Abstract
BackgroundIn brown algae, dioicy is the prevalent sexual system, and phenotypic differences between male and female gametophytes have been found in many dioicous species. Saccharina japonica show remarkable sexual dimorphism in gametophytes before gametogenesis. A higher level of phenotypic differentiation was also found in female and male gametes after gametogenesis. However, the patterns of differential gene expression throughout gametophyte development and how these changes might relate to sex-specific fitness at the gamete stage in S. japonica are not well known.ResultsIn this study, differences in gene expression between male and female gametophytes in different developmental stages were investigated using comparative transcriptome analysis. Among the 20,151 genes expressed in the haploid gametophyte generation, 37.53% were sex-biased. The abundance of sex-biased genes in mature gametophytes was much higher than that in immature gametophytes, and more male-biased than female-biased genes were observed in the mature stage. The predicted functions of most sex-biased genes were closely related to the sex-specific characteristics of gametes, including cell wall biosynthesis, sperm motility, and sperm and egg recognition. In addition, 51 genes were specifically expressed in males in both stages, showing great potential as candidate male sex-determining region (SDR) genes.ConclusionsThis study describes a thorough investigation into differential gene expression between male and female gametophytes in the dioicous kelp S. japonica. A large number of sex-biased genes in mature gametophytes may be associated with the divergence of phenotypic traits and physiological functions between female gametes (eggs) and male gametes (sperm) during sexual differentiation. These genes may mainly come from new sex-biased genes that have recently evolved in the S. japonica lineage. The duplication of sex-biased genes was detected, which may increase the number of sex-biased genes after gametogenesis in S. japonica to some extent. The excess of male-biased genes over female-biased genes in the mature stage may reflect the different levels of sexual selection across sexes. This study deepens our understanding of the regulation of sex development and differentiation in the dioicous kelp S. japonica.
Highlights
In brown algae, dioicy is the prevalent sexual system, and phenotypic differences between male and female gametophytes have been found in many dioicous species
S. japonica is a dioicous species in which sex is expressed during the haploid gametophyte generation and remarkable sexual dimorphism is shown in microscopic gametophytes and gametes
In contrast to the results in S. japonica, in the isogamous brown alga Ectocarpus, less than 12% of the sex-biased genes were detected during haploid gametophyte generation, and more sex-biased genes were found in immature gametophytes than in fertile gametophytes; Ectocarpus exhibits low-level sexual dimorphism in gametophytes [8]
Summary
Dioicy is the prevalent sexual system, and phenotypic differences between male and female gametophytes have been found in many dioicous species. Photoautotrophic and multicellular marine organisms, belong to the group Stramenopiles, which has been evolving independently of the well-studied animal, fungal and green plant lineages for over a billion years [1, 2]. Due to their unique evolutionary history, brown algae exhibit some remarkable characteristics. Recent studies have been conducted on the origin and evolution of UV chromosomes in the brown algae and on the regulation of sex determination and sexual dimorphism in Ectocarpus, a model brown alga belonging to Ectocarpales [2, 8,9,10,11]. All of the above similar features shared by both U and V
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