Premise: Most dioecious angiosperms have homomorphic sex chromosomes, and non-recombining sex-linked regions (SLRs) likely comprise a small portion of the sex chromosome. The fountain of youth hypothesis posits that rare recombination within heterogametic SLRs may act to maintain homomorphy. Alternatively, turnovers of the sex-determining genes could also contribute to sex chromosome homomorphy.
Methods: We used reduced representation sequencing and mapping to the Salix purpurea reference genome to determine the size of the sex-linked region in three dwarf willows (Salix phlebophylla, S. reticulata, and S. nivalis). To test the fountain of youth hypotheses, we performed allelic state and linkage disequilibrium (LD) analyses.
Results: For all three species, sex-linked single nucleotide polymorphisms (SNPs) were identified on chromosome 15. Mean heterozygosity was higher in females, confirming that all three species were female heterogametic (ZW). A recombinant haplotype between the Z and W chromosomes was identified in S. nivalis, contributing to higher-than-expected mean heterozygosity in SLR and slow LD decay in males.
Conclusions: We propose that the uncommon ZW haplotype resulting from a recombination event identified here initiates the homogenization of Z and W chromosomes, leading to the maintenance of their homomorphy. Based on comparisons across all three SLRs, we further speculate that ongoing rare recombination events have influenced the shared patterns of sex-linked SNPs across S. phlebophylla, S. reticulata, and S. nivalis and is a process that influences sex chromosome evolution throughout the Salicaceae and flowering plants with homomorphic sex chromosomes.
Keywords: Salicaceae; dwarf willows; recombination; sex chromosomes; sex‐linked regions.
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