Resolving the taxonomic enigma of Nesocaryum stylosum (Boraginaceae): phylogenetic evidence for its reclassification as Cryptantha stylosa

Abstract

The classification of Nesocaryum stylosum (Boraginaceae) has remained unresolved for nearly a century. This species is endemic to Isla San Ambrosio, a small island located approximately 900 km due west of the coast of central Chile. Ivan M. Johnston transferred the species from the genus Heliotropium to the monotypic Nesocaryum in 1927 but noted that, despite its quite different and rather unique vegetative, inflorescence, and calyx morphology, its unit fruits (nutlets / eremocarps) resemble those of Cryptantha.

Here, we review the morphology and taxonomic history of N. stylosum and provide DNA sequence data that support its placement in Cryptantha, for which we propose the new combination Cryptantha stylosa. Our data also support the placement of this species within the Maritimae clade, a monophyletic group of Cryptantha species that is phylogenetically distinct from the bulk of the genus. We propose an expanded membership of the Maritimae clade comprising up to 19 species: eight species (13 minimum-rank taxa) from North America and 12 species (12 minimum-rank taxa) from South America, including Cryptantha stylosa, with one taxon occurring on both continents.

We further review evidence bearing on the biogeographic and evolutionary history of Cryptantha stylosa and its putative closest relatives and identify the need for additional research within the group.