Abstract
Large regions of genomic colinearity have been demonstrated among
grass species by recombinational mapping, but the degree of chromosomal
conservation at the sub-centimorgan level has not been extensively
investigated. We cloned the rice and sorghum genes homologous to the
sh2
locus of maize on bacterial artificial chromosomes
(BACs), and observed that a homologue of the maize
a1
gene was also present on each of these BACs. In sorghum, we found a
direct duplication of
a1
homologues separated by about
10 kb. In maize,
sh2
and
a1
are
approximately 140 kb apart and transcribed in the same direction, with
sh2
upstream of
a1
. In rice and sorghum,
this arrangement is fully conserved. However, the
sh2
and
a1
homologues are separated by about 19 kb in both
rice and sorghum. We found low-copy-number and repetitive DNAs between
the
sh2
and
a1
homologues of sorghum and
rice. The
sh2
and
a1
homologues
cross-hybridized, but the repetitive DNA and most low-copy-number
sequences between these genes did not. These results indicate that
maize, sorghum, and rice have conserved gene order and composition in
the
sh2
-
a1
region, but have acquired
extensive qualitative and quantitative differences in the sequences
between these genes.