J. Maynard Smith

Fertility, mating behaviour and sexual selection inDrosophila Subobscura

SummaryThe eggs laid by outbred females have been collected from the time of mating until the death of the females, and the proportion of eggs hatching recorded. Initially oyer 99 % of the eggs hatch after mating to outbred males; after about 30 days the quantity of sperm in the ventral receptacle is inadequate to ensure fertilization, and few or no eggs hatch. After mating to inbred males, the initial hatch varied from 43 to 84%, and a sharp decline in the proportion hatching occurred after about 10 days. Thus inbred males are inferior both in the quality and quantity of sperm produced.

The genetics and cytology ofDrosophila Subobscura

SummaryThe length, of imaginal life is given for two inbred lines ofDrosophila subobscura, and for the reciprocal hybrids between them. The expectation of life at eclosion of hybrids is approximately twice that of inbred flies.

The effects of inbreeding on rate of development and on fertility inDrosophila Subobscura

SummaryTwo brother-sister mated lines ofDrosophila subobscura, theO andNF lines, were established from wild-caught females. In each generation fast and slow developing flies were selected and mated together. There was no difference in either line between the rates of development of the progeny of fast and slow pairs in the first selected generation.

Continuous, Quantized and Modal Variation

Intraspecific variation may be continuous, or it may be quantized, if the number of structures present is always an integer. If there is some modal number of structures present in almost all individuals, variation is said to be modal. A developmental process is defined as one of ‘simple quantization’ if, first, it gives rise to an integral number of structures, and secondly, if the number of structures formed depends on the ratio between two continuous variables, for example the field size and the chemical wavelength in the model suggested by Turing (1952). Whether variation is quantized or modal will then depend on the accuracy with which these continuous variables are regulated. The larger the modal number, the more accurate must this regulation be. Data on the range of continuous variation within animal populations suggest that simple quantization cannot give rise to modal numbers greater than about 5 to 7. Yet modal numbers of 30 or more occur. Three processes which might account for this discrepancy are suggested, and evidence is presented to show that two of them occur. These are ‘multiplicative’ processes, involving successive processes of simple quantization, and ‘chemical counting’ processes, depending on qualitative differences between successively formed structures. The relevance of processes of quantization to the genesis of two-dimensional patterns is discussed.

Asymmetrical response to selection for rate of development in Drosophila subobscura

Starting from a geographically hybrid foundation population of Drosophila subobscura , selection for fast and for slow development has been practised without inbreeding on a diet with an unusually high level of protein. Realized heritabilities in the fast and slow lines were + 0·063 ± 0·029 and + 0·186 ± 0·031 respectively. A half-sib analysis of the foundation population and full-sib analyses of the first two and the last two selected generations were carried out. Hybrids between the two lines were approximately intermediate between their parents. Two types of genetic explanation of the asymmetrical response are discussed. The first assumes directional dominance of alleles for fast development. Such an assumption can explain the asymmetrical response, but runs into difficulties in explaining the nature of the genetic variance in the selected populations and the intermediacy of the hybrids between the two lines. A second assumption, which appears to fit the facts better, is that there exists a ‘developmental barrier’ preventing development at a rate appreciably faster than that of the foundation population. In physiological terms this implies that more rapid development requires a more profound modification of the population than could be achieved by a few generations of selection. In genetic terms, it implies epistatic interactions between genes at different loci: gene substitutions at a given locus which increase development rate on a genetic background causing slow development have little or no effect on a genetic background causing rapid development. In other words, there is a law of diminishing returns as more and more alleles for fast development are accumulated in the genotype. It is suggested that genetic situations of this kind may be common in populations which have been exposed to directional selection for a long time in reasonably large populations, either in nature or in domestication.

Sex-limited inheritance of longevity inDrosophila subobscura

7. SummaryAdult expectation of life ofDrosophila subobscura at 20°C. has been measured for nine inbred lines, for two kinds ofF1 hybrid between inbred lines, for theF1 from wild females caught in Kent, and for theF1 andF2 from wild females caught in Galilee.There were large differences between inbred lines, but most inbred populations had a lower mean life span than outbred ones, and were more variable than either theF1 hybrids or the populations derived from wild females.Significant sex differences in longevity were found in six of the nine inbred lines, in one of the twoF1 hybrid populations, and in the Galilee population. In all, four sex differences in each direction were found. This shows the importance of genes with sex-limited effects on longevity,i.e. with different effects on males and females.In the Galilee population, correlations between relatives of the same sex were higher than between relatives of unlike sex, suggesting that genes with sex-limited effects were responsible for an appreciable part of the differences between families in the Galilee population. No effects of such genes could be detected in the Kent population.Parent-offspring correlations in the Galilee population were as large as sib-sib correlations, so that there is no evidence for the presence in that population of genes with heterotic effects on longevity.

Genetics and cytology ofdrosophila subobscura

SummaryFive stocks ofDrosophila subobscura are described, in which the flies fall into two classes, recognized by genetic markers, respectively heterozygous and homozygous for genes on chromosome 5. In one of these stocks the genetic heterozygotes are also inversion heterozygotes.The relative viabilities of the two classes are determined by comparison with the expected 1: 1 ratio in the progeny of mass cultures. In all five stocks there is an excess of the heterozygous class; the experiment has been repeated at a higher temperature for two stocks, and a similar excess found. Eeasons are given why the excess of heterozygotes cannot be ascribed to the effect of the genetic markers present. The fact that an excess is found in all five stocks shows that the greater viability cannot be due to position effect in inversion heterozygotes. By outcrossing one such stock to an unrelated stock, it has been possible to compare the viabilities of two classes of flies, both heterozygous for genes on chromosome 5, and one of which is also composed of inversion heterozygotes. In this experiment there was a shortage of inversion heterozygotes. It is concluded that the greater viability is due to heterozygosity at unmarked loci on chromosome 5.Comparisons have also been made of the rate of development from egg-laying to eclosion at two different temperatures. A rise of 6° F. reduces mean eclosion time by 4.6 days. Males develop more rapidly than females, the mean difference being approximately half a day. The genetic markers present are found to have some effect on eclosion time, but when these effects have been allowed for, it is found that heterozygous flies develop more rapidly than homozygous ones. This difference is much greater at the higher temperature, so that heterozygous flies respond to a rise in temperature by a greater increase in rate of development. At a given temperature, however, the variance of eclosion time is always less in heterozygous than homozygous flies.The bearing of these facts on the polymorphism for gene order found in wild populations ofD. subobscura is discussed.The authors wish to express their gratitude to Prof. J. B. S. Haldane, F.R.S., and Dr H. Spurway, who have made many helpful suggestions in the course of the present investigation, and to Dr T. Koske for permission to quote the results of her as yet unpublished observations on the cytology of the stocks used.Five stocks ofDrosophila subobscura are described, in which the flies fall into two classes, recognized by genetic markers, respectively heterozygous and homozygous for genes on chromosome 5. In one of these stocks the genetic heterozygotes are also inversion heterozygotes. The relative viabilities of the two classes are determined by comparison with the expected 1: 1 ratio in the progeny of mass cultures. In all five stocks there is an excess of the heterozygous class; the experiment has been repeated at a higher temperature for two stocks, and a similar excess found. Eeasons are given why the excess of heterozygotes cannot be ascribed to the effect of the genetic markers present. The fact that an excess is found in all five stocks shows that the greater viability cannot be due to position effect in inversion heterozygotes. By outcrossing one such stock to an unrelated stock, it has been possible to compare the viabilities of two classes of flies, both heterozygous for genes on chromosome 5, and one of which is also composed of inversion heterozygotes. In this experiment there was a shortage of inversion heterozygotes. It is concluded that the greater viability is due to heterozygosity at unmarked loci on chromosome 5. Comparisons have also been made of the rate of development from egg-laying to eclosion at two different temperatures. A rise of 6° F. reduces mean eclosion time by 4.6 days. Males develop more rapidly than females, the mean difference being approximately half a day. The genetic markers present are found to have some effect on eclosion time, but when these effects have been allowed for, it is found that heterozygous flies develop more rapidly than homozygous ones. This difference is much greater at the higher temperature, so that heterozygous flies respond to a rise in temperature by a greater increase in rate of development. At a given temperature, however, the variance of eclosion time is always less in heterozygous than homozygous flies. The bearing of these facts on the polymorphism for gene order found in wild populations ofD. subobscura is discussed. The authors wish to express their gratitude to Prof. J. B. S. Haldane, F.R.S., and Dr H. Spurway, who have made many helpful suggestions in the course of the present investigation, and to Dr T. Koske for permission to quote the results of her as yet unpublished observations on the cytology of the stocks used.

The Expression of Hybrid Vigour in Drosophila subobscura

By hybrid vigour we mean the possession by outbred organisms of a number of characters which would confer fitness in a wide range of environments. This definition deliberately includes both cases where heterozygous individuals are fitter in a Darwinian sense in the wild, and cases of species or other distant hybrids showing vigour in, for example, efficient utilization of food, long life, or high resistance to disease, although of low fitness because of infertility or for some other reason. Both types of phenomenon are well known, but whether it is convenient to consider them under the same heading will depend on the aspect of the problem being studied. For example, Dobzhansky (1950) preferred to divide hybrid vigour into ‘euheterosis’ as found in wild populations, and ‘luxuriance’ as exemplified in species hybrids. Such a distinction is a helpful one when the aspect studied is the role of natural selection in maintaining a balanced polymorphism in the wild. However, the high level of heterozygosity maintained by natural selection in many wild populations suggests that for many pairs of alleles the heterozygote is fitter than either homozygote. Where we have some knowledge of the mode of action of the different alleles present in a wild population at a given locus, as we have, for example, for the blood groups in man, it appears that different alleles produc qualitatively different substances. This has led to the suggestion that outbred organisms may be biochemically more versatile (Haldane 1948, 1954; Robertson & Reeve 1952). It is such versatility that all ‘hybrids’ may have in common, whether they result from outbreeding in the wild or from distant crosses made in the laboratory.

Radiation and ageing in insects

Abstract It does not seem possible at present to decide whether radiation reduces the life span of insects primarily by causing mutations in the nuclei of somatic cells. But some tentative conclusions can be drawn: 1. (i) The theory that life-shortening is primarily due to recessive somatic mutations can be rejected, for two reasons. First, if the theory were true, the difference in radio-sensivity between haploid and diploid male Habrobracon, and between male and female Drosophila, would be greater than it is. Second, the theory predicts that in females reduction in life span should be proportional to the square or to some higher power of the dose, whereas it is approximately proportional to the dose. 2. (ii) The facts can be explained if it is assumed that life-shortening is due in part to dominant and in part to recessive mutations, but it does not follow that this assumption is true since the doses involved are large enough to cause cytoplasmic damage; if it is true, then only dominant mutations have a significant effect on individuals diploid at all loci. 3. (iii) It is true that adult cells are many times more resistant to the induction of recessive lethals than are sperm, or, as seems more likely, that only a small fraction of genes are needed in any particular adult tissue. 4. (iv) It is likely that many gene changes have a dominant lethal effect in a zygote but not in an adult cell. If dominant mutations are important in radiation-induced life-shortening, they are likely to be mutations in the systems which control the activities of genes.