- ESE, CNRS University Paris Sud, Orsay, France
- Evolutionary genomics, Population genomics
Genetic mapping of sex and self-incompatibility determinants in the androdioecious plant Phillyrea angustifolia
Identification of distinct YX-like loci for sex determination and self-incompatibility in an androdioecious shrubRecommended by Tatiana Giraud and Ricardo C. Rodríguez de la Vega based on reviews by 2 anonymous reviewers
A wide variety of systems have evolved to control mating compatibility in sexual organisms. Their genetic determinism and the factors controlling their evolution represent fascinating questions in evolutionary biology and genomics. The plant Phillyrea angustifolia (Oleaeceae family) represents an exciting model organism, as it displays two distinct and rare mating compatibility systems : 1) males and hermaphrodites co-occur in populations of this shrub (a rare system called androdioecy), while the evolution and maintenance of purely hermaphroditic plants or mixtures of females and hermaphrodites (a system called gynodioecy) are easier to explain ; 2) a homomorphic diallelic self-incompatibility system acts in hermaphrodites, while such systems are usually multi-allelic, as rare alleles are advantageous, being compatible with all other alleles. Previous analyses of crosses brought some interesting answers to these puzzles, showing that males benefit from the ability to mate with all hermaphrodites regardless of their allele at the self-incompatibility system, and suggesting that both sex and self incompatibility are determined by XY-like genetic systems, i.e. with each a dominant allele; homozygotes for a single allele and heterozygotes therefore co-occur in natural populations at both sex and self-incompatibility loci .
Here, Carré et al. used genotyping-by-sequencing to build a genome linkage map of P. angustifolia . The elegant and original use of a probabilistic model of segregating alleles (implemented in the SEX-DETector method) allowed to identify both the sex and self-incompatibility loci , while this tool was initially developed for detecting sex-linked genes in species with strictly separated sexes (dioecy) . Carré et al.  confirmed that the sex and self-incompatibility loci are located in two distinct linkage groups and correspond to XY-like systems. A comparison with the genome of the closely related Olive tree indicated that their self-incompatibility systems were homologous. Such a XY-like system represents a rare genetic determination mechanism for self-incompatibility and has also been recently found to control mating types in oomycetes .
This study  paves the way for identifying the genes controlling the sex and self-incompatibility phenotypes and for understanding why and how self-incompatibility is only expressed in hermaphrodites and not in males. It will also be fascinating to study more finely the degree and extent of genomic differentiation at these two loci and to assess whether recombination suppression has extended stepwise away from the sex and self-incompatibility loci, as can be expected under some hypotheses, such as the sheltering of deleterious alleles near permanently heterozygous alleles . Furthermore, the co-occurrence in P. angustifolia of sex and mating types can contribute to our understanding of the factor controlling their evolution .
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