ABSTRACTS 159

McCutnTock, BARBARA, University of Missouri, Columbia, Mo.: A method
for detecting potential mutations of a specific chromosomal region. Plants
having two deficient chromosomes-5 (the deficiency included the locus of
Bm, allele of 6m, brown mid-rib, producing a brown color in the lignified
cell walls) and a small ring-shaped fragment covering the deficiency and carry-
ing Bm, are mosaics of heterozygous and homozygous deficient tissues through
frequent losses of the ring fragment during somatic mitoses. This homozygous
deficient tissue is (r) much reduced in growth capacity, (2) contains no chloro-
plasts, (3) has the characteristics of 7; in its cell walls and (4) dries on expo-
sure to sunlight. Relatively infrequently, during somatic mitoses, the ring
chromosome increases or decreases in size through duplications or deficiencies
of segments of chromatin composing the ring. Duplicated segments produce
no obvious tissue modifications. If removal of specific regions from the ring
results in homozygous deficient tissues having specific modifications, several
types of mutant sectorials, depending upon the region removed, should be
repeatedly encountered in large populations of such plants. The following
types of “simple” mutant sectorials have been found: (1) transparent white
with colorless cell walls, no plastids; (2) opaque white with colorless plastids,
colorless cell walls; (3) deficiency-bm, similar in detail to normal dm, (4) pink
colored tissue with colorless cell walls, colorless plastids; (5) blotched chloro-
phyll pattern, colorless cell walls. The following types of “compound” mutant
sectorials have been found: (1) pink, deficiency-bm, viable in sunlight; (2)
pink, deficiency-bm, dries in sunlight; (3) opaque white, deficiency-bm; (4)
blotch, deficiency-bm, dries in sunlight; ( 5) blotch, dries in sunlight. On the
theory that compound mutant sectorials are the product of losses of several
adjacent regions of chromatin, the simple mutant effects are referred to the
chromosome in the following order: pink, deficiency-bm, dries in sunlight and
blotch, with translucent white removed from deficiency-6m and opaque white
close to it. Since a homozygous deficiency for the Bm locus produces the same

effect as the known gene bm, it is possible that these other mutants may even-
tually appear as “genes” closely linked to bam.