A Mutated Myb Transcription Factor Underlies YLD1 – a Major Yield QTL In Salix

There is increasing global interest in the use of Salix and Populus species as bioenergy feedstocks.  However, for economic viability and to assuage the food versus fuel land-use debate, there is a pressing need to increase the biomass yield obtained per unit area of land.  To this end, we have been mapping QTL for yield traits in Short Rotation Coppice (SRC) willow in several different mapping populations over the past decade.  One such QTL, designated YLD1, has a large effect on plant height, diameter and ultimately biomass yield.  The locus was identified in the large K8 mapping population (n=947) within which its effect is consistent at different UK trial sites and over multiple harvest and regrowth cycles.  In an attempt to identify the causal polymorphism, we exploited synteny with the P. trichocarpa genome sequence to map gene-targeted SNPs in the region and increase map resolution.  The QTL was resolved to a genomic region comprising ten putative genes.  When corresponding BAC clones representing the contrasting QTL haplotypes were sequenced, a ~3kb insertion in the exonic sequence of one segregating allele of a myb transcription factor was identified.  This insertion carried the signature of a recent transposition event that may ultimately be responsible for the natural variation in biomass yield attributed to segregation at the YLD1 locus.