abstracts of papers presented at the 1962 meetings - Genetics

ABSTRACTS 991
XI1I.-These observations can be explained by a hypothesis which assumes that: (1) the mt and
N loci control separate steps in a sequence which results in the production of the XIV mating
type substance; (2) the ability of the macronucleus to differentiate to control only one of two
alternative mating type potentialities reflects the ability of the wild-type allele at the mt locus
(or, less likely, at the N locus) to be repressed when differentiation occurs in the presence of a
hypothetical substance, “R’; (3) “R” is a precursor of the XIV mating type substance. (Sup-
ported by a grant from the Natisonal Science Foundation.)
THOMPSON, PETER E., Iowa State University, Ames, Iowa: Synaptic modification of dominant
lethal frequencies after irradiation of the Drosophila testis.-Increases in the mutability of unpaired
chromosomes have led to the interpretation (THOMPSON, Genetics 47:337, 1962) that the
synapsis of homologous chromosomes is a major factor in the repair of radiation induced chromosomal
damage. This hypothesis is subtantiated by the present data on the production of dominant
lethals among males in which complex structural heterozygosity (presence of multiple inversion
SM5) has created almost total asynapsis of one major pair of chromosomes. Egg development
among one-day broods from such males after X-ray treatment gave evidence of an appreciable
increase in dominant lethals of meiotic origin, in comparison with males lacking structural heterozygosity.-The
brood pattern of dominant lethals from irradiated control males shows a
plateau of maximum frequency from early meiosis to spermatid stages, i.e., among sperm sampled
from the fifth to the eighth day after irradiation. Broods from irradiated SM5 males gave
similar frequencies for premeiotic and postmeiotic stages, including the fifth and tenth brood
periods, but during days 6-8 about twice as many dominant lethals were scored among these
lines. If dominant lethals may be equated with highly aberrant rearrangement or loss of chromosome
fragments, asynapsis appears to contribute markedly to gross genetic damage during
meiosis. (Supported by Grant RG-8912 from the National Institutes of Health.)
TUVESON, R. W., Department of Botany, University of Chicago, Chicago 37, Illinois: Hyphal
tip isolation as a criterion for heterocaryosis.-Specific heterocaryons between auxotrophic strains
of Cephalosporium mycophilum exhibit selection for the incorporation of one of the nuclear
types into the conidia produced by the heterocaryon. Growing the specific heterocaryons on
supplemented minimal medium resulted in altered nuclear proportions and recovery of the
minor nuclear component. Hyphal tip isolations from one of the heterocaryons exhibiting selection
when grown on minimal medium were incapable of continued growth on minimal medium
although tips transferred to complex medium grew vigorously. These results suggested that the
hyphal tips were homocaryotic for the dominant nuclear component. However, the heterocaryotic
nature of the mycelium had been previously established by the isolation of a somatic diploid
from which recombinants were subsequently recovered. Manipulating concentrations of supplements
in the minimal medium on which the heterocaryon was grown has resulted in the isolation
of hyphal tips able bo grow on minimal medium. Supplementation of the medium presumably
altered nuclear proportions, thus allowing for the isolation of hyphal tips capable of growth on
minimal medium. In species of Fusarium, isolation of hyphal tips from specific hetercaryons
exhibiting selective incorporation of nuclear types into conidia revealed that a high proportion
of these tips were capable of continued growth on minimal medium. These results suggest that
selection was not severe enough in these heterocaryons to preclude the isolation of heterocaryotic
hyphal tips. These experiments indicate that the inability to isolate hyphal tips capable of continued
growth on minimal is not conclusive evidence against heterocaryosis, but may reflect
severe selection resulting in a high proportion of homocaryotic tips. The results with Fusarium
indicate that selective incorporation of nuclei into conidia need not result in homocaryotic tips.
UNDERKOFLER, JUDITH, and M. R. IRWIN, University of Wisconsin, Madison, Wisconsin:
Inieraction products of new combinations of genes in species crosses of Co1umbidae.-A hybrid
substance is defined as an antigenic product of the erythrocytes of an individual (species hybrid