in the remaining genera. However, seed coat ornamentation in Leiothrix and Comanthera has distinct origins, as in the former, it comes from projections of the external layer of the outer integument, forming a striate pattern, and in the latter, from projections of the inner layer, conferring a rough pattern. Comanthera subg. Comanthera shows a rugose undulatory pattern, whilst C. subg. Thysanocephalus has a rugose micropapillate pattern. The newly reported features of the seed coat provide support for the Comanthera subgenera, thus presenting taxonomic and phylogenetic value.