Implementing molecular marker technology in forage improvement

Abstract

Accelerated improvement of two cornerstones of New Zealand's pastoral industries, per ennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), may be realised through the application of markerassisted selection (MAS) strategies to enhance traditional plant breeding programmes. Genome maps constructed using molecular markers represent the enabling technology for such strategies and we have assembled maps for each species using EST-SSR markers - simple sequence repeat (SSR) markers developed from expressed sequence tags (ESTs) representing genes. A comprehensive map of the white clover genome has been completed, with 464 EST-SSR and genomic SSR marker loci spanning 1125 cM in total, distributed across 16 linkage groups. These have been further classified into eight pairs of linkage groups, representing contributions from the diploid progenitors of this tetraploid species. In perennial ryegrass a genome map based exclusively on EST-SSR loci was constructed, with 130 loci currently mapped to seven linkage groups and covering a distance of 391 cM. This map continues to be expanded with the addition of ESTSSR loci, and markers are being concurrently transferred to other populations segregating for economically significant traits. We have initiated gene discovery through quantitative trait locus (QTL) analysis in both species, and the efficacy of the white clover map for this purpose was demonstrated with the initial identification of multiple QTL controlling seed yield and seedling vigour. One QTL on linkage group D2 accounts for 25.9% of the genetic variation for seed yield, and a putative QTL accounting for 12.7% of the genetic variation for seedling vigour was detected on linkage group E1. The application of MAS to forage breeding based on recurrent selection is discussed. Keywords: genome map, marker-assisted selection, perennial ryegrass, QTL, quantitative trait locus, SSR, simple sequence repeat, white clover