Photosynthesis-Dependent Extracellular Ca2+ Influx Triggers an Asexual Reproductive Cycle in the Marine Red Macroalga Porphyra yezoensis

Abstract

Asexual propagation to increase the number of gametophytic clones via the growth of asexual haploid spores is a unique survival strategy found in marine multicellular algae. However, the mechanisms regulating the asexual life cycle are largely unknown. Here, factors involved in the regulation of production and discharge of asexual spores, so-called monospores, are identified in the marine red macroalga Porphyra yezoensis. First, enhanced discharge of monospores was found by incubation of gametophytes in ASPMT1, a modified version of the previously established synthetic medium ASP12. Comparison of the compositions of ASPMT1 and our standard medium, ESL, indicated that the Ca2+ concentration in ASPMT1 was three times lower than that in ESL medium. Thus, we modified ASPMT1 by increasing its Ca2+ concentration, resulting in reduction of monospore discharge. These findings demonstrate the role of reduced Ca2+ concentrations in enhancing monospore production and release. Moreover, it was also observed that initiation of asexual life cycle required illumination, was repressed by DCMU, and was induced by a Ca2+ ionophore in the dark. Taken together, these results indicate that photosynthesis-dependent Ca2+ influx triggers the asexual life cycle by promoting the production and discharge of monospores in P. yezoensis.