Identification and time dependence of quantitative trait loci for basal locomotor activity in the BXD recombinant inbred series and a B6D2 F₂ intercross

A complimentary two-phase strategy was used to detect and map quantitative trait loci (QTLs) associated with the basal locomotor response to a saline challenge (10 ml/kg). In phase 1, putative QTLs, significant at p < 0.01 or better, were identified by analysis of the strain means for 25 strains of the B x D recombinant inbred series. QTLs were identified on chromosomes 1, 3, 5, 9, 10, 16, and 18. Some of these QTLs were detected across the entire experimental period (0-20 min), while others were associated with specific 5-min blocks. Eighteen hundred C57BL/6J (B6) x DBA/2J (D2) F₂ intercross animals were phenotyped for the basal locomotor response, and of this group, 500 to 700 individuals, psuedo-randomly selected, were used for a genomewide scan to confirm the RI-generated QTLs and to detect new QTLs. No new QTLs were detected but the QTLs on chromosome 1 were confirmed at p < 10^-5 to p < 10^-9, depending on the time interval. In addition, the QTLs on chromosomes 5 and 9 were confirmed at p < 0.001, providing a combined probability (RI + F₂) which exceeds the threshold for a significant association. Two additional phenotypes which showed significant RI strain differences were examined adaptation and thigmotaxis. Adaptation mapped to the same region of chromosome 9 and thigmotaxis to the same region of chromosome 1 as the distance-traveled QTL. Overall, the data presented here and elsewhere (Flint et al., 1995; Gershenfeld et al., 1997) illustrate that QTLs for basal activity are both robust and reliable.