particularly reflected in the early postnatal kit mortality in mink females
with different fertility (fig. 2). The kits, being members of average,
optimal-size litters, are in more favourable conditions. It must be noted
that females which are heterozygous for recessive colour mutations have
more viable kits in large-size litters. It is very interesting to investigate
the mechanisms of the pleiotropic effect of certain mutations on fertility.
It was found that increase and decrease of actual fertility in most cases
was connected either with a change in the relative number of fertilized
eggs, or with embryonic death; potential fertility (number of maturing
eggs) does not actually change (7). The nuinber of ovulating eggs seems
to be the most strictly stabilized feature, setting evolutionary-justified
limits of potential fertility, Genetic stability of such a basic reprg-
ductive function as the number of ovulating eggs is expressed in low
heritability of fertility: in minks of different genotypes the heritability
coefficient (h2 ) ranges from 0, 1 to 0,3. The decrease of actual fer-
tility of many mutant minks, as compared with standard females, may be
explained by heightened embryonic death, which in turn, is a result of
prolonged embryonic diapause and the decreased number of heat periods
in the breeding season.which negatively affect the functions of the
corpora lutea. Thus, the effect of mutation on mink reproduction is
mediated by flexible physiological mechanisms which modify the mani-
festation of fertility in the limits of a strictly stabilized number of ovu-
lating eggs. This is possibly true for most mammals. Investigations of
the influence of mutations upon different components of reproductive
function help in understanding the relative role of certain physiological
processes in fertility formation at the population level. At the same
time these investigations permit the possibility for transformation of the
animal reproductive function. The most effective results in this respect
may be attained by working with the more flexible components of this
function. Even so, the possibilities of transformation of the reproductive
function are rather limited. Reorganization of the basic components of
the reproductive function (breeding season, number of ovulating eggs)
demands not only understanding of their genetic nature, but also know-
ledge of their set evolutionary connections with other properties of the
organism,

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