Hybridization Resulted in Shifts from Dioecy to Monoecy in Weeping Willows (Salix L.)

In flowering plants, hybridization is an important evolutionary force that might change sex distributions and sex determination systems (SDSs). However, little is known about processes in the first hybrid generations. Here, we study a cultivated putative hybrid cross of weeping willows (genus Salix, S.), S. alba × babylonica to gain insights into the effects of hybridization into SDSs.

We analyzed the genetic structure of pure S. alba, pure S. babylonica, and the putative hybrid crosses in Central Europe using RADSeq data and five independent methods (NeighborNet, genetic structure analysis, Principal Component Analysis, hybrid index and heterozygosity analysis, and hybrid class analysis). The genetic SDS was analyzed on male, female, and mixed (monoecious) phenotypes by detecting sex-specific genomic markers using RADSex.

Genetic analyses indicate that most of the weeping willows represent F1 hybrids (S. alba × babylonica), and only two putative S. alba backcrosses. Hybrid index, heterozygosity, and hybrid class analyses provided more interpretable results than the other methods. The parental species were consistently dioecious, whereas hybrids had male, female, and monoecious phenotypes. RADSex revealed a male heterogametic XY system for S. alba, and this was combined in the hybrids with the previously known ZW system of S. babylonica.

We confirmed the historical records stating that S. alba × babylonica are mostly F1 hybrids. We report for the first time that the combination of XY and ZW systems in primary hybrids results in regular shifts to monoecy.