Basic HTML Version
Triticeae Genomics and Genetics 2012, Vol.3, No.3, 25
-
37
http://tgg.sophiapublisher.com
31
seems that the intermediate ancestral grass genomes
with
n
= 12 had undergone chromosome reduction
(also known as dysploidy) giving
n
= 10 in sorghum
(Luo et al., 2009) and maize (Salse et al., 2008);
n
= 9
in finger millet (Srinivasachary et al., 2007);
n
= 7 in
Triticeae (Luo et al., 2009; Hackauf et al., 2009) and
n
= 5 in brachypodium (Qi et al., 2010; International
Brachypodium
Initiative, 2010).
In addition to the reduction in chromosome numbers,
perhaps structural differences also occur between
wheat and brachypodium genomes. Consequently,
with a view to decipher the structural similarities and
differences between the chromosomes of wheat and
brachypodium, we examined the findings of present
study in relation to the 12 chromosomes of the
intermediate ancestral genome reported earlier (Salse
et al., 2008; International
Brachypodium
Initiative,
2010). In this connection, it may be noted that the 12
chromosomes of the intermediate ancestral genome
underwent breakage, translocation and fusion events
and reassembled into seven chromosomes of wheat
and five chromosomes of brachypodium (Figure 3).
As a result, two of the wheat consensus chromosomes,
namely WC3 and WC6 were directly evolved from the
intermediate ancestral chromosomes A1 and A2, while
the remaining five wheat consensus chromosomes
(WC1, WC4, WC5, WC2 and WC7) were derived
following fusion of two ancestral chromosomes each.
Reduction (in the course of dysploidy) in chromosome
numbers i.e.
n
= 7 in wheat, centromeres of four
chromosomes of the intermediate ancestral genome
(A5, A3, A4 and A6) were lost and the centromeres of
intermediate ancestral chromosomes A9 and A12
perhaps combined into giving rise the centromere of
wheat consensus chromosome WC5 (Qi et al., 2010;
International
Brachypodium
Initiative, 2010).
Similarly in brachypodium, only one brachypodium
chromosome (Bd5) evolved directly from the inter-
mediate ancestral chromosome A4. The remaining
four brachypodium chromosomes evolved from two
or three intermediate ancestral chromosomes. For
example, three brachypodium chromosomes (Bd1,
Bd4 and Bd3) evolved from three intermediate
ancestral chromosomes each and the remaining one
chromosome Bd2 was derived from two ancestral
chromosomes (Figure 3). During the above process of
dysploidy leading to the evolution of five brachypo-
dium chromosomes from the 12 chromosomes of
intermediate ancestral genome, the centromeres of
five chromosomes (A1, A4, A6, A8 and A12) were
Figure 3 Evolution of the wheat (
n
= 7) and brachypodium (
n
= 5) chromosomes from a common intermediate ancestral genome with
n
= 12
Triticeae Genomics and Genetics Provisional Publishing