Salix hukanoana is a rare riparian tree endemic to Japan, presently categorized as threatened on the Red List. It shows a discontinuous distribution restricted narrowly to upper river basins in the northern part of Japan. Interestingly, its distribution is wholly within the range of S. rorida, which has a disjunct distribution in Hokkaido and central Honshu in Japan.

Moreover, although S. hukaoana is classified in an independent section Hukaoanae according to its distinctiveness in several morphological characteristics, it has some morphological affinities to S. rorida.

We previously analyzed phylogenetic relationships among these willows using chroloplast DNA, and suggested S. hukaoana was maternally derived from S. rorida.

In this study, we analyzed nuclear DNAs to infer the phylogenetic origin of S. hukaoana. We analyzed three loci of low-copy nuclear genes, D-6-phosphate-gluconate (6PG), chloroplast-expressed glutamine synthetase (ncpGS), and phosphoglucose isomerase (PGI). In each locus, S. hukaoana and S. rorida were placed in different clades, and S. hukaoana was sister to different willows including S. gracilistyla.

It can be concluded that S. hukaoana was derived from a diploid hybrid between S. rorida and an unknown paternal parent such as S. gracilistyla, and adapted to warmer climate and heavier snow-fall through hybridization.