Agriculture Reference
In-Depth Information
Repetitive DNA and mobile elements as
perpetual generators of diversity
and evolution
rDNA block will be in the telomeric position as
has been detected in many wheat and
Aegilops
species (Mukai et al., 1990; Dubcovsky and
Dvoˇák 1995; Badaeva et al., 1996; Baum et al.,
2004). When chromosomes are broken, the break-
points become highly unstable and acquire the
ability to fuse with other broken ends (McClintock
1941). However, the breakpoints are eventually
stabilized, and the reconstructed chromosomes
are transmitted to the daughter cells. This
phenomenon, known as healing of breakpoints,
involves the addition of repetitive telomere
sequences at the breakpoints by telomerase, the
enzyme that normally synthesizes the telomere
sequence at normal chromosome terminals
(Tsujimoto et al., 1997). According to Tsujimoto
et al. (1999), rDNA sequences provide insight
into the properties of telomerase activity at the
breakpoints. The telomere sequences initiate two-
to four-nucleotide motifs in the original rDNA
sequence. These motifs are also found in the
repeat unit of telomere sequences. Thus, it has
been documented in many plant species that
rDNA in terminal positions could stimulate
de
novo
rapid synthesis of telomeres.
Therefore, we can emphasize that single Class
I or II TEs constantly form distinct clusters in or
around regular and irregular rDNA sites, and that
the presence of TEs in or around rDNA sites
increases the possibility of recombination and sat-
ellite loss. Apparently this event is common in
plant karyotype evolution, since in many plant
species rDNA clusters in terminal positions have
been detected.
Speciation in wild diploid and polyploid wheat is
tightly connected with signifi cant repatterning of
rDNA sites (Dubcovsky and Dvoˇák 1995). Con-
sidering rDNA in terms of temporary genome
changes, we face a certain paradox. On one hand,
rDNA is the most conservative fraction in the
eukaryotic genome, and ribosomal RNA genes
undergo minimal changes over hundreds of mil-
lions of years. On the other hand, this conserva-
tism appears to be a source of genome instability.
Due to the similarity of rDNAs, any chromosome
that carries extended rDNA arrays has the poten-
tial for involvement in heterologous synapses and
recombination (Raskina et al., 2004b). Thus, any
rDNA cluster could consist of several layers of
different origins, especially in a polyploid species,
with a high level of interhaplome invasion
(Belyayev et al
.
, 2000). This is clearly seen in
wheat (Color Plate 2f), where differently labeled
nontranscribed spacers (NTS) of 5S rRNA genes
of different origins (short A1 and long G1) show
slightly different positions inside the rDNA
cluster on chromosome 1A of
T. dicoccoides
(Baum
et al., 2004).
Due to the known capability of mobile ele-
ments to provoke ectopic exchanges, the conse-
quences of TE-rDNA interaction make it possible
to propose that the collocation of different TEs
within recombinogenic hot spots could intensify
homologous and heterologous recombination.
Moreover, TE-mediated intragenomic transfer of
rDNA fragments and the inheritance of such
mutations may cause signifi cant evolutionary
changes in chromosomal distribution of rDNA
clusters (Raskina et al., 2004a,b).
Another possible consequence of the physical
association of rDNA-TE within the 45S rDNA
region (NOR) could be the loss of chromosome
satellites with all their genetic content. McClintock
(1946) suggested that TEs could cause chromo-
somal breakages. High concentrations of TEs
around 45S rDNA increase the fragility of this
site (Color Plate 2c, chromosomes 1 and 6). In the
case of a satellite loss, the remainder of the 45S
THE POTENTIAL OF WILD EMMER IN
WHEAT IMPROVEMENT
Studies on wild emmer (
T. dicoccoides
), the pro-
genitor of most tetraploid and hexaploid wheats,
have revealed rich genetic resources applicable to
wheat improvement, given its diverse single- and
multilocus adaptations to stressful abiotic and
biotic environments (Xie and Nevo 2008). The
available resources have been described (Zohary
1970; Feldman 1979; Lange and Jochemsen 1992;
Grama et al., 1983; Nevo 1995, 2001; Nevo et al.,
