Isolation and expression analysis of a candidate gametophyte sex determination gene (sjhmg) of kelp (Saccharina japonica)

Saccharina japonica undergoes an alternating life cycle during which the diploid sporophyte generation alternates with the happloid gametophyte generation. Saccharina japonica uses the UV sex determination system to determine the sex of its haploid gametophytes. However, the sex-determining genes and the sex-determining mechanisms of kelp gametophytes have not been thoroughly elucidated to date. In this study, a kelp HMG-box-containing gene (SjHMG), which is located within the sex determination region of S. japonica, was isolated and characterized. SjHMG contained an open reading frame of 1,266 bp in length and encoded a deduced protein of 421 amino acid residues with two HMG-box domains. Phylogenetic analysis showed the strongest relationship between SjHMG and its orthologs in brown algae. An alternatively spliced transcript (SjHMG isoform-2) encoding a protein of 256 amino acid residues was also identified. The two isoforms were specific for male gametophytes. A real-time quantitative PCR analysis showed significantly higher abundances of two isoforms in immature male gametophytes than in mature ones. These findings suggested that the SjHMG gene is a candidate male gametophyte determination gene of kelp. In addition, the abundance of SjHMG isoform-2 transcripts was significantly lower than that of SjHMG isoform-1 transcripts, and only an HMG-box domain was conserved among species in the order Laminariales, which indicated that the gene is specifically involved in sex regulation in some species of the order Laminariales by alternative splicing.