ISSN 10227954, Russian Journal of Genetics, 2015, Vol. 51, No. 3, pp. 231–237. © Pleiades Publishing, Inc., 2015.
Original Russian Text © A.V. Trukhina, N.A. Lukina, A.A. Nekrasova, A.F. Smirnov, 2015, published in Genetika, 2015, Vol. 51, No. 3, pp. 290–297.
Several models of primary sex determination were
suggested for Vertebrates at present. The most recog
nized is a Genetic Sex Determination (GSD) model,
which arises before or after fertilization. It is based on
the presence of sex chromosomes. In animals possess
ing male heterogametes (XX/XY), this trait is deter
mined after fertilization, while it is determined before
fertilization in animals with female heterogametes
(ZW/ZZ). Cutting et al.  presented the suggested
schemes of genetic sex determination in some animals
and systematic groups. However, numerous genes, the
majority of which have not been revealed at present
and were not included into the proposed scheme,
determine the sex. In addition, the interaction circuits
of the products of the identified genes associated with
sex are not clear.
Organisms with Environmental Sex Determina
tion (ESD) do not obtain gender immediately after
fertilization; it is induced during ontogenesis in
response to abiotic (temperature, pH, salinity) and
biotic (population density, pathogens, hormones) fac
tors. The best analysed is temperaturedepended sex
determination (TSD). It is realized during the ther
mosensitive stage of embryogenesis or at the beginning
of gonadogenesis. The understanding of TSD was
obtained from studies of such animals as crocodiles,
lizards, and turtles .
In some species, GSD is not fixed for the life span.
Their primary gender can alter during development
without genotype alteration. This phenomenon was
called ESR (Environmental Sex Reversal) and was
observed in insects, fish, amphibians, and reptiles. For
some fish species, sex determination occurs through
the developmental stage (posthatching) and belongs
to the socalled GSD–TE (GSD plus Temperature
Effects) system . A polygenic sex determination
(PSD), during which the gender decision is assumed
according to the allelic combination of several loci,
was also described. Such sex determination is charac
. This process is reminiscent of a
Complementary Sex Determination (CSD): a single
described in some insects.
Numerous loci are usually dispersed along the whole
genome, but they are located on special sex chromo
somes in some species of bony fishes .
Strictly speaking, two different mechanisms of sex
determination were revealed: genetic, which is based
on internal signals, and environmental, which has
environmental switching mechanisms. In addition, a
number of intermediate systems were observed. In
such systematic groups as turtles, TSD–GSD trans
mission occurs recurrently .
The most intriguing are external and internal con
trol factors and factors in the accurate launching of
various systems of sex determination. Particularly
interesting is the epigenetic control of this process :
DNA and histone methylation and the involvement of
noncoding RNAs for activation and inactivation of
sexdetermination genes .
SEX IN FISH
Fish represent the most complex animal group in
relation to mechanisms of sex determination. Pres
ently, 30 thousand species of bony fishes are known,
and this is the biggest group of vertebrates. According
to regularities of sex determination, they are divided
into three groups: gonochoristic species, in which sex
is determined genetically or by environmental factors;
sequential hermaphrodites (about 2% of all the exist
ing species), which exchange the male sex for female
(protandrous) or female sex for male (protogynous),
or in both directions (serial), in the course of ontoge
nesis; and synchronous hermaphrodites, which simul
taneously develop gonads of both sexes.
Genetic gender control in gonochoristic fish is
largely unknown, while functional hermaphroditism
Sex Inversion and Epigenetic Regulation in Vertebrates
A. V. Trukhina, N. A. Lukina, A. A. Nekrasova, and A. F. Smirnov
Department of Genetics and Biotechnology, St.Petersburg State University, St. Petersburg–Old Peterhof, 199034 Russia
email: email@example.com, firstname.lastname@example.org
Received July 7, 2014; in final form, September 9, 2014
—This review discusses issues related to the regulation of sex determination and differentiation in
various groups of Vertebrates. Special attention was paid to factors of external and internal control for various
genetic systems of sex determination, as well as to the epigenetic control of this process. Opportunities for sex
inversion in various animals were also discussed.
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