Resistance to Malaria in Hybrids between Swiss and Certain Other Strains of Mice

Abstract

Inbred strains of mice were found to fall into 3, or perhaps 4, categories with respect to the degree of infection of mature erythrocytes at the end of the first week after inoculation with Plasmodium berghei (Greenberg and Kendrick, 1957): (1) Swiss mice with a mean count of 47% (2) STR mice with a mean parasitemia of about 5% (3) several strains of mice, including C57B1, A, BALB and C3H, with mean counts of about 25% and possibly (4) DBA mice with mean counts of about 35% which may have been a high estimate due to insufficient sampling. F1 hybrids between the extremes (STR and Swiss) had counts averaging about 16%, lower than Swiss but not as low as STR (Greenberg and Kendrick, 1958). The F2 hybrids of this cross also had counts averaging about 16% and there was no good evidence of segregation and recombination of genes effecting parasitemia. It was felt that more information could be obtained by testing the response of hybrids between Swiss and strains of mice which fell into categories 3 and 4 above. It was also thought that further clarification might result from testing hybrids of at least one of the strains of category 3 or 4 with STR, and for this purpose C57B1 was selected. The use of these hybrids may also answer some questions about inheritance of resistance to malaria as reflected in survival. It has been observed (Greenberg, Nadel and Coatney, 1954; Greenberg and Kendrick, 1957) that 4 inbred strains of mice had nearly identical survival patterns: Swiss, C3H, A, and DBA. In all of these strains a large percentage of the mice died after infection with P. berghei in an early wave of death with a mode at about the seventh day. It also has been observed (Greenberg, Nadel and Coatney, 1953; Nadel, Greenberg, Jay and Coatney, 1955) that when mice with radically different survival patterns were crossed, the hybrids very often lived longer than either parent. This increased vigor could be attributed to a recombination of many genes favorable to survival, or heterosis could be taken to mean that the parent strains differed from each other in many genes somehow involved in resistance to malaria. If, then, phenotypically similar mice were hybridized and heterosis were observed in the offspring, it could be assumed that the parent mice were genetically different with respect to resistance to malaria. There was some indication from a small series (Greenberg, Nadel, and Coatney, 1954) that hybrids between DBA and A lived longer than either parent. The experiments to be described were designed to obtain data on the longevity of hybrids of mice, the parents of which were phenotypically similar with respect to survival with malaria.