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

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.

Comparative transcriptome analysis between abundant and deficient spore strains provides novel insight into gene regulatory networks and mechanisms of monospore production in bladed Bangiales

As an important seedling source,monospores closely associate with yields in nori farming.However,the molecular mechanism underlying differences in monospore production for different strains remains unknown.Comparative transcriptome analysis was performed to examine gene expression differences between the spore abundant wild-type strain(WT)and spore deficient mutant(Y1)of Pyropia chauhanii.The WT strain that produces monospores in abundance exhibited more differentially expressed genes(DEGs)in both number and higher fold-changes than the Y1 strain incapable of producing monospores,indicating that the specific regulation of genes is involved in monospore production.Three lists of DEGs were obtained between the two strains using intersection and displayed in Venn diagram:one expressed only in WT strain,another expressed only in Y1 strain,and the third shared in both strains.DEGs annotated as homologous genes of Arabidopsis thaliana in these 3 lists were curated for online functional enrichment analysis on Metascape website.Gene regulatory networks of WT were functionally enriched in the processing,proteolysis,and transport of proteins,especially within the small GTPase protein family,which might be account for the monospore production ability,whereas Y1 were functionally enriched in the metabolism of essential substance and utilization of indispensable energy,which might be account for the rapid growth of blades.We found the differentially enriched gene regulatory networks between strains might be the intrinsic mechanisms of the different monospore production traits.These findings provide novel insights into the genes and regulatory networks associated with monospore production abilities,which are essential for developing accurate breeding technologies for optimal release of monospores and increase of total nori production.