ISSN 1022-7954, Russian Journal of Genetics, 2009, Vol. 45, No. 1, pp. 23–29. © Pleiades Publishing, Inc., 2009.
Original Russian Text © L.N. Nefedova, A.I. Kim, 2009, published in Genetika, 2009, Vol. 45, No. 1, pp. 30–37.
Mobile genetic elements (MGEs) constitute a major
part of repetitive DNA in genomes of eukaryotes .
Much attention has been recently given to retroele-
ments (REs) that are likely to be ancestors of retrovi-
ruses . Their main structural component is the
reverse transcriptase gene responsible for transposition
of MGEs of this group via reverse transcription. Retro-
elements are widespread in genomes of eukaryotes and
are also found in prokaryotes .
Retrotransposons of the
occupy an important place among
eukaryotic REs. These retrotransposons have long ter-
minal repeats (LTRs) and three open reading frames
(ORFs). ORF1 (
) of retrotransposons encodes
capsid proteins, ORF2 (
) encodes protease, reverse
transcriptase, RNase H, and integrase. ORF3 (
encodes a product responsible for recognition of cell
receptors and for penetration of a virus in a cell .
Thus, retrotransposons of the
group are structur-
ally identical to the forms of vertebrate endogenous ret-
roviruses (proviruses) integrated in the genome. It is
not accidentally that retrotransposons of this group of
[4, 5] and then
, were found to have infectious properties.
retrotransposons are the
ﬁrst true retroviruses described not only in
but also in insects and in invertebrates in general. They
were named errantiviruses (endogenous retroviruses of
insects) and included under this name in the interna-
tional classiﬁcation of viruses .
The origin of retrotransposons as well as retrovi-
ruses remained an open question for a long time.
Recently, a possibility has appeared to study their
molecular evolution. Based on structural similarity and,
hence, on homology of individual genetic structural
components of retrotransposons and retroviruses, it
might be theoretically assumed that the evolutionary
process can proceed in two directions: transformation
of retrotransposons into retroviruses and formation of
retrotransposons on the basis of retroviruses.
To elucidate the situation, it is necessary to examine
in detail the structural features of both retroviruses and
retrotransposons, with special emphasis on the search
for intermediate forms (intermediate evolutionary ele-
ments). In our discussion we shall restrict ourselves to
two RE types: retrotransposons and retroviruses of the
as a typical
insect/invertebrate and the best genetically studied
object, especially as it is so far the only known repre-
sentative of invertebrates with retroviruses.
The studies performed permitted two genes,
, to be identiﬁed in the genome of
that are homologous to genes of ret-
rotransposons of the
group. A possible function
of these genes and their role in the evolution of retrovi-
ruses are discussed.
MATERIALS AND METHODS
The genome sequence of
is deposited in the databases Berkeley Droso-
phila Genome Project (BDGP) (http://www.fruit-
Molecular Evolution of Mobile Elements of the
A Homolog of the
L. N. Nefedova and A. I. Kim
Moscow State University, Department of Genetics, Moscow, 119992 Russia;
Received December 27, 2007
—Retrotransposons of the
that are structurally similar to ret-
roviruses of vertebrates occupy an important place among retroelements of eukaryotes. The infectious abilities
of some retrotransposons of this group (
) have been demonstrated experimentally, and
therefore they are true retroviruses. It is supposed that retrotransposons can evolve acquiring new components,
the sources of which remain to be elucidated. In this work, the
of retrotransposons of the
group has been identiﬁed in the genome of
gene product has a highly conserved structure in different species of the Drosophilidae
family and is under of purifying selection, which suggests its important genomic function in
view of the earlier data, it can be concluded that homologous genes of all components of
are present in the
genome. These genes can be both precursors and products of domestication of