Accumulation of lethals in highly selected lines of Drosophila melanogaster

Summary Four synthetic lines of D. melanogaster selected for low sternopleural bristle number for 50 generations were screened for lethals on chromosome III when their mean score equalled 2.5. Each line originated from a cross between line M (previously selected for the same trait during 130 generations) and a different unselected cage population. Line M was already known to carry a recessive lethal on chromosome III affecting the selected trait, such that the bristle score of the lethal heterozygote was lower than that of the viable homozygote. Tests revealed 18 lethals, 15 of these present in at least two lines. Each line carried from 10 to 16 lethals. All lines carried groups of lethals present on the same chromosome, and at least six lethals in each line were included in such an association with a frequency of 0.18 or higher. It appears that the lethal affecting bristle score in line M has protected a segment of chromosome III from natural selection and that the remaining 14 lethals have accumulated later in that line.     

Selection responses for the number of fertile eggs of the Brown Tsaiya duck (Anas platyrhynchos) after a single artificial insemination with pooled Muscovy (Cairina moschata) semen

A seven-generation selection experiment comprising a selected (S) and a control (C) line was conducted with the objective of increasing the number of fertile eggs (F) of the Brown Tsaiya duck after a single artificial insemination (AI) with pooled Muscovy semen. Both lines consisted of about 20 males and 60 females since parents in each generation and each female duck was tested 3 times, at 26, 29 and 32 weeks of age. The fertile eggs were measured by candling at day 7 of incubation. The selection criterion in the S line was the BLUP animal model value for F. On average, 24.7% of the females and 15% of the males were selected. The direct responses to the selection for F, and correlated responses for the number of eggs set (Ie), the number of total dead embryos (M), the maximum duration of fertility (Dm) and the number of hatched mule ducklings (H) were measured by studying the differences across the generations of selection between the phenotypic value averages in the S and C lines. The predicted genetic responses were calculated by studying the differences between the S and C lines in averaged values of five traits of the BLUP animal model. The selection responses and the predicted responses showed similar trends. There was no genetic change for Ie. After seven generations of selection, the average selection responses per generation were 0.40, 0.33, 0.42, 0.41 genetic standard deviation units for F, M, Dm, and H respectively. Embryo viability was not impaired by this selection. For days 2–8 after AI, the fertility rates (F/Ie) were 89.2% and 63.8%, the hatchability rates (H/F) were 72.5% and 70.6%, and (H/Ie) were 64.7% and 45.1% in the S and C lines respectively. It was concluded that upward selection on the number of fertile eggs after a single AI with pooled Muscovy semen may be effective in ducks to increase the duration of the fertile period and the fertility and hatchability rates with AI once a week instead of twice a week.     

Geographic variation and the evolution of reproductive allocation in the pitcher-plant mosquito, Wyeomyia smithii

We measured the egg size of six geographic populations of the pitcher-plant mosquito, Wyeomyia smithii, from Florida (30 degrees N) to Ontario (49 degrees N). Populations from northern latitudes produced larger eggs than populations from southern latitudes. Egg size increased with increasing latitude more rapidly when larvae were reared under low rather than high density. One southern (30 degrees N) and one northern (49 degrees N) population of W. smithii that persisted through 10 generations of selection for increased persistence under conditions of chronic thermal- and nutrient-limiting stress (conditions similar to southern rather than northern habitats) produced smaller eggs more rapidly than unselected control lines. However, there were no differences in lifetime fecundity or fertility between control and selected lines. Thus, laboratory evolution in an environment representative of extreme southern latitudes caused evolutionary changes consistent with geographic patterns of egg size. These results implicate temperature as a selective factor influencing the geographic variation of egg size in W. smithii, and demonstrate a novel trade-off in reproductive allocation between egg size and egg maturation time.     

The effect of selection for fertility of frozen-thawed semen on spermatozoa oxygen uptake, motility and concentration, and ejaculate volume in the chicken

The response to 3 generations of selection for duration of fertility of frozen-thawed semen on fertility and hatchability of fresh and frozen-thawed semen, spermatozoa oxygen uptake, motility and concentration, and ejaculate volume was measured in a broiler line of chickens. The selected line, as compared to the control, had significantly higher levels for all fertility traits of frozen-thawed semen but not hatchabilities. For fresh semen, the lines differed only for duration of fertility. The only difference in semen quality traits was in oxygen uptake by spermatozoa of frozen-thawed semen. The differences between lines for the other parameters were not significant but were in a direction that supported the hypothesis that selection had resulted in improved reproductive capacity. All estimates of fertility within an ejaculate were positively correlated (P < 0.01). Correlations between corresponding fertility estimates of fresh and frozen-thawed semen were positive, low and generally non-significant. The proportion of variation as determined by stepwise regression, in fertility estimates accounted for by the measurements of oxygen uptake and motility of fresh and frozen-thawed spermatozoa, ejaculate volume and spermatozoa concentration ranged from 12 to 19 percent.     

Selection response for part period egg number and egg mass in chickens — a comparison

Summary Two sublines from each of four flocks of White Leghorn were subjected to two cycles of selection for part year egg number and part year egg mass. A family index (combined selection) was the selection criterion for each of the two traits. The generation means of an unselected pedigreed randombred control, bred and maintained along with the selected sub-lines, were used for correcting short term environmental trends. The sublines selected for a common selection criterion were considered as replicates. The control corrected generation means, when averaged over replicates, gave the mean direct and correlated responses for the selected and different unselected traits. The average genetic change per generation was 2.16 eggs in egg number selected sublines and 146 g of egg mass in egg mass selected sublines. The direct response marginally exceeded the correlated response for both part period egg number and egg mass seen in the corresponding sublines. Except for egg weight the correlated responses for different unselected traits were in the same direction in both egg number and egg mass selected sublines. While the egg weight in egg number lines did not change, its response was positive in egg mass selected lines. A comparison between the lines revealed that the egg mass selected lines matured later and laid heavier but slightly less numerous eggs than the egg number selected sub-lines.     

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.     

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.     

Further evidence of normal fertility and the formation of balanced gametes in sheep with one or more different Robertsonian translocations.

Mating experiments are described for sheep with three different Robertsonian translocations in the single heterozygous t1, t2 and t3, homozygous t1t1 and t3t3 and double heterozygous t1t2 and t1t3 state. The experiments were designed to investigate several previously reported unusual chromosome segregation ratios in sheep, to test the fertility of translocation heterozygous ewes mated to rams of normal karyotype and to test both the fertility and segregation patterns of sheep which were double translocation heterozygotes. The fertility of the translocation heterozygous ewes was normal as assessed from conception to first service, numbers of non-conceiving ewes and lambing percentages. Two types of double translocation heterozygous rams mated to ewes of normal karyotype produced regular chromosome segregation patterns in their progeny and the matings were of normal fertility. Double translocation heterozygous ewes were also fertile. Four sheep were bred with 51 chromosomes. Two of these were triple heterozygotes with three different Robertsonian translocations 51,xy,t1t2t3 and 51,xx,t1t2t3 and two were homozygous for one translocation and heterozygous for the others, namely 51,xx,t1t2t3 and 51,xxt1t3t3. All sheep were phenotypically normal. It is concluded that the t1,t2 and t3 Robertsonian translocations of sheep do not affect reproductive performance significantly.     

Selection for Reduced Crossing over in DROSOPHILA MELANOGASTER

Selection was practiced for reducing crossing over between the third chromosome genes Sb and H(2) of Drosophila melanogaster; the method employed was to select the repulsion double heterozygotes Sb+/+H(2) every generation. Two replicate selection lines were maintained. After 24 generations of selection, Line 1 showed no significant difference from the control, although the regression of recombination value on generation was significant. In generation 20, Line 2 had a significantly lower recombination value than the control, as well as having a highly significant regression coefficient. No chromosome rearrangements were involved in the response. It was concluded that there was substantial genic variability in the frequency of crossing over between Sb and H(2) in the base population.     

Influence of B Locus Blood Groups on Adult Mortality and Egg Production in the White Leghorn Chicken

The influence of the B locus blood group on adult viability and egg production was studied in two White Leghorn populations (S1 and S2) synthesized from inbred line crosses. Each line segregated for four B alleles. Four homozygotes and six heterozygotes were produced in each line over a five-year period, and for an additional three years tests on certain blood-group combinations were continued. A total of 4371 birds were included in the study. Greatest differences in blood groups were found in the S1 line, with the B(2) and B(21) alleles seemingly having favorable effects and with B(1) having unfavorable effects. The B(1) homozygote was consistently the lowest in egg production (53.2%) and highest adult mortality (40.4%). The relative spread in standard deviation units between the B(1) and B(2) homozygotes was more than three times greater in adult mortality than in egg production; B(2) was incompletely dominant to B(1). Within the S1 line, the superiority of the heterozygotes was mainly a consequence of the poor fitness of the B(1) homozygote, suggesting that in a random-mated population B(1) would be maintained only by mutation and not by a polymorphic mechanism.-Over the eight years of the experiment, adult viability of the B(1) homozygote improved 4.4% per year (P<0.05). Assuming this regression results from natural selection, either of two hypotheses can account for the results: (1) The B locus is pleiotropic with natural selection for many B modifiers, and (2) the B locus is neutral but linked to a major fitness locus.     

Correlated responses of chickens to selection for production of antibodies to sheep erythrocytes

A bi directional selection experiment was conducted to measure 5-day antibody titers to sheep erythrocytes in White Leghorn chickens. There was an immediate response to selection with significant differences between lines for the selected trait found in the S₁ and all subsequent generations. Comparisons of S₆, S₇ and S₈ generation females revealed differences between lines in disease resistance and in certain reproductive traits such as age at first egg, percentage hen-day egg production, percentage fertility and duration of fertility. The implications of these correlated responses are important to selection programs for general disease resistance.     

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.     

The population genetics of reinforcing selection

A common outcome of disruptive selection experiments between two differentiated populations which produce disadvantageous hybrids is an increase in homogamy. Experiments reported here result in another outcome when ‘classical’ selection experiments are redesigned. In these modified experiments, frequencies of genotypes in the mating population were not artificially maintained at parity but were instead determined from progeny proportions in the previous generation. In these selection lines another outcome, apart from an increase in homogamy, was demonstrated. Under a high selection coefficient against heterozygotes, elimination of a homozygote and the corresponding fixation of the other was observed. No selection line demonstrated the maintenance of two differentiated populations concurrently with the selection process of heterozygote disadvantage. A high number of generations of selection under this population genetical process is necessary to increase differences between two populations. However, the instability of gene frequencies which results in fixation or elimination of a homozygote is shown to be extremely rapid by comparison. Classical experiments were repeated and after 21 generations of selection there was no increase in divergence. For lower selection coefficients, high levels of introgression are apparent, and hence the genetical distinctness of the two populations decreases over time. This is in addition to the problem of an unstable equilibrium under selection against heterozygotes. Both aspects are important but not previously considered in experimental evidence for speciation models for which their implications are discussed.     

Controlled crosses and cage experiments with a translocation in Drosophila

Predictions for the use of autosomal translocations for pest control have been made on the assumptions that translocation heterozygotes are semi-sterile and translocation homozygotes are fully fertile and have viabilities which are either equal to or slightly less than that of the wild type. These parameters were assessed in controlled crosses using a translocation in Drosophila and they showed good agreement with expectation. It was expected that in a mixed population of such translocations and wild types, the karyotype at the higher frequency would be favoured by selection. Two cage experiments were initiated with mixed populations of translocation homozygotes and wild types at a frequency of 9:1 in favour of the translocation. Contrary to expectation, the translocation was eliminated from both populations. During this process, there was a reduction in population fertility. Computer studies showed that the results were consistent with a reduction in fitness of the translocation homozygote to about 0.5, i.e. about the same as the semi-sterile heterozygote, so that negative heterosis did not exist and hence frequency dependent selection could not occur.     

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.     

Polygenic Mutation in Drosophila Melanogaster: The Causal Relationship of Bristle Number to Fitness

The association between sternopleural and abdominal bristle number and fitness in Drosophila melanogaster was determined for sublines of an initially highly inbred strain that were maintained by divergent artificial selection for 150 generations or by random mating for 180 generations. Replicate selection lines had more extreme bristle numbers than those that were maintained without artificial selection at the same census size for approximately the same number of generations. The average fitness, estimated by a single generation of competition against a compound autosome strain, was 0.17 for lines selected for high and low abdominal bristle numbers and 0.19 for lines selected for high and low sternopleural bristle number. The average fitness of unselected lines, 0.46, was significantly higher than that of the selection lines. The fitnesses and the relationships of bristle number to fitness in progeny of all possible crosses of high X high (H X H), high X low (H X L) and low X low (L X L) selection lines were examined to determine whether the observed intermediate optima were caused by direct stabilizing selection on bristle number or by apparent stabilizing selection mediated through deleterious pleiotropic fitness effects of mutations affecting bristle number. Although bristle number was nearly additive for progeny of H X H, H X L and L X L crosses among sternopleural bristle selection lines, their mean fitnesses were not significantly different from each other, or from the mean fitness of the unselected lines, suggesting partly or completely recessive pleiotropic fitness effects cause apparent stabilizing selection. The average fitness of the progeny of H X H abdominal bristle selection lines was not significantly different from the fitness of unselected lines, but the mean fitness of the progeny of L X L crosses was not significantly different from that of the pure low lines. This is consistent with direct selection against low but not high abdominal bristle number, but the interpretation is confounded by variation in average degree of dominance for fitness (on average recessive in the high abdominal bristle selection lines and additive in the low abdominal bristle selection lines). Neither direct stabilizing selection nor pleiotropy, therefore, can account for all the observations.     

Indirect selection for increased susceptibility to permethrin in diamondback moth (Lepidoptera: Plutellidae)

We have been exploring the behavioral response ot insect pests to heterogeneous distribution of toxins (low dose with refugia), and its genetic correlation with physiological tolerance to these toxins. A field-collected population of diamondback moth, Plutella xylostellu (L.) (Lepidoptera Plutellidae), from Celeryville, OH, was selected with permethrin to determine whether low heterogeneous doses could lead to increased susceptibility to permethrin by selecting indirectly on behavior. Two replicates of each of three selection regimes: uniform high concentration hypothesized to result in increased physiological tolerance, heterogeneous low concentration hypothesized to result in increased susceptibility through indirect selection on behavior, and a control with no exposure to permethrin, were maintained in 1-in3 cages in a greenhouse, for 33 generations. All life stages of the diamondback moth were exposed to the selection regimes, and new generations were started with a random selection of pupae from the previous generation. Lines selected with uniform high concentrations developed 76-fold levels of resistance to permethrin by the 17th generation, with little changes thereafter. For generations 1-20, lines selected with heterogeneous low concentrations remained slightly lower in LC50 but not significantly different from the unselected control lines. Based on confidence intervals from probit analyses, the LC50 of the lines selected with heterogeneous low concentration, however, were significantly lower than those of the control lines in generations 21-33. Our results demonstrate that selection on behavioral responses can result in greater susceptibility than no selection at all, despite exposure to the toxin and ample genetic variation and potential for increased physiological tolerance. The implications of our findings, which are based on selection scenarios that could take place in field situations, are that behavioral responses can prevent and even decrease the levels of resistance in insect populations, an important result with respect to resistance and resistance management.     

Viability and behaviour of translocations that suppress female recombination in the Mediterranean fruit fly,Ceratitis capitata (Wied.)

Summary The viability of a series of recombination suppressor (RS) strains in Ceratitis capitata, all previously found to contain a reciprocal autosomal translocation, was assessed for egg hatchability and adult emergence in both the homozygous and heterozygous state. Except in T 30C, which contains a Y-autosome translocation in addition to the A-A translocation, egg hatch was significantly reduced in all heterozygous translocation strains, and ranged from 42.4% to 58.5% in seeded eggs compared to a control value of 82.8%. Adult emergence from hatched eggs was affected to a lesser extent, but with a range of 59.5% to 84.2%, compared to the control value of 83.1%, remained significantly reduced in 4 of the 6 translocation strains, as well as in the male line of T 30C. In the homozygous configuration all strains, except T 19 and T 109, showed a significant reduction in egg hatchability, whereas adult emergence was not adversely affected. A significant reduction in the egg hatchability of the translocation heterozygotes compared to that of the homozygotes was observed in 5 of the 7 strains, the observed reduction in T 55/109 being non-significant while that of T 30C was significantly increased. The behaviour of translocations as recombination suppressors and their suitability for inclusion in breeding schemes for the isolation of induced recessive mutations is discussed.This work forms part of a Joint FAO/IAEA research programme on the development of genetic sexing mechanisms for the Mediterranean fruit fly, Ceratitis capitata (Wied.)     

Parental Nutrition and Chick Production in Captive Willow Ptarmigan (Lagopus L. Lagopus)

The breeding performance of captive willow ptarmigan on different diets has been studied. The nutritional factors tested were protein concentration, natural feed supplement and grass meal and flavonoid admixture, and effects on egg numbers, fertility, hatchability, chick weights at hatching and 0–14 days mortality have been recorded. The breeding performance of ptarmigan hen in captivity showed great individual variations. Egg numbers were not statistically different in groups fed the different diets. Hens fed a 15 % crude protein died tended to produce smaller chicks with significantly lower viability than chicks from hens fed a 20 % crude protein diet. Supplement of natural feed tended to increase the number of chicks hatched through a combination of tendency to higher egg numbers and improved fertility. These tendencies were, however, statistically nonsignificant. Inclusion of 34 % grass meal to the diet also tended (non-significantly) to improve fertility and hatchability, while inclusion of flavonoids had no positive effect on reproduction. Eggs from captive hens showed significantly lower fertility, and a tendency to lower hatchability than eggs from wild hens. The former difference was probably caused by the close cage confinements for the captive ptarmigan, while the latter condition probably was due to different start of incubation, most of the eggs from wild hens being started naturally.     

Artificial selection for 18-day pupa weight and opposing simulated naturel selection in Tribolium castaneum

Summary The effect of simulated opposing natural selection on the response to mass selection for 18-day pupa weight of Tribolium castaneum was studied for 10 generations of selection. Natural selection was simulated in replicated treatment lines by imposing a negative relationship between mid-parent genetic value for pupa weight and fertility. Responses to selection and realized heritabilities were smaller (P < 0.05 and P < 0.10, respectively) for the treatment lines than for control lines under selection for pupa weight only. One treatment, line E3G1, reached an intermediate selection plateau by generation 10, and responded linearly to 4 generations of artificial selection after natural selection had been discontinued. Possible explanations for the different behaviors of the replicate lines E3G1 and E3G2 were also discussed.