ISSN 10227954, Russian Journal of Genetics, 2011, Vol. 47, No. 7, pp. 864–873. © Pleiades Publishing, Inc., 2011.
Original Russian Text © A.S. Voronov, D.V. Shibalev, N.S. Kupriyanova, 2011, published in Genetika, 2011, Vol. 47, No. 7, pp. 975–985.
At present, four reptile orders are recognized,
including Sphenodontia (two species), Crocodilia
(23 species), Testudines (about 300 species), and the
largest order, Squamata, or scaled reptiles (about 800
species). In classical phylogeny, the tree is rooted at tur
tles . Phylogenetic analysis of amino acid sequences
of mitochondrial (mt) proteins pointed to the existence
of the archosaur (birds and crocodiles) and the lepido
saur (lizards and snakes) clades, as postulated by mor
phologists. Phylogenetic analysis suggests that turtles
are a sister group to archosaurs [2–4]. These results
change the classical concept that turtles represent the
only surviving group of primary anapsid reptiles and
point to probable secondary skull fenestration .
The main scaled reptile lineages display a great variety
of morphologic, behavioral, and ecological forms.
According to morphological classification by Estes et al.
, iguanians split from the other scaled reptiles (Sclero
glossa) in the late Triassic. However, molecular phylog
eny does not support such early separation [6–11].
Snakes, according to morphological classification,
occupy a place close to other limbless taxa, specifically,
dibamids and amphisbaenians. In some classifications,
snakes are treated as a sister taxon of varanids. Snakes are
even placed in the clade Anguimorpha, thereby denying
their existence as an independent clade among the Squa
mata [10–12]. More recently, it was suggested that Ser
pentes, Anguimorpha, and Iguania group into the single
clade Toxicofera, characterized by the presence of
venomsecreting glands in its members [13–15]. How
ever, such glands were identified in some acrodonts,
albeit not all Iguanidae possess them [7, 9, 14–16].
Analysis of the evolutionary relationships between the
infraorders of Squamata based the mitochondrial and
nuclear sequence comparisons, produced contradictory
results. In some cases, snakes were treated as a sister
group to all remaining Squamata [15–17], in the other
cases, as a sister group to amphisbaenians and
lizards [3, 17]. None of these concepts can be statistically
rejected . It should be noted that amphisbaenians
possess many unique characters, distinguishing them
from other reptiles. For instance, their right lung is
reduced, corresponding to the narrowness of their bod
ies, while in snakes usually the left lung is reduced. The
skeleton structure and skin type in amphisbaenians also
have some specific features, compared to other Squa
mata . These contradictory data indicate that the
markers used are not always sufficiently informative. It
has been already established that the evolutionary rates of
different genome regions are different . Solving of this
problem requires identification of the most robust and
informative DNA region.
Ribosomal DNA is a good candidate for this role. In
eukaryotes, ribosomal RNA (rRNA) is transcribed in
nucleolus as a single precursor, where the sequences of
mature rRNA are separated by internal transcribed spac
ers, ITS1 and ITS2, which are removed in the course of
processing. Yeast experiments pointed to the important
role of ITS2 for correct prerRNA processing [20, 21].
ITS2 contains at least three processing sites, providing
precise formation of the mature ends of 5.8S and 28S
rRNAs, which are critical for any living cell. The ITS2
sequence can be the marker of taxon evolution. Although
Evolutionary Relationships between Reptiles Inferred
from the Comparison of Their ITS2 Sequences
A. S. Voronov, D. V. Shibalev, and N. S. Kupriyanova
Institute of Gene Biology, Russian Academy of Sciences, Moscow, 119334 Russia;
Received December 3, 2010
—The reptile phylogeny is poorly studied, and many existing hypotheses are controversial. In this study,
the ITS2 regions of 43 species of lizards, snakes, turtles, and crocodiles were cloned and sequenced in addition to
eight ITS2 sequences of amphibians, reptiles, and birds already present in the database. The ITS2 of reptiles, sim
ilarly to other vertebrates, contain short conserved (consensus) regions, alternating with variable regions (DI, DII,
and DIII), which are potentially capable of forming stable secondary structures. These functionally neutral rDNA
regions, separating the consensus regions, are substantially different in size, as well as in the primary and secondary
structure. Sequences of the ITS2 variable regions were aligned using the GeneBee Molecular Biology Server soft
ware program with subsequent automated construction of prescribed trees. The trees for all three variable regions
were highly similar, enabling certain conclusions on the evolutionary history of reptiles.