Experimental study of the influence of errors of genetic correlation estimates on unrestricted,optimum, and desired gains selection indices

Summary Effects of errors in estimates of the genetic correlation on the accuracy of unrestricted, optimum, and desired gains selection indices were examined experimentally in Tribolium castaneum. Three lines were selected for three generations for pupal weight at 21 days and adult weight at 31 days, using unrestricted (I9), optimum (O9), and desired gains (G9) index selection methods. The genetic correlation between pupal and adult weights in the base population was 0.95. The optimum index was designed to set the response of pupal weight by a fixed amount, while in the desired gains index the responses of pupal and adult weights were specified as being equal to 31. Three other indices were constructed using a deliberately incorrect genetic correlation (0.25), i.e., unrestricted (I2), optimum (O2), and desired gains (G2). Responses observed in unrestricted index lines (I9 versus I2) and optimum index lines (O9 versus O2) did not differ significantly, even though lines I9 and I2 differed in a practical sense. Responses in desired gains index lines (G9 versus G2) differed significantly. Responses obtained for aggregate genotype (pupal weight + adult weight) and for the component traits were greater in line I9 than those obtained in line I2. Responses obtained in the O9 and O2 lines for pupal and adult weights were similar, while those obtained in the G9 and G2 lines were similar for pupal weight but not (P<0.05) for adult weight. Therefore, underestimation of the genetic correlation seems to affect the efficiency of a desired gains index more than that of unrestricted or optimum indices.     

Effect of directional selection for body size on fluctuating asymmetry in certain morphological traits in <Emphasis Type="Italic">Drosophila ananassae</Emphasis>

Variation in the subtle differences between the right and left sides of bilateral characters or fluctuating asymmetry (FA) has been considered as an indicator of an organism’s ability to cope with genetic and environmental stresses during development. However, due to inconsistency in the results of empirical studies, the relationship between FA and stress has been the subject of intense debate. In this study, we investigated whether stress caused by artificial bidirectional selection for body size has any effect on the levels of FA of different morphological traits in Drosophila ananassae. The realised heritability (h²) was higher in low-line females and high-line males, which suggests an asymmetrical response to selection for body size. Further, the levels of FA were compared across 10 generations of selection in different selection lines in both sexes for sternopleural bristle number, wing length, wing-to-thorax ratio, sex comb-tooth number and ovariole number. The levels of FA differed significantly among generations and selection lines but did not change markedly with directional selection. However, the levels of FA were higher in the G10 generation (at the end of selection) than G0 (at the start of selection) but lower than the G5 generation in different selection lines, suggesting that the levels of FA are not affected by the inbreeding generated during the course of selection. Also, the levels of FA in the hybrids of high and low lines were significantly lower than the parental selection lines, suggesting that FA is influenced by hybridisation. These results are discussed in the framework of the literature available on FA and its relationship with stress.     

An empirical comparison of selection methods for the improvement of biomass

Summary A single generation of upward truncation selection on families with 20% selected was carried out in each of five replicates using Tribolium castaneum as the test organism. The experiment involved eight lines: N — selected for offspring number; W — selected for pupal weight; B — selected for biomass; Q — quadratic index selected; L₂₁ — linear index selected with relative economic weights of 21 offspring number to pupal weight; L₁₁ — linear index selected with relative economic weights of 11 offspring number to pupal weight; L₁₂ — linear index selected with relative economic weights of 12 offspring number to pupal weight; C — an unselected control.Biomass (weight of offspring per family), offspring number, and pupal weight were measured. No differences in response to selection were found among the linear index lines and the pupal weight line with regard to any trait analysed. Generally, response to selection in the linear index lines and pupal weight line was small for offspring number and high for pupal weight. Selection pressure on offspring number in these lines seemed to be dependent on the correlation between offspring number and pupal weight. As a result, response to selection for biomass was poor in the linear index and pupal weight selected lines. In the case of the linear indices, poor response to selection for biomass appeared to be due to the violation of the assumption of additivity of the traits included in the definition of aggregate genotype.The responses in the quadratic index, biomass, and offspring number selected lines were equal with respect to selection for biomass. The response of the quadratic index selected line was less than the responses of the biomass and offspring number selected lines for offspring number, but the response in the quadratic index line was as large as that of any other line included in the experiment and greater than the biomass and offspring number selected lines where pupal weight was the criterion.Highly significant amounts of variation were found for all traits indicating that more replicates are needed for precise evaluation of selection systems.     

Experimental comparison of selection methods to improve a non-linear trait in Tribolium

Summary Five lines of Tribolium castaneum (flour beetle) were selected in each of three, replicates for biomass, using quadratic (Q) and linear (L) selection indices, direct selection for biomass (B) and indirect selection for each component trait, i.e. pupal weight (P) and family size (F), respectively, for five generations. The highest response in biomass was obtained in the Q line using an exact quadratic index in which the genetic value for each trait was based on information from both traits. The expected results were that Q selection would not exceed L selection while the L method would be superior with respect to direct selection for biomass (B line); P selection would be better and F selection worse than B selection. Selection was effective for pupal weight in the Q, L, B and P lines, but was not effective for family size, although the Q line increased slightly. The lines had no linear response for family size. Selection for biomass was influenced more by family size than by pupal weight. It is advantageous to include terms such as squares and products in the selection index, even though the response to selection may have been non-linear because of the low heritability of family size and an intermediate optimum for this component trait.     

Butterfly selected lines explore the hormonal basis of interactions between life histories and morphology

Hormonal mechanisms underlie many life-history traits and their interactions. We studied the role of ecdysteroids with regard to wing pattern and development time of the polyphenic butterfly Bicyclus anynana. Ecdysteroid titers and sensitivity to ecdysone injection were assayed for two-trait selected lines (ventral eyespot size and development time concurrently). These two traits are genetically and phenotypically coupled, having a common endocrinal basis. Two-trait selection had been applied both antagonistically (opposite the correlation) and synergistically (in the same direction as the correlation). Although selected lines had diverged most in eyespot size, the widest differences in timing of ecdysteroid titers were observed between the development time selection regimes; fast selected lines had an earlier hormonal increase after pupation than slow selected lines (even when corrected for differential pupal times). This endocrine peak was also earlier for females than for males. Furthermore, sensitivity to ecdysone injection as measured by a subsequent decrease in pupal time was significantly lower for slow selected lines than for fast or unselected lines. We conclude that the observed response in eyespot size to artificial selection must have been achieved via alteration of, or selection on, other developmental mechanisms, because the dynamics of the alternative, hormonal, pathway were dictated by development time selection. The developmental system is flexible enough to allow evolution in directions opposing the correlation between wing pattern and developmental time, and responses to selection are not constrained by a shared hormonal system.     

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.     

An Experimental Comparison of Individual, Family and Combination Selection

Two selection experiments comparing the relative efficiencies of individual, family, and combination selection were conducted. The expected results for larval weight of Tribolium (h(2) = 0.20) and for pupal weight (h(2) = 0.40) were that combination selection would be a more efficient method than family selection, and that family selection would exceed individual selection. In experiment I, individual selection produced more response (P < 0.05) than did combination or family, which was not in agreement with expectation. There was confounding of inbreeding levels and random drift due to differential effective population sizes in the lines selected by different methods. Experiment II consisted of ten single-generation selection tests. An advantage of this approach is that it eleminates the inherent problems of differential inbreeding levels and differential rates of genetic drift due to unequal population sizes among the methods of selection. There were no statistically significant differences in efficiency among the three methods of selection for both traits. This was contrary to theoretical expectations but does suggest that with traits of 20% h(2) or higher, and where feasible, one may be justified in basing selection decisions on the phenotype of the individual only. Other advantages of single generation testing are that it allows more precise testing of selection theory and unbiased standard errors for estimates of realized heritability.     

Simultaneous selection on two fitness-related traits in the butterfly Bicyclus anynana

Abstract. Theory about the role of constraints in evolution is abundant, but few empirical data exist to describe the consequences a bias in phenotypic variation has for micro evolution. Responses to natural selection can be severely hampered by a genetic correlation among a suite of traits. Constraints can be studied using antagonistic selection experiments, that is, two-trait selection in opposition to this correlation. The two traits studied here were development time and wing pattern (eyespot size) in the butterfly Bicyclus anynana , both of which have a clear adaptive significance. Rates of response were higher for eyespot size than for development time, but were independent of the concurrent selection (either in the same direction as the correlation or perpendicular to it). Regimes differed in both traits in all directions after 11 generations of selection. The uncoupling lines had higher relative responses than the synergistic lines in development time and equal relative responses in eyespot size. The patterns for eyespot size (reaction norms) were consistent across different rearing temperatures. Differences in lines selected for fast and slow development time were more pronounced at lower temperatures, irrespective of the direction of joint wing pattern selection. Furthermore, correlated responses in pupal weight and growth rate were observed; lines selected for a slower development had higher pupal weights, especially at lower temperatures. The response of the uncoupling lines was not hampered by a lack of selectable genetic variation, and the relative response in the development time was larger than expected based on response in the coupled direction and quantitative genetic predictions. This suggests that the structure of the genetic architecture does not constrain the short-term, independent evolution of both wing pattern and development time.     

INBREEDING: ITS EFFECT ON RESPONSE TO SELECTION FOR PUPAL WEIGHT AND THE HERITABLE VARIANCE IN FITNESS IN THE FLOUR BEETLE,TRIBOLIUM CASTANEUM

We report our studies of the effect of inbreeding on the response to selection for increased pupal weight in the flour beetle, Tribolium castaneum. We also report the effects of inbreeding and selection for pupal weight on the heritable variation in fitness and fitness components. We created replicate and independent inbred lines with F-values of 0.00, 0.375, and 0.672, by 0, 2, and 5 generations, respectively, of brother-sister mating of adult beetles from an outbred stock population. Subsequently, we imposed artificial within-family selection for increased pupal weight in each of 15 inbred lines for eight generations; each line had its own paired, unselected control. We compared the response to selection across the three levels of inbreeding with theoretical expectation, and investigated the effects of inbreeding and selection on fitness variation among families within all 30 selected and control lines. Among-line variation in pupal weight increased with increased inbreeding prior to selection but diminished with directional selection. Inbreeding reduced the realized heritability of pupal weight concordant with quantitative predictions of additive theory. Mean fitness, measured in several ways, declined with inbreeding and declined further with selection. In contrast, the genetic variation for fitness in the inbred and selected lines lines equalled or exceeded that of the outbred controls. Our results suggest that inbreeding and selection may affect traits in different ways depending on the relative amounts of additive and nonadditive genetic variation.     

Hormonal regulation and patterning of the broad-complex in the epidermis and wing discs of the tobacco hornworm, Manduca sexta

Expression of Manduca Broad-Complex (BR-C) mRNA in the larval epidermis is under the dual control of ecdysone and juvenile hormone (JH). Immunocytochemistry with antibodies that recognize the core, Z2, and Z4 domains of Manduca BR-C proteins showed that BR-C appearance not only temporally correlates with pupal commitment of the epidermis on day 3 of the fifth (final) larval instar, but also occurs in a strict spatial pattern within the abdominal segment similar to that seen for the loss of sensitivity to JH. Levels of Z2 and Z4 BR-C proteins shift with Z2 predominating at pupal commitment and Z4 dominant during early pupal cuticle synthesis. Both induction of BR-C mRNA in the epidermis by 20-hydroxyecdysone (20E) and its suppression by JH were shown to be independent of new protein synthesis. For suppression JH must be present during the initial exposure to 20E. When JH was given 6 h after 20E, suppression was only seen in those regions that had not yet expressed BR-C. In the wing discs BR-C was first detected earlier 1.5 days after ecdysis, coincident with the pupal commitment of the wing. Our findings suggest that BR-C expression is one of the first molecular events underlying pupal commitment of both epidermis and wing discs.     

Realized correlated responses to artificial selection on pre-adult life-history traits in a butterfly

We use artificial selection experiments targeted on egg size, development time or pupal mass within a single butterfly population followed by a common-garden experiment to explore the interactions among these life-history traits. Relationships were predicted to be negative between egg size and development time, but to be positive between development time and body size and between egg size and body size. Correlated responses to selection were in part inconsistent with these predictions. Although there was evidence for a positive genetic correlation between egg and body size, there was no support for genetic correlations between larval development time and either egg size or pupal mass. Phenotypic correlations among the three target traits of selection gave comparable results for the relationships between egg mass and development time (no association) as well as between egg mass and pupal mass (positive association), but not for the relation between development time and pupal mass (negative phenotypic correlation). In summary, correlated responses to selection as well as phenotypic correlations were rather unpredictable. The impact of variation in acquisition and allocation of energy as well as of the benign conditions used deserve further investigation.     

Developmental instability as phenodeviance in a secondary sexual trait increases sharply with thermal stress

Abstract We test for effects of thermal stress applied to pupal flies from Noumea (New Caledonia) and Taipei (Taiwan) on developmental instability (DI) in the male sex comb of Drosophila bipectinata, as well as on pre‐adult survivorship and adult body size. The temperature treatments were Low (25 °C), High (29 °C) and Variable (18 h at 29 °C, 6 h at 34 °C). Although the Variable treatment reduced survivorship and body size, absolute comb size and fluctuating asymmetry generally were invariant across treatments. In contrast, comb phenodeviance increased with stress in both populations. Phenodeviance in one comb segment (C2) increased sharply with stress, whereas phenodeviance in a second major segment (C1) also increased with stress but only in Noumea flies. A major conclusion is that phenodeviations induced in a secondary sexual trait reflect the developmental environment that also damages fitness components, a foundation stone of the hypothesis that expressions of DI reveal phenotypic quality in sexual selection.     

Comparison of Detection Methods for Legionella Species in Environmental Water by Colony Isolation,Fluorescent Antibody Staining,and Polymerase Chain Reaction

Three detection methods for Legionella species in water samples from cooling towers and a river were examined. Direct counting of bacteria stained with fluorescent antibody (FA) for L. pneumophila (serogroups 1 to 6) could detect the cell of 10⁴ to 10⁶ cell/100 ml in all 14 samples, while colony counting method detected 10 to 10³ CFU/100 ml only in 8 samples from cooling towers. Polymerase chain reaction (PCR) assay with primers to amplify 16S ribosomal DNA sequence of most Legionella species (LEG primer) detected legionellae in 13 samples, while species-specific primers for L. pneumophila detected the DNAs from 3 samples. In laboratory examination, LEG primers could amplify DNAs of 29 species of genus Legionella with high sensitivity, even from 1 cell of L. pneumophila GIFU 9134. The PCR assay with LEG primers was specific and sensitive methods to be satisfied the survey of legionellae. Thus, PCR assay is a suitable method to detect and monitor Legionella species in an environment.     

Genetic and phenotypic variation in juvenile development in relation to temperature and developmental pathway in a geometrid moth

Life histories show genetic population-level variation due to spatial variation in selection pressures. Phenotypic plasticity in life histories is also common, facilitating fine-tuning of the phenotype in relation to the prevailing selection regime. In multivoltine (≥ 2 generations per year) insects, individuals following alternative developmental pathways (diapause/direct development) experience different selection regimes. We studied the genetic and phenotypic components of juvenile development in Cabera exanthemata (Lepidoptera: Geometridae) in a factorial split-brood experiment. F(2) offspring of individuals originating from populations in northern and central Finland were divided among manipulations defined by temperature (14°C/20°C) and day length (24 h/15 h). Short day length invariably induced diapause, whereas continuous light almost invariably induced direct development in both regions, although northern populations are strictly univoltine in the wild. Individuals from northern Finland had higher growth rates, shorter development times and higher pupal masses than individuals from central Finland across the conditions, indicating genetic differences between regions. Individuals that developed directly into adults tended to have higher growth rates, shorter development times and higher pupal masses than those entering diapause, indicating phenotypic plasticity. Temperature-induced plasticity was substantial; growth rate was much higher, development time much shorter and pupal mass higher at 20°C than at 14°C. The degree of plasticity in relation to developmental pathway was pronounced at 20°C in growth rate and development time and at 14°C in pupal mass, emphasizing multidimensionality of reaction norms. The observed genetic variation and developmental plasticity seem adaptive in relation to time-stress due to seasonality.     

Stage and segment specificity of the secretory cell of the dermal glands of the tobacco hornworm, Manduca sexta

The pair of epidermally derived Verson's glands on each segment of the tobacco hornworm, Manduca sexta, secretes at ecdysis proteinaceous products which coat the epicuticle. These proteins are produced by a single secretory cell which displays both stage- and segment-specificity during development. Three major 12-kDa polypeptides are synthesized at the larval molts, while higher molecular weight (14-93 kDa) polypeptides are produced at the pupal molt. In the pupa, but not in the larva, there are three segment-specific protein patterns, each involving both qualitative and quantitative differences: (1) thoracic (T) segments 1 and 2; (2) T3 and abdominal (A) segment 1; (3) A2-A8. Larval-specific proteins were found to be synthesized in low amounts throughout the penultimate fourth instar, with enhanced synthesis occurring during the molt, coincident with the molting surge of ecdysteroids. Synthesis of the major pupal products commenced about the time of wandering, with enhanced synthesis occurring throughout prepupal development, coincident with the prepupal surge in ecdysteroids. The onset of synthesis of the major pupal products differed, both within and between segments. Culture of fifth instar Day 2 glands in vitro showed that this synthesis depended on 20-hydroxyecdysone. The differential regulation within and between segments observed in vivo was also seen in vitro.     

Constraint on di-nucleotides by codon usage bias in bacterial genomes

It has been reported earlier that the relative di-nucleotide frequency (RDF) in different parts of a genome is similar while the frequency is variable among different genomes. So RDF is termed as genome signature in bacteria. It is not known if the constancy in RDF is governed by genome wide mutational bias or by selection. Here we did comparative analysis of RDF between the inter-genic and the coding sequences in seventeen bacterial genomes, whose gene expression data was available. The constraint on di-nucleotides was found to be higher in the coding sequences than that in the inter-genic regions and the constraint at the 2nd codon position was more than that in the 3rd position within a genome. Further analysis revealed that the constraint on di-nucleotides at the 2nd codon position is greater in the high expression genes (HEG) than that in the whole genomes as well as in the low expression genes (LEG). We analyzed RDF at the 2nd and the 3rd codon positions in simulated coding sequences that were computationally generated by keeping the codon usage bias (CUB) according to genome G+C composition and the sequence of amino acids unaltered. In the simulated coding sequences, the constraint observed was significantly low and no significant difference was observed between the HEG and the LEG in terms of di-nucleotide constraint. This indicated that the greater constraint on di-nucleotides in the HEG was due to the stronger selection on CUB in these genes in comparison to the LEG within a genome. Further, we did comparative analyses of the RDF in the HEG rpoB and rpoC of 199 bacteria, which revealed a common pattern of constraints on di-nucleotides at the 2nd codon position across these bacteria. To validate the role of CUB on di-nucleotide constraint, we analyzed RDF at the 2nd and the 3rd codon positions in simulated rpoB/rpoC sequences. The analysis revealed that selection on CUB is an important attribute for the constraint on di-nucleotides at these positions in bacterial genomes. We believe that this study has come with major findings of the role of CUB on di-nucleotide constraint in bacterial genomes.     

Costs of insecticide resistance in Cydia pomonella (Lepidoptera: Tortricidae)

The obvious benefits associated with insecticide resistance for pest species may come at a cost to other life-history traits. In this study, we compared the larval and pupal developmental times, pupal mass wing surface area and wing fluctuating asymmetry (FA) in insecticide resistant and control strains of codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), collected from apple (Malus spp.) orchards in central Canada. Resistant strains had significantly longer larval developmental times and lower pupal masses compared with the susceptible strain. Although the forewings of resistant moths were smaller in resistant than control strain, no difference in wing FA was detected. Longer developmental times could increase exposure time to natural enemies, and reduced adult size could affect longevity and total reproductive output.     

Maternal body size as an evolutionary constraint on egg size in a butterfly

Genetic and developmental constraints have often been invoked to explain patterns of existing morphologies. Yet, empirical tests addressing this issue directly are still scarce. We here set out to investigate the importance of maternal body size as an evolutionary constraint on egg size in the tropical butterfly Bicyclus anynana, employing an artificial two-trait selection experiment on simultaneous changes in body and egg size (synergistic and antagonistic selection). Selection on maternal body size and egg size was successful in both the synergistic and the antagonistic selection direction. Yet, responses to selection and realized heritabilities varied across selection regimes: the most extreme values for pupal mass were found in the synergistic selection directions, whereas in the antagonistic selection direction realized heritabilities were low and nonsignificant in three of four cases. In contrast, for egg size the highest values were obtained in the lines selected for low pupal mass. Thus, selection on body size yielded a stronger correlated response in egg size than vice versa, which is likely to bias (i.e., constrain), if weakly, evolutionary change in body size. However, it seems questionable whether this will prevent evolution toward novel phenotypes, given enough time and that natural selection is strong. Correlated responses to selection were overall weak. Egg and larval development times tended to be associated with changes in maternal size, whereas variation in pupal development times weakly tended to follow variation in egg size. Lifetime fecundity was similar across selection regimes, except for females simultaneously selected for large body mass and small egg size, exhibiting increased fecundity. Multiple regressions showed that lifetime fecundity and concomitantly reproductive investment were primarily determined by longevity, as expected for an income breeder, whereas egg size was primarily determined by pupal mass. Evidence for a phenotypic trade-off between egg size and number was weak.     

Genetic effects on flight capacity in the beet armyworm,Spodoptera exigua (Lep., Noctuidae)

The beet armyworm, Spodoptera exigua, is an important migratory insect pest in tropical and subtropical regions worldwide. The current study investigated genetic variation in the flight capacity of both female and male moths, using a quantitative genetics approach. The offspring–parent regression showed that parents had a significant influence on the flight duration of offspring, and the heritability estimated as 0.302. The upward selection increased mean flight duration from 123.7 to 284.6 min in females and from 113.9 to 254.0 min in males during 8 h of flight test; by contrast, downward selection decreased it from 123.7 to 65.6 min in females and from 113.9 to 29.8 min in males, while it did not change significantly in either females or males of the control line over eight generations. The mean realized heritability was estimated as 0.432 based on upward selection but 0.130 on downward selection, suggesting the asymmetry of response to selection on flight capacity. Reciprocal crosses between the two selected lines confirmed the dominance of ‘long‐flying genes’ in the inheritance of flight capacity. A positive genetic correlation was found between increased flight duration and pupal weight. The presence of such additive genetic variance and covariance for flight capacity and the fitness trait, pupal weight, in the population of S. exigua may have underpinned the evolution of its migratory behaviour.