Created 29/06/2015 Updated 19/08/2019

Origin and evolution of the sexes: novel insights from the brown algae

The brown alga Ectocarpus represents a remarkable experimental system to study sex chromosome evolution because it has a very primitive sexual system (incipient anisogamy) and because sexuality is expressed in the haploid gametophyte generation rather than in the diploid phase of the life cycle (i.e. a UV rather than an XY or a WZ sex chromosome system). We have identified the U and V sex determining regions of Ectocarpus (Ahmed et al. Curr. Biol. 2014), and we are analysing this region in an evolutionary context, shedding light on sex chromosome evolution in a lineage that has been evolving independently from plants, animals and fungi for over a billion years.

 

Ectocarpus male and female gametes exhibit subtle size differences (anisogamy)
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Control of major life cycle and reproductive features by the sex chromosomes

The brown algae present an outstanding variety of types of reproductive and life cycle traits

We are using a combination of developmental genetics and genomic approaches and the Ectocarpus experimental system to understand how the sex determining region, together with the sex-regulated genes located outside the sex locus, controls key life cycle and reproductive processes of broad importance to all eukaryotes. These include sexual differentiation, asexual reproduction through parthenogenesis but also the control of gamete size and the regulation of developmental switches during the life cycle.

Evolution of sex-biased gene expression

spore release by sporophyte of Ectocarpus
spore release by sporophyte of Ectocarpus
© RL, SBR
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The brown algae represent a fascinating group for studies of the evolution of the sexes, particularly with regard to the evolution of sexual dimorphism as this group exhibits a high level of variability for this trait, ranging from isogamous, through anisogamous, to oogamous systems with different levels of male and female dimorphism. We are using RNA-seq to identify the effector genes involved in the transition to anisogamy, and employing molecular evolution tools to shed light on the evolutionary causes of sex-biased gene expression.