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Abstract:
In most brown algae, sex is determined in the haploid gametophyte stage by a female U and a male V chromosome, but sexually dimorphic traits are to a large extent a result of differential expression of (autosomal) genes that are present in both sexes. To understand the mechanisms underlying sexual differentiation in brown algae, we compared wild type and three feminized male mutant strains of the giant kelp Macrocystis pyrifera using morphometric, genomic and transcriptomic approaches. Morphometric analysis of mutant lines revealed strong signs of feminization in these genetically male individuals, including increased gametophyte cell size. Upon fertility, feminized males produced gametangia morphologically similar to oogonia which released immotile gametes. These were significantly larger (≤ 500%) than male sperm, but significantly smaller (≤ 52%) than female eggs, and were capable of parthenogenesis, which is a female-specific trait in kelps. The phenotypic feminization of these mutants was underlain by conspicuous transcriptome feminization (increased expression of female-biased genes) and de-masculinization (decreased expression of male- biased genes) compared to wild type males, concurrent with significantly reduced expression of a subset of male sex-linked genes. Remarkably, comparison of transcriptomes revealed that distinct sex-linked and autosomal genes underlie feminization in the different mutant lines, suggesting that diverse, alternative pathways may lead to the same feminized phenotype in these genetically male individuals. A subset of genes, however, exhibited consistent changes in their pattern of expression associated with the feminization, and we will discuss how these genes may provide clues to further understand male versus female sexual differentiation.
