W. Scharloo

Genetic variation in the timing of reproduction in the Great Tit

SummaryAbout 40% of the population variation in the initiation dates of first clutches within years is genetically determined. The onset of laying, which is determined by the female, is not detectably influenced by spatial heterogeneity of the study area.There is a variable selection favoring early, middle, or late laying in some years. Over the study period as a whole there is a slight net selection for laying relatively late.The implications for a potential rapid evolutionary change are discussed. The conclusion is reached that the population mean might change with rates of up to one week per five generations, which is approximately a decade.

Inbreeding in an island population of the great tit

The deleterious effects of inbreeding may have been recognized since the start of civilization, possibly based on experience in breeding livestock and the occurrence of congenital defects in the offspring of relatives in man. The basis of these deleterious effects could only be understood after the rediscovery of Mendels laws. The discovery of recessive lethal genes in mice and the realization that inbreeding promotes homozygosity followed soon. Because the relative difference between the proportion of homozygotes produced by inbred and by outbred pairs is larger when the frequency of the gene involved is smaller, the noxious effects of inbreeding can be explained by the increase in the level of homozygosity of rare recessive deleterious genes. The data available for man, domesticated animals and plants show that harmful effects of inbreeding are present in all crossbreeding organisms (Lerner, 1954; Cavalli Sforza and Bodmer, 1971; Wright, 1977) and suggest that deleterious recessive genes are numerous. The general occurrence of deleterious recessives was confirmed by studies in Drosophila species. With the help of chromosomes marked with dominant genes and carrying inversions to suppress crossing-over, chromosomes from wild populations were made homozygous. Up to 40% were found to carry recessive lethal genes (Dobzhansky, 1955). The technique which allows such a direct demonstration is presently only available in Drosophila. Only in humans has inbreeding been studied in any depth in situations with a natural partner choice. However, in man relatives seem to be taken as partners more naturally by some people than by others (Bittles, 1977). Little evidence exists on inbreeding depression in animal populations under natural conditions. Although cases of inbreeding have been reported from population studies in birds, where nestlings were ringed and parents identified on the nest, the numbers involved were too small to allow more than a qualitative statement on the effect of inbreeding (Richdale, 1957; Bulmer, 1973; Greenwood et al., 1978). Yet inbreeding avoidance, presumably selected for by inbreeding depression, is used to explain a wide variety of phenomena (see for instance Bateson, 1978; Bengtsson, 1978). There is clearly a need for quantitive data on inbreeding in natural populations (May, 1979). The data from the long-term population studies of the Great Tit (Parus major) carried out at the Institute for Ecological Research at Arnhem (The Netherlands) allow us to give quantitative estimates of the occurrence and the effects of inbreeding. We will compare the occurrence and the detectability in a mainland and in an island population. Reduced hatching success will be shown to exist both for inbred zygotes and in clutches of inbred females.

Heritability of body size in a natural population of the Great Tit (Parus major) and its relation to age and environmental conditions during growth

We have analysed data on weight and tarsus length collected during a long-term study of natural populations of Great Tits to evaluate the relative importance of genetic variation in body size. Some of our data were collected over a 25-year period, and therefore include a relatively large sample of naturally occurring environmental conditions. An overall heritability estimate calculated from the uncorrected mean weights of breeding birds amounts to 0-5. This estimate is unlikely to be influenced by resemblance in environmental conditions between relatives. Heritability estimates based on the size of fledglings vary between zero and the value for adults, depending on the environmental conditions during growth. If the feeding conditions for the nestlings are poor, no resemblance between parents and offspring is observed. Selection against small nestlings acts strongly on the environmental variance. This is concluded from the higher heritability estimates in the same cohorts after survival for at least three months after fledging, compared to measurements on nestlings. Such selection acting differentially on the genetic and environmental components of the phenotypic variance has important consequences for our ability to make predictions of phenotypic change from measured natural selection. Nevertheless, the amount of genetic variation would allow rapid response should selection on adult size occur.

Genetic and Environmental Variation in Clutch Size of the Great Tit (Parus major)

An analysis is made of genetic and non-genetic components in the number of eggs in the completed first clutch of the Great Tit. 1) It is doubtful whether a causal relation between density and clutch size exists. 2) There is hardly a systematic effect of age on clutch size. 3) There is an annual variation in clutch size with similar changes in individual females in the same population, but it is hardly correlated between populations. This emphasizes a lack of genotype-environment interaction. 4) Within populations there is no detectable variation in clutch size that can be attributed to differences in habitat quality. 5) About 40% of the total phenotypic variation in clutch size is genetic variation. Several ways of eliminating a possible resemblance through correlated environments yield the same result. 6) Selection for clutch size is demonstrated in several years. 7) The implications for rapid evolutionary change in mean clutch size are discussed.

Environmental effects on body size variation in Drosophila melanogaster and its cellular basis.

Eight isofemale lines of Drosophila melanogaster were raised at four temperatures and at four yeast concentrations in their food. Temperature and food show a significant interaction in determining wing length and thorax length, affecting mean size per line and genetic variation between lines. The combination of low temperature and poor food conditions leads to a sharp increase in the genetic variation over lines of both body size characters. The increase in genetic variation in wing length under less favourable conditions is due to an increase in genetic variation of both cell size and cell number. Changes in wing area in response to both temperature and food level follow a common cell size/cell number trajectory. Changes in wing size are obtained by line-specific changes in the cellular composition of the wing, rather than by changes specific for the environmental factor.

Genetic variation in egg dimensions in natural populations of the Great Tit

Length and breadth of eggs were measured in ringed populations of the Great Tit. During a part of the study volume and weight were also measured, but this did not give additional information, viz. variation in specific weight of fresh eggs and deviations from calculated volume were within the limits of precision. Only in small eggs are length and breadth positively correlated.In two populations, a major part (60–80%) of the variation in the clutch means of egg length, egg breadth, shape index and egg volume is only found between clutches of different females. The absence of correlation between different female partners of one male and the similarity of female repeatability to heritability estimates based on daughter-mother regression lead to the conclusion that 60–80% of the variation in egg dimensions is genetic.The implications for a potential rapid response to selection resulting in a micro-evolutionary change are discussed.

A dual function of alcohol dehydrogenase in Drosophila

Alcohol dehydrogenase (ADH) of Drosophila not only catalyzes the oxidation of ethanol to acetaldehyde, but additionally catalyzes the conversion of this highly toxic product into acetate. This mechanism is demonstrated by using three different methods. After electrophoresis the oxidation of acetaldehyde is shown in an NAD-dependent reaction revealing bands coinciding with the bands likewise produced by a conventional ADH staining procedure. In spectrophotometric measurements acetaldehyde is oxidized in an NAD-dependent reaction. This activity is effectively inhibited by pyrazole, as specific inhibitor of ADH. By means of gas chromatographic analysis a quick generation of acetate from ethanol could be demonstrated. Our conclusion is further supported by experimental results obtained with either purified ADHF enzyme or genotypes with or without ADH, aldehyde-oxidase, pyridoxal-oxidase and xanthine-dehydrogenase activity. These results are discussed in relation to ethanol tolerance in the living organism in particular with respect to differences found between ADH in Drosophila melanogaster and D. simulans, and in relation to the possible implications for the selective forces acting on ADH-polymorphism.

Critical weight for the induction of pupariation in Drosophila melanogaster: genetic and environmental variation

Timing of puparium formation in Drosophila melanogaster is set by reaching a critical stage at which larvae attain the ability to pupariate. This critical stage is reached at a relatively constant size characterized by the mean critical weight, i.e. the weight at which 50% of surviving larvae pupate without further feeding. The mean critical weight might be affected by larval growth conditions. This hypothesis was tested by determining the mean critical weight in larvae raised at three temperatures and two food levels, for two isofemale lines from two populations. Pupariation probability is a function of larval weight. The two environmental variables affect pupariation probability and mean critical weight differently. Food level does not affect critical weight but affects weight‐independent mortality; higher temperatures lead to a reduction of mean critical weight. Mean critical weight shows substantial differences between lines; the differences are maintained over temperatures. Genetic variation in mean critical weights has ecological and evolutionary implications.

Phenotypic plasticity in morphological traits in two populations of Drosophila melanogaster

Isofemale lines of two populations of Drosophila melanogaster, originating from France and Tanzania, were examined over a range of temperatures. Morphological traits showed distinct patterns in phenotypic plasticity; flies of the two populations differed in shape.

Dual function of the alcohol dehydrogenase of Drosophila melanogaster: ethanol and acetaldehyde oxidation by two allozymes ADH-71k and ADH-F.

SummaryUntil recently the alcohol dehydrogenase of Drosophila melanogaster was thought to act only in the first step of primary alcohol oxidation, producing an aldehyde. Instead, acetic acid is the main product of a two-step process.A rapid procedure was developed for the isolation and purification of two allozymes. The thermostability of the purified enzymes was found to be very different, t1/2 at 35°C, being 45 min and 130 min for ADH-F and ADH-71k respectively.The kinetic parameters of ethanol oxidation by the two purified allozymes were determined within physiological substrate and coenzyme ranges. The use of artificial electron acceptors has a notable influence on the ethanol oxidation: the apparent Michaelis constants increase: the oxidation rate with ADH-71k increases, whereas it decreases with ADH-F.Purified ADH is shown to be able to catalyze the oxidation of acetaldehyde solely in the presence of NAD+, and PMS and MTT as artificial electron acceptors.From the kinetic data the relative in vivo oxidation rates of ethanol by both ADH allozymes were calculated. ADH-F turned out to be somewhat less effective (30%–40%) than ADH-71k. The physiological consequences of these differences are discussed.