Identification of yeast genes involved in Sauvignon Blanc aroma ...

6.4 Volatile thiol synthesis of 3MH and 3MHA is a quantitative trait 173French grape musts. Furthermore, N. Deed in this laboratory could not detect any changein thiol production when DAP or urea was added to SB grape must fermented with wineyeast M2. It is important to note that this grape must was from the vintage 2007 andnot from the year 2006 as in my study, and that a different strain was also used. Thesediscrepancies are consistent with the hypothesis that the varying composition of the grapejuices–which could include thiol precursor quantity and/or form–were responsible forthese contradictory findings. However possible yeast strain effects, like differences innitrogen requirements of yeast, cannot be precluded either. The lack of consistency inthe direction of these changes, combined with our poor understanding of exactly what thethiol precursors are, mean that further studies on the influence of nitrogen on volatile thiolrelease may be difficult to interpret. However, further studies could entail the correlationof the nature and amount of the nitrogen source on the impact on volatile thiol release, inrelation to different commercial wine yeasts.6.4 Volatile thiol synthesis of 3MH and 3MHA is aquantitative traitA total of 48 segregating progeny of a cross between a laboratory yeast and a wine yeastshowed normally distributed 3MH and 3MHA phenotypes, which demonstrated for thefirst time that both traits are quantitative. Like the results presented in Chapter 4, previousauthors had shown that deletions in more than one gene can affect the production of4MMP (Howell et al., 2004b) and 3MH (Thibon et al., 2008), but to my knowledge,analysis of thiol production by the progeny of a single cross has not previously beenpublished. Furthermore, the results demonstrated that the genes responsible for variationsin 3MH release, 3MHA release and fermentation rates are not linked and are thereforedifferent genes. One QTL, explaining some of the 3MHA differences in a segregatingprogeny of a cross between a wine yeast and a laboratory yeast, could be located on theleft arm of chromosome 14. Finer genetic mapping, which would require the screeningof more progeny, could narrow down the number of candidate genes in this region andwould enable the utilization of allele replacement strategies for gene verification.The genetic dissection of complex quantitative genetic traits (QTL) into their responsiblegenes is a challenging exercise (Hu et al., 2007). Furthermore, QTL analysis relies ona high divergence of the studied trait. Considering the low phenotypic variances in 3MH