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KRISTAL AND ELEFTHERIOU
since in both rats and mice, the proportion of a group of nonpregnant females
manifesting placentophagia increased significantly with parturitional experience
(Kristal, 1973; Kristal and Williams, 1973). Kristal (1973) demonstrated that
previous parturitional experience, and not the pregnant condition was the key
factor in predicting the occurrence of placentophagia during aphagia produced
by lateral hypothalamic lesions.
Kristal and Williams (1973) investigated the influence of genotype
(strain) and of reproductive condition on placentophagia in nonpregnant mice
and found that strain differences contributed significantly to the attitude of
mice toward placenta under all three reproductive conditions tested (virgin,
primiparous without nursing, primiparous with nursing). Before investigating
the mechanisms and parameters of the acquisition of experience relevant to
placentophagia, the significance of placentophagia to maternal behavior, and
the shift in neural control of placentophagia with the acquisition of experi-
ence, it seemed necessary to further investigate the factors influencing the
base-level response to placenta (that manifested by virgin females). Since
strain-comparison studies provide only an indication of genetic differences, it
was decided to concretize the indication by conducting a genetic analysis of
the strain differences observed by Kristal and Williams in nonpregnant
nulliparous mice.
The subjects used in the present study were 16 virgin females of each of
two highly inbred strains (C57BL∕6By and BALB∕cBy), 16 virgin females of
each of the reciprocal Fj hybrids of these two strains (CB6Fι and B6CFι),
and 16 virgin females of each of seven recombinant-inbred (RI) strains derived
from the F2 generation of a cross between the C57BL∕6By and BALB∕cBy
inbred strains (CXBD, CXBE, CXBG, CXBH, CXBI, CXBJ, CXBK). The
Ri-strain battery, derived in this manner from a cross between two highly
inbred strains, and maintained independently on a schedule of strict inbreed-
ing (brother × sister), can be considered to be a replicable recombinant
population (Bailey, 1971). The strains of the Ri-strain battery comprise only
BALB∕c and C57BL∕6 genetic material, but the Ri-strains differ from one
another in the chance recombinations that have formed and which are fixed
during the inbreeding process as the strains approach full homozygosity.
Testing of all 11 strains of the battery (two progenitors, two F ɪ hybrids, and
seven Ri-strains) plus certain crosses and backcrosses yields a strain distri-
bution pattern from which information on the number of loci involved, the
existence of dominance, the direction of dominance, maternal postnatal
influences, and epistasis can be gleaned. If the strain distribution pattern of
the seven Ri-strains (the distribution of the progenitor alleles across those
seven lines) matches a cataloged strain distribution pattern for a known gene,
information regarding the locus of the gene or genes in question can be
obtained. Finally, as a cross check for locus, many lines are available that are
congenic for segments of chromosomes containing known genes whose strain