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KRISTAL AND ELEFTHERIOU
The progenitor strains BALB∕cBy and C57BL∕6By show an incidence of
ρlacentoρhagia of 25% and 0%, respectively. These values are in accord with
values found previously for the two strains (Kristal and Williams, 1973). The
difference between the progenitor strains in response to placenta has a
probability of occurring by chance of about .05 (P =0.0506), as determined
by the Fisher exact-probability test.
The absence of placentophagia in the F1 hybrids (CB6F1 and B6CF1)
indicates that if the progenitor strain difference is genetically determined,
C57BL∕6 alleles of the gene or genes of influence are dominant. In addition,
the absence of a difference between the reciprocal F1 hybrids indicates the
absence of an effect attributable to the genotype of the mother. Testing of a
backcross generation consisting of the offspring of B6CF1 females backcrossed
to BALB∕cBy males exhibited an incidence of placentophagia of 39.5%
(19/48).
Of the seven strains constituting the RI-strain battery, placentophagia
was seen in only three: CXBD, CXBG, and CXBK (see Table 1). The inci-
dence of placentop agia in the CXBD and CXBK strains was lower than
that of the BALB∕c progenitor strain, but 43.7% of the females of the CXBG
strain (7/16) manifested placentophagia. To obtain information about the
result of mating the CXBG strain (high incidence of placentophagia) with a
strain which did not manifest placentophagia, the CXBH strain, female
offspring of a cross between CXBH females and CXBG males were back-
crossed to CXBG males. The incidence of placentophagia in this back-
cross was 62.9% (see Table 1). The strain distribution pattern did not
match any of the cataloged strain distribution patterns for known loci across
this same Ri-strain battery. The inability to match strain distribution patterns
precluded verification of effective loci by the use of the congenic lines (Klein,
1973). It should be noted that one congenic line was tested. The B6.C√∕-25c
congenic line (C57BL∕6 mice with a substituted piece of Chromosome 9
containing the BALB∕c allele of the H-25 histocompatability locus) bears a
strain distribution pattern almost identical to that produced by testing the
Ri-strain battery for placentophagia. The difference lies in the fact that the
score for the CXBG Ri-strain is considerably greater than that of the
BALB∕c progenitor. To be identical to the distribution of alleles found in the
B6.C-7∕-25c congenic line the CXBG Ri-strain would have to have manifested
an incidence of placentophagia not different from the BALB∕c progenitor. The
similarity between the strain distribution pattern of B6.C-7∕-25c and that of
the Ri-strain battery in the present study seemed great enough to warrant
testing of B6.C-∕∕-25c, despite the fact that negative results were expected.
Seven virgin B6.C√∕-25c females (obtained from the colony of D.W. Bailey)
were tested for placentophagia; none manifested the behavior (see Table 1).