Construction of High-density Genetic Maps Defined Sex Determination Region of the Y Chromosome in Spinach

Spinach (Spinacia olracea L.) is a dioecious leafy vegetable with a highly repetitive genome of around 990 Mb, which is challenging for de-novo genome assembly. In our study, a segregating F (double pseudo-testcross) population from 'Viroflay' × 'Cornell-NO. 9' was used for genetic mapping by resequencing genotyping. In the paternal 'Cornell-NO. 9' map, 212,414 SNPs were mapped, and the total linkage distance was 476.83 cM; the maternal 'Viroflay' map included 29,282 SNPs with 401.28 cM total genetic distance. Both paternal and maternal maps have the expected number of six linkage groups (LGs). A non-recombining region with 5678 SNPs (39 bin markers) co-segregates with sex type which located at 45.2 cM of LG1 in the 'Cornell-NO. 9' map while indicates the sex determination region (SDR). Integration of two maps into a consensus map guided us to anchor additional 1242 contigs to six pseudomolecules from the published reference genome, which improved additional 233 Mb (23.4%) assembly based on spinach estimated genome size. Particularly, the X counterpart of SDR in our assembly is estimated around 18.4 Mb which locates at the largest chromosome, as consensus with sex-biased FISH signals from previous cytogenetics studies. The region is featured by reduced gene density, higher percentage of repetitive sequences, and no recombination. Our linkage maps provide the resource for improving spinach genome de-novo assembly and identification of sex-determining genes in spinach.