Genetic sex determination and evolution of sexual dimorphism in seaweeds – SEXSEAWEED

Ectocarpus has recently been developed as a model system for the brown algae, and a number of tools are therefore available for this organism. These include the complete genome of a male and, more recently, of a female strain. We are using a range of approaches to elucidate the mechanisms of sexual determination and differentiation in Ectocarpus within an evolutionary context. These approaches include not only classical genetic analysis but also genomics, transcriptomics and molecular evolution.

We have characterized the U and the V sex chromosomes of the brown alga Ectocarpus and showed that these regions present universal features of sex determining regions described in other eukaryotes such as recombination suppression, accumulation of junk DNA and few genes. However, this UV system has clearly had a distinct evolutionary history. Both U and V exhibit modest levels of gene degeneration, despite being exposed to selection during the haploid phase. Several lines of evidence indicate that this is a very ancient sex-determining system, which has nonetheless remained compact because of the low level of sexual dimorphism that has provided a limited number of sex-biased genes to drive its expansion.

We are studying the genes that are differentially expressed in males and females of several brown algae with different degrees of sexual dimorphism. We will also assess the role of the sex chromosome in the regulation of a number of features associated with brown algae reproduction, such as gamete size and parthenogenic capacity.

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In genetically controlled sexual systems, gender is determined either by defined chromosomal regions or by complete sex chromosomes. Both types of structure have emerged independently and repeatedly during evolution. The structure, function and evolution of a number of such sex determining regions (SDRs) have been studied in animals, plants and fungi, but little is known about how sex is determined in other eukaryotic lineages. The brown algae represent an interesting group to study sex because they exhibit a broad range of different sexual characteristics (isogamy/anisogamy, sex determination in either the diploid or the haploid phase, varying levels of sexual dimorphism, etc.). The filamentous brown alga Ectocarpus is particularly interesting because it has a very primitive sexual system, with minimal differences between male and female individuals. Moreover, sex in Ectocarpus is determined during the haploid, gametophyte phase (UV sex chromosomes, rather than XY or ZW), a feature that has important repercussions for SDR evolution. The SDR has recently been identified in the Ectocarpus genome and the objective of this project is to study the evolutionary history of this chromosomal region and to elucidate how the locus mediates sexual differentiation. This project is expected to provide important insights both into the emergence of sexual systems and into the evolution of SDRs that function during the haploid phase of the life cycle. The data generated by the project will increase our understanding of sex chromosome biology in a general sense.