Experimental comparison of methods for simultaneous selection of two correlated traits in Tribolium

Summary Two lines of Tribolium castaneum were selected in each of three replicates for egg laying between 7 and 11 days after adult emergency and for adult weight at 12 days, using theoretical (IT) and empirical (IP) index selection methods. Index coefficients were given empirically in the IP line and they were adjusted in the successive generations of selection according to the results obtained in the previous ones. Highly repeatable selection responses in all replicates occurred in both lines for the aggregate genotype (egg laying plus adult weight) and for each individual trait. The IP line tended to increase slightly more than the IT line for aggregate genotype and egg laying, while the highest response in adult weight was obtained with the IT method. The two methods gave consistently different responses in each replicate. The expected results were that IT selection should not exceed IP selection for the aggregate genotype and egg laying while theoretically the IT method should have been superior for increase adult weight. Theoretical expectations for adult weight have been fulfilled in practice. The IP method would be preferred in a practical sense because of its simplicity and freedom from need of parameter estimation.     

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.     

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.     

Egg lengths inDrosophila melanogaster and correlated responses to selection

Egg lengths were studied inDrosophila using three lines, namely a high line, where selection for increased egg length was carried out, a low line where selection for decreased egg length was carried out, and a control line. Selection was more effective in the low than the high line. Adult fly weight assayed in the 18th and 19th generations of selection responded similarly to egg length. Sternopleural chaetae number, however, responded more in the high than in the low line. In the high line the response occurred earlier than for egg length. This rather surprising result can probably be explained by assuming that the genetic architectures controlling chaetae number and egg length differ.     

Long-term selection for 8-week body weight in chickens — direct and correlated responses

Summary A bidirectional selection experiment for 8-week body weight in chickens was conducted. In addition to 27 generations of selection, random samples were chosen from each selected line in generations 6, 13, 19 and 26 to initiate lines in which selection was relaxed. Genetic change was evident in the selected high-weight line through the first 75% of the study after which response in the direction opposing selection occurred. Selection for low body weight resulted in considerable reduction in body size, particularly in the last quarter of the study. Correlated responses evaluated were body weight at several ages, conformation, age at onset of lay, various reproductive and egg quality traits and ratio (female: male) of some traits. Data from lines where selection was relaxed indicated that natural selection opposed artificial selection with the effects greater in later generations.     

The genetics of foraging behaviour: artificial selection for food choice in larvae of the fruitfly,Drosophila melanogaster

Artificial selection for choice between high protein content and high energy content of food was performed for 44 generations of fruitfly larvae. There were significant responses to selection both in a line selected to choose high energy food and in a line selected to choose high protein food. Both lines showed a significantly higher preference for the selected food type than did the control line. The line selected for high energy food also showed a significant increase in response over the generations. Since the selection resulted in lines that differed significantly from controls and from each other, there must be some genetic basis for food choice behaviour. Realized heritability was very low in both selected lines. Viability, as measured by larval weight, pupation time, emergence time and adult size, was significantly higher for flies raised on high protein food than for those raised on high energy food.     

Selection for High Adult Body Weight in Drosophila Populations with Different Structures

Selection for high adult body weight in Drosophila melanogaster was practiced for 18 generations in three selection lines. These lines were genetically similar and of equal size but different in population structure. One line represented a large mass-selected, random-mating population, while the other two lines simulated large populations that had been subdivided into partial isolates or demes. Mass selection and random mating occurred within each deme. These two subdivided lines were different only in the rate of effective migration among the demes (5% and 10%). Selection intensities of approximately 20% were applied to these populations. A fourth line served as a random mating control. Heritability of adult body weight in the base population was estimated to be 0.58± 0.22. The results indicate that significantly greater responses were achieved in the subdivided lines than in the large mass-selected line, in spite of the fact that larger selection differentials were applied to the latter. No significant differences in response were observed between the two subdivided lines. Wright (1930, 1931) postulated that selection would be most efficient in subdivided populations with limited interdeme migration. The present findings appear to support this theory.     

Life history trade-offs and response to selection on egg size in the polychaete worm Hydroides elegans

In order to examine the genetic relationships among life-history traits in a hermaphroditic species we used artificial selection for increased egg size and measured correlated responses across the life cycle of the serpulid polychaete Hydroides elegans, a protandrous sequential hermaphrodite. We recorded sex ratios across generations, and measured egg size, egg energy, larval volume at two time points, juvenile tube length, adult dry weight and fecundity after selection. Selection for larger eggs produced positive correlated responses in egg energy, fecundity and larval size at competence. Selection for increased egg size was also manifested by earlier sex change and this resulted in selected individuals spending less time as males relative to controls. We propose that egg size is negatively correlated with duration of andromorphy, that is, that female fitness trades off with male fitness.     

THE RESPONSE TO SELECTION FOR FAST LARVAL DEVELOPMENT IN DROSOPHILA MELANOGASTER AND ITS EFFECT ON ADULT WEIGHT: AN EXAMPLE OF A FITNESS TRADE-OFF

A selection experiment using Drosophila melanogaster revealed a strong trade-off between adult weight and larval development time (LDT), supporting the view that antagonistic pleiotropy for these two fitness traits determines mean adult size. Two experimental lines of flies were selected for a shorter LDT (measured from egg laying to pupation). After 15 generations LDT was reduced by an average of 7.9%. The response appeared to be controlled primarily by autosomal loci. A correlated response to the selection was a reduction in adult dry weight: individuals from the selected populations were on average 15.1% lighter than the controls. The lighter females of the selected lines showed a 35% drop in fecundity, but no change in longevity. Thus, there is no direct relationship between LDT and adult longevity. The genetic correlation between weight and LDT, as measured from their joint response to selection, was 0.86. Although there was weak evidence for dominance in LDT, there was none for weight, making it unlikely that selection acting on this antagonistic pleiotropy could lead to a stable polymorphism. In all lines, sex differences in weight violated expectations based on intrasex genetic correlations: Females, being larger than males, ought to require a longer LDT, whereas there was a slight trend in the opposite direction. Because the sexual dimorphism in size was not significantly altered by selection, it appears that the controlling loci are either invariant or have very limited pleiotropic effect on developmental time. It is suggested that they probably control some intrinsic, energy-intensive developmental process in males.     

Two Meridic Diets for Perillus bioculatus (Heteroptera: Pentatomidae), a Predator of Leptinotarsa decemlineata (Coleoptera: Chrysomelidae)

Two artificial diets, devoid of insect components, were developed for the predator Perillus bioculatus (Fabricius). The diets based on chicken liver and tuna fish were designed to approach the nutritional characteristics of the primary insect prey, Leptinotarsa decemlineata (Say) eggs. P. bioculatus was reared on diet 2 for 11 consecutive generations without cumulative detrimental effects on its biology. Developmental time and preovipositional period were significantly longer, and egg viability, survival from egg to adult, and fecundity were significantly lower in P. bioculatus individuals reared on artificial diets than in those reared on L. decemlineata eggs. A significant reduction in developmental time, increase in survival from egg to adult, and reduction in length of the preovipositional period were observed after 11 generations of in vitro reared P. bioculatus on diet 2. These changes may indicate that the predators experience some degree of adaptation to the diet after several consecutive generations of in vitro rearing. No changes in egg viability were observed after 11 generations of in vitro rearing. Adult female weight was unaffected when reared on artificial diet, but a significant increase was observed after 11 generations of in vitro rearing. Females under all treatments weighed an average of 11.6 mg more than the males.     

No adaptation of Haemonchus contortus to genetically resistant sheep.

A composite population of Haemonchus contortus was established with larvae from seven diverse sources, then maintained in Merino sheep bred to have either increased or decreased resistance to Haemonchus. After five, seven and 14 parasite generations, the two resulting lines of parasites were used to infect sheep from the increased resistance line, an unselected control line and the decreased resistance line. Line of sheep had a highly significant effect on average faecal egg counts 4 and 5 weeks after infection (geometric means 329, 735 and 1490 epg, respectively after the 14th generation), but the two lines of parasites yielded similar egg counts. There was no significant interaction between line of sheep and line of parasite, indicating that the parasite populations had not diverged significantly in their reproductive fitness, as measured by faecal egg count.     

Effects of mating system in Japanese quail

Summary A 17-generation selection experiment was conducted to study direct and correlated responses to mass selection under a mating system with alternating generations of full-sib inbreeding and wide outbreeding (population I) as compared with a mass selected, randomly mated population (population II). The selection criterion was an index of total egg mass to 78 days divided by adult female body weight. Estimates of heritabilities and genetic correlations are reported. Estimated heritabilities for the index were 0.38±0.04 and 0.29±0.05 in population I and II, respectively. Realized heritabilites were 0.10±0.05 and 0.12±0.03. For most traits studied the mean phenotypic values in the cyclic mated population decreased for inbred generations. Increased inbreeding levels also caused outbred generation means of population I to decrease through the first six or seven generations. After this period of adaptation to inbreeding selection response was positive for the index and positively correlated traits. Total response to selection under the cyclic inbred-outbred mating system did not exceed selection response made under random mating. However, the rate of response in the cyclically mated population exceeded that in the randomly mated population in later generations when the cyclically mated population had apparently adapted to inbreeding.Published with approval of the Director of the Montana Agricultural Experiment Station, Journal Series No. 1221     

SELECTION ON HERBIVORE LIFE-HISTORY TRAITS BY THE FIRST AND THIRD TROPHIC LEVELS: THE DEVIL AND THE DEEP BLUE SEA REVISITED

Abstract The timing of life‐history events in insects can have important consequences for both survival and reproduction. For insect herbivores with complex life histories, selection is predicted to favor those combinations of traits that increase the size at metamorphosis while minimizing the risk of mortality from natural enemies. Studies quantifying selection on life‐history traits in natural insect herbivore populations, however, have been rare. The purpose of this study was to measure phenotypic selection imposed by elements of the first and third trophic levels on variation in two life‐history traits, the timing of egg hatch and pupal mass, in a population of oak‐feeding caterpillars, Psilocorsis quercicella (Lepidoptera: Oecophoridae). Larvae were collected from the field throughout each of two generations per year for three years and reared to determine the effects of the date of egg hatch on both the risk of attack from parasitoids and the pupal mass of the survivors. The direction and strength of phenotypic selection attributed to aspects of the first and third trophic levels, as well as their combined effects, on the date of egg hatch was measured for each of the six generations. Heritabilities of and genetic correlations between pupal mass and the date of adult emergence from diapause (the life‐history trait expected to have the largest influence on the timing of egg hatch, and thus larval development) were estimated from laboratory matings. In four of the six generations examined, significant directional selection attributed to the first trophic level was detected, always favoring early‐hatching cohorts predicted to experience higher leaf quality than late‐hatching cohorts. Directional phenotypic selection by the third trophic level was detected in only one of three years, and in that year the direction of selection was in opposite directions during the two successive generations. The combined effect of selection by both trophic levels indicated that the third trophic level acted to either reduce or enhance the more predictable pattern of selection attributed to the first trophic level. In addition, I found evidence of truncation selection acting to increase the mean and decrease the variance of pupal mass during the pupa‐adult transition in the laboratory. Pupal mass and diapause duration were found to vary significantly among full‐sibling families; upper bounds for heritability estimates were 0.57 and 0.30, respectively. Furthermore, these two traits were found to be positively genetically correlated (families with larger pupae had longer diapause durations), resulting in a fitness trade‐off, because larger pupae enjoy higher survival through metamorphosis and female fecundity but emerge later, when average leaf quality for offspring is generally poorer.     

Genetic Analysis of a Strain of Mice Plateaued for Litter Size

A strain of mice (S1) was successfully selected for large litter size for 31 generations, increasing the mean by 4.2 young per litter. After generation 31, there was no further progress and it was concluded that a selection plateau had been reached. Realized heritability decreased during the course of the experiment from 0.16+/-0.06 for the first 15 generations to 0.00+/-0.03 for generations 30 through 45.--In order to explore the nature of the selection plateau, the following groups were derived from line S1 at generation 34 or 35: Upward selection with inbreeding (SF), random (relaxed) selection (SO), and downward selection (SR). Selections were carried out for 10 to 11 generations. The means of SO and SF were similar to those of S1, ruling out any major effect of natural selection or overdominance. SR decreased, the mean averaging 2.3 young per litter below that of S1 during the last three generations. The fact that SR responded to selection indicates that genetic variance was still present in the plateaued population. The SF sublines were crossed when the inbreeding was 95% and a new line, SX, was formed. SX was maintained for three generations and the difference of +0.7 young per litter above the contemporary generations of S1 was significant. The results from this experiment suggest that the selection plateau in line S1 was caused by reduction of additive genetic variance to a very low level. Some nonadditive genetic variance remained, however, and was attributed to recessive alleles at low frequency. In agreement with results reported by Falconer (1971), inbreeding with selection followed by crossing of the inbred sublines proved to be effective in overcoming a selection plateau in litter size.     

DNA polymorphism in various goose lines by RAPD-PCR

RAPD markers often primers were used to assess the polymorphism among pooled DNA of eight goose lines. The number of bands amplified by each primer ranged from 1 to 8, within a mean of 2.86. Some bands appeared specific for the line or genetic background. RAPD technique is an effective method for generating the polymorphic DNA marker in the goose. RAPD patterns from mixed DNA samples can reflect the genetic information of populations. The present study indicated that 10 generations selected for egg production and body weight under various pressure, resulted in genetic variation among goose lines as detected by RAPD. Selection for meat traits caused greater genetic diversity than selection for egg production.     

Milk yield in lines of mice selected for growth or maternal ability.

Two experiments were conducted to measure milk yield as a correlated response to selection for increased 12-day litter weight (maternal lines) for 13 generations and increased and decreased body weight gain from 21 to 42 days of age (postweaning gain lines) for 12 to 14 generations. Milk yield was measured from day 6 to day 21 of lactation by separating litters from their dams for 6 h, then weighing litters before and after a 1.5 h suckling period. Average total milk production was 30.3 +/- 0.9 g and 28.6 +/- 1.1 g in maternal and control lines, respectively. This difference was not significant (P greater than 0.05). Differences among gain lines were significant (P less than 0.01) in the second experiment with average production of 44.1 +/- 1.4 g for increased gain lines, 22.5 +/- 0.9 g for the decreased gain line and 31.2 +/- 1.0 g for the control. These results indicated that response to selection for increased 12-day litter weight was not associated with increases in milk yield, but sizable changes inmilk yield accompanied bi-directional selection for early postweaning gain.     

Inheritance of weight in Rhipicephalus appendiculatus ticks (Acari: Ixodidae) in the laboratory

A selection of the 10% lightest and 10% heaviest males and females of a population of individually weighed Rhipicephalus appendiculatus Neumann adults was made in two experiments. The offspring of homologous pairs were followed until the next adult stage (light x light, control x control and heavy x heavy). The engorged nymphal weights, unfed adult weights, engorged female weights of the parents, egg mass weights, egg weights, larval scutal lengths, engorged larval weights, unfed nymphal weights, engorged nymphal weights and adult weights of the progeny were determined. No significant differences could be demonstrated between the two lines for egg weight, larval scutal length, engorged larval weight and unfed nymphal weight. Significant differences were found between the egg masses, engorged nymphal weights and adult weights of the two lines. The heritability coefficients of body weight determined from adult to adult were 0.14 and 0.10, respectively, during the first and second experiments. Considering females and males separately, the coefficients were 0.10 and 0.18 during the frist experiment and 0.12 and 0.09 during the repeat experiment respectively.     

Laboratory adaptation of Bactrocera tryoni (Diptera: Tephritidae) decreases mating age and increases protein consumption and number of eggs produced per milligram of protein

A significant reduction in age of mating occurred during the first four generations (G1-G4) of laboratory adaptation of wild Bactrocera tryoni (Froggatt) and this was associated with the earlier attainment of peak egg load although no significant differences were detected in the peak egg load itself. A long term laboratory (LTL) strain had a significantly earlier mating age and higher peak egg load than flies of wild origin or those from the first four laboratory generations. The amount of protein consumed by females in the first week of adult life was significantly higher in the LTL strain than in flies of wild origin or G1-G4 but there were no significant changes (or only slight changes) with laboratory adaptation in the amounts of protein consumed up to the ages of mating and peak egg load. Laboratory adaptation resulted in no significant changes in egg size, egg dry weight, puparial fresh weight and the dry weight of newly emerged females. The large increase in fecundity with laboratory adaptation is associated with a 4- to 5-fold increase in the rate of conversion of dietary protein to eggs (i.e. eggs produced per mg of protein consumed).     

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.     

Ancylostoma duodenale: maintenance through six generations in helminth-native pups.

An Indian strain of Ancylostoma duodenale with a known propensity for arrested development in man was passaged serially in 2-month-old, helminth-naive, male beagles. The sixth passage was initiated about 500 days after laboratory maintenance began. Experimental animals were infected by stomach tube with 1500 larvae each. These infections became patent in 22–31 days. In a line passaged in dogs without the use of a corticosteroid (prednisolone), adaptation, as judged by fecal egg count, increased through the first three generations and then declined sharply. This line appears destined for extinction. In another line passaged in prednisolone-treated pups, the strain's original viability is being maintained. Infections were long lived, frequently surviving for 90 days or more. Maximum egg output in terms of eggs per gram of feces (EPG) was highly variable between generations and between individuals within generations, but commonly exceeded 1500 EPG. In pups necropsied between 44 and 55 days, harboring fifth- and sixth-generation infections, adult worm burdens ranged from 47–146 and 34–128 in the nonprednisolone and prednisolone lines, respectively. Arrested larvae have been found in the same pups and apparently contribute to a turnover in the population of adult worms. Although A. duodenale has yet to be fully adapted to dogs in the sense that indefinite survival without prednisolone is assured, the present system yields a variety of life history stages in quantities suitable for many experimental purposes. Thus it provides a utilitarian laboratory model for the investigation of one of man's major hookworms.