Investigation of the effect of genetic background on QTL expression using three connected RIL populations
--Alain Charcosset, Mathilde Causse and André Gallais

A set of maize recombinant inbred lines (RIL) has been developed to investigate the genetics of several quantitative traits. Three parental lines were chosen to provide a good representation of the germplasm that can be used in the north of France. F2 is an early flint line derived from the French population Lacaune, F252 is an early dent line developed from US and Canadian germplasm (F186*Co125), Io is a later dent line from the Iodent group. The three possible hybrids between these three lines have been selfed. Resulting F2 populations have then undergone a classical single seed descent process yielding F5 lines. 145, 129 and 152 lines were developed for populations Io*F2, F252*F2 and Io*F252, respectively. These three populations will be called respectively D, E and G in the following text.

The lines were analyzed for their RFLP and a synthetic map was built (see companion paper). Field data were observed on F6 families in years 1992 and 1993. Earliness data (days to silking) were recorded in three environments in 1992 and one in 1993. QTL analysis (ANOVA and Mapmaker QTL) was performed for each population. Results appeared to be very dependent on the population. In population D, 5 chromosomal regions showed significant effects (1% alpha level) in the two northern environments. This number was three and one for populations G and E, respectively. The number of detected QTLs appeared to be positively related to the difference in silking time of the parents. Most important effects were detected for population D, near probe umc67 (chr. 1) with a substitution effect of 3.6 days (15% of the variation explained), and near probe umc103 (chr. 8) with a substitution effect of 4.5 days (17% of the variation explained).

Two chromosomal segments were clearly common to populations D and G. However, consistently with the results of Beavis et al. (Theor. Appl. Genet. 83:141-145, 1991), other segments were involved in the variation of a single population. For instance, a QTL near umc67 was observed in population D and not in the two other populations. Several causes can be evoked to explain such a result: (i) the power of the statistical tests, (ii) allelic relationships between parents, and (iii) epistatic effects. A simultaneous analysis of variance was performed for the three populations (for loci that were polymorphic in the three populations). Significant (alpha 5% level) interaction effects between marker and population were observed for 18% of the tests, which suggests that epistatic effects play a role in the differences that we observed.

Further analyses will be carried out on the hybrids between the RILs and the three parental lines to investigate dominance effects for several traits. Selfing of the RILs is underway to develop F7 lines that will be available for cooperations. 

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