Orthologous adh regions of the sorghum and maize genomes were sequenced and
analyzed. Nine known or candidate genes, including adh1, were found in a 2
25-kilobase (kb) maize sequence. In a 78-kb space of sorghum, the nine homo
logues of the maize genes were identified in a colinear order, plus five ad
ditional genes. The major fraction of DNA in maize, occupying 166 kb (74%),
is represented by 22 long terminal repeat (LTR) retrotransposons. About 6%
, of the sequence belongs to 33 miniature inverted-repeat transposable elem
ents (MITEs), remnants of DNA transposons, 4 simple sequence repeats, and l
ow-copy-number DNAs of unknown origin. In contrast, no LTR retroelements we
re detected in the orthologous sorghum region. The unconserved sorghum DNA
is composed of 20 putative MITEs, transposon-like elements, 5 simple sequen
ce repeats, and low-copy-number DNAs of unknown origin. No MITEs were disco
vered in the 166 kb of DNA occupied by the maize LTR retrotransposons. In b
oth species, MITEs were found in the space between genes and inside introns
, indicating specific insertion and/or retention for these elements. Two ad
jacent sorghum genes, including one gene missing in maize, had colinear hom
ologues an Arabidopsis chromosome IV, suggesting two rearrangements in the
sorghum and three in the maize genome in comparison to a four-gene region o
f Arabidopsis. Hence, multiple small rearrangements may be present even in
largely colinear genomic regions. These studies revealed a much higher degr
ee of diversity at a microstructural level than predicted by genetic mappin
g studies for closely related grass species, as well as for comparisons of
monocots and dicots.