Identification of genetic regions of importance for reproductive performance in female mice.

REPRODUCTIVE success in mice, as in all mammals, is dependent on several environmental and genetic factors. These factors are normally very complex and are often difficult to define. Most scientists who have worked with mice in different animal houses have experienced that the same inbred strains can show different reproductive performance. This discrepancy can be due to differences in more or less definable environmental factors, such as differences in cages, food, bedding material, routines, health status, and handling of the mice. In research there is often a considerable problem when there is a low reproductive performance of genetically modified model mice. However, some strains of mice have, after many years of selection, become more resistant to different types of stress and produce large litters in most types of environments. Such strains are often denoted “high breeders,” and they carry genes ensuring large litter size, high stress resistance during pregnancy, and good nursing properties during the lactation period. The NFR/N strain is an example of an inbred high-breeder strain, while common C57BL strains normally produce lower numbers of litters and are often denoted “moderate breeders.” Females of the NFR/N strain are known to produce several large litters during a long period of time. Furthermore, they are known to be excellent mothers (good nursing properties, high milk production, rapid neonatal growth, etc.); it is likely that these females carry several genes that are valuable for successful reproduction in mice. A limited number of reported studies have applied genetic mapping and linkage analyses in their search for genetic regions of significant importance for successful reproduction in mice. In the early 1980s a classical genetic analysis aimed at finding genetic regions critical for litter size was performed, but no clear-cut results were obtained (Horstgen-Schwark et al. 1984). The discovery of new techniques for genotyping during recent years, together with the continued development of more advanced linkage analysis programs, has made it possible to perform more exact genetic analyses and eventually also to identify single genes. Kirkpatrick et al. (1998) applied modern methods to map gene regions critical for litter size in an F2 progeny between the outbred Quackenbush-Swiss mouse line and ordinary C57BL/6 mice. They found significant linkages at specific segments of chromosomes 2 and 4 (Fecq1 and Fecq2) and a suggestive linkage at a region of chromosome 9. Furthermore, Peripato et al. (2002, 2004) presented QTL data for maternal reproductive performance in an F2 progeny between LG/J and SM/J mice, and they particularly point out genetic regions on chromosomes 7 and 12 as critical for the litter size phenotype. This group also showed that several reproductive phenotypes are very complex and subjected to epistatic interactions from genetic regions of several chromosomes. Rocha et al. (2004) also performed a QTL study of pregnancy-associated traits in mice, and they report that loci on chromosome 2 are of particular importance. Furthermore, Everett et al. (2004) reported that loci on chromosomes 1 and 9 control the ovulation of primary oocytes in mice, a phenotype related to litter size. In this context it should also be mentioned that Spearow et al. (1999a,b; Spearow and Barkley 1999) have mapped genes critical for differences in hormone-induced ovulation rate (ORI genes) between A/J and C57BL/6 mice, and QTL for this trait were identified on chromosomes 2, 6, 9, 10, and X. Still, there is a need for identification of additional loci that are critical for successful breeding of mice. This study focuses not only on female-dependent differences in litter size in normal mating, but also on a number of other female-associated traits critical for reproduction in mice, such as vaginal plug frequency during 96-hr mating periods, ratio of number of pregnancies to the number of vaginal plugs, growth of pups, neonatal mortality, and amount of maternal IgG transmitted to the offspring, etc. A unique approach used in our study is that we have included an analysis of the possible influence of MHC differences between mother and fetuses. Mammalian allogeneic pregnancies deal with the classical immunological problem, i.e., that the mother should avoid immunological rejection of her genetically different offspring and at the same time she should mount an optimal defense against pathogens to provide the fetuses with passive immunity. Since the days of Medaware (1953) the immunological enigma of mammalian pregnancy has been highlighted several times, and a number of possible protective mechanisms have been presented, such as low placental MHC expression (Sunderland et al. 1981; Mattsson et al. 1992), protective properties of the placenta (Petroff et al. 2003; Aluvihare et al. 2005), placental complement inhibitory factors (Thurman et al. 2005), cytokine balance (Raghupathy 1997; Svensson et al. 2001), placental tryptophan catabolism (Munn et al. 1998; Mellor and Munn 2000), etc. Although many of the suggested mechanisms of protection might be of importance during different situations it is still uncertain which of these might be most critical, or if products of genes that have yet to be identified might fulfill a more significant protection. We have in this study chosen to study mouse strains to evaluate differences in pregnancy success depending on differences in MHC between mother and fetuses (allogeneic vs. syngeneic pregnancies by means of MHC). The two parental strains chosen (NFR/N and B10.Q) carry MHC alleles of the H-2q haplotype. The females of the genotyped N2 generation (backcross to B10.Q) have been allowed to mate both with B10.Q males (syngeneic pregnancy) and with B10.RIII males (allogeneic pregnancy). In this study we provide genetic marker information for the NFR/N strain and define genetic regions containing alleles that influence the reproductive performance in female mice.