Four ascospore-pigmentation mutants were crossed with their wild-types by methods giving a considerable degree of synchrony in perithecial development; segregation patterns were scored from perithecia at different stages of maturity. The observed second-division segregation frequencies for asco in N. crassa were at a maximum when little or no ascal dehiscence had occurred but decreased markedly as dehiscence proceeded. No significant change in this frequency with maturity occurred for tan spore in Neurospora but in S. fimicola the observed second-division segregation frequencies for gray and hyaline rose with increasing perithecial maturity. As similar changes in this frequency were observed when asci were mounted in water instead of the 2 m sucrose solution normally used, it was concluded that changes in this frequency with maturity generally resulted from a differential maturation and bursting of asci with different spore arrangements, rather than from changes in crossover frequency in successive meioses. Some evidence of the latter phenomenon was found in two of the Sordaria crosses.

In a cross of asco × rib-1 in Neurospora, the observed frequency of recombination between the two loci was significantly higher in dispersed spores collected from young perithecia than in those from more mature perithecia: this effect was probably an artifact resulting from the differential maturation and bursting of asci with different divisions of segregation for asco. In a cross of crisp × + in Neurospora, an excess of crisp progeny was obtained from dispersed spores and an excess of +-progeny from spores from harvested perithecia. These deviations from the expected 1:1 ratio for cr: + were thought to result from a differential maturation of asci with different segregation patterns for crisp and the failure of many dehiscing asci to expel all eight spores.

Hypotheses to explain the various phenomena observed were discussed. Suggestions were made concerning the avoidance of bias from the differential maturation and bursting of asci in experimental procedures used in the study of recombination and gene conversion. This bias may be responsible for some phenomena previously attributed to events at meiosis.

Four crosses were made between bacteriophages T4 and partially non-excludable T2 exr+4. Each cross was designed as a four-factor cross: in total seven pairs of homologous genes were used as markers, extending over half the circular map from the locus for adsorption specificity (h+) along the map segment for the early functions to osmotic-shock sensitivity (o).

The frequencies of T2 markers recovered in the progeny show polarized exclusion: the frequencies are low for h+2 at the beginning of the map segment for the early functions and increasing to o+2. Recombination of T2 exr+4 is restricted for all loci concerned and the recombinational events are probably non-reciprocal. Restriction of sequential replication from a fixed starting point is proposed to explain polarized exclusion of T2 genes. Non-homology is dicussed in relation to restricted recombination.

Selection for and against the canalized phenotype in scutellar bristles was attempted in two selection lines and a randomly selected line was used as control. The selection lines were the Decanalization line (D) and the Canalization line (N). The D line was maintained by matings of scute males (scwbl) with three scutellars with wild-type females (scwbl/yw) with five bristles, in the N line scute males with four bristles were mated with wild-type females also with four bristles, while in the C line males and females of the above genotypes were selected at random. The lines were established from a sample of flies taken from a line selected for high scutellar numbers.

After eighteen generations of selection the C line was characterized by a regression of mean bristle number without appreciable change in variance. Relative to the N line, the D population showed a lower proportion of flies having four scutellars, a higher variance in bristle numbers, and a higher proportion of four-bristle scute flies having abnormal patterns.

Two alternative hypotheses were advanced to account for the results of this experiment. The first postulated a relative change in the widths of the four-bristle canalization zones in the selection lines, while the second suggested a relative change in frequencies of specific modifier genes for scutellars in scute and in wild-type genotypes of the lines. The evidence favours the latter hypothesis.

The genetic consequences of autogamy have been analysed in the autogamous strain A-25 of E. minuta. During autogamy as well as during conjugation four pronuclei (two genetically identical pairs) are formed in each individual. In conjugating pairs, any two of the four pronuclei of the autogamous conjugant may participate in synkaryon formation. Consequently, an 8:13 segregation ratio of clonal and synclonal mating-type inheritance has been found to occur in the progeny obtained by crossing A-25 animals with non-autogamous animals. During autogamy, synkaryon formation appears to be brought about by a preferential karyogamy of genetically dissimilar nuclei more often than is expected on a random basis. Therefore, heterozygosity is usually maintained after autogamy in spite of the extreme inbreeding characteristics of this sexual process.

An investigation of mitotic segregation and recombination in A. niger gave the following results:

1. Thirty-one non-allelic markers have been assigned to six linkage groups (containing 11, 9, 6, 3, 1 and 1 markers respectively) by the analysis of haploid mitotic segregants from synthesized diploids.

2. The sequence of nine markers in one linkage group was determined and some of the map intervals were estimated by the analysis of haploids, recombinants for linked markers.

3. Almost all the haploid segregants were obtained on medium supplemented with the aminoacid analogue, p-fluoro-phenylalanine, the action of which is interpreted as an induction of chromosome losses.

4. The rates of mitotic crossing-over and haploidization are much higher than in the sexual species A. nidulans and the data support Pontecorvo's (1958) suggestion that the parasexual cycle can be a substantial alternative to the sexual cycle.

1. Complex hybrids were produced having twenty-nine chromosomes, consisting of one telocentric and twenty complete chromosomes of T. aestivum (2n = 6x = 42), seven complete chromosomes of Ae. speltoides (2n = 2x = 14) and one telocentric chromosome derived from A. elongatum (2n = 10x = 70). The presence of the Ae. speltoides genome permitted pairing between homoeologous chromosomes at meiosis and the behaviour of the two telocentric chromosomes was observed.

2. The A. elongatum chromosome was seen to pair with chromosomes homoeologous to those of group 6. There was no evidence that it paired with chromosomes of any other group.

3. When the A. elongatum telocentric and those of 6A and 6D occurred in the same configuration it was evident that the telocentrics 6A and 6D were for corresponding chromosome arms, and the A. elongatum telocentric for the opposite arm.

4. The average rate of pairing was much lower for the A. elongatum telocentric than for wheat telocentrics. Previous studies had indicated very good genetic compensation of the A. elongatum chromosome for chromosomes 6A and 6D. It was therefore indicated that genetic equivalence and pairing affinity were not closely related in this case. Some implications of this are discussed.

Twenty-four lines were bred from a base population of outbred Q mice by continued full-sib mating. Inbreeding depression in litter size at birth was observed. This decline in litter size was analysed in terms of ovulation rate, the incidence of preimplantation mortality and the incidence of postimplantation mortality. Pregnant females were dissected at 17½ days' gestation and the numbers of corpora lutea, or eggs, and of live and dead embryos were counted. Matings were arranged so that separate estimates of the effects of inbreeding in the mother and in the litter on the components of litter size could be obtained.

In the first generation of inbreeding when the inbreeding coefficient of the litter was raised from 0 to 25% decline in litter size was attributable to an increased incidence of preimplantation mortality.

In the second and fourth generations decline in litter size was attributable to (1) a reduction in the number of eggs ovulated by the inbred mothers, (2) an increased incidence of preimplantation mortality which resulted from inbreeding in the mother. No evidence of significant effects on mortality of inbreeding in the litter was obtained in the later generation of inbreeding.

These findings are discussed in the context of previous work on the effects of inbreeding and crossing on litter size and its components in mice and pigs.

1. Selection for long or short wings was carried out in three pairs of lines of which two were started from the same basic population. At the end of the selection experiment the frequencies of the chromosomal arrangements were determined in each line and compared with the corresponding original frequencies to see if the chromosomal polymorphism of Drosophila subobscura is related to the genetic variation of size in a regular way.

2. The three pairs of lines show an asymmetrical response to selection. Selection for long wings is less effective in changing the mean of the population than selection for short wings.

3. Heritability is lower in the lines selected for long wings, especially in the later generations of selection.

4. Viability shows a very small decrease during selection in most selected lines.

5. In no case does the coefficient of variability show statistically significant changes, but there is a general tendency for it to decrease.

6. Selection for long wings favours combinations heterozygous for the standard chromosome orders and specific complex inversion orders. Selection for short wings generally fixes in homozygous combination specific complex inversion orders.

7. The heterozygous combinations of the standard chromosomal orders, which are most frequent in northern populations, when combined with the complex inversion orders which are most frequent in southern populations, have a heterotic effect on size.

8. The complex inversion orders usually fixed in homozygous combinations in the lines selected for short wings are those most frequent in southern populations which have a smaller mean size than northern populations.

9. The blocks of genes integrated in ‘supergenes’ in the inverted segments of the chromosomes of D. subobscura differ in genetic composition with respect to the control of wing length.

Three parameters of diuretic response to water-load (diuretic lag, maximum rate of diuresis and ratio of total output to input volume) have each responded to selection from a genetically heterogeneous base stock with significant realized heritabilities. Selection has been continued for ten generations and has produced mice more extreme than any of the founder strains. In the selection lines for ratio of output to input, correlated responses in salt excretion during the course of the diuresis, relative adrenal weight, the relative size of the renal medulla, and the quantity of renal collecting ducts, have been observed. These correlated responses are accounted for in terms of renal and adrenal physiology. The elucidation of the individual genetic determinants of the responses is discussed.

We have isolated a number of λ dg HFT lysates which carry proximal fragments of the galactose operon. Most of these have been shown to be different, and each terminates in either the kinase or transferase cistron. They divide the kinase cistron genetically into seven blocks of mutants, and the transferase into eight.

When a λ dg is used to transduce a bacterium which carries a mutation in a cistron which is intact in the λ dg the transduction frequency is high in the presence of λ-helper. This is attributed to integration of the transducing fragment at the λ-attachment site and complementation between the two operons in the heterogenote. When the same λ dg transduces a mutation lying in the cistron in which the λ dg terminates, so that recombination within the galactose operon is obligatory, the transduction frequency is 10 to 1000 times less.

In such cases there is a general increase in transduction frequency between distal mutations (i.e. those lying near the termination of the deletion) and proximal mutations, but the relationship does not hold for many individual pairs of mutants, probably due to physiological differences between the bacterial strains.

A new neurological mouse mutant, Nijmegen waltzer (symbol nv), that causes circling and head-shaking without deafness, a marked tendency for aggressiveness, and reduced weight, is described. It is a single autosomal recessive, with complete penetrance and variable expression, and is probably located in linkage group I.

Recombination within the Y locus in Ascobolus immersus

By A. Kruszewska and W. Gajewski

Vol. 9, No. 2, p. 159 (1967)

In the legend to Fig. 1, p. 165, for ‘(in %)’ on lines 4 and 5 substitute ‘(× 103)’.

Recombination and incompatibility in Schizophyllum

By G. Simchen

Vol. 9, No. 2, p. 195 (1967)

On p. 207, line 9, for ‘½(nα + 1)’ read .