1. The four possible kinds of heterokaryon of Coprinus lagopus with no, one or both mating-type factors in common (dikaryon, common A, common B and common AB) were produced. Analysis of hyphal tips of common A and common AB heterokaryons has shown that both nuclei may be present in the same hypha.

2. All four heterokaryons are prototrophic when synthesized from two auxotrophic components with different requirements.

3. When synthesized in this way compatible heterokaryons were stable in all tests, but the other heterokaryons showed different degrees of stability. Common B heterokaryons were the most stable and rarely gave rise to monokaryotic mycelia. Dissociation of the common A and the common AB heterokaryon into either component took place much more easily.

4. Comparisons of the growth-rates of wild-type heterokaryons on complete medium show that common A heterokaryons are less vigorous, and dikaryons more vigorous than their monokaryon components. On minimal medium both compatible and common A heterokaryons are less vigorous than their wild-type monokaryon components. The possible reasons for this are discussed.

5. Fruit-bodies have been obtained from both common A and common B heterokaryons. Both types showed normal segregation at the heterozygous locus (B or A), but showed in addition the segregation of new reactions at the ‘homozygous’ locus.

We report here the occurrence of a new spontaneous mutation at the tufted locus present within the crossover-suppressed region of the complete t haplotype fwLub1 This new tufted allele shows complete expressivity of the tufted phenotype without any apparent effect on viability or reproduction. The presence of a tufted mutation within a t haplotype that is cytologically marked by a Robertsonian translocation is particularly useful in genetic experiments aimed at understanding the structure and function of t haplotypes.

The future value of mitochondrial DNA (mtDNA) sequence information to studies in population biology will depend in part on understanding of mtDNA transmission genetics both within cell lineages and between animal generations. A series of stochastic models has been constructed here based on various possibilities concerning this transmission. Several of the models generate predictions inconsistent with available data and, hence, their assumptions are provisionally rejected. Other models cannot yet be falsified. These latter models include assumptions that (1) mtDNA's are sorted through cellular lineages by random allocation to daughter cells in germ cell lineages; (2) the effective intracellular population sizes (nM's) of mtDNA's are small; and (3) sperm may (or may not) provide a low level ‘gene-flow’ bridge between otherwise isolated female lineages. It is hoped that the models have helped to identify and will stimulate further empirical study of various parameters likely to strongly influence mtDNA evolution. In particular, critical experiments or measurements are needed to determine the effective sizes of mtDNA populations in germ (and somatic) cells and to examine possible paternal contributions to zygote mtDNA composition.

The generalized transducing phage F116 has been used to prepare lysates from fast- and slow-growing cultures of Pseudomonas aeruginosa strain 1. These lysates have been used to transduce a number of auxotrophic markers to prototrophy and the ratios of the numbers of transductants obtained with each lysate have been determined. Since the markers are those which have been mapped by conjugation in previous studies it has been possible to compare the ratios obtained for each marker with the relative position of the marker on the chromosome map. If the assumption is made that there is only one circular chromosome in P. aeruginosa strain 1 it is possible to suggest a way in which two apparently unlinked segments might be joined together. It is also possible to suggest that the chromosome replicates sequentially in two directions from a fixed origin.

The duodenal mucosa of normal (+/+) and heterozygous tortoiseshell (Moto / +) female mice was analysed by an ultrastructural histochemical copper localization technique. Copper was rarely detected along the duodenal microvilli of + / + female mice. However, copper was localized on the surface of the brush border of the duodenal mucosa and within pinocytotic vesicles at the base of the microvilli of Moto /+ female mice. The mottled defect may involve a defective intestinal uptake of copper, as well as a faulty copper transport mechanism.

Twenty-six strains of mice were tested for their reaction to four different sweet substances; saccharin, acesulfame, dulcin and sucrose. There was considerable strain variation in the degree to which they found the sweet substances preferable to water. The variation in preference for any one sweet substance is very highly correlated with the variation in preference for the other sweet substances. This is interpreted to mean that there is only one sweetness receptor, although an alternative explanation in terms of variation in psychological motivation is not discounted. The difference between C57BL/6Ty and DBA/2Ty is largely due to a single gene, Sac.

Eight mouse mutants with altered charge or activity of lactate dehydrogenase-1 have been detected in offspring derived from mutagen-treated spermatogonia. Using two chromosome-7 marker genes pooled recombination frequencies are estimated as c−14·4±0·8−p−6·9±0·6−Ldh-1.

Effects of a recessive gene causing high growth (hg) were studied on two major components of the growth axis in mice. Plasma and pituitary levels of growth hormone and plasma levels of insulin-like growth factor I (IGF-I) were measured in three lines homozygous for hg, each compared with a control line of alike genetic background but wild type for the hg locus (Hg). Line Gh (hghg) and line GH (HgHg) are from a line which had undergone long-term selection for high postweaning weight gain; line Ch (hghg) and line CH (HgHg) were extracted from the second backcross of Gh to C57BL/6J; line L54 (hghg) was from the sixth backcross to C57BL/6J (B6) (HgHg). Pituitary GH levels and plasma IGF-I levels were measured in both sexes at 3, 4·5, 6 and 9 wk of age. Plasma growth hormone was measured in 8- to 12-wk-old males at hourly intervals from 08.00 to 17.00. Body weight in lines homozygous for hg at 6 and 9 wk of age was 10–30% greater than in control lines. The ontogeny of this increased growth depended on genetic background. Pituitary growth hormone content was 52% lower in the two hghg lines measured (lines Ch and Gh) than in control lines at 4·5, 6 and 9 wk. Plasma growth hormone levels were also much lower in hg mice, with values only 20–30% of those in their respective controls, hg lines showed consistently low plasma growth hormone levels throughout the 9 hr sampling period, while control lines expressed the characteristic pulsatile hormone secretion. In contrast, plasma IGF-I levels were greater in line Ch (hghg) than in line CH (HgHg) at 3, 4·5 and 9 wk, and were also greater in line Gh (hghg) vs. line GH (HgHg) at 6 wk of age. The results suggest that the growth enhancing effect of the hg gene occurs through an IGF-I-mediated process. In addition, the genetic background itself is also a factor in the phenotypic expression of the gene.

The structural changes induced by X-rays in cells at different stages of spermatogenesis were analysed in salivary gland chromosomes of Drosophila melanogaster and compared with the changes induced by formaldehyde added to the food (FF) of the larvae.

The different stages of spermatogenesis vary in sensitivity to X-rays when measured by the percentage of sex-linked lethals, by the percentage of spermatozoa carrying structural changes, and by the number of changes in 100 spermatozoa. The proportions of the different types of change (T, In, Rp, Df), however, are fairly similar in all stages of spermatogenesis, but entirely different from those found after FF treatment. This suggests that it is the mutagen and not the sensitive stage which is responsible for the characteristic pattern of the FF effects.

The differences between the effects of X-rays and of FF are attributed to the different proportions of potential breaks induced by these two mutagens. Evidence has been presented indicating that while most of FF induced breaks are potential (about 73%), most of the X-rays induced breaks are immediate. For the dose rate used in the present experiment (below 1000 r/min.) only a small proportion (4–10%) of breaks induced by X-rays was found to be potential.

Recessive transformants (col) obtained from v (vermilion) embryos treated with v+ DNA are shown to map at 1−0·02, a position not distinguishable from that of su(s) (suppressor-of-sable) and in agreement with observations indicating phenotypic allelism of col with su(s) mutants. Recombination in the y–col–gt segment of the X chromosome, over a total map length of O·3 units, was studied among the progeny of colv1/y1gtE6cvv1f and y1colgtE6cvv1f/v1 females. The data from both crosses exhibit the following features: (1) recovery of reciprocal recombinants between y1 and col; (2) recovery of reciprocal recombinants between col and gtE6 and (3) striking negative interference in the y–col–gt segment. These results allow three alternative interpretations: (1) that recombination in the y–col–gt segment results from conventional crossing-over, with high coincidence of crossovers in the two subsegments; (2) that it results from symmetrical gene conversion at the col site (col to col+, and col+ to col), which may be accompanied by single cross-overs in either of the adjacent regions; (3) that col behaves like a transposable element, formally symbolized su(s)+·col, and that recombination in su(s)+·col/su(s)+ (i.e. col/col+) heterozygotes results from transposition of the col element from homolog to homolog, accompanied sometimes by crossing-over, either in the y–col subsegment or in the col-gt subsegment.

The heritability (h2) and frequency of the neural tube closure defects, cranium bifidum (CB) and spina bifida (SB), have been estimated for a population of 9-day-old hamster embryos through half-sibling analysis. The average frequency of the total affected embryos per litter is approximately 17% while the pooled estimate for h2 based on between sires and between dams within sires components was 4%. This value points to the importance of environmental factors in contributing to the variance in defect frequencies observed within this population.

The genes responsible for u.v. sensitivity in ten sensitive mutants of E. coli strain B (Bs strains of R. Hill) have been mapped by transduction. The uvr genes of all the mutants able to reactivate u.v.-irradiated phage (HCR+), including Bs2, 4, 5, 6, 7, 9 and 10, were linked, 50–95%, with malB. The gene of Bs12 (HCR) was also linked to malB as is uvr A. The gene of Bs8 (HCR) was linked to gal, that of Bs3 (HCR) to his. Transduction of mal+ from a strain with uvr A, or a mal+ derivative of Bs12, to Bs2, 4, 5, 6, 7, 9 or 10, yielded about 30% u.v.-resistant transductants. A mal+ transduction with Bs2 mal+ as donor and other Bs strains as recipients yielded < 0·1% u.v.-resistant transductants. The malB-linked uvr genes of all mutants (except Bs3 and Bs8) were transducible with metA. The quasi-reciprocal crosses and three-point tests suggested the order of markers as metA malB uvr (HCR+) uvr (HCR). The sensitivity gene of Bs9 was exceptional in that it appeared to lie between metA and malB. The sensitivity gene of Bs11 could not be mapped because transductants to that strain were not possible, nor did it act as a recipient in sexual recombinations.

A method is proposed for estimating effective population size (N) from data on linkage disequilibrium among neutral genes at several polymorphic loci or restriction sites. The efficiency of the method increases with larger sample size and more tightly linked genes; but for very tightly linked genes estimates of N are more dependent on long-term than on recent population history. Two sets of data are analysed as examples.

Over 100 temperature-sensitive mutants of mt-h (apogamic) strains of Physarum polycephalum were isolated either by testing clones of mutagenized amoebae (ATS mutants) or by the more laborious method of testing plasmodia derived from such clones (PTS mutants). When amoebae and plasmodia of each mutant were tested for growth temperature sensitivity on different media (to give optimum growth of each phase), only 21% of 73 ATS mutants and 32% of 31 PTS mutants appeared to be temperature-sensitive in both phases, suggesting that the majority of mutants are phase-specific, as concluded from several similar studies by previous authors. When the mutants were tested on a third medium which allows growth of both amoebae and plasmodia, many of the mutants no longer had a temperature-sensitive phenotype in either phase. Among the remainder, 51% of ATS mutants and 67% of PTS mutants were temperature-sensitive in both phases. It was suggested that certain media have a remedial effect on some temperature-sensitive mutants so that the phenotype is apparently normal. Thus, the proportion of phase-specific mutants may be over-estimated if tests of temperature-sensitivity are done on the different media commonly used for culture of amoebae and plasmodia respectively. It was concluded that the most efficient procedure for isolation of temperature-sensitive mutants expressed in plasmodia is to screen clones of amoebae on a medium resembling as closely as possible that which is to be used for testing plasmodia.