Skripkina, T.A.                                                                              Institute of Cytology and Genetics
                                                                                                                     Novosibirsk 630090, Russia

Mutant line TS34 was obtained following treatment of the seeds of the cultivar Ramonsky-77 with 0.15% EMS. Line TS34 is characterised by a fasciated stem, very convex veins on leaflets and stipules, deformed flowers at the lower nodes, and an increased number of root nodules. The latter had an initial phenotype: they were roundish, very small (up to 1.5 mm in diameter), yellowish, and mostly without leghaemoglobin. Only a few nodules may attain further stages of development to acquire leghaemoglobin, nitrogenase, etc, and to be capable of nitrogen fixation. The number of nodules exceeded by several-fold the number on normal plants and it did not depend on the amount of nitrate in the soil. Thus the characteristics are those of a nitrate-resistant hypernodulation (nod++, nts) mutant.

All the mutant traits except hypernodulation showed stable expression under various growing conditions. However, hypernodulation showed incomplete penetrance as it was observed only in about half of mutant fasciated plants in a greenhouse, regardless of inoculation by effective strains of Rhizobium leguminosarum.

Two hypernodulation loci are so far known in the pea: nod3, located in the d-Idh region of the genome (4), and nod4, linked to the genes gp and cp in group V. The recessive hypernodulating allele of nod4 also confers a fasciated phenotype (like that of TS34) as a pleiotropic effect (3). In addition, Duc and Messager (1) reported induction of two hypernodulating, fasciated mutants which appeared to be allelic. Among 500 plants of the F2 population of the cross with an initial cultivar, they found no case of decoupling of hypernodulation and fasciation. The authors supposed either a mutation with a pleiotropic effect or simultaneous mutation of two very closely linked genes (1). Mapping of these mutations, and their relationship to nod3 and nod4, was not reported.

Line TS34 was crossed with line “nod3”, homozygous for this gene, and line K301, the type line for nod4. F1 plants of the former cross had a normal phenotype. Thus the TS34 mutation is not allelic to nod3.

In the cross TS34 Ґ K301 both parents had a fasciated phenotype. All 11 F1 plants, grown in a greenhouse on the ceramsite substrate with a full nitrogen nutrition, had a fasciated phenotype while hypernodulation was observed only in one of them. All 48 plants of the subsequent F2 generation, grown in the same conditions, were fasciated but lacked nodules. In these conditions the cultivar Ramonsky-77, an ancestor of both lines, either lacks nodules or produces a few small, yellowish, ineffective nodules. The same experiment was carried out in the field. The parental lines TS34 and K301 in these conditions differ to some extent: K301 plants had about a thousand very small, yellowish, ineffective nodules, while TS34 had a smaller amount of larger nodules, some of which contained leghaemoglobin. All the F1 plants had fasciation and ineffective nodulation; the number of nodules corresponded to the potential of the original cultivar (about 300) and was much less than in both parents. These data show that TS34 carries a mutation at the nod4 locus but it is not identical to the one carried by line K301. The new allele is characterized by a less affected nodulation, which is restored in the heterozygote with the earlier obtained mutation.

Line TS34 was crossed with a testerline carrying recessive alleles gp and cri (derived by V.A. Berdnikov from WL1238, a mutant line SGE182 (cri), and WL1255). The nod4
Table 1. Joint segregation data in the F2 population of the cross between line TS34 and a testerline cri gp.

¹A/a, first locus; B/b, second locus. ***P < 0.0001. The calculations were made using S.M. Rozov’s program CROS.

mutation was traced by stem fasciation. The analysis of the F2 data is given in Table 1. Linkage of the nod4 locus with gp corresponds well to the results reported by Sidorova and Uzhintseva (2).

It is worth mentioning an intriguing circumstance that four mutations were induced in total, referring to one or two loci, which produce both hypernodulation and fasciation. This suggests that these two traits share some way of developmental regulation.

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