Vle CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. I

Genetic aspects of the reproductive performance in the mink (Lutreola
vison Brisson).

V.l. EVSIKOV, D.K. BELYAEV
Institute of Cytology and Genetics, Siberian Division of the USSR
Academy of Sciences, Novosibirsk, U.S.S.R.

The evolution of the reproductive function in mammals has resulted
in a rather stable hereditary basis, providing the maintenance of fertility
from generation to generation in strict, evolutionary-justified, and most
optimal limits. On the other hand, such evolution resulted in the for-
mation of systems of physiological regulation which provide the gptimal
fertility level in concrete environmental conditions. The Ranch mink is
a convenient subject for studying the genetic-physiological mechanisms
of fertility regulation. 20 mutant coat colour genes have been found
in this species which have a pleiotropic effect on the reproductive
function (1, 2, 3); the physiology of reproduction in mink has also
been well investigated (4, 5, 6). The role of separate components of
the reproductive function in the formation of optimal mink fertility
at the population level may be well shown when the relationships be-
tween the date of mating, gestation length (or more exactly, the length
of embryonic diapause), and litter size (see the table) are analyzed.

The shortening of the gestation length results in an increase of litter
size. At the same time there is an inverse correlation between the date
of mating and gestation length: the earlier a female is mated, the
longer is her gestation. Corresponding correlation coefficients range
from 0,5 to 0,8; no differences have been found in the value of this
correlation in minks of various genotypes. One might expect that early-
mated females (those which have a more prolonged gestation length)
would have smaller litters than those females which were mated later in
the breeding season. However, no such relationship has been noted.
This may be explained by the fact that by the end of the breeding season
the number of ovulating eggs is compensated by a better survival, and
females having a last mating at the end of the breeding season have, on
the average, a greater number of active corpora lutea, a fact which also
favours better embryonic survival (7). As for early-mated females, some
of them have short gestation length and as a result, heightened fertility.
In connection with this fact, it should be noted that the average fertility
of early-mated females increases, and approaches the average female
fertility of the rest of the population. Thus, fertility in the mink depends
on at least two mechanisms: the number of maturing eggs in the breeding
season and embryonic death, particularly preimplantation loss of blasto-
cysts. Close interaction of these mechanisms is one of the most signifi-
cant factors in the maintenance of optimal mink fertility at the popu-
lation level (fig. 1). The action of mechanisms supporting the optimal
number of progeny may also be observed in the postnatal period. It is

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