Sequence-tagged high-density genetic maps of Zoysia japonica

Zoysiagrass, a warm-season turfgrass, has received increased interest due to its low maintenance and excellent tolerance to salt, shade, heat, and cold stress conditions. Japanese lawngrass(Zoysia japonica) is an allotetraploid species (2n = 4x = 40) that belongs to the Chloridoideae subfamily. We constructed high-density genetic maps of Z. japonica using restriction site associated DNA (RAD) sequencing approach and an F1 mapping population derived from a cross between ‘Carrizo’ and ‘El Toro’. A total of 60 F1 individuals were genotyped along with their parents by RAD sequencing with Nsi I and Mse I double-digestion. Two linkage maps were constructed, one for each of the parents. A map consisting of 2,408 RAD markers distributed on 21 linkage groups was constructed for 'Carrizo'. Another map with 1,230 RAD markers mapped on 20 linkage groups was constructed for 'El Toro'. The average distance between adjacent markers of the two maps was at 0.56 cM and 1.4cM, respectively. Comparative genomics analysis was carried out among zoysiagrass, rice and sorghum genomes. Our results indicated that zoysiagrass, rice and sorghum genomes were mostly collinear in the genic regions, and sorghum genome had undergone great expansions surrounding centromeric regions since its divergence from zoysiagrass. Two homologous chromosomes of each of the 10 sorghum chromosomes were found in zoysiagrass genome, indicating an allotetraploid origin of zoysiagrass. Pairwise comparison between zoysiagrass and sorghum, and zoysiagrass and rice genomes revealed that the reduction of the basic chromosome number from 12 to 10 in Chloridoids had taken place via an independent single-step nested chromosome fusion (NCF) event after the divergence of zoysiagrass from sorghum.