RAD-seq linkage mapping and patterns of segregation distortion in sedges: meiosis as a driver of karyotypic evolution in organisms with holocentric chromosomes.

Abstract

Meiotic drive, the class of meiotic mechanisms that drive unequal segregation of alleles among gametes, may be an important force in karyotype evolution. Its role in holocentric organisms, whose chromosomes lack localized centromeres, is poorly understood. We crossed two individuals ofCarex scoparia (Cyperaceae) with different chromosome numbers (2n=33II =66×2n=32II =64) to obtain F1 individuals, which we then self-pollinated to obtain second-generation (F2) crosses. RAD-seq was performed for 191 individuals (including the parents, five F1 individuals and 184 F2 individuals). Our F2 linkage map based on stringent editing of the RAD-seq data set yielded 32 linkage groups. In the final map, 865 loci were located on a linkage map of 3966.99cM (linkage groups ranged from 24.39 to 193.31cM in length and contained 5-51 loci each). Three linkage groups exhibit more loci under segregation distortion than expected by chance; within linkage groups, loci exhibiting segregation distortion are clustered. This finding implicates meiotic drive in the segregation of chromosome variants, suggesting that selection of chromosome variants in meiosis may contribute to the establishment and fixation of chromosome variants in Carex, which is renowned for high chromosomal and species diversity. This is an important finding as previous studies demonstrate that chromosome divergence may play a key role in differentiation and speciation in Carex.