Genetic analysis of low survival rate of pups in RR/Sgn inbred mice

Newborn offspring of the inbred mouse RR/Sgn strain have a low survival rate prior to weaning. We hypothesized that this is a consequence of an inferior nurturing ability of RR/Sgn mothers and that RR/Sgn mothers have a tendency to lose their pups. We performed quantitative trait locus (QTL) mapping for inferior nurturing ability and tendency to lose pups in RR/Sgn mothers. The number of pups was adjusted to 6 per dam on the day of delivery, and the number of surviving pups and their total weight (litter weight) were scored at 12 days after birth. Nurturing ability was evaluated by litter weight, and tendency to lose pups was evaluated by scoring whether or not the mothers lost their pups. For litter weight, we identified one significant QTL on chromosome 4 and three suggestive QTLs on chromosomes 7, 9 and 17. The RR/Sgn allele was associated with lower litter weight at all loci. For the tendency to lose pups, we identified three suggestive QTLs on chromosomes 4, 9 and 16. The RR/Sgn allele was associated with an increased tendency to lose pups at all loci. These results supported our hypothesis that the low survival rate phenotype was attributable, at least in part, to a phenotype whereby mothers display inferior nurturing ability and a tendency to lose pups. Thus, it suggests that these two traits share genetic basis.