EMS-induced lethal in the vicinity of the locus lf
|Kosterin, O.E., Berdnikov, V.A.
and Gorel, F.L.
Institute of Cytology and Genetics
In a communication by Gorel et al. (1) in this issue we described an experiment performed to obtain EMS-induced mutations linked to the breakpoint of the Hammarlund translocation. We report here another mutation from that experiment and characterize the mutation as a sporophytic lethal near the locus lf.
Among the M3 families (for experimental design see ref. 1) we found a family of 16 individuals, none of which had the phenotype cri gp a. This observation suggests the presence of a lethal located near the breakpoint on the normal chromosome set. Thirteen of the plants were tested for the His(2-6) phenotype. Six appeared to be heterozygous for this locus and, hence, for the translocation; seven were homozygous for His(2-6), and these should be homozygous for the translocated karyotype. The progeny from the selfing of three heterozygous plants were sown and produced 85 Cri plants (44 structural heterozygotes and 41 homozygotes for the translocation) and two cri plants (structural homozygotes for normal karyotype). No crossover events between the breakpoint and the markers a, cri, and gp were observed. The progeny of 11 structural heterozygotes were again planted and produced 76 Cri plants (50 structural heterozygotes and 26 homozygotes for the translocation) and 2 cri plants. One of the structural heterozygotes had phenotype gp, indicating that a crossover event had taken place between this marker and the breakpoint.
Combining all data from the M3 to the M5 generation gives a total of 100 structural heterozygotes, 74 homozygotes for the translocation, and 4 homozygotes for normal karyotype (with phenotype cri). We conclude that the line with the normal karyotype bears a sporophytic lethal closely linked to the Hammarlund translocation breakpoint. This lethal seems to reduce competing ability of the male gametophytes for fertilization, because the observed ratio (100:74) deviates significantly from the expected 2:1 (Chi-square=6.62; P<0.025). The crossovers observed suggest that the lethal may be either on linkage group II or on linkage group V distal to cri but not on linkage group V proximal to this gene. To test the first option we crossed a structural heterozygote with the testerline WL1238 and obtained four F1 plants. One of them had semi-sterile pollen phenotype and presumably was heterozygous for the translocation. Three had fully fertile pollen and were assumed to have normal karyotype. The F2 populations derived from these three plants (110 individuals in total) were grown and analyzed for deviations from the expected segregation ratios of markers cri, His(2-6), a, lf, and His7. The normal karyotype of the hybrid Tau-2 has a genotype His72 lf a His(2-6)1123 cri; the line WL1238 has His73; Lf A His(2-6)1221 Cri.
The following phenotype ratios were observed:
for the gene cri:
88 Cri : 22 cri
Although deviation of the segregation for the gene cri from the expected 3:1 is not significant (c2=1.47 P>20%), segregation for the other markers, belonging to the linkage group II, is greatly disturbed. No crossovers were registered between the lethal and the gene lf. All the plants started flowering at a node not lower than 14, while the plants with a normal karyotype segregating from the hybrid Tau-2 always started flowering at node 8 or 9. The number of crossovers between the lethal and other markers of linkage group II correspond to their distance from lf. We derived equations for a maximum likelihood estimation of the crossover distances between a lethal and a dominant or co-dominant marker, based on segregation deviations of a single gene segregation in F2 from 3:1 and 1:2:1 models, respectively (2). Such calculations gave the following linkage relationships:
His7_______ ___________lt, lf___________a___________His(2-6)
These distances correspond closely to the average map of this segment (3), with only the distance lf—a somewhat reduced.
This mutation appears to be the first mapped lethal in pea that stops sporophyte development at an early stage (before the seed is formed) and thus exhibits no visible phenotype. A question arises what nomenclature would be best for mutations of this kind. We propose to adopt, with modification, the system accepted for Drosophila and to symbolize such lethals as l(x)y, where x is the linkage group number and y is the order number of the lethal. Hence, the lethal described is to be named l(2)1.
This lethal can be useful for constructing some balancer systems in the pea, and we already have used it in some of our stocks.
Acknowledgement: This work was partly supported by the Russian State Program “Russian Fund for Fundamental Research”.
1. Gorel, F.L., Kosterin, O.E. and Berdnikov, V.A. 1999. Pisum Genetics 31:5-8.
2. Kosterin, O.E., Berdnikov, V.A. and Gorel, F.L. 1999. Pisum Genetics 31:33.
3. Kosterin, O.E., Bogdanova, V.S., Gorel, F.L., Rozov, S.M., Trusov, Y.A. and Berdnikov, V.A.
1994. Plant Science 101:189-202.