The number of blastoderm cells in Drosophila whose descendants form adult structures has frequently been estimated from genetic mosaics. Data from somatic recombination (method I) and gynandromorph (method II) mosaics both yield very low estimates, e.g. about 10–20 progenitor cells for the eye and antenna, wing or leg.

In gynandromorphs the mosaic dividing line has a random orientation on the blastoderm. In the 6000 cell blastoderm it should be very unlikely that the mosaic dividing line passes through any small patch of only 10–20 cells. Yet it has been reported that 10–25% of eye/antenna, wing or leg disks in gynandromorphs are mosaic. Thus the frequency of mosaicism data seems to be in contradiction to the progenitor population estimates. Similar discrepancies are found in the data for other adult structures.

In this paper we derive a formula for estimating the number of cells in a blastoderm patch from the frequency with which the gynandromorph dividing line passes through it (method III). In a second method (method IV) we use the maximum distances inside the progenitor areas on a fate map to estimate the progenitor patch size. These two estimates agree closely with each other. We find, e.g. that 50–100 cells are in the patches from which the eye/antenna, wing or leg disks derive.

We examine a number of possible explanations for why the first two estimates are so much smaller than the last two. The former estimates refer to the number of progenitor cells which actually have descendants in the adult structure; the latter estimates refer to the total patch area in which the progenitor cells sit. With the present information the most reasonable conclusion is that the progenitor cells for the adult structures are dispersed among other cells which have different developmental fates. If confirmed by experiment, this result has many implications for the process of determination.

A male common shrew trapped at Bonn/BRD was found to have a karyotype differing from the normal karyotype by a dissociation of a large metacentric chromosome. Similar findings are hitherto unknown in mammals.

1. Mate-killer paramecia (derived from stock 540, P. aurelia) were treated with ribonuclease and 8-azaguanine to determine the effect of these two substances on the mu particles and metagons.

2. Ribonuclease destroyed the metagons in living paramecia but had no direct effect on the viability of the mu particles. The latter were however destroyed precisely after one fission of cells whose metagons had been eliminated by ribonuclease treatment.

3. Re-synthesis of metagons was found to take place between the second and third fissions after treatment with ribonuclease, if the dominant gene M2 was present.

4. Increasing the concentration of ribonuclease in the external medium, or in the duration of exposure of paramecia to ribonuclease, resulted in destruction of increasing proportions of metagons.

5. 8-azaguanine did not destroy metagons already present, but destroyed the mu particles immediately.

6. It is concluded that the results obtained with these two substances are consistent with the view that RNA is an essential constituent of metagons.

A method of using information on the location of markers to improve the efficiency of markerassisted selection (MAS) in a population produced by a cross between two inbred lines is developed. The method is closer to mapping QTL than the selection index approaches to MAS described by previous authors. We use computer simulations to compare our method with phenotypic selection and two selection index approaches, simulations being performed on three genetic maps. The simulations show that whilst MAS can be considerably more efficient than phenotypic selection differences between the three MAS methods are slight. Which of the MAS methods is best depends on a number of factors: in particular the genetic map, the time scale under consideration and the population size are of importance.

The killing action by mu toxin, which is contained in the cytoplasm of stock 540, Paramecium primaurelia, was demonstrated against the various stocks of paramecia by means of microinjection. Most of the toxin is present in the soluble fraction of the host cytoplasm. The toxin was precipitated by ammonium sulphate at 50–80% saturation, and was almost completely inactivated by incubation at 60 °C for 30 min. Pre-autogamous paramecia were more sensitive than post-autogamous ones to the toxin. Paramecia which bear endosymbionts were generally resistant to the mate-killer toxin.

1. By the use of intervarietal chromosome substitution lines, the poor crossability with rye (S. cereale, 2n = 14) of the wheat (T. aestivum, 2n = 42) variety Hope was shown to be determined by chromosomes 5A and 5B. The genes Kr1 and Kr2, which are responsbile for poor crossability (Lein, 1943), are probably located on chromosomes 5B and 5A respectively.

2. Crossability is actively inhibited by the dominant alleles, Kr1 and Kr2, of Hope and is apparently not enhanced by the recessive alleles, kr1 and kr2, of the readily crossable variety Chinese Spring.

3. The inhibition of crossability with rye conferred evolutionary and agricultural advantage upon wheat by preventing the production of sterile wheat-rye hybrids which could be regarded as weeds generated from within the crop.

An experimental evaluation of the effect of mating systems and selection upon an additive trait thought to be highly heritable was made. There were two similar replications. Each consisted of a mass selected and randomly selected group, with five mating systems within each group.

Realized heritabilities in the mass selected lines were considerably less than was expected prior to the initiation of the experiment, and averaged approximately fourteen percentage points less than heritability estimated from the zero generation. This in turn resulted in smaller correlations between the genotypes of mates than had been previously expected in the assortatively and disassortatively mated lines.

The average response of the mass selected, assortatively mated lines was slightly more than the mass selected, randomly mated lines, though not statistically significant. This result seems to conform to theoretical expectations.

In the mass selected lines, estimates of phenotypic and genetic variance declined regardless of mating systems. There was a tendency for phenotypic variances to decrease in the randomly selected lines, but this was not the case for estimates of genetic variance.

As an aid to selection, it seems that assortative mating would be of little value with traits of low or intermediate heritability but might be useful if the trait is highly heritable.

An isolation method for N-methyl-N′-nitrosoguanidine-induced catalase negative mutants of P. shermanii based on replica plating is described. In contrast to previous methods, it extends to the early stages of tetrapyrrole biosynthesis which are common in both corrins and porphyrins. It may thus aid in elucidating the mechanism and control of porphyrin and corrin biosynthesis. Some preliminary results are discussed.

A model of genetic variation of a quantitative character subject to the simultaneous effects of mutation, selection and drift is investigated. Predictions are obtained for the variance of the genetic variance among independent lines at equilibrium with stabilizing selection. These indicate that the coefficient of variation of the genetic variance among lines is relatively insensitive to the strength of stabilizing selection on the character. The effects on the genetic variance of a change of mode of selection from stabilizing to directional selection are investigated. This is intended to model directional selection of a character in a sample of individuals from a natural or long-established cage population. The pattern of change of variance from directional selection is strongly influenced by the strengths of selection at individual loci in relation to effective population size before and after the change of regime. Patterns of change of variance and selection responses from Monte Carlo simulation are compared to selection responses observed in experiments. These indicate that changes in variance with directional selection are not very different from those due to drift alone in the experiments, and do not necessarily give information on the presence of stabilizing selection or its strength.

Mitochondrial DNA (mtDNA) variation in Drosophila simulans was studied to determine whether the cytoplasmic state of mtDNA heteroplasmy persists in natural populations in Réunion. For this purpose, 172 isofemale lines, newly collected from two local populations, were examined, among which three types of mtDNA (siII, siIII and siIII′) were found, based on the Hpa II restriction pattern. Ten of the lines were heteroplasmic for a combination of siII and siIII, as determined by autoradiography. The same type of heteroplasmy had been noted in one of the two local populations 8 years before (Satta et al. 1988). The present results suggest that the heteroplasmic state occurs recurrently in natural populations of D. simulans in Réunion.

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.

A synthetic lethal system involving homozygous combination of the non-allelic eye mutants eye-gone and eyeless is described. The major lethal crisis is towards the end of the pupal instar although a number of double homozygotes die at pupation. The pupal lethal crisis can be overcome in a number of individuals by the experimental removal of the operculum from differentiated pupae. However, only a few of the resulting enclosed double homozygotes proved fully viable, the remainder dying almost immediately after emergence. Dissection of the surviving adults revealed that the brain is located in the anterior thorax.

There is a strong genetic component to fertility differentials among individuals of ponderosa pine. Prolific cone-producers as a group were markedly different from low cone-producers at the three protein loci which were monitored. The two groups did not differ significantly in age, but trees with high cone production had slower growth rates and smaller diameters than trees with low cone production. To our knowledge, these results provide the first demonstration of fertility differentials associated with specific genes in a woody plant.

Different types of chimerism result from double fertilization. Differences result from fertilization of first and second polar bodies and from early fusion of two zygotes. These three types of chimerism have different phenotypes or different proportions of possible phenotypes, and variation is greater in respect of gene loci close to the centromere. The phenotype frequencies in the different types of chimerism are tabulated and the phenotype frequencies expected in different mating types are shown. Studies of such cases offer the only method of autosomal gene localization relative to the centromere at present available in man.