QTL mapping for growth and leaf characters in bread wheat

In bread wheat, QTL interval mapping for four growth characters (early growth habit, days to heading, days to maturity and plant height), and association studies for two leaf characters (leaf colour and leaf waxiness) were conducted utilising the International Triticeae Mapping Initiative reference population (ITMIpop) that was used in a number of earlier studies on molecular mapping in this crop. Using QTL Cartographer, composite interval mapping (CIM) for all the four growth characters and multitrait composite interval mapping (MCIM) for three correlated traits (excluding plant height) were conducted. For growth characters, CIM suggested the presence of 16 QTL (LOD = 2.0-12.7), of which only six were common with those among the 18 QTL identified by MCIM. This suggested possible presence of some false positives among QTL identified by CIM. Fourteen (14) molecular markers that were closest to the 14 QTL identified by CIM were also tested for marker-trait association using regression and t-tests. Five markers showed significant association, and therefore, are recommended for marker-assisted selection (MAS). Incidentally, the QTL associated with these five markers were identified by both CIM and MCIM thus placing higher level of confidence in these markers. Some of the QTL identified by CIM and joint MCIM also affected more than one trait each, suggesting that the observed correlation may be either due to tight linkage or due to pleiotropy. During CIM for individual traits, effects of all QTL (phenotypic variations explained or PVE) that were identified at LOD score of 2.0 or above, together accounted for approximately 17-91% of the phenotypic variation. However QTL effects, when measured irrespective of LOD score, exhibited characteristic L-shaped distribution, suggesting that there are many minor QTL, which should be taken into account during MAS. The two leaf characters exhibited 100% correlation. Consequently, the 14 markers that were identified showed significant marker-trait association with both the traits. Some of these markers are the same, which also exhibited association with some growth and yield traits, studied by us earlier, thus adding to their utility in wheat breeding through MAS. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.