II. 14. Two incomplete or semi-dominant mutants of barley.
C.A. Foster, Welsh Plant Breeding Station, Aberystwyth, Wales, UK.
Mutations which exhibit incomplete or semi-dominance are uncommon in barley. Two mutants recently isolated are as follows:
1. A semi-dominant chloroplast mutant (WPBS Cb 3369)
This mutant segregates in the ratio 1:2:1 for normal green, light green and albino respectively. The intermediate plants are viable but less vigorous, later emerging and smaller than the normals. Leaves vary in color with plant age; initially being viridis or viridoxantha, becoming progressively darker green until at ear emergence plant color is almost normal.
The total segregation observed is 65:125:58 (X2 = 0.53; P 0.90 - 0.50). The progeny of normal green plants are all green. The progenies of light green plants (viridis) all segregate in the 1:2:1 ratio. A test cross between a light green and a normal plant (as male) gave a 1 normal:l light green ratio. It is concluded that a single gene semi-dominant chloroplast mutant is present, the genetic constitution of the segregants being AA normal, Aa viridis, aa albino.
The origin of this stock (WPBC Cb 3369) was a single plant found in a yield trial plot of a Station selection. At the time the plant had a single tiller, the leaves of which were green in one half and yellow in the other, the change in color being almost exactly at the midrib of the leaf laminae. Each successive leaf on the tiller was of this type, while subsequent tillers were uniformly viridis or viridoxantha. Four ears were formed and the progeny of each ear segregated in the 1:2:1 ratio. It appears that the collected plant was the one in which this spontaneous mutation first occurred.
A similar mutant induced by EMS and symbolized Xa was reported by Takahashi et al. (1972) but this segregated 1 green:2 chlorina: 1 xantha, the xantha being lethal and the chlorina viable.
2. An incomplete or semi-dominant dwarf mutant (WPBS Cb 3370)
This mutant, induced in Carlsberg II by diethyl sulphate, segregates in the ratio of 1 normal:2 intermediates:l grass dwarf. The "intermediate" plants are slightly less vigorous than normal, have ears which are lighter green than normal, with lax, fragile awns. In many mature grains the lemma is split at the base, exposing the embryo. The grain itself, which is slightly shrunken, readily sprouts on the ear in wet harvesting conditions. The grass dwarf plants are severely stunted, having many small, thin tillers a few which produce a small ear but no seeds.
The progeny of normal plants are all normal. The M5 progenies of 14 M4 intermediate plants segregated 48:100:56 grass dwarf, closely fitting a 1:2:1 expectation (X2 = 0.71; P 0.70). In the following generation 19 normals gave all normal progeny, while 54 intermediates each segregated into three classes, with an overall total (homogeneous across families) of 526:770:437, a ratio which deviated significantly from a 1:2:1 expectation (X2 = 30.64; P 0.001). This deviation was due to an excess of normals approximately equivalent to the deficit of intermediates, the fit of normals + intermediates:grass dwarf to a 3:1 ratio was close (X2 = 0.035; P 0.90 - 0.80), indicating misclassification rather than differential viability or survival. Classification at ear emergence is easier and more reliable than that at harvest. It is provisionally concluded that a semi-dominant single gene is present in the homozygous recessive state in the grass dwarf phenotype and in the heterozygous state in the intermediate phenotype.
Reference:
Takahashi, R., J. Hayashi, T. Konishi, and I. Moriya. 1972. Inheritance
and linkage studies in barley. Ber. Ohara, Inst. Landw. Biol. Okayama Univ.
15, 3, 147-168.