GrainGenes

II. 14 The genetics of the six-row/two row character.

P.T. Gymer, Rothwell Plant Breeders Ltd., Rothwell, Lincoln, LN7 6DT, United Kingdom.

We are not aware of any account in the literature which clearly describes the genetic control of the character which distinguishes the main botanical broups of barley, viz. lateral floret type.

It has long been recognised that two independent loci control the six-row/two-row difference, and that pure-breeding intermediate types may be obtained from crosses between them (Harlan and Hayes, 1920; Robertson, 1933).

The loci are of course Vv on chromosome 2, and Ii on chromosome 4. Normal two-rowed barley is VVii, and normal six-rowed barley is vvII. The genotype VVII is a fixed intermediate, having lateral florets which are fully male-fertile and sometimes partially female-fertile; these florets are awnless and distinctly larger than in normal two-rows. The cultivar Nigrinudum is an example of this genotype.

Several authors have recognised the existence of a vvii genotype ("regressive six-row"), but have been unable to recognise it except by its breeding behaviour (Leonard, 1942; Woodward, 1949; Murty and Jain, 1960). In recent years however, many barley breeders have recognised the existence of two distinct six-row phenotypes, one having reasonably large lateral grains on extremely short pedicels ("sessile"), and the other fhaving grains which are much smaller (less than half the size of the central grains), apparently because pedicels 1 to 3 millimetres long restrict the flow of nutrients to the developing grain.

We have crossed the different types in all possible combinations at Rothwell, and identified the classes which can be obtained as pure-breeding lines. Considering the four possible types of six-row x two-row crosses, we find that:

Sessile six-row x normal two-row gives sessile six-rows, pedicelled six-rows, normal two-rows and intermediates,

Pedicelled six-row x normal two-row gives pedicelled six-rows and normal two-rows only,

Sessile six-row x intermediate gives sessile six-rows and intermediate only, and

Pedicelled six-row x intermediate gives all four types.

Thus we can allocate genotypes as follows:

Sessile six row    vvII

Pedicelled six-row    vvii

Normal two-row    VVii

Intermediate (staminate-lateralled two-row) VVII

A third allele at the Ii locus is also known, "I^h" (Leonard, 1942). In a VV genotype this produces even more fertility of the laterals than the I allele. In a vv genotype, its effect is as far as we know indistinguishable from the I allele. The genotypes vvII and vvI^hI^h IhIh both seem to occur frequently among cultivated six-rowed barleys.

Two additional alleles have been described at the Vv locus. The first is the deficiens allele, "V^t" (Woodward, 1947). This is a more extreme version of the normal two-row allele, and behaves in the same way, except that the lateral florets are so small that the i,I and I^h alleles are virtually indistinguishable in the V^tV^t genotype. The second allele is "V^d ", described in Svanhals by Woodward (1949). This was said to confer larger lateral florets than the normal V allele. However, at Rothwell we have grown Svanhals from several different sources, and found it to be identical with normal two-rows. Woodward's data do not in fact distinguish it from the normal VVii genotype. His interpretation may have been confused because he considered the variety Nudideficiens to have the genotype VVii, rather than the more probable V^tV^tii. We feel therefore that the V^d allele does not exist.

The dominance of the different alleles of the Vv and Ii series has been the subject of diverging opinion. In fact none of these alleles is fully dominant, although the V^t allele is nearly so. Thus Fl's of any cross are somewhat intermediate in phenotype.

We are aware that other alleles may exist, particularly at the Vv locus. Some such alleles which appear to confer intermediate phenotypes are present in the breeding program at Rothwell, and we hope to report on them in the future.

References:

Harlan, H. V. and H. K. Hayes, 1920. Occurrence of the fixed intermediate, Hordeum intermedium haxtoni, in crosses between H. vulgare pallidum and H. distichum palmella. Jour. Agr. Res. 19: 575-591.

Leonard, W. H. 1942. Inheritance of fertility in the lateral spikelets of barley. Genetics 27: 299-316.

Murty, G. S., and K.B.L. Jain, 1960. Genetic studies in barley - II. Inheritance of fertility of the lateral florets and certain other characters. J. Indian Botany Soc. 39: 281-308.

Robertson, D. W., 1933. Inheritance in barley. Genetics 18: 148-158.

Woodward, R. W., 1947. The Ih, I, i alleles in Hordeum deficiens genotypes of barley. J. Amer. Soc. Agron. 39: 474-482.

Woodward, 1949. The inheritance of fertility in the lateral florets of the four barley groups. Agron. J. 41: 317-322.

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