Genetic heterogeneity and accelerated responses to directional selection in Drosophila

Summary Strains set up from single inseminated founder females of D. melanogaster from the same population have been previously shown to differ genetically for the incidence of flies with more than four scutellar chaetae (additional chaetae). Therefore the base population is polymorphic for this trait. This was exploited by carrying out directional selection in lines derived from those strains initially having a high mean chaeta number. This led to far more rapid responses to selection than were obtained in lines derived from strains with lower means, such that in one line a continuous accelerated response was observed for the first 12 generations of selection. A correlated response was found for sternopleural chaeta number at generation 19 of selection, showing that at least some genes may affect both traits.     

Selection for six scutellar chaetae in Drosophila melanogaster

Summary 1. From a base population showing some flies with more than the normal 4 scutellar chaeta phenotype directional selection was carried out and lead to a line with many flies having 6 chaetae. 2. Selection was then practised for 6 chaetae such that the extra 2 chaetae were in the anterior left and anterior right positions on the scutellum. This lead to a line with most flies having this chaeta number and pattern, therefore showing some canalization. 3. Additive genetic activity controlling the increased chaeta number was found on all the 3 major chromosomes.     

Effects of different base populations on selection response in Drosophila

Of two laboratory strains of Drosophila melanogaster used in this study, the +3 strain had slightly higher mean abdominal bristle number and estimated heritability of this character than the Oregon‐R. Their F₁ hybrid exhibited 5 % heterosis. Fourteen generations of the two original strains and the F₃ of the hybrid were selected for high and low numbers of abdominal bristles on the 4th and 5th sternites, at a selection intensity of 20%. A mass‐mated unselected control was maintained for each population. The +3 population responded considerably more to selection for low numbers of bristles than high, and the Oregon‐R population showed a similar, though less marked, tendency; the Crossbred population responded more strongly to selection for high numbers. Except for the Crossbred high selection line, all lines declined in response rate, phenotypic variance, and realised heritability. The average realised heritability of the Oregon‐R and +3 high and low selection lines over 14 selection generations fell short of their predicted base population heritabilities. The deviation from the predicted was particularly pronounced with selection for high bristle number in the +3 line.     

A pair of closely linked genes controlling high scutellar chaeta number in Drosophila

Summary 1. A line selected for high scutellar chaeta number reached a mean of about 16 chaetae in females and 13.5 in males at the 69th generation of selection following an accelerated response to selection which commenced at generation 65 and added five chaetae. 2. The accelerated response can probably be explained in terms of two recessive high chaeta number genes 1.05 cM apart, and which are located between po and vg on chromosome II. The gene closest to vg was found to be scabrous, sca, which causes rough eyes when homozygous and has a pleiotropic effect on scutellar chaeta number. The gene was found in one of the strains used in setting up the selection lines. 3. The results are discussed in relation to other theories of control of the scutellar chaeta system.     

Directional selection in lines founded from different parts of the phenotypic distribution of sternopleural chaetae number in Drosophila melanogaster

Summary Divergent directional selection lines were initiated from base populations founded from parents taken from different parts of the sternopleural chaetae distribution in a cage population of Drosophila melanogaster. Lines founded from parents taken from the central part of the distribution showed greater response and higher realised heritability than lines derived from parents with extreme high or extreme low chaetae number. The results suggest that centrally derived phenotypes have higher heterozygosity for chaetae factors than extreme phenotypes and that these factors have a large effect on the character.     

Morphological variation and preadult viability at one-generation inbreeding in Drosophila melanogaster: A family selection hypothesis

Wing length, thorax length, sterno-pleural and sternal chaetae numbers were determined in offspring of sib crosses in a Drosophila melanogaster population, in relation to egg-to-adult viability. It appears that high between- and within-family variabilities for wing and thorax length together with lack of developmental homeostasis are associated with a high rate of embryonic and larvo-pupal mortality. Effects on chaetae numbers are more controversial. Results suggest that families which are homozygous for particular gene combinations may be eliminated by natural selection. Maintenance of high levels of heterozygosity might result in populations where circumstances foster substantial inbreeding.     

Association of allozyme heterozygosity and sternopleural chaetae number inDrosophila melanogaster

Variation at two polymorphic allozyme loci (Adh and Gpdh) has been assayed in relation to sternopleural chaetae number in flies derived from a cage population ofD. melanogaster known to be under stabilising selection with respect to chaetae number. Central phenotypes were significantly more heterozygous at these loci than more extreme phenotypes. Analysis of the base popylation and of divergent directional selection lines derived from it suggested that alleles atAdh and Gpdh have a pleiotropic effect on chaetae number or are in strong linkage disequilibrium with chaetae factors. In addition, homozygotes atAdh and Gpdh were observed to have a higher variance in chaetae number than heterozygotes. It is concluded that additive and homeostatic models of gene action might both partly explain the higher heterozygosity of central phenotypes.     

SELECTION, PREDICTION AND RESPONSE

1. The biometric approach to selection experiments has been outlined, and found to be rather deficient because it is based on excessively restrictive models which cannot take into account the complex architectures of quantitative traits as are being revealed today. 2. The nature of polygenes is discussed in detail from the theoretical point of view. In out breeding species, some form of the balanced polygenic complex is likely, showing polymorphism for the constituent genes. Although polymorphism is implicit in the argument, definitive evidence for poiymorphisms has only just appeared. 3. There is no evidence that polygenes differ from any other gene. 4. Several artificial selection experiments are described, in particular in Drosophila. By means of appropriate breeding techniques after obtaining responses to selection, genetic activity controlling quantitative traits can be located to chromosomes, and even specific loci found. Such few studies as have been carried out reveal, in general, the types of genetic architecture predicted on theoretical grounds. 5. Selection for behavioural traits is considered briefly and it appears that no new principles are needed, except that careful environmental control and objective measurement present problems. 6. The results of selection for quantitative traits in micro-organisms reveal similarities to results in higher organisms in the few cases where definitive work has been carried out. 7. Work on the simulation of models by computers has not greatly advanced selection experiment theory, mainly because, with few exceptions, linkage has been ignored. 8. The existing theory on which selection experiments are based is inadequate for several reasons. It cannot predict the rate of response to selection nor the ultimate limits to selection, the nature of correlated responses to selection, nor the nature of gene segregation underlying the observed variability. 9. Strains set up from single inseminated founder females from the same population of Drosophila have been shown to differ genetically for several quantitative traits. Therefore the base population is polymorphic for genes controlling these traits. This was exploited by carrying out directional selection on lines derived from those strains showing a high incidence of scutellar chaetae. This led to far more rapid responses to selection than lines derived from strains where the incidence of scutellar chaetae was lower. 10. Ultimately, one can envisage the selection experiment as it is known today being partly replaced by the manipulation of located genes controlling quantitative traits into certain combinations.     

Evolutionary genetics of Drosophila ananassae. I. Effect of selection on body size and inversion frequencies

Adaptive importance of inversion polymorphism has been discussed in Drosophila species at several levels but no study has been carried out demonstrating the individual and combined effects of polymorphic inversions on the fitness of flies through bi‐directional selection. Therefore, artificial bi‐directional selection for thorax length in Drosophila ananassae was carried out for 10 generations. Both, Tukey test for selection difference and regression coefficients of offspring on mid‐parent are highly significant. The realized heritability (h²) in males of both high and low selection lines is more or less similar but is more pronounced in low line females, which suggests the asymmetrical response. This asymmetry in selection is discussed in the light of evidence provided by the study of chromosome inversion frequencies in different selection lines at different generations of selection. Interestingly, chromosome inversion frequency changes towards homozygosity for different gene arrangements in different selection lines. Tests of correlations at G₆ and G₁₀ among different gene arrangements as well as with mean thorax length suggest that 2L‐ST gene arrangement is negatively correlated, while 3L‐ST gene arrangement is positively correlated with thorax length. Furthermore, the present study shows the significant effects of 3L‐ST and 2L + 3L (positive correlation) on thorax length, while 3R‐ST and 2L + 3R show significant effect (negative correlation) on thorax length, which was not evident in the previous study. Present results also suggest how polymorphic inversions and their combinations affect the body size differently in different selection lines. These results suggest that thorax length in D. ananassae is under polygenic control and inversion polymorphism plays crucial role in maintaining body size by modifying genotypic frequency under various selection pressures.     

Sexual dimorphism and interpopulation differences in lizard hind limb length: locomotor performance or chemical signalling?

Intraspecific variation in morphology has often been related to fitness differences through its effects on performance. In lizards, variation in hind limb length can be shaped by natural selection for increased locomotor performance, sexual selection on the number or size of femoral pores involved in chemical signalling, or both. Here, we analyse the selective forces involved in sexual dimorphism and differences in hind limb length between two populations of Psammodromus algirus living at different elevation. Males were more robust and had longer hind limbs and limb segments than females, and low‐elevation lizards had longer limbs than high‐elevation lizards. However, differences in locomotor performance were small and non‐significant, making natural selection for faster runs an unlikely explanation for the observed pattern. On the other hand, males had more femoral pores than females, and lizards had more pores at lower elevation, although the difference was significant only for males (which invest more in chemical signalling). In males, the number of pores, which remains constant along a lizard's life, was not correlated with hind limb length. However, femur length was positively correlated with mean pore size, allowing low‐elevation males to have larger than expected pores, which could increase the effectiveness with which they spread their signals in a dry and warm habitat where chemicals become volatile rapidly. Also, saturation of the sexual coloration of the head was higher for low‐elevation males, suggesting that sexual selection pressures may be more intense. Overall, our results indicate that sexual selection plays a significant role in shaping intraspecific variation in hind limb length. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 318–329.     

The functional morphology and possible taxonomic significance of the parapodia of the maldanid polychaetes Clymenella torquata and Euclymene oerstedi

The morphology of the chaetae of the maldanid polychaetes Clymenella torquata (Leidy) and Euclymene oerstedi (Claparède) (= Caesicirrus neglectus Arwidsson, '11) are described and related to movements observed in the laboratory. Graphs are constructed of the number and length of the neuropodial chaetae of each chaetiger throughout the body of Clymenella and of species of the genera Euclymene, Macroclymene and Axiothella, and show a characteristic and relatively constant pattern for each species. This work suggests that in making taxonomic decisions, more attention should be paid to the overall pattern of the neuropodial chaetae and less to the confusing distinction between aciculae and rostral uncini.     

Genetic control of B chromosome transmission in Aegilops speltoides (Poaceae)

The transmission of B chromosomes in Aegilops speltoides was studied in 1B and 3B plants. 1B plants tend to lose the B, while the Bs are accumulated in 3B plants with respect to Mendelian expectation. The experimental conditions permitted us to deduce the number of Bs transmitted in the egg cells. The quantitative character “mean number of Bs transmitted in the egg cells” shows a large variation in both types of plants. We have carried out a mass selection experiment, selecting those plants with the highest and the lowest values for this character. A genetic component of such variation was demonstrated because significant selection gain was obtained for high transmission rate in 1B plants, and low transmission rate in 3B plants. We have also estimated some fitness variables and their relationship with B transmission by the egg cell. We found that vigor variables are not correlated with B transmission variation. However, in 3B plants there is a significant negative correlation between B transmission rate and fertility, measured as grains per plant and grains per spikelet. It seems that natural selection acts at this level, balancing the strength of the drive and preventing the excessive increase of B number.     

Experimental study of the efficiency of a reduced index in Tribolium

The relative efficiency of a reduced index (IR) relative to a full index (IF) was compared in an experiment with Tribolium. The selection objective included pupal length, adult weight, and egg mass. The reduced index was based on pupal length and adult weight, whereas the full index was based on these traits and also on egg mass. There were five generations of selection with four replicates, and a selected proportion of 20%. For each replicate, an unselected control was produced. Responses were significant in the IF and IR lines. Responses for the selection objective differed significantly between lines (p < 0.01). The efficiency of the IR line relative to the IF line was 0.52, similar to the expected efficiency of 0.51. The IF and IR lines did not differ significantly for pupal length nor adult weight, whereas the response for egg mass in the IF line was significantly different from the response in the IR line. Realized heritability was greater in the IF line (0.63 ± 0.05) than in the IR line (0.37 ± 0.16). The deleted trait (egg mass) has high heritability and genetic and phenotypic correlations nearly equal to zero with the other two traits included in the selection criterion. The results show the importance of including index traits with high genetic value that are independent of other traits, and they could be useful in breeding programs simultaneously considering production and reproduction traits with nearly zero correlations between them.     

Bone length and muscle weight in mice subjected to genetic selection for the relative length of the tibia and radius

Mice which had undergone 5 generations of selection for high and low values of the tibia length/radius length ratio were compared with unselected controls at 10 weeks of age. The ratio responded to selection in both directions. The length of the radius was increased in the low line while the response in the high line was due to an increase in the length of the tibia and a small but statistically insignificant decrease in radius length. High and low line mice were heavier than the controls. The responses of the tibia and femur and of the radius and humerus were generally similar, suggesting the existence of an association between the genetic control of the lengths of the bones of a limb. There were also correlated responses in the weight of the tibialis anterior, biceps brachii and sternomastoid, and there is some evidence to suggest that the weight of a muscle may be influenced by the length of a bone to which it is attached.     

Genetic heterogeneity among the founders of laboratory populations of Drosophila IV. Scutellar chaetae in different environments

The incidence of flies with more than four seutellar chaetae (additional chaetae) has been followed for up to 3 years in 4 lines at 15°C, 20°C, 25°C and 27.5°C derived from each of 3 strains, initially set up from single inseminated females of Drosophila melanogaster, collected in the wild single the same locality at the same time. Within each set of temperature lines, the incidence of additional chaetae was highest in one strain, intermediate in another strain, and lowest in the third. This shows that the differences between lines can be at least partly attributed to differences between the founder females of the three strains, although some divergence occurred between lines within strains in some cases. With respect to quantitative traits, the results indicate that in situations where new habitats are occupied by a small number of colonists, the nature of the founder female is possibly of greater importance than the different environments of the new habitats.Some of this work was supported by the Australian Research Grants Committee.     

ARTIFICIAL SELECTION ON EGG SIZE PERTURBS EARLY PATTERN FORMATION IN DROSOPHILA MELANOGASTER

Pattern formation in Drosophila embryogenesis has been widely investigated as a developmental and evolutionary model of robustness. To ask whether genetic variation for pattern formation is suppressed in this system, artificial selection for divergent egg size was used to challenge the scaling of even‐skipped (eve) pattern formation in mitotic cycle 14 (stage 5) embryos of Drosophila melanogaster. Three‐dimensional confocal imaging revealed shifts in the allometry of eve pair‐rule stripes along both anterior–posterior (A–P) and dorsoventral (D–V) axes as a correlated response to egg size selection, indicating the availability of genetic variation for this buffered trait. Environmental perturbation was not required for the manifestation of this variation. The number of nuclei at the cellular blastoderm stage also changed in response to selection, with large‐egg selected lines having more than 1000 additional nuclei relative to small‐egg lines. This increase in nuclear number in larger eggs does not scale with egg size, however, as nuclear density is inversely correlated with egg length. Nuclear density varies along the A–P axis but does not correlate with the shift in eve stripe allometry between the selection treatments. Despite its macroevolutionary conservation, both eve stripe patterning and blastoderm cell number vary genetically both within and between closely related species.     

The genetic basis of egg lay response to conditioned medium in the flour beetle,Tribolium castaneum

Summary The rate of egg laying in Tribolium castaneum is affected by the quality of the environment and can thus serve not only as a component of fitness of the individual, but also as an adaptation to the long-term survival of the population. In an attempt to determine whether it is an independent genetic character, selection for high and low rates of response was carried jut in beetles from three wild populations. When tests were done on virgin females, and responsiveness, corrected for scale effects, was used as the criterion for selection, separation between the lines was achieved in one generation. It is suggested that in natural populations of Tribolium, the responsiveness of the rate of egg laying to the quality of the environment is controlled by a small number of genes.     

Relative efficiency of selection methods to improve a ratio of two traits in Tribolium

Summary Three generations of upward selection for the egg mass/adult weight ratio were carried out in Tribolium castaneum. The experiment involved five lines: E — selected for increased egg mass; W — selected for decreased adult weight; R — selected for egg mass/adult weight directly; L — linear index selected with economic weights of m ₂: -m ₁ egg mass to adult weight (m ₁ and m ₂ are the means for adult weight and egg mass, respectively); NL — nonlinear index selected. Adult weight (7 days after adult emergence) and egg mass (between 7 and 11 days) were measured. The NL, E, and L lines had the greatest observed responses for the ratio; the R and W lines were not effective in improving the egg mass/adult weight ratio. It was expected that the NL line would be superior to the E, L, and R lines, and that the W line would respond the least. Observed response was significant for egg mass in the NL line, and for adult weight in the E, W, and R lines. Strong selection to increase egg mass seems to represent the optimal criterion for the ratio to be improved. The usefulness of nonlinear indices as selection criteria to improve a nonlinear trait, previously found to be optimal for a trait defined as the product of two component traits, appears to hold also for the selective improvement of the ratio of two traits. Serious limitations expected for direct selection of the ratio have been confirmed in this experiment.     

Selection for divergent emigration rates in laboratory populations ofMusca domestica

High and low emigration rates through a laboratory system were selected for in populations of house flies (Musca domestica L.). Emigration consisted of movement of flies from on plastic box to another by way of a connecting tube. Selection was carried out by first dividing a wild population of flies into two lines and then selecting for movement from the box in one line and against movement from the box in the other line. The selection experiment was performed twice. In both experiments a statistically significant difference between the two strains was obtained in three to four generations of selection. In the second experiment, after 5 genrations the mean number of high emigration rate flies emigrating in 1 hour was 4.37 times the mean number of low emigration rate flies and in 24 hours was 2.81 times the number of low emigration rate flies. The second experiment was terminated after 6 generations, but the first experiment was continued for 35 generations. In this case, divergence ceased in roughly 15 generations. The possible relevance of the findings to laboratory population experiments in which spatial discontinuities are included is discussed.     

The relation of radiation sensitivity to pronuclear chromosome structure

After treatment of spermatozoa of Phryne cincta from a Berlin population (bs) with low crossing-over and from an Alpine population (as) with high crossing-over frequency, egg mortality was ascertained and chromosomal mutation rate determined by examination of the polytene chromosomes of the F₁ progeny. — The as stock exhibits a 1.33 times greater radiation sensitivity than the bs stock, both in chromosomal mutation and in egg mortality rates. There is no evidence that factors other than structural ones account for the differences in radiation sensitivity. From inversion length measurements, it was proposed that a reduction of the spiral diameter of the paternal pronucleus chromosomes is responsible for a higher site number and thereby for a greater radiation sensitivity.Dedicated to Prof. Dr. B.E. Wolf, initiator of this work, in honour of his 70th birthday on September 27, 1978