Showing papers on "Heterosis published in 1995"

Dominance Is the Major Genetic Basis of Heterosis in Rice as Revealed by Qtl Analysis Using Molecular Markers

Abstract: A set of 194 F7 lines derived from a subspecific rice cross showing strong F1 heterosis was backcrossed to the two parents. The materials (388 BC1F7 lines, 194 F8 lines, two parents, F1) were phenotyped for 12 quantitative traits. A total of 37 significant QTLs (LOD > or = 2.0) was detected through 141 RFLP markers in the BC1F7 populations. Twenty-seven (73%) quantitative trait loci (QTLs) were detected in only one of the BC1F7 populations. In 82% of these cases, the heterozygotes were superior to the respective homozygotes. The remaining 10 (27%) QTLs were detected in both BC1F7 populations, and the heterozygote had a phenotype falling between those of the two homozygotes and in no instances were the heterozygotes found to be superior to both homozygotes. These results suggest that dominance complementation is the major genetic basis of heterosis in rice. This conclusion was strengthened by the finding that there was no correlation between most traits and overall genome heterozygosity and that there were some recombinant inbred lines in the F8 population having phenotypic values superior to the F1 for all of the traits evaluated--a result not expected if overdominance was a major contributor to heterosis. Digenic epistasis was not evident.

Hybrid Performance in Wheat as Related to Parental Diversity

Abstract: The probability of recovering a superior individual from the cross of 2 inbred parents is greater if both parents are similar in performance as opposed to one parent being inferior. Heterosis, coefficients of parentage, and molecular markers have been advocated to assess genetic diversity directly between parents. This study was done to determine the extent of DNA polymorphisms among a set of 7 hard red spring wheats and the relationship between genetic diversity based on these molecular markers and coefficients of parentage, and to examine the relationship between diversity and hybrid performance in a diallel cross of these parents. Parents plus the F1 hybrids were grown in 3 field environments (one irrigated and 2 dry-land) near Bozeman. Grain yield, grain weight and protein concentration were measured. Coefficients of parentage, r, were computed for all pairwise combinations of parents. Parents were assayed with 63 sequence tagged site-polymerase chain reaction (STS PCR) primer sets. Genetic similarities, GS, were computed using 27 STS PCR primers sets that showed polymorphism. Rank correlations were computed between the 1 - r and 1 - GS measures of genetic diversity and F1 performance, mid-parent heterosis and SCA effects. The only significant associations between diversity and hybrid performance were those between 1 - r and grain weight SCA effects and heterosis, and protein concentration heterosis. Genetic diversity was inversely related to heterosis and SCA effects for protein concentration. Correlations between the 1 - r and 1 - GS measures of diversity and hybrid performance traits were of the same sign and about equal in magnitude. Coefficient of parentage and GS were significantly correlated, and cluster analysis based on the 1 - r and 1 - GS measures of diversity grouped the genotypes similarly. Neither diversity measure showed promise for predicting F1 performance, but both measures similar information for grouping and differentiating genotypes

Interval mapping of viability loci causing heterosis in Arabidopsis.

Abstract: The genetic basis of heterosis has implications for many problems in genetics and evolution. Heterosis and inbreeding depression affect human genetic diseases, maintenance of genetic variation, evolution of breeding systems, agricultural productivity, and conservation biology. Despite decades of theoretical and empirical studies, the genetic basis of heterosis has remained unclear. I mapped viability loci contributing to heterosis in Arabidopsis. An overdominant factor with large effects on viability mapped to a short interval on chromosome I. Homozygotes had 50% lower viability than heterozygotes in this chromosomal region. Statistical analysis of viability data in this cross indicates that observed viability heterosis is better explained by functional overdominance than by pseudo-overdominance. Overdominance sometimes may be an important cause of hybrid vigor, especially in habitually inbreeding species. Finally, I developed a maximum likelihood interval mapping procedure that can be used to examine chromosomal regions showing segregation distortion or viability selection.

Molecular divergence and hybrid performance in rice

Abstract: This study was undertaken to determine the relationship between genetic distance of the parents based on molecular markers and F1 performance in a set of diallel crosses involving eight commonly used parental lines in hybrid rice production. The F1s and their parents were measured for five traits including heading date, plant height, straw weight, grain yield and biomass. The parental lines were assayed for DNA polymorphisms using two classes of markers: 140 probes for restriction fragment length polymorphisms (RFLPs) and 12 simple sequence repeats (SSRs), resulting in a total of 105 polymorphic markers well spaced along the 12 rice chromosomes. SSRs detected more polymorphism than RFLPs among the eight lines. A cluster analysis based on marker genotypes separated these eight lines into three groups which agree essentially with the available pedigree information. Correlations were mostly low between general heterozygosity based on all the markers and F1 performance and heterosis. In contrast, very high correlations were detected between midparent heterosis and specific heterozygosity based on the markers that detected significant effects for all the five traits; these correlations may have practical utility in predicting heterosis. The analyses also suggest the existence of two likely heterotic groups in the rice germplasm represented by these eight lines.

Molecular aspects of heterosis in plants

Abstract: Since Shull's original description of heterosis, breeders have made wide use of this phenomenon. However, while agronomists have been utilizing heterosis as a means of improving crop productivity, the biological basis of heterosis remains unkown. It is generally believed that our understanding of heterosis will enhance our ability to form new genotypes which may be used directly as F1 hybrids or form the basis for future selection programmes. While the original concept of heterosis resulted from studies at the phenotypic morpohological level, they were soon followed by biochemical data with the advent of electrophoresis and the consequent ease of accumulation of data related to isozyme variability. However, the large number of restriction fragment length polymorphisms from more recent studies has allowed the development of linkage maps with a high degree of resolution useful in locating and manipulating quantitative trait loci (QTL). When substantial numbers of such neutral markers were used to measure genetic distance in large numbers of maize inbreds, very significant correlations were recorded between parental genetic distance and hybrid performance. Through the same approach, a relatively small number of QTLs dispersed through the maize genome were identified which show clear overdominance expression controlling heterosis. The hypothesis was made that some QTLs could code for regulatory proteins since these proteins are able to control a vast array of other structural genes, the products of which are necessary for the expression of complicated characters such as yield and heterosis for yield. The few such proteins identified thus far are all multimeric proteins with the heteropolymers exhibiting significantly different activities in comparison with the homopolymers, that is in compliance with the clear overdominance manifestation of the few QTLs analyzed so far. In addition, parameters derived from the variability of genome expression assessed through studies of polymorphisms in the amounts of individual proteins or mRNAs show numerous significant correlations between these indices and hybrid vigor. These correlations supported the conclusion that QTLs could be loci controlling the amount of mRNAs or proteins synthesized from a number of structural genes, and stress the significance of both the regulatory proteins (and their encoding genes) and the structural genes, being regulated, in manifestation of complicated characters, such as heterosis.

Heterosis and inbreeding depression in upland rice crosses

Abstract: F1 heterosis over midparent (MP) and better parent (BP) and F2 inbreeding depression (ID) were studied in 8 crosses of rice for grain yield/plant and its four other component traits, i.e., plant height, effective tillers/plant, No. of filled grains/panicle and 1000-grain weight. Highly significant and positive MP and BP heterosis for grain yield was expressed byfour hybrids and ID in each case invariably accompanied in F2. Such high heterosis for yield was due to additive heterotic effect of one or more component traits. In all the cases, heterosis for effective tillering was found to be the major contributor and number of filled grains/panicle contributed less to grain yield heterosis. High heterosis accompanied by high ID for effective tillers/plant and number of filled grains/panicle indicated the presence of non additive gene effects governing the inheritance of these traits. Since growth vigour was not of retentive nature as indicated by high ID in F2 for grain yield, it should be exploited in F1 itself.

Relationship between genetic distance and heterosis for yield and morphological traits in winter canola (Brassica napus L.).

Abstract: Genetic distance among canola cultivars was estimated through multivariate analysis. Thirty cultivars from various sources were analyzed and clustered based upon five morphological characteristics and yield components-crown diameter, number of branches plant-1, number of pods plant-1, number of seeds pod-1 and yield plant-1 -and placed in three distinct clusters. Two cultivars from each cluster were selected as parents and 15 partial-diallel inter- and intra-cluster crosses were made between the six selected parents and evaluated at two locations in Michigan in 1990/1991. The association between genetic distance and mid-parent heterosis was investigated. The correlation between genetic distance and heterosis was positive and highly significant for seed yield, number of pods plant-1, and number of seeds pod-1. Clustering, based on yield and yield-component traits, demonstrated that inter-cluster heterosis was greater than intra-cluster heterosis in the majority of cases.

Alternative models for allozyme-associated heterosis in the marine bivalve Spisula ovalis.

Abstract: Correlations between allozyme heterozygosity and fitness-related traits, especially growth, have been documented in natural populations of marine bivalves. However, no consistent pattern has been exhibited, because heterotic effects on size vary with age and individual growth parameters are generally unknown. No consensus has emerged on the genetic basis of allozyme-associated heterosis. The species studied here, Spisula ovalis, displays annual shell growth lines, which allows us to compute individual age and growth dynamics over the whole life span. Our morphological study was coupled to a protein electrophoresis study at seven polymorphic loci. While the maximum size gained is not related to heterozygosity, the age at half maximum size, t1/2, is significantly negatively correlated with heterozygosity, indicating an heterotic effect on initial growth. The correlation between heterozygosity and size is expected to vanish when age increases, due to the form of the growth function. This decreasing correlation is consistent with previous studies. We compare the relative performances of five linear models to analyze the genetic basis of heterosis. Surprisingly, the largest part of variance in t1/2 is due to additive effects, the overdominant components being much weaker. Heterosis is therefore due to general genomic effects rather than to local overdominance restricted to allozymes or small neighboring chromosomal segments. A significant dependence of individual heterotic contributions of the enzyme loci upon expected heterozygosities, rather than metabolic function, further supports the hypothesis of enzymes acting as markers. General genomic effects can hold only if allozyme heterozygosity is positively correlated with heterozygosity at fitness-related genes scattered throughout the genome. This hypothesis is supported here by heterozygosity correlations between enzymatic loci.

Analysis of Diallel Crosses among Colombian Landraces and Improved Populations of Maize

Abstract: Few of the world's maize (Zea mays L.) germplasm collections have been systematically evaluated in spite of the recognized necessity of broadening the germplasm base of this crop. Knowledge of new heterotic patterns and the breeding potential of maize collections stored in gene blanks can contribute to a widening of the genetic base. The objectives of this study were to (i) evaluate the breeding potential of eight tropical maize germplasms, and (ii) provide further information to clarify the relationships among them. Five Colombian landraces («Andaqui», «Comun», «Costeno», «Puya», and «Yucatan») and three improved maize populations («Tuxpeno-1», «Suwan-1», and «ETO») were crossed in a dialled mating system. The parents and the 28 hybrids were evaluated in six environments for grain yield, days to 50% silking, prolificacy, and ear height. Cultivar (additive) and heterotic effects were highly significant (P ≤ 0.01) for all traits except ear height. Significant (P ≤ 0.05) cultivar heterosis and specific heterotic effects were found for grain yield, with an average 10% high-parent heterosis. Cultivar effects were positive for Suwan-1 (0.86**) and Tuxpeno-1 (0.62**) and negative for Andaqui (−0.42*) and Puya (−0.78**). The same materials showed significant cultivar heterotic effects (0.29**, 0.12*, −0.23**, and −0.22**, respectively). Specific heterosis was found in crosses between Tuxpeno-1 and Comun (0.44**), Yucatan (0.35*), and Suwan-1 (−0.67**). The best materials were improved populations, Suwan-1, and Tuxpeno-1. The ETO population did not perform well per se or in hybrid combinations. Yucatan outyielded ETO and was a parent in two of the three highest yielding crosses. The hybrid Tuxpeno-1 × Ycatan showed the highest high-parent heterosis value (25%**). Results generally agreed with the reported relationships among the parents and pointed to Yucatan as a potentially useful genetic resource for tropical maize breeding programs

Detection and prediction of heterosis in Eucalyptus globulus

Abstract: Significant specific combining ability effects (SCA) for two year growth were detected in a factorial of Eucalyptus globulus ssp. Globulus. This factorial was established across 5 locations in Australia and involved crossing between and within two provenances (King Island and Taranna) in addition to crossing males from both provenances to a single female from south Flinders Island. We attempted to predict heterosis using genetic distance between parents calculated using RAPD markers. We isolate DNA from all 8 females and 19 out of 26 male parents. Data were collected from 99 different RAPD bands. Sixty-six out of the 99 RAPD bands scored were polymorphic. A UPGMA dendrogram based on Nei’s distance resulted in parents within provenance being clustered together in all but one case. The degree to which genetic distance predicted heterosis was statistically significant but accounted for less than 5% of the variation in SCA, and mainly reflected the greater genetic differentiation between parents involved in interprovenance crossing. SCA effects within provenances were unpredictable.

Combining ability and heterosis estimates for kernel cadmium level in sunflower

Abstract: On certain soil series, nonoilseed sunflower (Helianthus annuus L.) yields kernels with a cadmium (Cd) concentration in excess of some international Cd limits. The purpose of this study was to determine the relative importance of general combining ability (GCA), specific combining ability (SCA), and heterosis estimates needed to develop low Cd hybrid cultivars of nonoilseed sunflower. The kernel Cd accumulation of 36 sunflower F 1 hybrids, obtained by crossing six inbred cytoplasmic male-sterile lines to six inbred male-fertile lines in a factorial mating design, was evaluated from field tests conducted at four locations with different soil types. Although the genotype x location interaction was statistically significant, the correlations for Cd among locations were significant over F 1 hybrids and parental lines, indicating that inbred parental lines and hybrids performed consistently across the four locations. GCA was significant and accounted for 80% of the hybrid sums of squares. The interaction between males and females also was significant. These results indicate that additive genetic effects predominantly influence the expression of kernel Cd accumulation in hybrids. The regression of actual hybrid kernel Cd on expected hybrid kernel Cd, based on parental GCA values, was highly significant. Although the regression of actual hybrid kernel Cd on midparent also was statistically significant, the R 2 value was only 0.21. These results indicated that GCA was more effective than midparent for describing hybrid performance. The levels of heterosis varied widely among crosses, ranging from -50 to 50%. Twenty-seven of the 36 crosses registered double-digit negative heterosis, indicating breeding for low kernel Cd in sunflower hybrids should be feasible.

Magnitude of hybrid vigor retained in double haploid lines of some heterotic rice hybrids.

Abstract: Sixty one double haploid (DH) lines were evaluated for yield components and yield and compared with the three corresponding hybrids from which they were derived through anther culture. Analysis of a 6×6 diallel experiment led to the selection of these hybrids based on their high heterosis and revealed the nature of gene action of the characters under evaluation. The DH lines along with the hybrids and parents were planted following a simple lattice design with two replications. The results show that in DH lines the values of the characters expressing predominantly additive genetic effects could reach the heterotic level of the hybrids. For characters including yield showing predominantly dominance effects, values in the DH lines were significantly lower than those of the corresponding hybrids. The promising DH lines, however, possess a higher yield potential than the better parents.

Genetic distance and heterosis in Indian mustard: developmental isozymes as indicators of genetic relationships.

Abstract: The use of isozymes as indicators of genetic diversity and as markers for the selection of agronomic traits has been proposed in different crop species. The present investigation was conducted to study the use of isozyme-derived genetic distance between parents in predicting the F1 heterosis in Indian mustard. In addition, the interaction of isozyme-based diversity with quantitative trait and pedigree-based diversity measures, and its role in predicting hybrid heterosis has also been examined. Sixteen Indian mustard lines and their 48 crosses (12 × 4, line x tester crossing) were evaluated over two environments for isozyme and quantitative morphological characters. The results from this study suggest that the heterotic response to isozymic changes is more responsive in crosses derived from morphologically and pedigree-wise related parents in comparison to crosses derived from unrelated parents. It was possible to improve heterosis predictions by partitioning the isozyme-based genetic distance into general genetic distance and specific genetic distance and correlating the latter with the specific combining ability of morphological traits. The possible reasons for these observations are discussed.

Heterosis breeding for grain yield and other agronomic characters in rice (Oryza sativa L.)

Abstract: Thirty six hybrids evolved from nine high yielding and widely adapted cultivars were evaluated with parental lines and the standard variety, Jaya, to determine the nature and extent of heterosis for seven characters including grain yield per plant. Most of the crosses manifested significant heterosis for grain yield and panicle number per plant, panicle length, 1000-grain weight, grains per panicle, and days to maturity. Very few hybrids manifested significant heterosis for plant height. The range of heterosis for seed yield per plant was -96.7 to 258.2% over the better parent, -96.1 to 268.2% over midparent and -96.3 to 301.6% over the standard variety. The highest heterotic effects for grain yield were observed in the crosses Prasad x PP 72, Govind x PP 72, Govind x Sita, Prasad x Mahsuri and Govind x Jaya hybrids, which was, respectively, 258.2, 216.8, 177.8, 161.6 and 108.8% over better parent, 268.2, 250.8, 178.1, 204.2 and 119.4% over mid parent, and 301.6, 255.2, 151.6, 285.0 and 108.8% over the best check variety Jaya. A comparison between heterotic and nonheterotic hybrids for yield and its components indicated that heterosis for grain yield was due to two or more direct yield contributing characters. Implications of the nature and magnitude of heterosis for commercial exploitation in rice are discussed.

Heterosis and combining ability analysis of some quantitative characters in Turkish tobacco

Abstract: Six Turkish tobacco cultivars and their 15 F1 hybrids, excluding reciprocals, were evaluated for five agronomic and four chemical traits. With the exception of ash content, average heterosis was significant for all characters studied. The average yield of hybrids was 15.2% above that of the parents. Number of leaves per plant, total alkaloid and total sugar exhibited negative heterotic effects. The analysis of combining ability revealed that the variances of GCA were significant for all traits evaluated. On the other hand, variances of SCA were significant for ash content, total nitrogen, total alkaloid and for total sugar, indicating that non-additive gene action was important as well for these traits. A comparison of the relative magnitudes of the GCA and SCA variances suggests that the attributes, except total nitrogen, were governed more by additive gene action than by dominance. The line 40-18B was identified as being the most suitable combiner overall parent in the experiment for improving the characters investigated. Based on significant SCA effects, together with significant heterotic estimates, the crosses 3x4, 3x6, and 2x3 would be suitable combinations

Divergence and heterosis for fodder attributes in pearl millet

Abstract: No relationship was observed between the geographical diversity and genetic divergence in the fifteen parental lines of fodder pearl millet. The clustering pattern was effected by environment and role of different characters varied with shift in season. The size of D2 statistic had no effect on the magnitude of heterosis for the attributes studied.

Studies on heterosis in sponge gourd

Abstract: 28 F1 hybrids of sponge gourd in a diallel set involving 8 parental lines were studied to assess the extent of hybrid vigour in the yield contributing traits. The mean values of hybrids was greater than those of parents for all the characters except the fruit maturity. The increase yield in heterotic hybrid ranged from 20.0 to 90.7 percent. The maximum and significant heterosis was recorded in P2×P4 for vine length, P2×P9 for fruit length, P2×P5 for fruit/plant, P4×P7 for average fruit weight and P2×P4 for total yield. In all the characters, the best performing hybrid was significantly better than the top parent. In order of merit P2×P4 P6×P5 and P2×P8 were best F1 hybrid for total yield and its many component characters. The best parental combination was found to be P2×P4 (IIHR-9 and IIHR 23) which exhibited 90.78 percent increase over better parent and 72.78 percent increase over top parent. Thus, IIHR 76 can be utilised for exploitation of hybrid vigour in sponge gourd.

Heterosis for yield and yield contributing characters in safflower (Carthamus tinctorius L.)

Abstract: High heterosis over better parents was recorded in 32 hybrids pooled over three environments for yield and its components in safflower. The hybrids exhibited the maximum heterosis for seed yield/plant followed by seeds/capitulum on secondary branch, secondary branches/plant and seeds/capitulum on primary branch. Highest value of heterosis over better parent (92.3%) was recorded for seed yield/plant in the hybrid MS-1 X NS-1016. In general, hybrids derived from genic male sterile lines gave better performance for seed yield and other characters.

Variation and covariation for quantitative resistance to head blight (Fusarium culmorum) in two testcross series of S2 lines in winter rye

Abstract: The amount of genetic variation among inbred lines and testcrosses, and covariation between both genetic materials, are of crucial importance for selection efficiency in hybrid breeding. To estimate these quantitative genetic parameters for resistance of winter rye (Secale cereale) to head blight caused by Fusarium culmorum, 88 three-way cross hybrids, produced by crossing each of 44 S2 Carsten inbred lines with two unrelated Petkus single-cross testers, were evaluated along with the parental lines over 2 years. Resistance traits were head-blight rating and grain weight per spike relative to the non-inoculated control. Significant genotypic variation occurred among lines and in both testcross series. S2 lines displayed considerably more variation than testcross series. Genotype × environment interaction was more marked among the inbred lines, while estimates of heritability were similar for both genetic materials. Testcrosses showed heterosis for head-blight resistance. No relationship existed between S2 lines and the two testcross series for any resistance trait. This might be caused by an association between inbreeding and Fusarium-head-blight susceptibility and different inbreeding depression among the S2 population. The phenotypic correlations between the testcross series were moderate for both traits (r = 0.58, P < 0.01). In conclusion, Fusarium-head-blight resistance has to be selected at the respective heterozygosity levels.

Identification and transfer of heterotic chromosome blocks for forage yield in short-day exotic pearl millet landraces

Abstract: Photoperiod sensitivity of pearl millet, Pennisetum glaucum (L.) R. Br., exotic accessions has limited their use in nontropical environments due to excess height and late maturity problems. Objectives of this research were (i) to describe and evaluate a modification of a direct method to screen non-converted short-day Burkina Faso landraces (BFL) of pearl millet for large heterosis-creating heterotic chromosome blocks (HCBs) that can replace a small HCB in the male parent T383 of tbe top producing F 1 hybrid Tifleaf 2 and to increase its forage yield; and (ii) to determine the variation in the size of HCBs in a single landrace. Short early T383-like F 2 plants in IBFL x T383) F 2 populations were screened for HCBs by crossing them with T23Rp 1 , which carries the red marker gene, crossing those F 1 s on T85A [cytoplasmic male sterile (cms) green 23], growing 100 spaced plants of the cross T85 x (BFL x T383) F 2 , cutting and weighing 50 of the red and 50 of the green plants in each population, and analyzing the data with the SAS General Linear Model. Percent heterosis for plant forage yield in 100-plant populations of each cross ranged from 0 to 51%. The average HCB heterosis for T383 was 4%. Sister plants within a single BFL were variable enough to make it important to cross a single BFL plant with T23Rp 1 to measure HCB heterosis and with T265 (T383Rp 1 ) to start the transfer of the top HCB to T383. The modification method requires an extra year before measuring HCBs, but requires a less variable 100-plant screening population and should require fewer backcrosses to transfer the top HCB to T383.

Heterosis and combining ability in hybrids among 12 commercial varieties of sesame (Sesamum indicum L.).

Abstract: Heterosis and combining-ability effects were studied in the parents and 66 F 1 hybrids from a diallel cross of 12 elite sesame varieties in five different environments in Venezuela. The mean yield of the hybrids was clearly superior to the parental mean in all five locations. There were three heterotic hybrids that showed a higher yield than controls in all environments. The best hybrid in each place yielded significantly better than the best cultivar, the differences ranging from 28 to 109%. Specific combining-ability (SCA) effects were more important than general combining-ability (GCA) in four of the environments. A large genotype x location interaction was found for all traits, suggesting that hybrids should be produced for specific locations. Arawaca and Piritu should be chosen as parents for any sesame-breeding programme in view of their large GCA effects.

Combining ability among fifteen early cycle maize populations in Brazil.

Abstract: Early maturity maize (Zea mays L.) populations are used in short growing seasons areas, and their use is gradually increasing in tropical areas. The objective of this study was to determine the genetic potential of 15 early maturity maize populations in a diallel crossing system. The 15 parents, 105 diallel crosses among parents, and selfed generation of parental were evaluated in a triple lattice design at 13 locations in Brazil. The combined analysis of variance for grain yield, time to tassel, and plant height indicated highly significant (P < 0.01) differences for entries, parents, heterosis, parent heterosis, specific heterosis and for the first-order interactions with environments. Average heterosis was highly significant for grain yield, significant(P < 0,05) for time to tassel, and not significant for plant height. Mean yield ranged from 3187 to 5213 kg/ha for populations, and ranged from 3041 to 6017 kg/ha for the population crosses. Population Pool 17 had the largest inbreeding depression effects, whereas population ACMS 52 had the smallest inbreeding depression effects. The highest specific cross for grain yield was for Across 8528 x Pool 18. None of the population crosses was superior to the hybrid checks for the three traits. No association was found between endosperm type and heterosis. The results suggest that either the population themselves or the synthetic composition of selected populations can be used for breeding purposes.

Local variation in the genetic basis of morphological traits in the salamander ambystoma talpoideum.

Abstract: Two local populations of the mole salamander Ambystoma talpoideum (Caudata: Ambystomatidae) were previously found to have similar phenotypic values for per capita fecundity and life-history pathway (paedomorphosis and metamorphosis), but to have different genetic bases for those traits (Harris et al. 1990). Because one adaptive phenotype could be specified by more than one genotype, it was concluded that independent evolution of these traits had occurred (Harris et al. 1990). The genetic basis of fecundity and life-history pathway was found to be less divergent between two other pairs of populations. We now expand on those results by presenting evidence of both similar and dissimilar genetic bases for a number of external and internal morphological traits among the three isolated populations of A. talpoideum that were studied earlier. The degree of genetic compatibility among local populations depends on their evolutionary histories. Lack of genetic compatibility is due to independent evolution from different populations or divergence from a common ancestral population. Genetically compatible populations are either linked by gene flow or have not diverged from a common ancestral population. Evidence of genetic compatibility among isolated populations can be inferred by analyzing the F1 and especially the F2 generations of hybrid and nonhybrid populations. Phenotypic dissimilarity of hybrid and nonhybrid trait values, referred to here as hybrid dissimilarity, is taken as a measure of genetic incompatibility (Dobzhansky 1970). Hybrid dissimilarity is caused by at least two factors: heterosis and epistasis (Wright 1931; Falconer 1989). Heterosis is proportional to the amount of dominance present at all loci that affect the trait. If the parental populations contain dominant alleles at different loci, then hybrids will have dominant alleles at a greater number of loci. This generally leads to a more fit hybrid or the phenomenon of hybrid vigor. Alternatively, unique epistatic interactions, the functional interaction of alleles at different loci, may come to characterize each parental population. When crossed, the epistatic structure of each population is altered, leading to hybrid dissimilarity or often hybrid breakdown. Both heterosis and epistasis reflect nonadditive genetic variation in the parental populations. Traits more closely related to fitness are expected to be underlain by epistatic interactions (Wright 1977, 1982; Travis et al. 1987). By expanding on previous results with additional traits, we are able to associate the pattern of genetic compatibility of traits among populations to the likely relationship of those traits to fitness. MATERIALS AND METHODS

Heterosis in soybean (Glycine max (L.) Merrill)

Abstract: Twenty one soybean hybrids derived from a seven-parent half-diallel set along with their parents were evaluated to estimate heterosis. Heterosis was significant positive for yield in 16 hybrids over midparent and in 9 hybrids over better parent. Heterosis for yield was generally accompanied by heterosis for yield components. For protein, five and for oil one hybrid exhibited significant positive heterosis ovtr midparent. In view of the availability of genetic male sterility, the study revealed good scope for commercial exploitation of heterosis for yield and protein contents in soybean.

Genotype x environment interactions in a 10 × 10 diallel cross of quality protein maize (Zea mays L.)

Abstract: A 10×10 diallel cross experiment involving white modified opaque-2 maize inbred lines was grown at four sites. A stability analysis, based on both a combining ability and a heterotic pattern model, was developed. The stability analysis provided valuable information on the genotype x environment interaction properties of the 10 inbred lines. The Gail & Simon (1985) test for qualitative interactions provided a means of determining the nature of these interactions. The inbred, SO507W(M), is shown to have the best potential for use in a hybrid breeding programme, in terms of having the highest weighted general combining ability and line heterosis, and the best general adaptability to all four sites. The single cross, SO713W(P) x PO558W(F), has been identified as one of the genotypes to be used in a recurrent selection programme that favours specific combining ability.