Jan 9, 1990 - The four lines Aha, Isaria, Niimberg, and WMR were crossed to a marker line with three ... periment of the study, ten different barley lines of.
He!-editas 112 171-178 (1990)
Genetic control of recombination in barley 11. Variation in linkage between marker genes TORBJORN SALL Department of Crop Genetics and Breeding, Swedish University of Agricultural Sciences, Svalov, Sweden
SALL,T. 1990. Genetic control of recombination in barley. 11. Variation in linkage between marker genes. -Hereditas 112: 171-178. Lund, Sweden. ISSN 0018-0661. Received January 9, 1990. Accepted January 26, 1990 The recombination frequencies over a segment of chromosome 1 were investigated in four different barley genotypes. The four lines Aha, Isaria, Niimberg, and WMR were crossed to a marker line with three linked markers. The recombination frequencies were calculated from the segregation of the marker genes in the F2 families. The experiment was repeated over two different years with only three combinations the second year. The results show significant differences in recombination frequencies among the lines, with Alva having the lowest recombination frequencies and WMR the highest. The results support earlier experiments with the same lines, in which the frequency of recombination was estimated from the reduction in fertility of heterozygotes for an inversion in the same chromosome arm. Intraplant variation between tall and short spikes and among different parts of the spike were investigated but no such variation was found. In the first year negative interference was observed in all crosses. In the second year the estimated interference was not negative but unusually low.
Torbjorn Sail, Department of Crop Generics and Breeding, Swedish University of Agricultural Sciences, S-268 00 Svalov, Sweden
Recombination is an almost universal feature of meiotic organisms and a vital process in animal and plant breeding. Because of this, we have initiated a study of the genetic control of recombination in cultivated barley, Hordeum vulgare. In the first experiment of the study, ten different barley lines of Northern European origin were investigated (SALL et al. 1990), and it was shown that genetic variation for recombination rates exists in these lines. The experiment was based on using sterility in inversion heterozygotes as an indication of recombination, i.e., we regarded the frequency of sterility in inversion heterozygotes as proportional to the amount of recombination taking place inside the inversion loop. While the use of an inversion has several advantages, the most important of which is the relatively small number of plants needed to be scored, there are some potential problems with using sterility as a method to study recombination. Sterility is a difficult indicator, which may be affected by many facors; we believe, however, that this was not a major problem in our study (SALLet al. 1990). In the experiments using the inversion, there were no signs of any additional sources of sterility acting
differently on different genotypes. The pattern of segregation and the variances also gave a good fit to the assumption that recombination within the inversion was the main cause of the observed sterility (except in one family which was excluded from the analysis). It has also been shown in several studies that inversions themselves have an influence on the pattern of recombination in the genome; for a review see LUCCHESI and SUZUKI(1968). This potential problem could not be controlled within the former experiment and we have therefore complemented our studies with a regular recombination analysis. In order to do so, four of the ten lines used in the full investigation (SALLet al. 1990) were crossed to a triple marker line. The markers in this line reside in the same chromosomal region as the inversion used in the earlier study (EKBERG 1974). Thus, we have two different ways of studying recombination in the same chromosome region. Two experiments are presented in this paper. The first was performed in 1984 and included 4 combinations. This experiment produced two important results: first of all we found differences in recombi-
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nation frequencies among the different combina- MING 1930). All three marker loci are situated on tions and, secondly, negative interferences were ob- the short arm Of chromosome 1 (S0GAARD 1977; EKBERG1974) and the mutants have been backserved. In 1986 a second experiment with three of the crossed to a Bonus background (PERSON and HAGcombinations was performed in order to repeat the BERG 1972). The triple mutant line was produced by earlier results and to make a detailed study of the crossing the two double marker lines, fi. - cer-a' negative interferences. One possible cause of inter- and cer-a' - hi.. produced by SOgaard (lines 1 and D and then selecting the triple ference process, suggested by SALLand BENGTSSON 6 in S ~ C A A R1978), (1989), is that there is heterogeneity of recombina- mutant recombinant phenotype in the F2 generation. tion frequencies within the experimental population in which recombination behaviour is studied. This would cause negative interference because flovrers with high recombination frequencies produce a certain proportion of double cross-overs at the same E.\-periment of I984 time as the average recombination frequency is de- The F1 plants from the crosses between the four creased by the flowers having low recombination lines and the triple mutant line were grown out of frequencies. Since interference is calculated from doors in a benchyard during the summer of 1984, the ratio between the observed and the expected so that the relevant meioses during which recombidouble cross-overs. and the number of expected nation took place occurred simultaneously and undouble cross-overs is calculated as the product of der identical conditions to those studied using the the recombination frequencies in the investigated inversion, reported as the 1984 experiment by SALL intervals. negative interference will appear. et al. (1990). The plants were harvested individually Such meiotic heterogeneity within an experiment at full maturity. After threshing, the F2 plants were could be due to meiotic differences either between grown in a greenhouse. The chlorina character was the F, mother plants or within the mother plants scored at the two-leaf stage and the other two mubecause of. e.p.. temporal variation or variation be- tant phenotypes were determined after heading. Out tween different parts of the plants. In the second of 5955 plants sown, 5581 were scored for all experiment. we therefore tested whether any recom- characters. bination differences could be found among different Fz families, between early and late spikes (called A and B spikes). and between different parts (low. mid, upper) of the ear. Some of the results reported in this paper have E.\peiYmeiit oj' I986 earlier been described by BENGTSSON and SALL In the 1986 experiment F1plants from the combinai 1987 ). tions of Alva, Isaria and WMR with the triple mutant line were sown to produce F2 seed. The Fl plants were grown out of doors in a benchyard durMaterial and methods ing the summer of 1986 simultaneous with the Four barley lines were compared in this investiga- plants that were scored for sterility in inversion hetion: A h a , Nurnberg, Isaria (Ackennan Isaria) and terozygotes (S.41.i~et al. 1990). The plants were WMR ( Weihenstephancr Mehltauresistente I). For har\,ested at full maturity. The spikes were then :i detailed description of the line\ see L I ~ I E - divided into A-spikes (from early. tall tillers) and B-spihes (from later, smaller tillers). Furthermore, et al. (1982) and SALL. et al. (1990). ur lines were crossed as mother plants with each ear was divided into three approximately equal parts. upper, middle and lower. Each spike-type and :I triple mutant line. which was homozygous for the mutant alleles cer-u' (eceriferum; waxless spike and ear-part was threshed separately. The F2 plants were grown in a field during 1987. straw!, A. (chlorina: light green when young!. and hr- tbrachytic, short awns and stature). All three mu- The chlorina character was scored during the twotants are recessive. The cw-u' mutant was originally to three-leaf stage, and the two other mutant properinduced by X-rays in Bonus (LCINDQVIST and VQY ties were scored after heading. Kecombination inWETTSTEIN 1962). The hr- mutant was found in the formation was collected for each investigated line, variety Himalaya (POWERS 1936). and the J;. mutant each family (the offspring from one individual FI was found in Colsess barley (ROBERTSON and LIE- plant). spike-type and ear-part.
I i r l e d m ~l l 2 (19901
GENETIC CONTROL OF RECOMBINATION IN BARLEY II
Results Experiment of 1984 The numbers of piants in each of the eight possible phenotypic classes are shown in Table 1. The onefactor segregations, shown in Table 2, indicate that more than three times as many normal plants as mutant phenotypes were observed in all cases. This effect is especially marked in the combination with WMR. The table also shows the result of a chisquare test of the 3:1 segregation. It is seen that in WMR the deviation from a 3: 1 segregation is highly significant for all three genes, while in the Isaria combination, two of the factors have significantly different segregations from 3:l. Within each family, segregation was distorted in a fairly uniform way over all three markers. However, mortality associated with one of the markers would, due to the close linkage, cause a skewed distribution also for the two other markers. Since higher incidences of mortality have been observed in chlorina mutant lines (U. Lundqvist pers. commun.), we have chosen to introduce a mortality parameter for the chlorina homozygotes in the estimates of the recombination frequencies. Table 1. The distribution of plants over the phenotypic classes in the F, populations derived from crosses between the investigated lines and a triple marker strain. Results from 1984 experiment Phenotype
Investigated line
WMR
Alva
Isaria
Numberg
+ + + + f, + + + br + f, br cer + + cer f, + cer + br
930 3 19 12
1191 8
880 2 30 21 22 21
4
1 I24 5 30 12 24 33 2
cer f,
259
320
313
182
1254
1624
1543
1160
br
Sum
12 18 1
42
13 21 25
2
173
The recombination frequencies were estimated by the maximum likelihood method, see, e.g., Rao 1947, and the estimates with their standard errors are shown in Table 3. There are consistent differences among the combinations. Alva shows the lowest recombination frequencies for all three segments and WMR clearly has the highest values. Isaria and Numberg are intermediate. A test of homogeneity of recombination was performed according to RAO(1947). The result shows a significant difference among the lines at the 5 % level (x2=19.4, df=9; p=0.02). Interference was measured by the coefficient of coincidence, c = (r1+r2-r12)/2r3r2,where rl is the recombination frequency in the first segment, r2 is the recombination frequency in the second segment, and r12 is the recombination frequency over both segments. The results are shown in Table 4. In all four cases, the coefficients are larger than 1, which is equivalent to what is called negative interference. A homogeneity test does not show any significant variation among the combinations with respect to the coefficient of coincidence (x2=7.62, df=3, p=0.054). In the combined material there is a highly significant difference from 1 (x2=10.6, df= 1, p=O.OOl).
Experiment of 1986 The numbers of individuals scored per cross-combination, spike-type and ear-part are given in Table 5. Considerably fewer seeds were sown from Bspikes than from A-spikes, since B-spikes are smaller and produce fewer seeds. The number of seeds from the different parts of the ear should be equal; however, due to poorer germination of the smaller top and bottom seeds, there are usually slightly more plants from the middle section of the ear. The percentage of individuals with a recombinant phenotype is shown in Table 6. The proportion varies between 5.1 % and 10.2 %. It can be seen
Ttrbk 2 . Segregation ratios observed for the different factors, with chi-square values for the tit to the expected 3:l segregation and the associated p-value. All chi-square test are with one degree of freedom. Results from 1984 experiment Investigated line
Factor
chlorina
eccr@rum +:cer Alva Isaria Numberg WMR
x2
P
+:fc
3.3:l 3.4:I 3.2: I
2.4 4.3 0.7
0.13 0.04 0.42
3.3:1 3.4:l 3.2:1
4.1:1
18.2
