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

Abstract

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.

Differential gene expression patterns during gametophyte development provide insights into sex differentiation in the dioicous kelp Saccharina japonica

“…Many studies have shown that the HMG domain gene in Ectocarpus, Ec-13_001750, is a particularly strong male sex-determination candidate gene [2,10,55]. Our previous study on the characteristics and expression analysis of SJ05808 also confirmed their potential role in sex determination in S. japonica [17]. A ste20 protein kinase has been reported to be necessary for haploid cells to respond to sex pheromones in the yeast Saccharomyces cerevisiae [56]; this kinase can transduce the signal induced by the binding of pheromones to membrane receptors in the protein kinase cascade.…”

“…Information on sex-specific markers is listed in Additional file 1 : Table S1. To facilitate gametogenesis, the gametophytes in each group were broken down into short fragments and placed in PES medium as described previously [ 17 ]. After 12 days of culture, the gametophytes attained sexual maturity (production of oogonia or antheridia) and were collected as two other groups, representing the mature female gametophyte group (SMf) and the mature male gametophyte group (SMm) (Additional file 16 : Figure S8).…”

“…S. japonica belongs to the Laminariales order, and the sexes of its haploid gametophytes are also determined by a UV sexual system. During the haploid generation, remarkable sexual dimorphism was exhibited in female and male gametophytes, and a high level of phenotypic differentiation was also found in female and male gametes after gametogenesis [ 16 , 17 ], which contrasted with the limited levels of sexual dimorphism in Ectocarpus gametophytes and gametes. Sex chromosome and SDR genes can play key roles in the evolution of phenotypic differences and speciation [ 18 ].…”

Discerning the Putative U and V Chromosomes of Saccharina japonica (Phaeophyta) by Cytogenetic Mapping of Sex-Linked Molecular Markers

“…Meanwhile, MSj68-16-2 was also chosen for the present study since it was proposed to be a male-linked marker which was developed by Zhang et al (2018) on the basis of S. japonica transcriptomic and genomic sequences. It shared 100% similarity with partial coding sequence (170 bp in length) of the kelp HMG-box-containing gene (SjHMG) (Zhang et al, 2019), suggesting that this marker was a part of DNA sequence of this gene. Moreover, Zhang et al (2019Zhang et al ( , 2021 provided experimental evidence that the HMG domaincontaining protein, a transcription factor (Ec-13_001750) in Ectocarpus as annotated by Ahmed et al (2014), was expressed exclusively in the kelp male gametophytes and was thus proposed to be a candidate male-determining gene.…”

“…It shared 100% similarity with partial coding sequence (170 bp in length) of the kelp HMG-box-containing gene (SjHMG) (Zhang et al, 2019), suggesting that this marker was a part of DNA sequence of this gene. Moreover, Zhang et al (2019Zhang et al ( , 2021 provided experimental evidence that the HMG domaincontaining protein, a transcription factor (Ec-13_001750) in Ectocarpus as annotated by Ahmed et al (2014), was expressed exclusively in the kelp male gametophytes and was thus proposed to be a candidate male-determining gene. In addition, MSj68-58-2, a developed female-specific marker in S. japonica by Zhang et al (2018), was selected in the present study as a candidate for sex-linked marker.…”