ISSN 1022-7954, Russian Journal of Genetics, 2008, Vol. 44, No. 7, pp. 786–792. © Pleiades Publishing, Inc., 2008.
Original Russian Text © Vl.A. Brykov, A.D. Kukhlevsky, E.A. Shevlyakov, N.M. Kinas, L.O. Zavarina, 2008, published in Genetika, 2008, Vol. 44, No. 7, pp. 906–913.
The sex ratio is an important parameter largely
determining adaptation, as well as the direction and rate
of genetic processes in animal populations . The sex
ratio in ﬁsh populations is usually close to 1 : 1; how-
ever, it is sometimes shifted towards predominance of
one or the other sex [2–4]. Changes in the sex ratio, if
they occur in anthropogenically unaffected natural pop-
ulations, may be determined by different population
parameters and external factors.
The pink salmon (
chum salmon (
) are the most abundant species of
Paciﬁc salmons at the Asian coast . Both species are
characterized by considerable annual variations of the
population size, whose cause is not always clear. We
analyzed data on the effect of the population size on the
sex ratio in pink salmon and chum salmon populations
accumulated for a period of many years. It has been
demonstrated that the sex ratio in populations of both
species varies; this parameter is largely determined by
the population size and is essentially adaptive. The
mechanisms underlying the interdependence between
the population size and sex ratio in salmon populations
remain poorly understood. Sex inversion is a possible
mechanism of changes in the sex ratio in populations. It
has been found in many species of ﬁsh, including
Paciﬁc salmons. The existence of this mechanism is
conﬁrmed by the discrepancy between the sex identi-
ﬁed by molecular markers and by morphological char-
acters of ﬁsh in populations of both species. In addition,
our data on the relationship between the proportion of
sex-inversed ﬁsh in populations and the size of the next
generation also indicate the importance of sex inversion
for the control of population size.
MATERIALS AND METHODS
Determination of population density at spawning
grounds and the sex of fry.
The ﬁsh population density
at spawning grounds was determined from the area of
spawning grounds in a speciﬁc river (Ostroumov,
archives of the Kamchatka Research Institute of Fish
Farming and Oceanography) and the numbers of ﬁsh
spawning there according to the results of annual aerial
surveys (archives of the Kamchatka Research Institute
of Fish Farming and Oceanography).
The sex of fries in samples of more than 100 indi-
viduals each was determined visually under a micro-
). Females had yellowish orange ribbonlike
ovaries 0.4–0.6 mm in length, where developing
oocytes varying in diameter from 38 to 96
m are seen.
In males, testes had a form of whitish threadlike cords.
Molecular identiﬁcation of the sex.
Total DNA was
isolated from the heart tissue of mature ﬁsh by the stan-
dard method . Polymerase chain reaction (PCR) was
Sex Ratio Control in Pink Salmon (
and Chum Salmon (
) Populations: The Possible Causes
and Mechanisms of Changes in the Sex Ratio
Vl. A. Brykov
, A. D. Kukhlevsky
, E. A. Shevlyakov
, N. M. Kinas
, and L. O. Zavarina
Zhirmunsky Institute of Marine Biology, Far East Division, Russian Academy of Sciences, Vladivostok, 690041 Russia;
Kamchatka Research Institute of Fisheries and Oceanography, Petropavlovsk-Kamchatskii, 683000 Russia
Academy of Ecology, Marine Biology, and Biotechnology, Far Eastern State University, Vladivostok, 690000 Russia
Received May 3, 2007
—Long-term changes in the sex ratio have been studied in pink salmon (
chum salmon (
) populations of Kamchatka and Sakhalin. It has been demonstrated that these changes
are an adaptation to population dynamics: an increase in the population size is accompanied by a shift towards
a higher proportion of males; a decrease in population size, by a shift towards a higher proportion of females.
The correspondence between morphological and molecular characters in populations of the two species has
been analyzed in order to determine the mechanism of sex ratio control. In some pink salmon and chum salmon
populations, there is a discrepancy between sex identiﬁcations based on morphological characters and mole-
cular markers. This discrepancy is assumed to be accounted for by sex inversion mechanisms, which may be
population- or region-speciﬁc. In two cases, it has been found that the sex ratio discrepancy in populations is
related to the numbers of ﬁsh in subsequent generations. These ﬁndings suggest that sex inversion may be
related to population size control.