Nature Of Gene Action Research Articles

Nature of gene action and genetic parameters for yield and its components in chickpea

To determine the gene action and genetic parameters of agro-morphological traits in chickpea, five genotypes of chickpea as a half diallel crossed with each other in 2008, five parents and 10 progenies were planted as randomized complete block design with three replications and some traits including days to flowering and maturity were recorded on row basis when 50% plants flowered or matured. Plant height, biomass, harvest index, number of primary branches, number of pods per plant, number of seeds per plant, 100-seed weight and seed yield were recorded for each plant (10 observation). Based on analysis of variance, variance due to additive gene effects showed significant differences for plant height and number of primary branches per plant. Both additive and dominance genetic effects were significant for days to flowering, days to maturity, biomass, 100-seed weight, harvest index, number of pods, seeds and seed yield per plant. Higher values (>1) of the average of dominance (H1/D) 1/2 , a measure of over dominance, were observed in this study except for plant height and harvest index. Higher values of narrow-sense heritability was also observed for harvest index (67%), 100-seed weight (56%) and plant height (42%) indicating that huge genetic gain could be achieved for these traits. Key words: Additive, chickpea, diallel, dominance, heritability.

THE NATURE OF GENE ACTION AND THE PERFORMANCES OF HYBRIDS WITH RESPECT TO EARLINESS AND YIELD TRAITS UNDER NORMAL AND LATE SOWING DATES IN WHEAT (Triticum aestivum , L.).

This investigation was made to evaluate three bread wheat hybrids and their six populations generated from each of them under normal and late sowing dates. The three hybrids were obtained where hybrid 1 was a result of crossing (Late × Late) cultivars; hybrid 2 was a result of crossing (Early × Early) cultivars and hybrid 3 was a result of crossing (Late × Early) ones. The early parental in breed lines included: Line1, Line2 and Line3 while, thelate parental onesincluded: Misr2 and Gemmeiza9. The two sowing dates were: normal sowing on 29 November 2011 and the late sowing on 29 December 2011. Therefore, the total genetic materials were obtained from the three hybrids included 18 genotypes which were evaluated at Sakha Agricultural Research Station Farm, Wheat Section, Egypt. Earliness traits showed that late sowing date required fewer days than normal sowing date for days to heading; days to maturity and grain filling period. This trend was present for all populations of the three hybrids. It appeared that late sowing shorten the life span of plants. Yield component characters which included: plant height; 100- kernel weight and grain yield/ plant showed the advantage of hybrid 1 for plant height and grain yield/plant than the other two hybrids. These two characters showed a modest amounts of heterosis for the three hybrids. Nature of gene action which estimated from the six populations and evaluated using the scaling test analysis proposed by Mather and Jinks (1971). The results indicated the importance of additive genetic variance than dominance genetic variance one for most studied traits. These results were also supported by the results obtained from the scaling test analysis as the quantities A,B and C indicated the adequacy of the additive dominance model.

Determination of gene action for some biometrical traits in Lens culinaris Medik. under mid-hill conditions of northwestern Himalayas

in south Asia.High protein percentage (22–34.6%) along with significantconcentration of limiting amino acid lysine and its fast cook-ing characteristics make it an important part of diet in manyparts of the world, especially west, south and southeast Asia.This crop is valued for its role in the cropping system as itneeds few inputs and has beneficial effects on the soil fer-tility, because of symbiotic nitrogen fixation. The contribu-tion of India to global lentil area and production is 39.52 and42.42%, respectively. However, the productivity of lentil inIndia is very low (619 kg per ha) in comparison to the worldaverage of 887 kg per ha because it is normally grown underrainfed conditions by resource-poor farmers with dearth ofpotential high yielding cultivars.Being an autogamous crop, recombinant breeding is themost appropriate approach to combine various desirableattributes like high yield and other important yield contribut-ing and quality attributes. The choice of most suitable breed-ing procedure, among several available, depends to a largeextent on the nature of gene action involved in the con-trol of character(s) in which the plant breeder is interested(Cockerham 1961). The detection, estimation and interpre-tation of epistasis has progressed much faster at the levelof first degree statistics (Mather and Jinks 1982) which hascertain limitations due to the cancellation of genic effects.An experimental method known as triple test-cross to over-come these limitations has been developed by Kearsey andJinks (1968), which besides overcoming these limitations has

Combining Ability for Yield and Fruit Quality in Luffa

The mating design (Line and Tester) has been extensively used to estimate general combining ability (GCA) and specific combining ability (SCA) variance and their effects. This design is also used to understand the nature of gene action involved in expression of economically important traits. Diallel analysis and combining ability in Luffa acutangula (L.) Roxb have not been extensively reported. Combining ability of 13 traits was examined in 45 Luffa hybrids derived from a 10 × 10 diallel mating design (excluding reciprocals) at two sowing dates and two locations. Additive and nonadditive genes were important for traits. The interaction of SCA × Location was not significant for traits except for days to opening of first female flower. Additive gene effects were more important for vine length, days to opening of the first male flower, number of male and female flowers/plant, number of nodes, number of fruit/plant, fruit length, fruit diameter, and number of seed/fruit. ‘Slumber Long’ was the best combiner, among 10 parental lines, for fruit yield/plant and fruit size. Crosses involving diverse parents and showing high SCA effects for fruit yield/plant have the potential to be utilized in Luffa improvement through simple recurrent selection.

Combining ability and gene action studies for yield-contributing traits in crosses involving winter and spring wheat genotypes

The success of winter × spring wheat hybridization programmes depends upon the ability of the genotypes of these two physiologically distinct ecotypes to combine well with each other. Hence the present investigation was undertaken to study the combining ability and nature of gene action for various morpho-physiological and yield-contributing traits in crosses involving winter and spring wheat genotypes. Five elite and diverse genotypes each of winter and spring wheat ecotypes and their F 1 (spring × spring, winter × winter and winter × spring) hybrids, generated in a diallel mating design excluding reciprocals, were evaluated in a random block design with three replications. Considerable variability was observed among the spring and winter wheat genotypes for all the traits under study. Furthermore, these traits were highly influenced by the winter and spring wheat genetic backgrounds, resulting in significant differences between the spring × spring, winter × winter and winter × spring wheat hybrids for some of the traits. The winter × spring wheat hybrids were observed to be the best with respect to yieldcontributing traits. On the basis of GCA effects, the spring wheat parents HPW 42, HPW 89, HW 3024, PW 552 and UP 2418 and the winter wheat parents Saptdhara, VWFW 452, W 10 and WW 24 were found to be good combiners for the majority of traits. These spring and winter wheat parents could be effectively utilized in future hybridization programmes for wheat improvement. Superior hybrid combinations for one or more traits were identified, all of which involved at least one good general combiner for one or more traits in their parentage, and can thus be exploited in successive generations to develop potential recombinants through various breeding strategies. Genetic studies revealed the preponderance of additive gene action for days to flowering, days to maturity and harvest index, and non-additive gene action for the remaining six traits.

COMBINING ABILITY AND HETEROSIS IN MAIZE (Zea mays L.)

A line × tester analysis was conducted in maize involving 12 lines and 3 testers for grain yield and its components to determine the heterosis as well as general combining ability (GCA) and specific combining ability (SCA) effects. Highly significant genotypic differences were observed indicated wide range of variability present among the genotypes. GCA and SCA variance for yield per plant number of kernels per row and 100-kernels weight were observed significant, which indicated importance of additive as type of gene action for these characters. The ratio of SCA and GCA variances were high for the all character studied that revealed the preponderance of non additive type of gene action. Standard heterosis ranged from -28.29 to 28.41%; -12.29 to 24.38%; -1.11 to 24.44%; -14.75 to 6.67%; -17.24 to 11.26% and -10.94 to 20.83% for grain yield per plant, number of grains per row, number of rows per ear, ear length, ear diameter and 100-kernel weight, respectively. The lines IPB 911-16, IPB 911-12, IPB 911-2, IPB 911-18 and IPB 911-47 showed significant positive GCA effect and simultaneously possessed high mean value indicating that the per se performance of the parents could prove as an useful index for combining ability. The crosses exhibited significant SCA effects involved high x high, high x low, low x high, average x low and low x low general combining parents. The cross combinations with significant positive SCA effect having high mean values might be used for obtaining high yielding hybrids. The information on the nature of gene action with respective variety and characters might be used depending on the breeding objectives.DOI: http://dx.doi.org/10.3329/bjpbg.v21i1.17045

Combining ability analysis to identify suitable parents for heterosis in seed cotton yield, its components and lint % in upland cotton

Combining ability and heterosis were studied in a 6 × 6 diallel cross to see the nature of gene action in Upland cotton ( Gossypium hirsutum L.) during 2002 to 2004. Analysis of variance revealed highly significant differences among all the F 1 and F 2 hybrid means and their respective six parental values for all the traits examined. In both generations, the mean squares due to general combining ability (GCA) and specific combining ability (SCA) were also highly significant. SCA genetic variances were greater than GCA and more important for the traits, i.e. boll weight, boll number and seed cotton yield per plant, showing the predominance of non-additive gene action. Lint % in both generations and boll weight in F 2s only were controlled by additive type of gene action due to maximum GCA variances. Cultivar CIM-1100 was found to be the best general combiner and its utilization produced valuable hybrids with desirable SCA in both generations. F 1 and F 2 hybrids, viz., CIM-1100 × CRIS-9, CIM-1100 × FH-682, CIM-1100 × BH-36 and CIM-109 × CIM-1100 as high × low and low × high parents performed well in SCA determination, outstanding mean performance and heterosis. Better SCA effects associated with useful heterosis were more pronounced for yield traits. In F 1 hybrids, maximum heterosis was observed for seed cotton yield followed by boll number, boll weight and lint %. The heterosis over better parent was +3.13 to +65.63% for bolls per plant, +0.75 to +24.40% for boll weight, +0.82 to +115.22% for seed cotton yield and +0.27 to +3.88% for lint %. Involvement of CIM-1100 in most of the F 1 and F 2 hybrids resulted in the synthesis of superior genotypes for most of the traits studied. Inbreeding depression was elevated in good performing hybrids and was the highest for seed cotton yield. Highest yielding F 1 hybrids yielded lesser in the subsequent generation due to over-dominance and inbreeding depression, whereas moderate yielding F 1 hybrids were found more stable even passing through process of segregation due to additive gene action. The combined performance of F 1 and F 2 hybrids could be a good indicator to identify the most promising populations to be utilized either as F 2 hybrids or as a resource population for further selection.

GENETIC ANALYSIS OF SOME QUANTITATIVE TRAITS IN COTTON

The main objective of the present investigation is to obtain wide genetic variation by using lee it tester analysis involving two seeded testers as male parents, namely, AugUraliall and Karashnelry and four Egyptian cotton culbvars as females (lines) namely Gza80, Giza85, Gza90 and Gia87 (G.barbadense). Eight Ft's, the two tester varieties and the four lone cultivars were grown in a randomized complete block design with three replications in 2006 season to evaluate combining abilities and nature of gene action. Genotypes mean squares were highly significant for all traits except for int percentage (L %), lint index (LI) and uniformity index (U.I). Parents mean squares revealed highly significant differences for all studied traits except for lint percentage (L %) and ant index (U). Higher estimates of dominance (02d) variance than additive valance (02A) were recorded and low narrow sense heritability values (h%n.$) and low magnitude of the ratio Q2gCa/Q25ca (less than unity) were found for first fruaing node (FFN), seed cotton yield (KY), lint yield (LY), lint percentage (1%), seed index (SI), lint index (U), micronalr value (Nie), plessly index (PI) and Upper half mean (U.H.M). On the other hand days of first flower (DFF), boll weight (8W) and uniformity index (U.I) recorded lion estimates of additNe variance (02A) and narrow sense heritability (h%n.$). For the testers, Australian and Karashneky varieties were good combiners for first fruiting node (FFN), days of first flower (OFF), seed cotton yield (SCY), lint yield (LY) and uniformity index (VI). Among female parents the variety Giza85 was the best combiner for days of fist flower (OFF), seed cotton yield (SCY), lint yield (LY), seed index (SI) and uniformity index (MI). Giza87 was the best general combiners for fiber traits. Gza90 was the best general combiner for boa weight (avo, seed index (SI) and uniformity index (U.I). Gza80 was the best combiner for first fruiting node (FIN), days of first flower (OFF), boll weight (BW), seed talon yield (SCY), ant yield (LY), micron* value (Mic) and con* index (F1). It oduld be concluded that (tie hybrid (Australian it Giza85) and (Karashneky it Gza85) may be used for the improvement of seed cotton yield (SCY), lint yield (LT), seed Index (SI) and Upper half mean (URA), while (Australian it Gza80) and (Karashneky x Gza80) hybrids were the best for the Improvement of seed cotton yield (SCY), lint yield (1Y) and pressty Index (PI).

Biphasic survival analysis of trypanotolerance QTL in mice

A marker-assisted introgression (MAI) experiment was conducted to transfer trypanotolerance quantitative trait loci (QTL) from a donor mouse strain, C57BL/6, into a recipient mouse strain, A/J. The objective was to assess the effect of three previously identified chromosomal regions on mouse chromosomes 1 (MMU1), 5 (MMU5) and 17 (MMU17) in different genetic backgrounds on the survival pattern following infection with Trypanosoma congolense. An exploratory data analysis revealed a biphasic pattern of time to death, with highly distinct early and late mortality phases. In this paper, we present survival analysis methods that account for the biphasic mortality pattern and results of reanalyzing the data from the MAI experiment. The analysis with a Weibull mixture model confirmed the biphasic pattern of time to death. Mortality phase, an unobserved variable, appears to be an important factor influencing survival time and is modeled as a binary outcome variable using logistic regression analysis. Accounting for this biphasic pattern in the analysis reveals that a previously observed sex effect on average survival is rather an effect on proportion of mice in the two mortality phases. The C57BL/6 (donor) QTL alleles on MMU1 and MMU17 act dominantly in the late mortality phase while the A/J (recipient) QTL allele on MMU17 acts dominantly in the early mortality phase. From this study, we found clear evidence for a biphasic survival pattern and provided models for its analysis. These models can also be used when studying defense mechanisms against other pathogens. Finally, these approaches provide further information on the nature of gene actions.

Gene system governing stomatal number and specific leaf weight in bread wheat ( Triticum aestivum L.)

Gene effects were analysed using mean stomatal number and specific leaf weight of 12 populations, consisting of both parents (P 1 and P 2 ), F 1 , F 2 , first backcross generations (BC 1 and BC 2 ), second backcross generations (B 11 , B 12 , B 21 , B 22 ) and backcross selfed generations (B 1 s and B 2 s) of four crosses involving three drought-tolerant and three drought-susceptible cultivars of Triticum aestivum L. to determine the nature of gene action governing stomatal number (SN) and specific leaf weight (SLW) through generation mean analysis in moisture stress (E 1 ) and moisture non-stress (E 2 ) environments. The digenic epistatic model was found to be inadequate for stomatal number and the additive-dominance model was found to be adequate for specific leaf weight in most of the crosses. Additive gene effects were predominant for SLW, while for SN both additive and dominance components of variance were important. Epistatic effects, particularly the additive × dominance (j) type of interaction, were present for both the characters. The duplicate type of epistasis was observed for stomatal number in the cross VL421/HS240 in the moisture stress environment. Significant heterosis was observed for the crosses Hindi 62/HS240 and VL421/HS240 over the standard check (SC) in the moisture stress environment (E 1 ) for both the characters. Genotype-environmental interactions and/or differential gene expression appeared to account for the different results found between environments. Hybridization systems, such as biparental mating and/or diallel selective mating, could be useful for the improvement of these traits, which would help in identifying drought-tolerant progenies.

Inheritance of resistance to spotted stem borer, Chilo partellus, in sorghum, Sorghum bicolor

The spotted stem borer, Chilo partellus, is one of the most important pests of sorghum, and host plant resistance is an important component for the management of this pest. Most of the sorghum hybrids currently under cultivation are based on cytoplasmic male-sterility (CMS). In order to develop a strategy for resistance to stem borer, we studied the traits associated with resistance, and their nature of gene action in F1 hybrids derived from resistant, moderately resistant, and susceptible CMS and restorer lines. The hybrids based on stem borer-resistant, moderately resistant, or susceptible CMS and restorer lines were equally resistant or susceptible as the parents for leaf feeding [Damage rating (DR) 5.8 to 6.6 vs. 5.9 to 6.6], and had significant and decreasing trend in deadheart formation (resistant CMS × resistant restorer lines < moderately resistant CMS × moderately resistant restorer lines < susceptible CMS × susceptible restorer lines), respectively. Proportional contributions of restorer lines were greater than those of the CMS lines for leaf feeding, deadhearts, recovery and overall resistance, stalk length, nodes per plant, stem borer holes per plant, and peduncle tunneling. The general (GCA) and specific combining ability (SCA) estimates suggested that leaf feeding score, number of nodes, overall resistance score, panicle initiation, recovery score, and stalk length (dominance type of gene action) have been found to be associated with resistance to spotted stem borer, governed by additive type of gene action, their correlation and direct effects in the same direction, and explained 65.3% of the variation in deadhearts, and thus could be used as marker traits to select and breed for resistance to C. partellus in sorghum. The parents having significant SCA effects for two or more resistance traits for either or more parents have also been discussed for their use in the stem borer resistance breeding.

Gene Action and Combining Ability for Grain Yield and Its Components in Rice

ABSTRACT In this study, an effort was made to identify good parents and nature of gene action governing different characters of rice by line (4 restorers) × tester (2 cytoplasmic male sterile lines) analysis. The trial comprising 8 hybrids and their parents were laid out in randomized block design with 3 replications and characterized for yield and nine yield component traits. In the present study, the ANOVA for combining ability revealed significant differences between the treatments (genotype) for all the traits studied and amongst the treatments the mean sum of square (MSS) due to parents showed significant differences for all the traits. The grain yield, grain number per panicle, plant height and days to fifty percent flowering appears to be the key determining traits and showed significant differences for the lines, testers, line × testers. Parents with low × high general combining ability (GCA) effects resulted in heterotic hybrid combination like the IR68281A/IR68749R. Interestingly, we found low × low GCA to give the best heterotic combination, i.e., IR68281A/IR60966R. However, it is worthy to note that the line IR60966R showed non-significant GCA effects for grain yield, hundred grain weight, flag leaf length and grain width while significant but negative GCA effects for grain number per panicle and a low GCA tester IR68281A for all the traits. Perhaps, it implies that the GCA only identifies better parental lots but it will be unwise to discard the low GCA types. Therefore, it can be concluded that the prediction of heterosis on the basis of GCA of the parents may not always be accurate. Based on the specific combining ability (SCA) effects only three combinations were found to be highly significant for grain yield, i.e., IR58025A/IR60819R, IR68281A/IR42686R, IR68281A/IR60966R. Such hybrid combinations will be utilized for exploitation of hybrid vigor after assessing their cooking quality features. Heterobeltiosis for yield was positive ranging from 4.1 to 32.2% for six out of eight hybrids. The grain length and width improved for hybrids over the better parents. Predominance of dominant gene action for all the traits studied except for plant height and hundred grain weight thus suggesting that the hybrid breeding must be strongly pursued for the exploitation of the yield advantage over the inbred varieties.

“A Gene for. . .”:The Nature of Gene Action in Psychiatric Disorders

A central phrase in the new “GeneTalk” is “X is a gene for Y,” in which X is a particular gene on the human genome and Y is a complex human disorder or trait. This article begins by sketching the historical origins of this phrase and the concept of the gene-phenotype relationship that underlies it. Five criteria are then proposed to evaluate the appropriateness of the “X is a gene for Y” concept: 1) strength of association, 2) specificity of relationship, 3) noncontingency of effect, 4) causal proximity of X to Y, and 5) the degree to which X is the appropriate level of explanation for Y. Evidence from psychiatric genetics is then reviewed that address each of these criteria. The concept of “a gene for” is best understood as deriving from preformationist developmental theory in which genes—like preformationist anlagen—“code for” traits in a simple, direct, and powerful way. However, the genetic contribution to psychiatric disorders fails to meet any of the five criteria for the concept of “X is a gene for...

“A Gene for…”: The Nature of Gene Action in Psychiatric Disorders

A central phrase in the new "GeneTalk" is "X is a gene for Y," in which X is a particular gene on the human genome and Y is a complex human disorder or trait. This article begins by sketching the historical origins of this phrase and the concept of the gene-phenotype relationship that underlies it. Five criteria are then proposed to evaluate the appropriateness of the "X is a gene for Y" concept: 1) strength of association, 2) specificity of relationship, 3) noncontingency of effect, 4) causal proximity of X to Y, and 5) the degree to which X is the appropriate level of explanation for Y. Evidence from psychiatric genetics is then reviewed that address each of these criteria. The concept of "a gene for..." is best understood as deriving from preformationist developmental theory in which genes-like preformationist anlagen-"code for" traits in a simple, direct, and powerful way. However, the genetic contribution to psychiatric disorders fails to meet any of the five criteria for the concept of "X is a gene for Y." The impact of individual genes on risk for psychiatric illness is small, often nonspecific, and embedded in complex causal pathways. The phrase "a gene for..." and the preformationist concept of gene action that underlies it are inappropriate for psychiatric disorders.

Genetics of spike length in durum wheat

Gene effects were analyzed using mean spike length of 12 populations, viz., both parents, F1, F2, first back cross generation, BC1 and BC2, second backcross generations, BC11,BC12, BC21 and BC22 along with BC1 self and BC2 self derived by selfing BC1 and BC2populations of three crosses involving six diverse cultivars of Triticum durumto determine the nature of gene actions governing spike length through generation mean analysis under normal and late sown environments. The six-parameter model was adequate in most of the cases to explain genetic variation among the generation means under both the sowing environments. Additive (d) gene effect was significant in all the cases, whereas dominance (h) gene effect was not so frequently observed significant. Epistatic effects, particularly digenic types were predominant over additive and dominance effects in most of the cases under both normal and late sown environments except in the cross Cocorit 71 × A-9-30-1 (normal sown).Additive × dominance × dominance (y), trigenic interaction played significant role in controlling the inheritance of this trait in the cross HI 8062 × JNK-4W-128under late sown condition. Duplicate epistasis was observed in the cross HI 8062× JNK-4W-128 (normal sown). Non-fixable gene effects were of higher magnitude than fixable gene effects in almost all cases, confirmed the major role of non-additive gene effects to control the inheritance of spike length in durum wheat. Significant heterosis over better parent was not observed. Similarly, inbreeding depression was not commonly observed. Favourable and suitable environment must be considered before finalizing breeding programme for its simple inheritance to get desirable improvement for high grain yield. Hybridization systems, such as biparental mating and / or diallel selective mating, which exploit both additive and non-additive gene effects, simultaneously, could be useful in the improvement of spike length in durum wheat.

Genetic Effect on Amylose and Protein Contents in the Crossed Rice Seeds

SummaryThe nature of gene action may be revealed and desirable parents may be found by analyzing the combining ability and estimating the degree of heterosis in an early generation. Currently, varieties with a good eating quality are preferred. This study was carried out to obtain information on the action of genes using the analysis of variance components estimated by minimum norm quadratic unbiased estimation (MINQUE(1)) and to estimate the genetic effects on amylose and protein contents, as criteria for selection of good eating quality by adjusted unbiased prediction (AUP). The mean values of amylose content, protein content and grain yield in the parents were 18.4%, 7.78% and 19.8 g plant–1, respectively, and those in crossed F1 seeds were 17.4%, 10.8% and 23.6 g plant–1, respectively. The effects of additive were significant for both amylose and grain yield, but not for protein content, indicating the importance of the additive gene action on amylose content and grain yield. The effects of dominance were highly significant for amylose content, protein content and grain yield. Especially, the effect of dominance was greater than that of additive on protein content. The effect of maternal was detected in only amylose content, and L 42 and L 53 showed significant and negative maternal effect on amylose content. Ansanbyeo, Hinohikari, L 42 and L 53 exhibited negative additive effects on amylose content, which suggested the possibility of a decreased amylose content in their crossed progenies. Mid-parent heterosis for amylose content ranged from –44.7% (L 42 × Hinohikari) to 32.9% (L 53 × Lemont) with an average value of –7.2% for all crossed F1 hybrids. However, all of the crossed seed showed positive mid-parent heterosis for protein content in this study and the differences were not significant among the hybrids.

EFFECTS OF DIFFERENT LEVELS OF INBREEDING ON PROGENY FITNESS IN PLANTAGO CORONOPUS.

Inbreeding depression (δ) is a major selective force favoring outcrossing in flowering plants. Many phenotypic and genetic models of the evolution of selfing conclude that complete outcrossing should evolve whenever inbreeding depression is greater than one-half, otherwise selfing should evolve. Recent theoretical work, however, has challenged this view and emphasized (1) the importance of variation in inbreeding depression among individuals within a population; and (2) the nature of gene action between deleterious mutations at different loci (epistasis) as important determinants for the evolution of plant mating systems. The focus of this study was to examine the maintenance of inbreeding depression and the relationship between inbreeding level and inbreeding depression at both the population and the individual level in one population of the partially self-fertilizing plant Plantago coronopus (L.). Maternal plants, randomly selected from an area of about 50 m2 in a natural population, were used to establish lines with expected inbreeding coefficients (f) of 0, 0.25, 0.50, 0.75, and 0.875. Inbreeding depression was estimated both in the greenhouse and at the site of origin of the maternal plants by comparing growth, survival, flowering, and seed production of the progeny with different inbreeding coefficients. No significant inbreeding depression for these fitness traits was detected in the greenhouse after 16 weeks. This was in strong contrast to the field, where the traits all displayed significant inbreeding depression and declined with increased inbreeding. The results were consistent with the view that mutation to mildly deleterious alleles is the primary cause of inbreeding depression. At the family level, significantly different maternal line responses (maternal parent × inbreeding level interaction) provide a mechanism for the invasion of a selfing variant into the population through any maternal line exhibiting purging of its genetic load. At the population level, evidence for synergistic epistasis was detected for the probability of flowering, but not for total seed production. At the family level, however, a significant interaction between inbreeding level and maternal families for both traits was observed, indicating that epistasis could play a role in the expression of inbreeding depression among maternal lines.

The nature of gene action in a Nicotiana rustica cross revealed by the recombinant inbred and second-cycle hybrid analysis

A unique set of data recorded on 60 randomly extracted single-seed-descent (F∞) lines of a highly heterotic cross between two varieties of Nicotiana rustica and their 870 reciprocally produced pairwise crosses, the second-cycle hybrids (SCH), are analysed to investigate the true nature of genetical control in the cross and the results are compared with those in earlier publications. The analysis revealed that epistasis, genotype-by-micro-environmental interaction, maternal effects and linkage are significant for several characters and the additive and non-additive components of variation take large values for all of the traits. Epistasis is predominantly duplicate and not complementary. Dominance is high but partial, all estimates of dominance ratio lying between 0.5 and 0.9. Dominance is predominantly unidirectional for leaf length, leaf width and final height, while for the remaining traits, some genes show ambidirectional dominance, although the incidence of unidirectional dominance is much higher throughout. The direction of dominance is predominantly for the increased score, except for flowering time where alleles conferring earliness are up to five times more frequently dominant. The present study has also confirmed that the F2 and SCHi distributions are very similar and that the former can be used to predict the transgression in the latter with confidence. The reduced range of the SCHi families compared to the recombinant inbreds, further indicated that heterosis among many of the SCHi is due to gene dispersion and there is little evidence for the presence of over-dominance.

COMBINING ABILITY IN WINTER SORGHUM

The nature of gene action through combining ability analysis for grain yield and its components was studied in 5 X 5 diallel cross of sorghum. Estimates of gea and sca indicated the influence of both additive and non additive gene action for the inheritance of grain yield, stem girth and plant height. The cultivar SPV-462 was the best general combiner for all the traits under study. The best specified crosses viz, SPV-462 X M35- 1, SPV-472 X Moti, SPV-913 X M35-1 and M35-1 X Moti for grain yield resulted from non-additive gene action could be utilised in heterosis breeding programme.

A quantitative genetic model for mixed diploid and triploid hybrid progenies in tree breeding and evolution

Interspecific hybridization has played a critical role in tree evolution and breeding. The findings of triploidy in forest trees stimulate the development of a quantitative genetic model to estimate the nature of gene action. The model is based on clonally replicated triploid progenies derived from a two-level population and individual-within-population mating design in which offspring have a double dose of alleles from the parent and a single dose of alleles from the other parent. With the same genetic assumptions of a diploid model, except non-Mendelian behavior at meiosis, and the experimental variances estimated from a linear statistical model, total genetic variances in the triploid progenies are separated into additive, dominance, and epistatic components. In addition, by combining the new model with the already existing model based on disomic expression, the partitioning of additive, dominant, and epistatic variances can be obtained for a mixed diploid/triploid F1 progeny population. This paper provides an alternative technique to study the modes of quantitative inheritance for outcrossing, long-lived forest trees in which inbred lines cannot be easily generated. The accuracy for estimating gene action using this technique is discussed.