A probable codominant allele of the locus bt with a striking effect on the pod.
Kosterin O.E., Berdnikov V.A., Rozov S.M., Gorel F.L.
Institute of Cytology and Genetics, Novosibirsk 630090, Russia
In 1970 N.A. Sobolev and V.P. Bugriy (3) reported on a pea with a striking pod shape. In 1961 the latter of the cited authors found an anomalous oblong seed among those obtained from the cultivar Sever (Vulgatum 63) in the field in the Tomsk region. The resulted plant had very long, narrow and curved pods which had inside a highly developed spongeous layer and usually cracked before seed ripening. Its progeny segregated for the characters in question, their expression being highly variable. V.P. Bugriy gave to this syndrome the name "Originalnyi" (that means "peculiar, original").
Thanks to the courtecy of Dr. Alexei Kravchenko, we obtained seeds of the "hybrid Originalnyi". They were of both round and oblong shape, heterogeneous for green and yellow cotyledon colour (the gene i), and lack anthocyanine pigmentation. We planted 10 seeds which exhibited three phenotypic classes:
i) 3 plants had normal pods.
ii) 5 plants had long pods curved in a sigmoid manner, with enlarged distance between the ovules and stretched out tips ending with a small hook. In the distal part of some pods their leafs opened at the ventral seam with seed growing, as if the pods were too narrow, and the seeds finished their growth in open air. The pods had a considerable spongious layer inside. The first pods on the plants were closer to normal, the subsequently appearing pods became more transformed, the pods on lateral branches being most narrw and curved. Thanks to their narrow pods, these plants gave oblong seeds.
iii) 2 plants had extraordinarily narrow pods with great interovular distances, which were either straight or strongly curved upwards or winding, with the very tip strongly bended ventrally. Almost all pods cracked and most of the seed developed freely. The spongious layer inside was heavily expressed. Many ovules were sterile or gave abortive seeds, the viable seeds being of cylindrical shape.
This picture might be explained by the presence of a codominant factor which gives the phenotype ii) in a heterozygous state and the phenotype iii) in homozygote. Several following experiments proved this supposition.
First, we planted the progeny of selfing of a single plant of the phenotype ii) and obtained 9 plants of (normal) phenotype i), 25 plants of phenotype ii), and 5 plants of phenotype iii). Next, we crossed a plant with phenotype iii) with one of our original testerlines "fas-bt-n" and in the F2 progeny of a single fully fertile F1 plant, showing phenotype ii), obtained 3 plants of phenotype i), 12 plants of phenotype ii), 9 plants of phenotype iii) (Fig. 1). If to pool the results of segreration of progenies of these two plants we obtain the ratio 12:37:14, that is quite close to 1:2:1 (Chi-square being 1.35; P>0.20). We concluded that the pod syndrome is determined by a codominant genetic factor and gave it a preliminary name Orig.
Figure 1. The pods of the F2 plants of the cross of a plant heterozygous for Orig from the initial stock and a testerline "fas-bt-n". Left(i): normal; middle(ii): heterozygous for Orig; right(iii): homozygous for Orig.
i ii iii
To attempt its linkage analysis we crossed a plant homozygous for Orig with the testerline WL1238 (=JI73) and obtained a single vigorous fully fertile F1 plant which exhibited a phenotype heterozygous for Orig. 91 F2 plants were grown up and showed the linkage of Orig with markers r and tl of the linkage group V (Table 1).
Table 1. Joint segregation data of the F2 progeny of a single plant resulted from the cross of a plant bearing the factor Orig and the testerline WL1238.
+-----------+------------------------+-----------------------+ | | Number of progeny with | | | Gene | designated phenotype | Joint Recomb. St. | | pair +------------------------+ Chi-sq. frac. Error | | | AB Ah Ab aB ah ab | % % | +-----------+------------------------+-----------------------+ | gp tl | 22 30 24 1 8 6 | 3.31 42.97 6.31 | | Orig tl | 22 26 12 1 12 18 | 18.13*** 26.11 5.24 | | r tl | 23 38 3 0 0 27 | 78.06*** 3.14 1.85 | | gp Orig| 52 -- 24 8 -- 7 | 1.27 45.09 7.42 | | gp r | 55 -- 21 9 -- 6 | 0.92 44.53 7.37 | | Orig r | 51 -- 9 13 -- 18 | 18.17*** 24.41 5.32 | +-----------+------------------------+-----------------------+ 1 - A,a - first gene; B,b - second gene; h - heterozygous. Capital letter stands for dominant alleles of Gp, R, for the factor Orig, and for allele Tl of a codominant gene tl. In this experiment we considered Orig as a dominant gene and pooled together homozygotes and heterozygotes. All dominant alleles are in a coupling phase. *** - probabilities less than 0.0001. Here and in Table 4 calculation were made with the aid of the program "Cros" by Dr. S.M. Rozov. Note: A strong distortion of monogenic segregations resulted from suffering of the plants from the Fusarium vilt, which in this experiment happened to kill a great number of seedlings, mostly of the phenotype R.
The results obtained implied the following linkage relationship:
Besides, we crossed a plant heterozygous for Orig from the initial stock (with genotype R Det His12 SCAs Curl) with our original testerline "r-det-curl" bearing the linkage group V markers r, det, His11, SCAf, curl. Among 12 F1 plants 6 had heterozygous phenotype with respect to Orig. They were pollinated with the same testerline. Results of this testcross are presented in Tables 2 and 3.
Table 2. Segregation of progeny of the testcross "Orig" x ("r-det-curl") x ("det-r-curl"). Orig has alleles Orig, Det, R, His12, SCAs, Curl; the testerline "r-det-curl" has alleles orig, det, r, His11, SCAf, curl. In phenotype designations + stand for phenotypes corresponding to heterozygotes, - stand for phenotypes corresponding to homozygotes for the testerline alleles. n is the number of progenies, their total number being 78.
Phenotype n Phenotype n
Orig r det His1 SCA curl Orig r det His1 SCA curl
non-recombinant non-recombinant
+ + + + + + 23 - - - - - - 21
single cross-overs single cross-overs
+ + + + + - 1 - - - - - + 0
+ + + + - - 2 - - - - + + 2
+ + + - - - 2 - - - + + + 8
+ + - - - - 1 - - + + + + 0
+ - - - - - 9 - + + + + + 6
double cross-overs double cross-overs
+ + + - + + 1 - + + - - - 1
+ - - + + + 1
Note: the testerline "r-det-curl" was obtained by V.A.Berdnikov and F.L. Gorel' by combining mutations in the loci r and det, induced by X-rays in a genotype SG (1), with a mutation in the locus curl induced by EMS in the same genotype (unpublished). The line was then purified by eight generations of selfing.
Table 3. Linkage data of the testcross "Orig" x ("r-det-curl") x ("r-det-curl") (see Table 2).
Gene pair Joint P Rec.Fract. S.E.
Chi-sq. % %
Orig r 25.0 <0.0001 21.8 4.7
Orig det 22.9 - " - 23.1 4.8
Orig His1 7.3 <0.01 34.6 5.4
Orig SCA 4.1 <0.05 38.5 5.5
Orig curl 0.7 <0.5 39.7 5.5
r det 74.1 <0.0001 1.3 1.3
r His1 32.5 - " - 17.9 4.3
r SCA 25.4 - " - 21.8 4.7
r curl 23.0 - " - 23.1 4.8
det His1 35.4 - " - 16.7 4.2
det SCA 28.0 - " - 20.5 4.6
det curl 25.4 - " - 21.8 4.7
His1 SCA 59.3 - " - 6.4 2.8
His1 curl 55.8 - " - 7.7 3.0
SCA curl 74.1 - " - 1.3 1.3
They suggest the following relationships:
One can see that the factor Orig is projected to the map region where the locus bt is thought to locate. This locus also affects the pod shape, its recessive allele bt determining the apical angle of the pod to be acute. A question arises whether Orig is allelic to bt or is a gene linked to the locus bt. The normal plants from the initial stock had the Bt phenotype. If Orig is an allele of the locus bt, then in the progeny of the cross of an Orig/Orig genotype with a bt/bt genotype we would never observe the phenotype Bt, but eiter Orig or bt. If Orig and bt are different loci (and hence the original stock was homozygous for the allele Bt), then we could expect the phenotype Bt resulted from a cross-over event between Orig and Bt.
We crossed an Orig homozygote (with the genotype Orig Det R Tl His12, SCAs) from the original stock with a testerline RT-6 (bt det r tlw His11, SCAf). The results are presented in Table 4.
Table 4. Joint segregation data of the F2 progeny of the cross of a plant (Orig Det R Tl His12, SCAs) with the testerline RT-6 (bt det r tlw His11, SCAf)
---------+------------------------------------+-----------------------+
Gene | Number of progeny | Joint Recomb. St. |
pair |------------------------------------+ Chi-sq. frac. Error |
| AB Ah Ab hB hh hb aB ah ab | % % |
---------+------------------------------------+-----------------------+
r Orig| 16 66 16 -- -- -- 2 12 16 | 16.92** 27.45 4.52 |
det Orig| 16 66 16 -- -- -- 2 12 16 | 16.92** 27.45 4.52 |
tl Orig| 9 19 2 7 44 14 2 15 16 | 20.86** 29.90 3.58 |
His1 Orig| 12 16 3 17 45 7 3 17 8 | 10.64 34.69 3.89 |
Orig SCA | 8 6 4 17 44 17 3 15 14 | 12.34 34.33 3.87 |
det r | 97 -- 1 -- -- -- 1 -- 29 | 117.10*** 1.61 1.12 |
r tl | 30 64 4 -- -- -- 0 1 29 | 102.92*** 3.93 1.75 |
r His1| 27 63 8 -- -- -- 1 6 23 | 59.02*** 12.93 3.14 |
r SCA | 27 61 10 -- -- -- 1 4 25 | 61.90*** 12.76 3.12 |
det tl | 30 65 3 -- -- -- 0 0 30 | 112.80*** 2.35 1.35 |
det His1| 27 64 7 -- -- -- 1 5 24 | 66.58*** 11.23 2.93 |
det SCA | 27 62 9 -- -- -- 1 3 26 | 69.42*** 11.09 2.92 |
tl His1| 25 5 0 2 58 5 1 6 26 | 153.41*** 8.17 1.79 |
tl SCA | 25 5 0 2 56 7 1 4 28 | 154.90*** 8.15 1.79 |
His1 SCA | 28 3 0 7 62 0 0 0 28 | 205.37*** 3.99 1.25 |
+--------+------------------------------------+-----------------------+
A,a - the first gene; B,b - the second gene; h - heterozygous.Alleles come from the first parent are denoted with capital letters, those from RT6 - with lower case letters. All dominant alleles came from the first patent and are in a coupling phase.
**,*** - probabilities less than 0.001 and 0.0001, respectively.
Note: the line RT6 were derived by S.M. Rozov with colleagues from the lines WL1018 and WL2132 and the accessions VIR320 and VIR7036 and purified by six generations of selfing. Linkage relationsips between Orig and markers of linkage group V were similar to the above given:
In this cross we observed no plants with the phenotype orig Bt. Of course, the absence of such plants would not confidently prove Orig and bt to be allelic since a possibility retains that they are alleles of very tightly linked loci. But the supposition of allelism seems to be more probable.
Linkage relationships of the linkage group V markers resulted from the crosses studied correspond in general to the map assumed for them (2), except for the orientation of the gene pair r and det with respect to other markers. It should be noted that no crosses have been reported so far where this orientation would be studied in a direct experiment. In ref. 2 it was inferred indirectly from cross-over distances derived from different crosses.
So, we have observed the factor Orig to behave as a codominant
factor locating at the extreme of the so far known linkage group V and
most probably being an allele of the locus bt. Orig sems not to be accompanied by strucrtural rearrangements of chromosomes, as in all crosses the F1 hybrids were fully fertile and cross-over was not supressed in a chromosome region where Orig is located. We don't propose a special gene symbol for it and preliminarily designate it as the allele BtO.
The factor considered locates at the extreme of a known linkage map and hence can be useful in genetical work.
The authors express their gratitude to Dr. A. Kravchenko for providing us with the initial Orig stock and to L.P. Romkina or technical assistance. The work was partly suppported by the Russian State Program "Russian Fund for Fundamental Research".
References:
1. Gorel', F.L., Berdnikov, V.A., and Temnykh, S.V. 1994. Pisum Genetics 26:16-17.
2. Rozov, S.M., Temnykh, S.V., Gorel', F.L., and Berdnikov, V.A. 1993. Pisum Genetics 25: 46-51.
3. Sobolev, N.A. and Bugriy, V.P. 1970. In: Otdalennaya gibridizatsiya rastenii. Zernovye i zernobobovye kultury. [Far Hybridisation in Plants. Corns and Legumes]. Kolos, Moscow, pp. 415-421 [in Russian]
