Life cycle and reproduction dynamics of Bangiales in response to environmental stresses

Abstract

Red algae of the order Bangiales are notable for exhibiting flexible promotion of sexual and asexual reproductive processes by environmental stresses. This flexibility indicates that a trade-off between vegetative growth and reproduction occurs in response to environmental stresses that influence the timing of phase transition within the life cycle. Despite their high phylogenetic divergence, both filamentous and foliose red alga in the order Bangiales exhibit a haploid–diploid life cycle, with a haploid leafy or filamentous gametophyte (thallus) and a diploid filamentous sporophyte (conchocelis). Unlike haploid–diploid life cycles in other orders, the gametophyte in Bangiales is generated independently of meiosis; the regulation of this generation transition is not fully understood. Based on transcriptome and gene expression analyses, the originally proposed biphasic model for alternation of generations in Bangiales was recently updated to include a third stage. Along with the haploid gametophyte and diploid sporophyte, the triphasic framework recognizes a diploid conchosporophyte—a conchosporangium generated on the conchocelis-phase and previously considered to be part of the sporophyte. In addition to this sexual life cycle, some Bangiales species have an asexual life cycle in which vegetative cells of the thallus develop into haploid asexual spores, which are then released from the thallus to produce clonal thalli. Here, we summarize the current knowledge of the triphasic life cycle and life cycle trade-off in Neopyropia yezoensis and ‘Bangia’ sp. as model organisms for the Bangiales.