Transcriptome map of perennial ryegrass ( Lolium perenne L.). The EST-based SNPs developed in this study were used to map 495 ryegrass unigenes in the VrnA mapping population using the Haldane mapping function of JoinMap version 4.0 . Linkage groups (LG) were numbered according to the nomenclature accepted for Triticeae, scale units are given in centi Morgan (cM). The resulting VrnA transcriptome map contained 838 DNA markers, ranging from 87 on LG 5 to 168 on LG 4 with an average of 120 markers per LG. Out of these, 767 are EST-derived SSRs, SNPs or CAPS marker. The total map length was 750 cM, spanning from 63 cM on LG3 to 151 cM on LG 2 (mean LG length of 107 cM). The average marker distance was less than 0.9 cM.