Evidence of sterility of the male sporophytes of the brown alga Saccharina japonica (Phaeophyceae) in culture irrespective of their ploidy levels.

Abstract

Monoclonal female gametophytes of Saccharina japonica, when cultured independently, can develop into female sporophytes. Previous research has shown that the chromosomes in female sporophytes of S. japonica may naturally duplicate, forming diploids, and these diploid female sporophytes are capable of forming sori and releasing zoospores. In contrast, male sporophytes derived from culturing monoclonal male gametophytes failed to form sori and produce meiospores; thus, it was hypothesized that these male sporophytes are haploid and unable to perform meiotic division. In order to explore whether the chromosomes in male sporophytes can naturally duplicate and whether such diploid male sporophytes can effectively form sori and produce meiospores, we obtained 62 male sporophytes and cultivated them into adults. Only male-specific DNA markers were detected in all of these male sporophytes, indicating their male nature. Ten microsatellite markers were used to estimate the relatedness of the parental gametophytes and the corresponding sporophytes. Results revealed that the genotypes of the male sporophytes matched exactly with the corresponding male gametophytes. Both diploid and haploid sporophytes were detected in these 62 male sporophytes when analyzed by flow cytometry. After 16 months of cultivation, none of the male sporophytes formed sori regardless of their ploidy. As controls, both female and hybrid sporophytes developed sori and released viable zoospores. These findings suggest that the sterility of male sporophytes in S. japonica is not related to their ploidy; rather, it is attributed to the absence of essential elements for sori formation that may be present only in the female.