Ronald L. Phillips
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some 1 as defined by groups 9 and 10 is represented by seven
independent breakage events. This break is marked proximally
by the SSR marker p-mmc0041 and distally by the SSR marker
p-bnlg1720 (Fig. 2). The portion (7 of 21) of lines that break at
the same point or very similar points indicates a preferential
breakage and
͞or transmission of the chromosome segments to
offspring. On the short arm, five independent events (5 of 21)
demonstrate a common break in the range that is marked by the
SSRs p-umc1071 and p-umc1727 defining segment 1. All other
breakpoints are defined by one or two events. Most striking is
that we did not observe a centric break resulting in either a
telocentric maize chromosome or a centric maize–oat translo-
cation. This situation left intact a large area spanning the
centromere and the proximal regions on both arms, segment 7
(Fig. 2), and it is in strong contrast to earlier results from the
production of maize chromosome 9 RHs (12) that showed that
the level of chromosome 9 breakage across the chromosome was
relatively constant.
The physical sizes of the segments differ remarkably as shown
by GISH experiments (Fig. 3). In the line 1.07.3-001.3-03 (a
sibling from group 7), the maize chromosome shows a primary
constriction that defines the deficient short arm to
Ϸ80% of its
regular WT metaphase length. This result would mean that the
missing element (20%) represents a genetic size of at least 445
map units separated into 6 distinct segments by 8 of 10 groups.
On the other hand, GISH of line 1.07.2-007.3-04 (a sibling from
group 9) shows the distal maize chromosome fragment translo-
cated to an oat chromosome. The fragment length corresponds
to
Ϸ20% of the long-arm WT length in metaphase and visualizes
segment 8. Even considering that the definition of the single
segment by two markers varies over a considerable genetic
distance, the segment 7 spans approximately the proximal 80%
of the short and the proximal 80% of the long arm of the genetic
map of maize chromosome 1. The line 1.07.1-020.3-01 (sibling of
group 3) shows by GISH analysis a fragment of
Ϸ15% of the WT
short-arm length translocated to an oat chromosome. This
fragment visualizes the length of the segments 1 and 2 together
marked by SSRs p-umc1397 and p-umc1479.
Summary
The current set of disomic oat–maize addition lines involves all
maize chromosomes in different oat backgrounds with the
exception of chromosome 10. The maize chromosome 10 addi-
tion progeny has only the short arm; a fertile disomic telocentric
addition line is available. The whole chromosome 10 added to
haploid GAF-Park oat does allow the availability of DNA.
Although not fertile, and therefore not capable of producing
disomic addition offspring, we continue to maintain the original
plant vegetatively by tiller cloning under short-day growing
conditions. The leaves show remarkable somatic stability for the
added maize chromosome over a period of
Ͼ3 years. The plant
serves as a source for chromosome 10 genomic DNA and RNA.
The complete series of DNAs made from each maize chro-
mosome addition has been used as a powerful tool to allocate
genes and markers to chromosome. Ananiev et al. (31) used the
oat–maize chromosome 9 addition line as the DNA source to
construct a chromosome-specific cosmid library allowing the
isolation of maize-specific repetitive DNA families. The low level
of cross-hybridization under standard conditions between oat
and maize genomic DNA makes it possible to screen libraries for
maize species-specific sequences (31).
Oat–maize addition lines are ideal for mapping gene families
and markers that have more than one copy on different chro-
mosomes likely because of the duplicative nature of maize. For
example, Okagaki et al. (32) mapped 350 ESTs and sequence
tagged sites to chromosomes by a presence vs. absence test and
Fig. 2.
Panel of the first RH lines for maize chromosome 1. Shown are the 15
SSR markers that frame the seven breakpoints, hence define the RH segments
between p-umc1354 (most distal on the short arm) and p-umc2244 (most distal
on the long arm) markers representing a genetic distance of more than 1,120
map units according to the IBM2 map.
Fig. 3.
GISH of metaphase chromosomes from root tips of three RH plants of the maize chromosome 1 panel. (A) Plant BC
1
F
2
, 1.07.3-001.3-3 (sibling of group
7), arrow points to the deficient short arm of maize chromosome 1; the chromosome lost
Ϸ20% of its short arm. p-umc1626 is the most distal present marker
tested (see also Fig. 2). The yellow-painted chromosome visualizes the segments 7 and 8 representing the genetic distance of 656 – 675 map units (B) Plant BC
1
F
2
1.07.2-007.3-4 (sibling of group 9), arrow points to the translocation fragment visualizing the RH segment 8. The translocation fragment accounts for
Ϸ20% of
the long-arm length representing the genetic distance of 261–332 map units (
C) Plant BC
1
F
2
1.07.1-020.3-1 (sibling of group 3), arrow points to the translocation
fragment visualizing the RH segments 1 and 2 accounting for
Ϸ15% of the short-arm length representing the distance of 226–257 map units.
Kynast et al.
PNAS
͉
June 29, 2004 ͉ vol. 101 ͉ no. 26 ͉
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