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.     

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.     

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.     

Directional and stabilizing selection for developmental time and correlated response in reproductive fitness in Tribolium castaneum

Summary Directional and stabilizing selection for developmental time (DT) were done for seven generations in replicated lines of Tribolium castaneum. There were no significant differences between sexes or among replicates in means or coefficients of variation. For directional selection, there were significant responses in both directions, measured as deviation from control, viz. -0.35±0.15 day per generation for Fast (F) and 0.73±0.15 day for Slow (S). The unselected control (C) and the stabilizing selection (I) lines were similar, with average response per generation not significantly different from zero. — Phenotypic variation, from the first generation, was larger in the S line than in the other three lines. The I line showed a significant decrease in phenotypic variation, due mainly to a decrease in genetic variance. The realized heritability was 0.219±0.045 for F and 0.324±0.036 for S, the difference being highly significant. — Correlated response in reproductive fitness (number of pupae produced) was significant only for S (r_p=-0.88 and r_{G realized}=-0.79). Regression of the correlated response on DT in this line was-19.28±4.77 pupae per day (phenotypic) and -28.77±10.06 pupae per day (genetic).     

Correlated response in male and female sterility to selection for pupa weight in Tribolium castaneum

Summary The correlated responses in male and female sterility to 50 generations of individual selection for pupa weight in Tribolium were analyzed. Two replicate lines (S-lines) were selected for heavier pupa weight and stabilizing selection for pupa weight was practiced in two replicate control lines (C-lines). There was close agreement between replicates in both sets of lines for direct and correlated responses. The rate of inbreeding has been constant for all lines (approximately 0.5% per generation).Regression of generation means for pupa weight on generation of selection indicated a significant linear regression in the direct response for both lines. The linear increases of 46 and 55 g. per generation in the S-lines accounted for 98% of the variation among generations and the linear decreases of 5 and 10 g. per generation in the C-lines accounted for 70–90% of the variation in the generation means.Maximum likelihood estimators were used to calculate the frequency of male and female sterility for each generation and line. Average sterility in the base population ranged from about 4 to 12% for both sexes. Polynomial regressions of percent sterility on generation of selection showed that quadratic and higher order regressions were occasionally significant but accounted for a relatively small fraction of the total variation. In the two S-line replicates, linear regression coefficients of percent sterility on generation number were 0.16±.09 and 0.20±.07 for males and 0.72±.08 and 0.54±.08 for females, suggesting a larger correlated response in female than in male sterility. In the C-lines, linear regression coefficients were 0.02±.08 and –.12±.05 for males in the two replicates and –.05±.05 and –.05±.05 for females. Estimates of realized genetic correlations between pupa weight and sterility in the S-lines ranged from 0.04 to 0.14 for males and from 0.14 to 0.37 for females when the heritability of sterility was allowed to take on values from 0.05 to 0.25.Supported by NSF Grants G-1238 and GB-5987, NIH Grant GM-16074 and NIH Fellowship 1 FO2 GM4 5130-01.     

Responses of wheat plants to nutrient deprivation may involve the regulation of water-channel function

The sap flow (Jv) and the osmotic hydraulic conductance (L₀) of detached, exuding root systems from wheat (Triticum aestivum L. cv. Chinese Spring) plants deprived of nitrogen for 5 d (— N) or of phosphorus for 7 d (—P), were measured and compared with controls receiving a complete nutrient supply. In the roots of — N and — P plants, Jv and L₀ decreased markedly, but between 4 and 24 h after resupplying N to — N plants (NRS plants) and P to — P plants (PRS plants), Jv and Lo recovered to values similar to those of control plants. Values of Jv and L₀ were always greater during the light period than during the dark, due to the diurnal variation of these parameters. Reducing transpiration in the light had no effect on Jv and L₀ of — N and — P plants. Sap flow and L₀ were also determined using individual axes from plants which had been grown with their roots divided between nutrient-deficient (- N or- P) solution and a complete nutrient solution. Differences were observed in Jv and L₀ between axes of the same plant, but stomatal conductance (Gs), which was also measured, was not affected in these split-root experiments. In control plants, Jv and L₀ declined sharply to values similar to those of roots from — N and — P plants after HgCl₂ treatment (50 M), but were restored by treating with 5 mM dithiothreitol. In plasma membranes from — N and — P roots, the amount of stigmasterol increased relative to sitosterol compared with control roots. The degree of unsaturation of bound fatty acids also increased, compared with controls, as a result of a decline in the relative amounts of 160 and 180 and an increase in 182. Plasma-membrane fluidity, estimated by steady-state fluorescence polarisation using 1,6-diphenyl hexatriene, showed that the plasma membranes from nutrient-deprived plants were less fluid than those from control plants, measured during both the light and dark periods and in split-root experiments. In NRS plants, the relative abundance of sitosterol increased, so that the stigmasterol/sitosterol ratio returned to a value similar to that of controls. However, in PRS plants, the difference in stigmasterol/sitosterol ratio was maintained, compared with controls. The degree of unsaturation of bound fatty acids, membrane fluidity and the hydraulic conductivity of root systems also recovered in NRS and PRS plants to values similar to those of control plant plasma membranes. The results obtained suggested that — N and — P treatment decreased L₀, by reducing either the activity or the abundance of Hg-sensitive water channels. Also, there may be an interaction between the increase in membrane lipid ordering and the decrease in L₀.Abbreviations DTT dithiothreitol - Gs stomatal conductance - Js solute flow into the sap - Jv sap flow - L₀ hydraulic conductance - - N nitrogen-deprived for 5 d - NRS nitrogen resupplied after 5 d deprivation - - P phosphorus-deprived for 7 d - PRS phosphorus resupplied after 7 d deprivation M.C. was funded by a grant from CSIC, Spain. IACR receives grant-aided support from the Biotechnology and Biological Sciences Research Council of the United Kingdom.     

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.     

Genetic studies of self-fertility in rye (Secale cereale L.). 1. The identification of genotypes of self-fertile lines for the Sf alleles of self-incompatibility genes

Segregation for self-fertility has been studied in progenies from the crosses of self-sterile (SS) plants with interline hybrids obtained by a diallel scheme of pollinations between seven self-fertile (SF) lines (nos. 2–8) and with F₁ (SS plant x SF line) hybrids. All the offspring families from the SS plant x F₁ (SS plant x SF line) crosses demonstrated a 1SF1SS segregation. The crosses of SS plants with some interline hybrids gave only self-fertile plants, whereas the crosses with other interline hybrids gave a segregation of 3SF:1SS expected in the case of digenic segregation. The data obtained permitted us to identify three different S loci (S1, S2, S5) and to estimate the genotypes of self-fertile lines for their Sf alleles: lines 5, 6, 7 and 8 are S1f/S1f S2n/S2n S5m/S5m, line 4 is S1n/S1n S2f/S2f S5m/S5m, and lines 2 and 3 are S1n/S1n S2m/S2m S5f/S5f(Sn, Sm designate active alleles of the incompatibility genes). The identification of the particular S gene which is presented by the Sf allele in each line has been made on the basis of our data concerning the linkage of the Sf mutation with isozyme markers of particular rye chromosomes, which is reported in an accompanying paper.     

Characterization of agronomic traits and markers of recombinant inbred lines from intra- and interracial populations of Phaseolus vulgaris L.

The value of intra- and interracial populations in common bean (Phaseolus vulgaris L.) needs to be determined in order to create useful genetic variation for maximizing gains from selection, broadening the genetic base of commercial cultivars, and making efficient use of available resources. Five large-seeded parents of race Nueva Granada (N), two small-seeded race Mesoamerica (M), and one medium-seeded race Durango (D) were hybridized to produce one intraracial (N x N) and three interracial (two N x M and one N x D) populations. Seventy-nine F₂-derived F₆ lines randomly taken from each population along with their parents were evaluated for agronomic traits and markers at Palmira and Popayán, Colombia, in 1990 and 1991. Variation for agronomic traits and for morphological, protein, and isozyme markers was larger in interracial populations than in the intraracial population. Mean seed yield of all lines as well as yield of the highest yielding line from two interracial populations were significantly higher than that of the intraracial population. The highest ( 0.80±0.15) heritability was recorded for 100-seed weight. Values for seed yield varied from 0.19±0.17 to 0.50±0.16. Gains from selection (at 20% selection pressure) for seed yield ranged from 3.9% to 11.4%. Seed yield was positively associated with biomass yield, pods/m², and days to maturity, but harvest index showed negative correlations with these traits and a positive value with 100-seed weight. Polymorphism was recorded for phaseolin, lectins, protein Group-1 and protein Group-2 fractions, and six isozyme loci. Lines with indeterminate growth habit had significantly (P < 0.01) higher seed yield than lines with determinate growth habit in a Redkloud x MAM 4 population. Also, 23 other associations of markers with agronomic traits other than seed yield were recorded. Of these associations, lines with T phaseolin, the Diap1 ² allele, and lilac flower color tended to possess greater seed weight.     

A computer simulation evaluation of the role of mutations in finite populations on the response to directional selection: The generations required to attain maximum genetic variance

Summary The role of mutations in finite populations on response to artificial selection was investigated by a computer simulation model designed to mimic the biological model of pupal weight of Tribolium. Given the model, the results showed that with selection about 25–55 generations were needed for genetic variances to reach a maximum value depending on population size, selection intensity, and gene number. When effective population size was larger than 40 or the intensity of selection was high (less than 50% selected), selection had a dramatic effect in reducing the time to approach the maximum point of genetic variance. Furthermore, the genetic variance after that point often declined as a function of selection instead of remaining at a steady state in the subsequent generations.     

Genetic analyses of photoresponsiveness in the Djungarian hamster,Phodopus sungorus

Summary Endotherms living at temperate and arctic latitudes must adjust their physiology and behavior in order to survive seasonal change. The Djungarian hamster uses photoperiod to cue annual cycles of reproduction and thermoregulation, and its responses to short photoperiod include loss of body weight and change in pelage color. Some individuals do not exhibit these responses when exposed to short days.In this study individual variation in photoresponsiveness is quantified, and four lines of evidence for a genetic component to that variation are provided. First, two separate breeding stocks differed in both the percent of animals responding to a short-day lighting regimen (SD) and in the degree and timing of their response. Second, analysis of variance within and between families of full sibs for a photoresponsive index, PI (body weightloss +2 (molt index –1)) following 12 weeks in SD demonstrated a significant family resemblance (intraclass correlation of 0.36±0.03). Third, heritability estimates from regression of offspring scores on parent scores for body weight loss, molt index and PI after 12 weeks in SD were 0.340.13, 0.36±0.10 and 0.37±0.12, respectively, indicating a strong additive genetic component for the three characters. Finally, a significant response occurred after one generation of artificial selection for and against photoresponsiveness.Abbreviations PI photoresponsive index (body weight loss+2 (molt index –1)) - SD short day light regimen     

Photoregulation of germination in seed of transgenic lines of tobacco and Arabidopsis which express an introduced cDNA encoding phytochrome A or phytochrome B

Photoinduction and photoinhibition of germination in seed from a homozygous tobacco (Nicotiana tabacum L.) line containing an introduced oat phyA cDNA (encoding phytochrome A) is compared with that of isogenic wild-type (WT) tobacco. Under continuous irradiation by a light source with a low redfar-red (RFR) ratio the transgenic tobacco seed appeared to be less susceptible to photoinhibition of germination compared with WT seed. However, induction of germination following a short pulse by R (666 nm) was not enhanced in the genotype transformed by oat phyA cDNA compared with the WT; neither did germination of the transgenic tobacco seed show an increased sensitivity to saturating pulses of light of longer wavelengths (666–730 nm). In seeds of transgenic Arabidopsis thaliana (L.) Heynh. which contained an introduced phytochrome-B-encoding cDNA, levels of dark germination were enhanced, consistent with mediation of response by phytochrome B-P_fr. The germination behaviour of Arabidopsis genotypes wich contained an introduced cDNA encoding phytochrome A, however, did not significantly differ from that of the WT.Abbreviations ABO seed transformed with Arabidopsis phyB - cDNA; CaMV cauliflower mosaic virus - FR far-red light - P_fr far-red-absorbing form of phytochrome - P_tot total phytochrome - P_fr/P_tot phytochrome photoequilibrium - R red light - RBO seed transformed with rice phyB cDNA - RFR quantum ratio of red and far-red light - WL white light - WL + FR whitelight supplemented with far-red light - WT wild type The authors wish to thank R.D. Vierstra (Department of Horticulture, University of Wisconsin-Madison, USA) for providing the transgenic tobacco line, and M.T. Boylan, D. Wagner and P.H. Quail (U.C. Berkeley/USDA Plant Gene Expression Center, Albany, Calif. USA) for providing the transgenic Arabidopsis lines. The work presented in this paper was funded by grants from the Agricultural and Food Research Council (H.S., A.C.M., G.C.W.).     

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.     

General organization of the genes specifically involved in the diaminopimelate-lysine biosynthetic pathway of Corynebacterium glutamicum

Summary To obtain animal cell lines carrying nonsense mutations and the corresponding suppressors, we used a supersuppressor selection strategy on the CHO cell line. The wild-type strain is resistant to the aminopterin present in HAT medium (i.e., it is HAT^r) because it contains the enzymes hypoxanthine-guanine phosphoribosyl transferase (HPRT) and thymidine kinase (TK), whereas both HPRT mutants — selected by their resistance to 6-thioguanine (TG^r) — and TK^- mutants — selected by their resistance to 5-bromodeoxyuridine (BrdUrd^r) — are HAT^s. Therefore, from HPRT^- TK^- double nonsense mutants, whose phenotype would be TG^r BrdUrd^r (HAT^s), simultaneous HPRT⁺ TK⁺ double phenotypic revertants could be obtained by selecting HAT^r (TG^s BrdUrd^s) variants carrying the corresponding nonsense supersuppressors. Through ethylmethane sulfonate (EMS) mutagenesis of the CHO cell line we obtained 65 TG^r variants, 53 of which were HAT^s and the rest HAT^r. Among 36 TG^r (HAT^s) variants tested, 23 did not revert to HAT^r, 4 reverted spontaneously and with EMS, and 9 reverted only with EMS. Some of the latter were probably HPRT^- nonsense mutants because they were very stringent (had less than 2% of wild-type [³H]hypoxanthine incorporation and HPRT enzyme activity), and did not complement genetically. The introduction of a second marker (BrdUrd^r) in 7 of these strains allowed us to isolate 29 TG^r BrdUrd^r (HAT^s) double drug-resistant lines. Through one-step mutagenesis and selection in HAT medium, from two double resistant strains we could isolate HAT^r (TG^s BrdUrd^s) wild-type phenotypic revertants, each of which probably carries suppressible HPRT and TK nonsense (or missense) alleles and the corresponding supersuppressor. Our strategy could now be extended to obtain variants carrying suppressors in other cell lines.     

Modification of the properties of a translocation in the German cockroach by selection

Summary A stock of Blattella germanica bearing the interchange T(3; 12)/3;12 was subjected to close inbreeding with selection for random disjunction at metaphase I. After 3–4 generations of selection, interchange quadrivalent chiasma frequency decreased, variability in free bivalent chiasma frequency increased sharply, and individuals with either random or directed disjunction were present in the stock. Random disjunction was modified from a ratio of 2112 (adj.-1; alt.-1; adj.-2; alt.-2) to a ratio of 1111. After 7–8 generations of selection, chiasma frequency appeared to stabilize at lower than normal levels and variability decreased for both quadrivalents and free bivalents. Directed disjunction was modified from a ratio of 2114 to 1112, and no individuals with the original high level of directed disjunction were detected. Chains-of-four tended to orient randomly, especially in individuals where the ring quadrivalents showed directed disjunction. Relaxation of inbreeding, but not selection, produced an increase in chiasma frequency and variability in both free bivalents and quadrivalents, but the modified ratios for both random and directed disjunction were retained. These results are discussed with respect to inbreeding effects and genetic control of chiasma frequency and metaphase I disjunction in interchange quadrivalents.     

Improvement of Rice Biomass Yield through QTL-Based Selection

Biomass yield of rice (Oryza sativa L.) is an important breeding target, yet it is not easy to improve because the trait is complex and phenotyping is laborious. Using progeny derived from a cross between two high-yielding Japanese cultivars, we evaluated whether quantitative trait locus (QTL)-based selection can improve biomass yield. As a measure of biomass yield, we used plant weight (aboveground parts only), which included grain weight and stem and leaf weight. We measured these and related traits in recombinant inbred lines. Phenotypic values for these traits showed a continuous distribution with transgressive segregation, suggesting that selection can affect plant weight in the progeny. Four significant QTLs were mapped for plant weight, three for grain weight, and five for stem and leaf weight (at α = 0.05); some of them overlapped. Multiple regression analysis showed that about 43% of the phenotypic variance of plant weight was significantly explained (P < 0.0001) by six of the QTLs. From F₂ plants derived from the same parental cross as the recombinant inbred lines, we divergently selected lines that carried alleles with positive or negative additive effects at these QTLs, and performed successive selfing. In the resulting F₆ lines and parents, plant weight significantly differed among the genotypes (at α = 0.05). These results demonstrate that QTL-based selection is effective in improving rice biomass yield.     

Effects of pollen selection on progeny vigor in a Cucurbita pepo x C. texana hybrid

We examined the effects of pollen selection for rapid pollen-tube growth on progeny vigor. First, we crossed a wild gourd (Cucurbita texana) to a cultivated zucchini (Cucurbita pepo cv Black Beauty) to produce an F₁ and then an F₂ generation. Half of the F₁ seeds were produced by depositing small loads of C. texana pollen onto the stigmas of C. pepo. These small pollen loads were insufficient to produce a full complement of seeds and, consequently, both the fast- and the slow-growing pollen tubes were permitted to achieve fertilization. An F₂ generation was then produced by depositing small loads of F₁ pollen onto stigmas of F₁ plants. The F₂ seeds resulting from two generations of small pollen loads are termed the non-selected line because there was little or no selection for pollen-tube growth rate on these plants. The other half of the F₁ and F₂ seeds were produced by depositing large pollen loads (>10 000 pollen grains) onto stigmas and then allowing only the first 1% or so of the pollen tubes that entered the ovary to fertilize the ovules. We did this by excising the styles at the ovary at 12–15 h after pollination. The resulting F₂ seeds are termed the selected line because they were produced by two generations of selection for only the fastest growing pollen tubes. Small pollen loads from the F₂plants, both the selected and the non-selected lines, were then deposited onto stigmas of different C. pepo flowers, and the vigor of the resulting seeds was compared under greenhouse and field conditions. The results showed that the seeds fertilized by pollen from the selected line had greater vegetative vigor as seedlings and greater flower and fruit production as mature plants than the seeds fertilized by pollen from the non-selected line. This study demonstrates that selection for fast pollen-tube growth (selection on the microgametophyte) leads to a correlated increase in sporophyte (progeny) vigor.     

Pollen irradiation and gene transfer in Capsicum

Summary The aim of the experiment was to study the possibility of facilitating the gene transfer and reducing the number of required backcrosses through pollen irradiation and subsequent selection of F₁M₁ plants containing a very high proportion of sterile pollen as male parent for backcrossing. Anthers of a donor line, C-3-1, were irradiated with 1,500 rad -rays and the pollen used for pollination of a recipient genotype W-8 which posseses a number of recessive marker genes. Five F₁M₂ plants containing more than 80% sterile pollen grains and one semi-sterile plant were selected and used for backcross to W-8. The segregation pattern of four characters expressed in the first backcross generation [W–8×(W–8×C–3–1)] was assessed and compared with the non-irradiated control. A changed segregation pattern was observed (in some cases even non-transfer of a paternal allele) as well as a shift towards more plants possessing the investigated maternal alleles. A scheme for backcross procedure in combination with pollen irradiation is discussed.     

Correlated responses in growth and body composition of replicated single-trait and index selected lines of mice

Summary Correlated responses in growth, body composition and efficiency were evaluated in lines of mice selected in the following ways: W⁺T _i ^o , increased six-week body weight (WT6); W ° T _i ⁺ , increased six-week tail length (TL6); W⁺T _i ^– , increased WT6 and decreased TL6; W^–T _i ⁺ , decreased WT6 and increased TL6; M16, increased three-to six-week postweaning gain (PWG). Each of the first four selection treatments had two replicate lines (i = 1, 2) selected for 13 generations and the fifth treatment had one line selected for 30 generations. All lines were derived from a randombred ICR albino population which served as a control. Additional traits studied were three-week body weight and tail length, postweaning gain in tail length, percent body composition (ash, fat, moisture and protein) at six weeks of age, and three-to six-week feed consumption (CONS) and efficiency (EFF = PWG/CONS). Efficiency of body constituent gains (ash, fat, protein and caloric value) were determined by dividing each constituent by CONS. Relative to selection treatments, replicate variation in the array of traits was small and was primarily attributable to the effects of genetic drift; more frequent significant replicate differences among traits in W⁺T^– were associated with a replicate difference in cumulative selection differentials. Selection for different criteria involving WT6 and TL6 did not change the allometric relationship between tail length and body weight in the three-to six-week age interval. The significant divergence between W⁺T ° and W °T⁺ and between W⁺T^– and W^–T⁺ was as expected for WT6 and TL6. Significant asymmetry of selection response between W⁺T^– and W^–T⁺ for WT6 and TL6 was attributed to maternal effects. In agreement with theory, antagonistic index selection generally yielded smaller genetic responses than single trait selection. Positive correlated responses in CONS and EFF were found for M16 and W⁺T °. Significant correlated changes in CONS (positive in W °T⁺ and negative in W^–T⁺) were not accompanied by a significant change in EFF. In contrast, W⁺T^– evinced an increased EFF and no change in CONS. Percent fat increased significantly in W⁺T ° and M16. For W⁺T^o, W⁺T^– and M16, an increased energetic, fat and ash efficiency was observed, whereas M16 exhibited a positive increment in protein efficiency as well. Among selection treatment means, there were high positive correlations between WT6 and fat weight, protein weight, percent fat, CONS and EFF and a high negative correlation between WT6 and percent protein.Paper No.4916 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh, N.C. 27607. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Experiment Station of the products named, nor criticism of similar ones not mentioned.