New observations on the sexual and asexual reproductive stages of Staurastrum gracile (Desmidiaceae, Zygnematophyceae)

Abstract

ABSTRACT Staurastrum gracile is a highly polymorphic species that overlaps morphologically with other species of the genus. The observation of some reproductive features, such as zygospore development, is necessary for correct identification of some Staurastrum species. This study describes the sexual and asexual reproduction of S. gracile in culture and emphasises the importance of characters observed in the fully developed stage of vegetative individuals and zygospores. The description of the life cycle is based on isolated populations from Mexico studied in light and scanning electron microscopy. We describe morphology in the asexual reproductive stages, resulting from an early division before complete cell development, but also results from the number (3, 4, or 7), the length, and the plane of projection of the arms (divergent, moderately divergent to straight, and slightly convergent). We describe gamete fusion type in S. gracile and establish that meiosis occurs in early stages of zygospore formation, characterising the morphology of all stages until germination. The phylogenetic position of the Mexican populations was confirmed by sequence analysis of the ATPase beta-subunit gene (atpB), and the resulting phylogeny was compared with zygospore characters of species. The result suggests that zygospore morphology can be a significant phylogenetic signal in desmid taxonomy.