Agriculture Reference
In-Depth Information
ectopic chromosomal exchanges when homolo-
gous and/or nonhomologous chromosome recom-
bination moves sequences within and between
genomes. Furthermore, insertions of TEs may
create new crossing-over “hot” spots that provoke
transposable element-mediated homologous or
nonhomologous chromosome rearrangements.
The latter include spontaneous translocations,
inversions, and deletions and are potential mecha-
nisms for rapid genome reorganization during
speciation and stabilization of any allopolyploid.
For example, in the wheat 4AL-7BS transloca-
tion (Naranjo 1990), a cluster of Ty1-
copia
ret-
rotransposons was detected (Color Plate 2b), and
chromosomes 4A and 7B were also involved in a
4AL-7BL translocation, which was detected in a
natural population (Raskina et al., 2002).
The manipulation of repetitive DNA com-
plexes plays an important role in evolutionary
genome transformation. Changes in repetitive
DNA may cause chromosomal rearrangements
and, in turn, chromosomal rearrangements
may cause repetitive DNA change through
mechanisms of concerted evolution (Elder and
Turner 1995). Therefore, these processes are
interdependent.
(for review, see Heslop-Harrison 2000). There is
evidence that rDNA repeat sites may alter chro-
mosomal location without the involvement of
translocations or other chromosomal rearrange-
ments (Dubcovsky and Dvoˇák 1995). Schubert
and Wobus (1985) examined the mobility of
nucleolar organizing regions (NORs) in
Allium
and proposed TE activity as one of the possible
sources for rDNA movement. Recent studies
proposed that transposons (
En/Spm
-like; ele-
ments of Class II that move by extinction and
reintegration) might be involved in rDNA repat-
terning (Raskina et al., 2004a). The ability of
some classes of transposons (Pack-MULES,
Helitrons) to capture entire genes and move them
to different parts of the genome has been docu-
mented (Jiang et al., 2004; Lai et al., 2005).
Therefore, any interaction of ribosomal genes and
TEs relative to evolutionarily signifi cant chromo-
somal repatterning appears to be of tremendous
interest yet remains largely unexplored. Certain
remodeling of chromosome-specifi c repetitive
DNA patterns may lead to meiotic abnormalities.
In extremes, these abnormalities are capable
of causing reproductive (postzygotic) isolation
(Grant 1981).
A series of
in situ
hybridization (ISH) experi-
ments revealed permanent clustering of different
TEs in the NOR (which contains 45S rDNA loci)
as well as near or within clusters of 5S rDNA
(Belyayev et al
.
, 2001, 2005; Raskina et al
.
,
2004a,b). Therefore, we can suggest that the pos-
sible association of TEs and rDNA loci arise due
to, fi rst, the insertion preference of the TEs in the
rDNA arrays. Indeed, rDNA arrays are common
targets for several LINE retrotransposons (Eick-
bush and Eickbush 2003; Averbeck and Eickbush
2005) and also for some Class II transposons
(Penton and Crease 2004). Second, these two
components may accumulate preferentially within
the same genomic context, perhaps driven over
time by selection against insertions elsewhere in
the genome (e.g., heterochromatin in the case of
retroelements). Third, a possible functional rela-
tionship exists between the dispersion of TEs and
rDNA genes. Additional molecular-bioinformatic
studies may further explain TE-rDNA gene
interactions.
Repatterning of rDNA arrays in
the wheat genome
In addition to the direct detection of major chro-
mosomal rearrangements, it is also possible to
indirectly estimate the level of microevolutionary
genomic change by evaluating the repatterning of
well-determined chromosomal markers and by
the mobility of rDNA clusters. It is obvious that
speciation-related chromosome structure change
establishes further increases or decreases in the
number of rDNA sites or their repositioning, but
the dynamics of rDNA clusters may be regarded
as an indicator for signifi cant intragenomic pro-
cesses (Jiang and Gill 1994a; Dubcovsky and
Dvoˇák 1995; Raskina et al., 2004b).
The location, number, and mobility of rDNA
clusters have been described in many plant species
and may involve major loci, small numbers of
copies of the repeat unit, or fragments of a repeat
unit, which are often known not to be transcribed
