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Eleven of 14 (80%) middle repetitive sequences (class 10–100
copies) are also maize-specific. Two among the remaining
three showed only faint cross-hybridization to oat genomic
DNA. Highly repetitive sequences were preselected in this
study, and the process of selection showed that most highly
repetitive maize sequences are also maize-specific (Fig. 4 b–d).
DISCUSSION
Maize Species-Specific Sequences.
The cross between oat
and maize, two species from different subfamilies of the
Graminae, represents the widest combination of crop species
to date yielding stable, fertile partial hybrids (37). However,
little is known about the comparative structure and composi-
tion of the two genomes. Highly conserved sequences such as
rRNA genes or tubulin cDNAs cross-hybridize to correspond-
ing genes in the maize and oat genomes. Unique maize RFLP
probes also have been used to detect sequences in the oat
genome (9). A series of cDNA probes has shown 71% con-
servation of linkage associations between maize and oat
according to Van Deynze et al. (38); however, almost 50% of
maize PstI unique genomic probes do not hybridize to oat
genomic DNA under standard conditions (39).
We report herein that the proportion of maize and oat
nucleotide sequences that cross-hybridize to each other is low
under standard hybridization conditions (65
ЊC, 6ϫ SCC).
Among the set of low-copy-number sequences isolated and
tested, more than 50% appeared to be maize specific. The
proportion of maize-specific sequences among the middle or
highly repetitive maize elements was about 80–95% and
among those that do hybridize to oat, most hybridize only
weakly; therefore, even total genomic DNA of maize can serve
as an efficient probe to identify maize-specific cosmids in an
oat genomic background. But total maize DNA is less reliable
for this use than the described multiprobe because it gives
more false positive clones during primary screening of the
cosmid library. The oat–maize chromosome addition lines are
especially attractive as a source for isolation of region-specific
maize DNA fragments. It is possible to identify even small
portions of a maize chromosome in an oat genome by probing
with total maize genomic DNA, a multiprobe of maize re-
peated nucleotide sequences, or many other types of cloned
maize sequences. In the case of the highly conserved nucleo-
tide sequences, which are common in both maize and oat
genomes, RFLPs may be used to differentially identify maize
DNA in the oat genome.
Efficiency of Recovering Maize-Specific Cosmid Clones.
The efficiency of identifying maize-specific cosmid clones
depends on copy number and the pattern of distribution of
repeated sequences chosen to comprise the multiprobe. Stud-
ies of genome structure in maize (28, 40), especially analysis of
repeated DNA sequences around several maize genes (30, 31),
show that unique sequences with a median size of 2.1 kb are
surrounded by a long, complex array of repeated sequences
that belong to many different families, many of which appear
to be of retrotransposon origin (31).
Our multiprobe highlighted EcoRI subfragments in each of
the 29 cosmid clones of chromosome 9 analyzed, detecting
about 100 of the total of more than 200 fragments. Each clone
contained one or more of the repeated sequences present in
the multiprobe. The same group of EcoRI fragments were
classified as highly repetitive nucleotide sequences because
they showed strong hybridization to labeled total maize
genomic DNA. In the same set of clones at least 50 additional
EcoRI fragments showed a strong hybridization signal to
maize genomic DNA and may be classified as additional highly
repetitive nucleotide sequences. These additional sequences
could be included in our collection of maize-specific repeated
nucleotide sequences to increase the efficiency of the multi-
probe used in searching for maize-specific cosmids.
The number and diversity of repeats differ among clones.
Some of the cosmids consist entirely of several different
repeated sequences; others have only one copy of the tested
repeats. Regions of maize chromosomal DNA consisting only
of low-copy-number sequences with a composite length ex-
ceeding 40 kb, the size of the DNA insertion in most cosmids,
would not be detected in a cosmid library by screening with the
multiprobe.
Because the 29 analyzed clones of maize chromosome 9 were
selected with the help of the multiprobe, they may carry more
repeated DNA sequences than a random set of clones. There-
fore, a control experiment was conducted to analyze the
distribution of repetitive sequences in a set of 38 cosmid clones
randomly chosen from a cosmid library constructed from
DNA of the parental maize line Seneca 60. Only 2 of 38 maize
cosmid clones revealed cross-hybridization to oat genomic
DNA, and they were found to be composed of rDNA se-
quences. Thirty-four of the remaining 36 cosmid clones carried
maize-specific highly repetitive sequences detected by the
multiprobe (data not shown). This experiment indicates that
the probability of recovering maize-specific cosmid clones in a
genomic library of maize chromosome 9 with the help of the
primary multiprobe is around 95%.
Oat as a Host for Cloning Large Maize Chromosomal DNA
Fragments.
Impressive achievements have been made in clon-
ing large (
Ͼ1 Mb) DNA fragments in yeast artificial chromo-
somes (41). However, the difficulties encountered make this
method of cloning a eukaryotic genome expensive and prob-
lematic for wide use. Radiation hybrids (derivative cell lines
from irradiated somatic cell hybrids) are an attractive alter-
native for cloning subfragments of a chromosome. Irradiation
to produce chromosome breakage and segregation of genetic
material has been applied to a number of mammalian somatic
cell hybrids containing individual alien chromosomes (10). A
disadvantage of this approach in some cases is that a host
genome and an alien chromosome have too many in-common
nucleotide sequences, thus making difficult the direct analysis,
identification, and isolation of the alien chromosome frag-
ments.
The results of the research reported here allow us to propose
the use of oat as a host for cloning maize genomic DNA. The
substantial differences in the repetitive DNA composition of
oat and maize genomes and the high level of genetic stability
of the maize-addition chromosomes make it possible to apply
conventional molecular methods to the study of maize genetic
material directly in the oat genome. This approach would be
modeled after that used in mammalian systems (10). Radiation
would be used to induce translocations of maize segments into
the oat genome, from which maize DNA could subsequently be
isolated. In comparison with other possible maize-cloning
systems using phages, bacteria, or yeast, the advantage of this
approach is that all maize chromosome fragments will derive
from a known chromosome. The high proportion of maize
sequences that may serve as maize-specific DNA probes in this
system allows the identification of almost any maize chromo-
some segment in oat or any cloned maize DNA fragment in a
library. The current availability of an extensive collection of
mapped DNA markers for all maize chromosomes (42) may
allow us to identify the boundaries of cloned subfragments and
to generate a collection of overlapping subfragments repre-
senting the whole chromosome. This approach would allow the
cloning in oat of subfragments of maize chromosomes of a
wide size range. Chromosomal subfragments 5–30 Mb in size
may be considered optimal because they can be aligned relative
to the genetic map and, if cloned in bacterial artificial chro-
mosomes or even in cosmid vectors, may be arranged in contigs
to generate detailed physical maps for each subfragment.
Thus, 10–60 subchromosomal fragments may comprise a
collection of overlapping large DNA fragments of one partic-
ular maize chromosome. Taking into account the high stability
3528
Agricultural Sciences: Ananiev et al.
Proc. Natl. Acad. Sci. USA 94 (1997)
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