Agriculture Reference
In-Depth Information
As has been observed in the early endosperm
development of wheat-rye hybrids, deletions
within the rye genome were clearly detected
(Gustafson and Bennett 1982; Bennett and Gus-
tafson 1982). Deletions and increases in DNA
have even been detected within heterochromatic
regions of the genus
Secale
(Gustafson et al.,
1983). Variations in DNA content have been
observed in polyploids of the Triticeae tribe
(Feldman et al., 1997; Ozkan et al., 2001; Kash-
kush et al., 2002; Liu and Wendel 2002; Han et
al., 2003; Ma et al., 2004; Ma and Gustafson 2005,
2006), in rice (Wang et al., 2005), in maize
(Messing et al., 2004), in a few
Hordeum
species
(Jakob et al., 2004), and in synthetic polyploids of
Brassica
(Song et al., 1995). In some induced di-
haploids of
Nicotiana
(Dhillon et al., 1983; Leitch
et al., 2008) and
Gossypium
species (Grover et al.,
2007), an increase in DNA has actually been
detected. For an excellent review of polyploids
showing a decrease in DNA over time, see Leitch
and Bennett (2004). Any genome cell cycle differ-
ences could easily be the major cause of genome
variation in DNA content between an allopoly-
ploid and its parental species.
combination of sequences from two genomes, but
still less is known about the interaction between
sequences from different arrays in chromatin
fractions (Wendel 2000). In this section we draw
attention to several critical points of speciation-
related chromosomal changes.
Chromosomal rearrangements and
repetitive DNA
Major structural chromosome rearrangements
including deletions, duplications, translocations,
and inversions are often associated with cytoge-
netically detectable heterochromatic regions com-
posed of repetitive DNA, and they frequently
appear in heterochromatin-euchromatin borders
(Badaeva et al., 2007). Chiasmata in meiosis
appear very close to the terminal and intercalary
C-bands and mark the point of exchange (Loidl
1979). Well-studied intraspecifi c C-banding poly-
morphisms can be regarded as a manifestation
of this interdependence. The diploid-polyploid
Aegilops
-
Triticum
complex exemplifi es abundant
C-banding polymorphism based on chromosomal
rearrangements (Badaeva et al., 1996, 1998, 2002,
2004, 2007; Friebe and Gill 1996; RodrÃguez
et al., 2000a,b; Maestra and Naranjo 1999, 2000).
A good example of this is where the combination
of C-banding techniques and fl uorescence
in situ
hybridization (FISH) with ribosomal RNA genes,
5S and 18S-5.8S-26S rDNA (45S rDNA),
and with a D-genome-specifi c repetitive DNA
sequence pAs1 revealed species-specifi c patterns
of heterochromatin, rDNA, and pAs1 clusters for
six D-genome-containing allopolyploid
Aegilops
species:
Ae. cylindrica
,
Ae. ventricosa
,
Ae. unia-
ristata
,
Ae. crassa
,
Ae. vavilovii
, and
Ae. juvenalis
(Badaeva et al., 2002). A wide spectrum of chro-
mosomal rearrangements, particularly species-
specifi c, and genome-specifi c redistribution of
repetitive DNA clusters led to hypothesizing the
phylogenetic relationships in this group of poly-
ploid
Aegilops
species.
MECHANISMS FOR CHROMOSOME
EVOLUTION
As stated previously, the evolution of the genus
Triticum
serves as a good model of polyploidy, one
of the most common forms of plant evolution
(Elder and Turner 1995; Soltis and Soltis 1999).
From a practical perspective, large stores of
simply inherited genes that confer different types
of resistance are available in wheat and its wild
relatives via germplasm collections. Knowledge
of the mechanisms of polyploidization will help
plant breeders to enrich the gene pool of culti-
vated wheat. The origin and co-evolution of A
and B genomes of tetraploid wheat has long been
controversial (Feldman and Sears 1981). Unknown
are the details of the co-evolution of A and B
genome repetitive sequence arrays in allotetra-
ploid wheat. There is no reason to regard the
process of allopolyploidization as a mechanical
Heterochromatin
An inherent feature of heterochromatin is the
complex composition of tandem repeats of various
