3. Suppressor genes concerned with chloroplast biosynthesis.

A method has been devised for the demonstration and isolation of suppressor mutations through the use of a pleiotropic gene. The pleiotropic gene alters the endosperm color (from normal yellow to light yellow) and plant color (from normal green to albino) when present in the homozygous recessive state. A suppressor mutation (defined as "a sudden change occurring at a particular chromosomal locus such that a gene is formed capable of nullifying the effect of a particular major gene") which affects the plant color without affecting the endosperm color can be detected and isolated in the presence of the pleiotropic gene by growing light endosperm seeds. An example will serve to clarify the use of this method. The widely used inbred, Ohio 28, was crossed with a tester stock which was heterozygous for the pleiotropic gene. Six plants from F₁ progeny of this cross were selfed and one ear which segregated 390 dark to 110 light kernels (a satisfactory 3:1 segregation) was harvested. Forty seeds of both the dark and light types were germinated in seedling flats under greenhouse conditions. Color classifications of these plants are summarized in table 1.

Table 1

Plant Color	Endosperm Color		Expected (on basis of incompletely dominant supressor gene mutant
	Dark	Light
Normal green	40	0
Light green	0	12		9.75
Very light yellow green	0	16		19.50
Albino	0	11		9.75
% Germination	100	98
			Chi square=	1.445
			D.F.=	2
			p=	30 - 50%

The good fit indicates that an incompletely dominant suppressor gene mutant is segregating in this progeny. Since the tester stock has no such gene, it is evident that the inbred Ohio 28 is carrying a suppressor gene mutant and is presumably homozygous for the dominant allele.

In a similar manner several other inbreds have been tested and classified as shown in Table II.

Of the 25 individual inbreds tested 11 (or 44%) have segregating progeny indicative of suppressor mutations. If frequent presence of suppressor genes compensating for the deleterious action of this major gene's recessive allele is found to be rather typical the evolutionary significance of suppressors is obvious. One line, Connecticut C106, seems to have two suppressor genes which are not well fixed. In this line 15:1 as well as 3:1 segregations have been found in light seed progeny. The other inbreds seem to be fixed (homozygous) for the suppressor genes. However, it has not been possible to grow extra progeny of all these F₂'s.

Work is in progress to ascertain the linkage relationship of the pleiotropic gene and to utilize the various mutant forms in studies of chloroplast biosynthesis. Small amounts of seed are available upon request.

Table 2. Suppressor gene classification.

Inbred	Has none	Has one incompletely dominant		Has one completely dominant
Ohio 56A	x
West Va. W1455		x
Wise. W16	x
Id. 50	x
Conn. 106		x ?	and	x
South Dakota 105	x
Ind. P8		x
Conn. 103				x
Conn. 14	x
Conn. 102	x
Ill. Hy 2	x
Ohio 56‑1				x
Ind. Hs 8
Kansas K155	x
Ohio 61		x
Ind. DC 6				x
Iowa LK				x
Minn. A25	x
Iowa I198		x	or	x
Rogers Flint		x
Wise. W23	x
Iowa T1				x
Iowa Os 420	x
Minn. A158	x
USDA C.I. 4‑8	x

H. L. Everett