abstracts of papers presented at the 1962 meetings - Genetics

ABSTRACTS 985
infected. Because virus nucleic acid, unprotected by virus protein, is known to be extremely
labile to the action of ribonucleases, the tobacco seedlings exhibiting symptoms were tested for
ease of transmission of the infection to local lesion hosts in the presence of a small amount of
pancreatic ribonuclease. Of 155 tobacco seedlings inoculated, 72 exhibited visible signs of virus
infection and of these two behaved peculiarly in that it was difficult to transfer the infection by
standard techniques. Phenol extracts of these two plants contained infectivity whereas buffer
extracts contained little or none. Extracts made by the technique of DIENER (Virology 16: 140,
1962) which is designed to yield both intact virus and free infectious ribonucleic acid, yielded
only ribonuclease-sensitive infectivity. The lack of ablity to induce the synthesis of virus protein
does not, apparently, limit the replicating ability of the virus nucleic acid in the host cell nor the
spread of the infectious principle from cell to cell.
SIMANTEL, G. M., J. G. Ross, C. C. HUANG, and H. D. HAENSEL, South Dakota State College,
Brookings, South Dakota: Somatic chromosome reduction in sorghum after colchicine treatment.
-Following treatment of sorghum seedlings of the variety “Experimental 3” with colchicine,
diploid mutant plants arise that breed true in subsequent generations. The production of diploid
mutants was found to be dependent upon the genotype as well as the presence of certain environmental
conditions after colchicine application. To explain the homozygosity of the mutants, it
has been proposed that a reduction of chromosomes to a haploid condition occurred with later
doubling to restore the diploid number. This was tested by treating seedlings heterozygous for
chromosome markers (reciprocal translocations) .-After treatment of 80 seedlings heterozygous
for the chromosome marker, three diploid mutants occurred. Since these mutant plants when
examined at diakinesis showed no configuration of four chromosomes, they must have been
homozygous at least for the two pairs of marked chromosomes. They had uniformly high seed
set in comparison to the semisterility noted in the other plants. The marked chromosomes of
two of these mutants were demonstrated to be same as the normal parent (male). The homozygosity,
therefore, could not have arisen as a result of se1fing.-The presence of homozygous
normal chromosomes identified in the mutants obtained after colchicine treatment demonstrates
the occurrence of somatic chromosome reduction followed by doubling, of at least the two marked
chromosomes followed in this study. It would be expected that the whole chromosome complement
would be involved in such a somatic reduction to explain the phenomenon of the appearance
of true-breeding mutants after colchicine treatment. (This investigation was supported in
part by research grants from the National Cancer Institute, U. S. Public Health Service (C2086)
and the National Science Foundation.)
SMITH, HAROLD H., Department of Biology, Brookhaven National Laboratory, Upton, L. I.,
N. Y.: Studies on the origin, inheritance and mutation of genic-cytoplasmic male sterility in
Nicotiana.-Second generation populations of the cross Nicotiana langsdorffii x N. sanderae
yield approximately ten percent male sterile segregants. None appear in the parental species,
reciprocal backcrosses of the F, to either parent, or in F, progeny of the reciprocal F, hybrid.
Experimental results were compatible with the hypothesis that male sterility is controlled in this
material by a cytoplasmic factor [ms], present only in N. langsdorffii; and two independent recessive
alleles, one contributed by each parent species. The dominant alleles, Rf, and Rf,, restore
pollen fertility. N. langsdorfii is [ms] Rf,Rf,rf,rf,; N. sanderae is [MF] rflrflRfeRfZ, and only
[ms] rf,rf,rf,rf, segregants are male sterile. The rf, allele is linked with approximately 13%
crossing-over to p, a gene from N. 1angsdorfiCii that inhibits anthocyanin pigmentation. Pollen
carrying the Rf, allele is at a disadvantage of about 1:4 in competition with rf, pollen. This
accounts for certain anomalous ratios observed as, e.g. ten percent than 6.25% male steriles in
the F,. Two genotypes were synthesized to study induced mutation of the cytoplasmic factor,
namely: male fertile [MF] rf,rf,rf,dz and male sterile [ms] rf,rf,rf,rf,. Plants of each were
exposed to chromic gamma irradiation of 100dday and seeds to 10,OOOr of acute gamma radiation.
Results of efforts to mutate [MF] rjlrflrferj, to [ms] rf,rj,rj,rj, and vice versa will be reported.