Selection efficacy at intraspecific and interspecific scales: insights from haplo-diplontic plants

Introduction

Natural selection is a pillar of evolutionary biology: it tends to fix beneficial mutations and remove deleterious ones. But the factors that determine selection efficacy are yet poorly understood. A proposed factor is the interaction between mutations' dominance and ploidy level: recessivity should reduce selection efficacy in diploids but not in haploids. Methodological limits make this challenging to test in species with a diploid-dominant life cycle.

Background

Yet, a whole phylogenetic clade under-explored in genomics gathers all the attributes to tackle this question: Bryophytes. Their life cycle is characterized by alternating a long haploid phase (gametophyte) with a short diploid phase (sporophyte), both phases being macroscopic. It is ideal for shedding light on what drives selection efficacy within and between species; thus, improving our understanding of adaptation and speciation.

Research Questions

1. How does ploidy affect the efficacy of adaptive and purifying selections within species?
   Using comparative genomics, I will empirically test:
   i) if selection is more efficient for haploid-biased genes (i.e., expressed primarily in the gametophyte) than diploid-biased genes, and
   ii) if that difference is reinforced in species with a more complex sporophyte.

2. How does ploidy affect the efficacy of selection against hybridization between species?
   With theoretical developments, I will quantify the extent to which the length of gametophytic and sporophytic phases influences the rate of accumulation of reproductive barriers against interspecies introgression. These predictions will be confronted with genomic estimates by developing a statistical inference method.

3. How efficient is selection in non-recombining genomes?
   I will take advantage of the Bryophytes' U/V sexual system to assess the extent to which selection efficacy is reduced on their sex chromosomes. I will also tackle an overlooked issue: are U/V sex chromosomes efficient barriers to introgression?