Agriculture Reference
In-Depth Information
Drosophila
that gene structure and expression
were infl uenced by the location of genes proximal
to heterochromatin and were evolving at a rate in
response to their chromosomal location.
No fully satisfactory explanation has been sug-
gested for why these major evolutionary genome
modifi cation, deletion, and addition processes take
place mainly in the noncoded portion of the genome.
Wicker et al. (2003) and Gaut et al. (2007) have
made a strong case for illegitimate recombination
having a major infl uence on genome evolution. Ille-
gitimate recombination is capable of generating
deletions, inversions, gene conversions, and dupli-
cations within any chromosome of any genome.
However, it is diffi cult to envision illegitimate
recombination as the main cause for such a sizeable
DNA deletion, of up to about 10% or more in the
genomes of many allopolyploid cereals. It is likely
that no single explanation will answer the question
of why the cereal genomes vary so much in size. It
will most certainly require a number of working
hypotheses and a large body of new evidence and
knowledge bearing on the problems associated with
the evolution of genome size in grasses to resolve
this question. See a recent review on synteny and
colinearity in plant genomes by Tang et al. (2008).
We can propose one possible cause for many of
the observed vast changes in grass genome com-
position. Clearly every grass genome goes through
its cell cycle at a specifi c rate, which varies with
each genome. Van't Hof and Sparrow (1963) fi rst
proposed the existence of a relationship between
DNA content, nuclear volume, and mitotic cell
cycle, and suggested that any mitotic cell cycle is
greatly infl uenced by the amount of DNA present
in the genome. They made it clear that the amount
of DNA present in a genome does affect cell cycle,
and ultimately plant growth, regardless of whether
or not it was coded. Recently, Francis et al. (2008)
concluded that the speed of DNA replication was
identifi ed as the limiting factor in the cell cycle.
Therefore, it follows that individual genome cell
cycle differences cause problems of maintaining
their synchrony when two or more genomes, with
different volumes of DNA, are placed together in
a cell.
For example, in the wheat-rye hybrid triticale
(×
Triticosecale
Wittmack), Bennett and Kaltsikes
(1973) showed that the meiotic duration of wheat
and rye differed from that observed in the hybrid,
and the hybrid had a meiotic cell cycle closer to
the wheat parent. Their observations made it
clear that if one genome of a hybrid has not com-
pleted its cell cycle by the time cell wall formation
has initiated, the possibility of breakage-fusion-
bridges occurring in the genome with the lagging
cell cycle will be greatly increased, most likely
resulting in DNA elimination or addition. This is
what happens in a wheat-rye hybrid and can be
readily seen in the formation of large aberrant
nuclei that are readily visible in the early ceno-
cytic stages of endosperm development before
cellularization takes place (Fig. 1.2). The forma-
tion of cell walls at the fi rst division of the embryo
would defi nitely cause breakage-fusion-bridges to
occur immediately and lead to the decrease—or
even increase—of DNA present in the genome
with the lagging cell cycle.
(a)
(b)
(c)
(d)
Fig. 1.2
(a) A wheat-rye hybrid (triticale) seed only 48 hours after pollination with a cenocytic endosperm and a cellular
embryo (arrow); (b) a nuclear division (24 hours after pollination) showing bridges that have formed during anaphase; (c)
nuclear divisions (48 hours after pollination) showing rye telomeres that have formed bridges during anaphase; and (d) nuclear
divisions (72 hours after pollination) showing rye telomeres that have formed bridges during anaphase.
