Ronald L. Phillips
981
Dissecting the maize genome by using chromosome
addition and radiation hybrid lines
Ralf G. Kynast*, Ron J. Okagaki*, Mark W. Galatowitsch*, Shannon R. Granath*, Morrison S. Jacobs*, Adrian O. Stec*,
Howard W. Rines*
†
, and Ronald L. Phillips*
‡
*Department of Agronomy and Plant Genetics and Center for Microbial and Plant Genomics, University of Minnesota, 1991 Upper Buford Circle, St. Paul,
MN 55108; and
†
Plant Science Research Unit, Department of Agriculture–Agricultural Research Service, St. Paul, MN 55108
Contributed by Ronald L. Phillips, May 13, 2004
We have developed from crosses of oat (Avena sativa L.) and maize
(Zea mays L.) 50 fertile lines that are disomic additions of individual
maize chromosomes 1–9 and chromosome 10 as a short-arm
telosome. The whole chromosome 10 addition is available only in
haploid oat background. Most of the maize chromosome disomic
addition lines have regular transmission; however, chromosome 5
showed diminished paternal transmission, and chromosome 10 is
transmitted to offspring only as a short-arm telosome. To further
dissect the maize genome, we irradiated monosomic additions
with
␥ rays and recovered radiation hybrid (RH) lines providing
low- to medium-resolution mapping for most of the maize chro-
mosomes. For maize chromosome 1, mapping 45 simple-sequence
repeat markers delineated 10 groups of RH plants reflecting dif-
ferent chromosome breaks. The present chromosome 1 RH panel
dissects this chromosome into eight physical segments defined by
the 10 groups of RH lines. Genomic in situ hybridization revealed
the physical size of a distal region, which is represented by six of
the eight physical segments, as being
Ϸ20% of the length of the
short arm, representing
Ϸone-third of the genetic chromosome 1
map. The distal
Ϸ20% of the physical length of the long arm of
maize chromosome 1 is represented by a single group of RH lines
that spans >23% of the total genetic map. These oat–maize RH
lines provide valuable tools for physical mapping of the complex
highly duplicated maize genome and for unique studies of inter-
specific gene interactions.
P
lants with one chromosome (monosomic) or one pair of
homologous chromosomes (disomic) of an alien donor spe-
cies added to the entire recipient species chromosome comple-
ment serve to dissect the donor genome into individual chro-
mosome entities and separate them from their own genome
remnant. The transfer liberates the added chromosome (pair)
from the interactive gene expression network of the donor
genome and puts the chromosome’s genes into the environment
of the host genome. This new structural and functional situation
can create novel orthologous and nonhomologous gene-to-gene
interactions and, hence, helps to answer fundamental questions
about gene expression control, inheritance, and syntenic corre-
spondence among different plant species, especially those with
large genomes, including maize, with a 1C content of
Ϸ2.7 billion
base pairs [Plant DNA C-Values Database (Release 2.0, January
2003), M. D. Bennett and I. J. Leitch, http:
͞͞rbgkew.org.uk͞
cval
͞homepage.html] and a subgenome structure reflecting an-
cient tetraploidy (1).
By crossing maize to oat, (oat
ϫ maize)F
1
proembryos were
generated, of which 5–10% could be rescued
in vitro. Molecular
and cytological analyses showed retention of one or more maize
chromosomes in addition to the haploid oat genome in 34% of
the F
1
plants (2–7). Because haploid oat frequently develops
unreduced gametes (8), subsequent self-fertilization of (oat
ϫ
maize)F
1
plants with one maize chromosome added to the
haploid oat genome (n
ϭ 3x ϩ 1 ϭ 22) can produce F
2
offspring
with one homologous maize chromosome pair added to the
doubled haploid (hexaploid) oat genome (2n
ϭ 6x ϩ 2 ϭ 44)
among other euploid and aneuploid types (9).
A complete series of oat–maize chromosome addition lines
(10) has enabled markers and genes to be physically allocated to
maize chromosomes without a need for detectable polymor-
phisms. These unique plant materials confirmed interchromo-
somal duplicate loci on a large scale, one of the obstacles to
whole-genome sequencing. Numerous locus duplications com-
plicate the reassembly of a set of shotgun DNA sequences.
Oat–maize chromosome addition lines, however, physically sep-
arate these interchromosomal maize orthologs and paralogs
from each other and make them accessible to mapping, sequenc-
ing, and cloning, even in cases where the duplicated loci of
interest carry genes with monomorphic allelic sequences.
A second obstacle that impedes sequencing of the complete
maize genome by today’s technology is the repetitive nature of
Ϸ85% of the maize DNA. Thus, sequencing strategies are being
tested that accomplish the targeted sequencing of less repetitive
gene-rich regions (11, 12). These strategies must involve tech-
nologies that are capable of arranging those gene islands along
the chromosomes and bridging long gaps between contigs.
Generating random breaks in the maize chromosome in an
identified monosomic oat–maize chromosome addition line and
maintaining diminutive maize chromosomes or pieces translo-
cated into oat can provide DNA panels of radiation hybrid (RH)
lines, which allow for a presence vs. absence test of markers
without the need for polymorphisms (13). With sufficient reso-
lution that is determined by the number and distribution of
breaks along the maize chromosomes, RH lines can contribute
to placing contigs in the correct order. A panel of RH lines for
maize chromosome 9 has demonstrated the efficient mapping of
molecular markers (14).
This report summarizes the status for the oat–maize chromo-
some addition line production and characterization, including
the irregular transmission behavior of maize chromosomes 5 and
10 in oat, the last two maize chromosomes recovered as fertile
oat–maize addition lines. We also illustrate the development of
oat–maize RH lines from addition plants for maize chromosome
1. We show the use of these RH lines for physical mapping and
relating genetic map distances to physical chromosome segment
sizes.
Materials and Methods
Plant Material.
Plants of oat (Avena sativa L.) cultivars GAF-Park,
Kanota, Preakness, Starter, Stout, Sun II, and the MN-hybrid
(MN97201-1
ϫ MN841801-1) were grown and crossed by maize
This report was presented at the International Congress ‘‘In the Wake of the Double Helix:
From the Green Revolution to the Gene Revolution,’’ held May 27–31, 2003, at the
University of Bologna, Bologna, Italy. The scientific organizers were Roberto Tuberosa,
University of Bologna, Bologna, Italy; Ronald L. Phillips, University of Minnesota, St. Paul,
MN; and Mike Gale, John Innes Center, Norwich, United Kingdom. The Congress web site
(www.doublehelix.too.it) reports the list of sponsors and the abstracts.
Abbreviations: BC, backcross; GISH, genomic in situ hybridization; RH, radiation hybrid; SSR
marker, simple-sequence repeat marker.
‡
To whom correspondence should be addressed. E-mail: phill005@umn.edu.
© 2004 by The National Academy of Sciences of the USA
www.pnas.org
͞cgi͞doi͞10.1073͞pnas.0403421101
PNAS
͉
June 29, 2004 ͉ vol. 101 ͉ no. 26 ͉
9921–9926
PLANT
BIOLOGY
Copyright (2004) National Academy of Sciences, USA.
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