Agriculture Reference
In-Depth Information
types (Sharma and Raina 2005) and transposable
elements (TEs), predominantly retrotransposons
(elements of Class I that transpose via RNA inter-
mediates) (Lipman et al., 2004). Three groups
of retroelements—Ty1-
copia
, Ty3
-gypsy
, and
LINE
—were found in large quantities in hetero-
chromatin of the diploid B/G-genome progenitor
Ae. speltoides
(Belyayev et al., 2001). Clusters of
tandem repeats and TEs form species-specifi c
and chromosome-specifi c heterochromatin pat-
terns. There is a certain correlation between
distribution-clustering of retroelements and
chromosome location of tribe-specifi c and species-
specifi c sequences within cereal genomes (Color
Plate 2). Thus, the independently discovered
tribe-specifi c tandem repeat
Spelt
52
(Anamtha-
wat-Jonsson and Heslop-Harrison 1993; Friebe
et al., 2000; Giorgi et al., 2003) and
Ae. speltoi-
des
-specifi c tandem repeat
Spelt 1
(Salina et al.,
1997; Pestsova et al., 1998) cluster together with
retroelements at the same chromosomal locations
corresponding to AT-enriched heterochromatin.
Moreover, this complex of distal-terminal chro-
mosomal regions enriched by TEs of different
types and tribe- and species-specifi c tandem
repeats could be classifi ed as a faster-evolving part
of the genome (Belyayev and Raskina 1998) (Color
Plate 2, note the green signal on the distal regions
of chromosome 4 after GISH).
Similar differences have been observed between
populations of
Ae. speltoides
involving a series of
distal-terminal chromosomal rearrangements
(Raskina et al., 2004a,b)
.
Multiple translocations
and deletions occurred, which led to the current
heterochromatin pattern (Color Plate 2e) and a
majority loss of
Ae. speltoides
-specifi c tandem
repeat
Spelt 1
clusters (A. Belyayev, pers. comm.).
Since new allopolyploids continue to occur in the
periphery distribution areas of existing species
(Grant 1981), we will continue to observe allo-
polyploidization involving the
Ae. speltoides
genome containing numerous chromosomal rear-
rangements and mobile elements (Raskina et al.,
2004a,b). The present-day B genome of wild and
cultivated wheat carries from zero to two
Spelt 1
clusters per haploid genome in contrast to the G
genome of existing allopolyploids, which contains
up to six
Spelt 1
clusters (Salina et al., 2006).
These data are in accordance with the purported
independent origin of the B and G genomes
of allopolyploid wheat (Jiang and Gill 1994b;
Rodríguez et al., 2000a).
Ongoing permanent intragenomic mutagenesis
in plant populations is a generator of heterozygos-
ity leading to intraspecifi c genetic variability and
creates the basis for natural selection under chang-
ing environments. Signifi cant inter-B/A-genomic
interactions, in allotetraploid wild emmer wheat,
revealed major substitutions of part of the A-
genome heterochromatin clusters by satellite
DNA from the B genome (Color Plate 2c) (Bely-
ayev et al., 2000). Enrichment of these clusters
with mobile Ty1
-copia
retroelements suggests an
important role of TEs in rebuilding and homog-
enizing the allopolyploid genome, leading to
stabilization of
T. dicoccoides
as a new species.
Retroelements are known to play a large role in
gene and genome evolution (Flavell et al., 1997;
Kidwell and Lisch 2001; Bennetzen 2002). In
T.
aestivum
, substitution of part of the heterochro-
matin from the “youngest” D genome by repeti-
tive DNA from the A and B genomes was revealed
to a far lesser degree (Color Plate 2d, 14 red D-
genome chromosomes marked by asterisk).
Transposable elements can directly change
molecular composition and/or DNA amount in
the regions of insertions. They also can mediate
Repetitive DNA
The repetitive DNA fraction plays an impor-
tant role during polyploidization and post-
polyploidization changes (Dvoˇák and Zhang
1990, 1992; Dvoˇák and Dubcovsky 1996; Feldman
and Levy 2005; Ma and Gustafson 2005, 2006). In
the genomes of allopolyploid wheat,
T. dicoccoides
(B and A genomes) and
T. aestivum
(B, A, and D
genomes), the distribution pattern of highly repet-
itive DNA clusters and Ty1
-copia
retroelements
differs from those of its diploid progenitors,
T.
urartu
and
Ae. speltoides
(Color Plate 2a-d) (Raskina
et al., 2002). Signifi cant intercalary repositioning
and decay of a majority of distal-terminal clusters
of AT-positive heterochromatin were observed in
the B genome of allopolyploid wheat in contrast to
the S genome of
Ae. speltoides
(Color Plate 2c).
