Identification Of Sex Chromosome In Salix Suchowensis And Analysis

Compared with animals, sex-determining system in plants is more complicated which leads to an unclear situation for plants sex-determining mechanism. Populous and Salix are ideal models of sex evolution study. Relevant papers have already determined that sex determination genes are located on chromosome XIX in Populous, while specific genes haven’t been reported. Furthermore, sex-determining mechanism of Salix is rarely studied. In this study, we used SSR marker techonolgy to identify the sex chromosome in Salix suchowensis based on established genetic maps. Gender locus was mapped as a 1:1 segregating morphological marker.106 and 90 pair of SSR primers are successfully designed for chromosome XIX and XV respectively. MAPMAKER(version 3.0) following the two-way pseudo-test cross mapping strategy was employed to perform the genetic linkage analysis, using SSR loci amplified in present study together with 749 AFLP loci(420 were maternally informative; 329 were paternally informative) by our lab in previous. Mapping results showed that gender locus could only be mapped on the maternal map of chromosome XV, but was unmappable on the paternal map, indicating that the female was the heterogametic gender in willow. Notably, the marker density in the region near gender-associated locus was significantly higher(P=0.05 with a Poisson calculator), indicating severe suppressed recombination in this region.Synteny analysis revealed high collinearity between chromosome XIX in S. suchowensis and chromosome XIX, XIII in Populus trichocarpa;high collinearity between chromosome XV in S. suchowensis and chromosome XV, XII in Populus trichocarpa. Remarkably, chromosome XV in S. suchowensis shared no syntenic segments with chromosome XIX in P. trichocarpa. It was the same between chromosome XIX in S. suchowensis sharing no syntenic segments with chromosome XV in P. trichocarpa, indicating that sex chromosomes of poplar and willow originated from different ancestral chromosomes.Selective pressure analysis showed the general evolutional pressure for genes on chromosome XIX and XV in S. suchowensis had divergenced(Mann-Whitney U test,P=0.05). In addition, two rapidly evolving genes related with calcium ion binding on chromosome XIX were detected. Further gene ontology annotation results showed no genes on chromosome XV related with sex determiantion in willow was detected, suggesting that either sex determination genes in Salicaceae had no homology with sex determination genes in known species, or the gene was buried deep in repeat sequences, so that it was too difficult for the sequencing technology to cover.In conclusion, sex of S. suchowensis was found to be determined by the ZW system in which the female was the heterogametic gender. Chromosome XIX in poplar and chromosome XV in willow originated from different ancestral chromosomes. After the divergence of willow and poplar, chromosome XIX in poplar and chromosome XV in willow evolved independently into sex chromosomes. Although there is no evidence of morphological difference based on cytological studies, sex chromosome in willow has already begun to diverge from autosomes.