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

 


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