Contrasting Mechanisms of Sex Determination in the Salicaceae

Sex determination in Populus is controlled primarily by loci on chromosome 19 across all species that have been studied in the genus. Furthermore, this chromosome has some of the hallmarks of a young sex chromosome, including regions of suppressed recombination, high density of repetitive elements, and strong haplotype divergence. However, it is currently unclear which gender is heterogametic, with evidence supporting both XY and ZW systems.  Here we report some surprising findings from the Salix purpurea genome. First, based on mapping in an F₂ intraspecific cross as well as in an association population of unrelated individuals, sex determination occurs on Chromosome 15. This is quite surprising, especially given the high collinearity of the vast majority of the Populus and Salix genomes. Furthermore, examination of the genotypes of sex determination loci in male and female trees suggests that Populus trichocarpa has a predominantly XY sex determination system, while Salix purpurea has a ZW system.

A comparative analysis of the sex determination regions of the two genera reveal many shared characteristics, including structural complexity, high repeat density, and suppressed recombination. Although gene content is mostly divergent, there is a small piece of chromosome 19 translocated to chromosome 15 in S. purpurea, and this region contains several interesting sex determination candidate genes.