Characiopsis; Eustigmataceae group; Eustigmatophyceae; phylogeny; Pseudocharaciopsis; pyrenoid; taxonomy; Xanthophyceae

Characiopsis, established by Borzì in 1895, is the largest genus traditionally classified in the class Xanthophyceae. However, Characiopsis-like algae studied over the last five decades using transmission electron microscopy and molecular phylogenetics have all proved to belong to a different class, the Eustigmatophyceae. Despite this, Characiopsis is still treated as a xanthophyte taxon by most algal taxonomy resources, partly because of uncertainties concerning the identity of the type of the genus. Here we document the morphology of 20 morphologically diverse and (mostly) previously unstudied Characiopsis isolates, and establish their phylogenetic position using 18S rRNA and rbcL gene sequence data. We show that these algae all belong to a single clade within the eustigmatophyte subgroup referred to as the Eustigmataceae group. From careful re-examination of previous taxonomic accounts concerning the genus Characiopsis we conclude that its type is undoubtedly Characiopsis minuta (Braun) Borzì (basionym Characium minutum Braun). In view of the loss of the holotype of this species, we designate a neotype, and also a supporting epitype (a cryopreserved culture of one of the studied strains). Our results convincingly show that Characiopsis must be transferred from the Xanthophyceae to the Eustigmatophyceae.