QTL Mapping for a Trade‐Off between Leaf and Bud Production in a Recombinant Inbred Population of Microseris douglasiiand M. bigelovii(Asteraceae, Lactuceae): A Potential Preadaptation for the Colonization of Serpentine Soils

Abstract: The different response to growth on serpentine soil is a major autecological difference between the annual asteracean species Microseris douglasiiand M. bigelovii,with nearly non‐overlapping distribution ranges in California. Early flowering and seed set is regarded as a crucial character contributing to escape drought and thus is strongly correlated with survival and reproductive success on serpentine as naturally toxic soil. M. bigelovii(strain C94) from non‐serpentine soil produces more leaves at the expense of bud production in the first growing phase than M. douglasii(B14) from serpentine soil. A QTL mapping study for this trade‐off and for other growth‐related traits was performed after six generations of inbreeding (F7) from a single interspecific hybrid between B14 and C94 on plants that were grown on serpentine and alternatively on normal potting soil. The trade‐off is mainly correlated with markers on one map region on linkage group 03a (lg03a) with major phenotypic effects (phenotypic variance explained [PVE] = 18.8 ‐ 31.7 %). Plants with the M. douglasiiallele in QTL‐B1 (QTL‐NL1) produce more buds but fewer leaves in the first 119 days on both soil types. Three modifier QTL could be mapped for bud and leaf production. In one modifier (QTL‐B2 = QTL‐NL4) the M. douglasiiallele is again associated with more buds but fewer leaves. QTL mapped for bud set in the F6 co‐localize with QTL‐B1 (major QTL) and QTL‐B3. Two additional QTL for leaf length and red coloration of leaves could be mapped to one map region on lg03a. Co‐localization of the two QTL loci with major phenotypic effects on bud and leaf production strongly suggests that a major genetic locus controls the trade‐off between the two adaptive traits. The importance of mutational changes in major genes for the adaptation to stressful environments is discussed.