Fort Collins, Colorado
Colorado State University
Recombination frequency for maize inbred
line KYS using recombination nodules
--Anderson, LK, Stack, SM
Recombination nodules (RNs) have been demonstrated
to faithfully reflect crossovers in a wide range of organisms. In order
to examine the distribution of crossing over at the highest possible cytological
resolution (using electron microscopy), we are preparing a map of RNs on
synaptonemal complexes (SCs) from the inbred line KYS. To date, we have
identified more than 1000 SCs (approximately 100 of each of the 10 SCs)
that we are using to map RNs. While the mapping effort is still underway,
there is sufficient data to present a summary of the results (Table 1).
The average number of RNs per SC set is 20.2. This compares well with estimates
of 18-27 chiasmata per cell (Beadle, G.W. 1933, Cytologia 4:269-287; Darlington,
C.D. 1934, Z. Indukt. Abstammungs Vererbungsl. 67:96-114). In addition,
SC length is a good predictor of average RN number (y = 0.12x + 0.78; r2
= 0.92), an observation that is consistent with prior observations made
in a number of different organisms. In general, regardless of SC length,
if there is only one RN on an SC, it is more often in the long arm than
in the short arm. If there are two RNs on an SC, usually there is one RN
in the long arm and one RN in the short arm. If there are three RNs on
an SC, the most common pattern is for one RN to be in the short arm and
two in the long arm. If there are four RNs on an SC, two RNs in each arm
or three in the long arm and one in the short arm occur at about the same
frequency. SCs with more than four RNs are rare. Overall, most RNs occur
in the distal third of each SC arm. The predicted map length for maize
KYS based on RN frequency is (# RNs X 50 map units =) 1010 map units. In
comparison, the genetic map for maize is almost twice as long at 1856 map
units (Maize DB). This difference may be due to genetic versus cytological
techniques and/or differences in the crossover rate in an inbred (KYS)
compared to hybrids. To determine the basis of this difference, we will
examine the number and distribution of RNs on SCs from B73, Mo17, and B73
X Mo17. This work was supported by NSF grant MCB-9728673.
Table 1. Average number of RNs for each
SC of maize and a comparison of the map lengths predicted from the RN frequency
with the map lengths determined by genetic methods (Maize DB).
|
SC
|
Relative length
|
Average Number of RNs
|
Predicted map length - RNs
|
Map length -genetic map
|
|
1
|
14.9
|
2.54
|
127.0
|
258
|
|
2
|
11.9
|
2.39
|
119.5
|
224
|
|
3
|
11.0
|
2.20
|
110.0
|
216
|
|
4
|
10.7
|
2.09
|
104.5
|
172
|
|
5
|
10.9
|
2.20
|
110.0
|
185
|
|
6
|
8.9
|
1.75
|
87.5
|
144
|
|
7
|
8.6
|
1.77
|
88.5
|
128
|
|
8
|
8.6
|
1.81
|
90.5
|
177
|
|
9
|
7.7
|
1.83
|
91.5
|
178
|
|
10
|
6.7
|
1.62
|
81.0
|
174
|
|
Total
|
99.9
|
20.2
|
1010
|
1856
|
Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.
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