Inheritance of grain filling duration in spring wheat

The genetic basis of grain-filling duration (GFD, days from anthesis to maturity) in six spring wheat hybrids involving nine varieties (‘Son-alika’/‘Bobwhite’, ‘Sonalika’/‘Glennson 70’, ‘Lelija’/‘Bobwhite’, ‘Lelija’/‘Mitacore’, ‘Buckbuck’/‘Dugoklasa’, and ‘Vesna’/‘Radu?a’) and their F₁, F₂, BC₁, and BC₂ generations was studied in the field near Sarajevo, Yugoslavia. Parental means differed in four of the six crosses. Generation mean analyses of genetic effects indicated that an additive-dominance model was sufficient for only two crosses: Lel/Bow and Lel/Mco. One or more types of epistasis were significant in the remaining crosses. The F₁ and F₂ means were either intermediate, closer to the mean of the parent with the longer GFD, or closer to the mean of the Parent with the shorter GFD. Even though different modes of gene action controlled GFD among the six crosses, the heritabilities were reasonably high (narrow sense, 39-59) range for six crosses), indicating that progress could be made from selection in these crosses for either long or short GFD. The parents were selected to have a range in days from planting to anthesis and to maturity. The relationship between dates of anthesis or maturity and GFD was not consistent, but the two latest-maturing varieties had the longest GFD, indicating that anthesis or maturity dates are not a good criteria for choosing parents for modifying GFD. Additive genetic effects predominated in the crosses studied here, but epistasis involving dominance gene action was sufficiently important. To eliminate confounding epistatic dominance effects and to take advantage of favourable additive × additive effects during selection for GFD, a breeding strategy involving rapid approach to homozygosity followed by selection after the achievement of homozygosity was suggested.     

Inheritance of resistance to Fusarium wilt (race 2) in chickpea

The inheritance of resistance to Fusarium wilt (race 2) of chickpea was studied in a set of three crosses, i.e. ‘WR315’בC104’ (resistant × susceptible), ‘WR315’בK850’ (resistant × tolerant) and ‘K850’בGW5/7’ (tolerant × tolerant) in order to investigate the number of genes involved, their complementation and to find out whether resistant segregants are possible in a cross between two tolerant cultivars. Tests of F₁, F₂ and F₃ generations of these crosses under controlled conditions at ICRISAT, Patancheru, India, indicated involvement of three loci (two recessive and one dominant alleles). The homozygous recessive form at the first two loci conferred resistance whereas susceptibility occurred when the first two loci were in the dominant form. A dominant allele at the third locus can complement the dominant alleles at the other two loci to confer tolerance. Occurrence of resistant segregants in a cross between two tolerant cultivars was observed.     

陆地棉纤维细度相关性状的遗传及相关性分析

 利用主基因与多基因混合遗传模型的P₁、P₂、F₁、F₂四世代联合分析方法,研究了棉花纤维细度相关性状的遗传,进一步明确了主基因存在的普遍性。同时,对麦克隆值密切相关的纤维线密度及成熟度比率进行了F₂、F_2:3家系的主基因 多基因单世代分析,初步判定线密度也存在一对主基因。性状间的遗传相关分析表明,麦克隆值及成熟度比率与整齐度分别呈极显著正相关和不相关,该两个性状与其它性状的相关性两个世代表现不一致;线密度与整齐度呈正相关,与伸长率呈极显著负相关,与长度、强度及衣分不相关。线密度与各性状的相关性要比麦克隆值和成熟度比率稳定,受环境影响小,有必要加强线密度的选择及深入研究。     

Study of heritable variation and genetics of earliness in mungbean (<Emphasis Type="Italic">Vigna radiata</Emphasis> L. Wilczek)

Fifty-five mungbean lines were evaluated for days to maturity and grain yield per plant. This material showed considerable range of variability for the target traits. Eight genetically diverse parents were selected and used for a full diallel set of crosses to study the mode of inheritance for earliness related parameters (days to flowering, days to maturity and length of reproductive phase) during summer 2005. The F₁ generation of these crosses was sown during the spring of 2006 and the selfed seeds were used to raise the F₂ generation during kharif season. The data recorded from two generations were subjected to genetic analysis. The formal ANOVA showed the significance of both additive and dominance effects for all the traits in both generations. Significance of D, H₁ and H₂ components also confirmed the contribution of both additive and dominance effects in controlling the inheritance of these traits. The estimates of narrow sense heritability were low to moderate except higher estimates for days to maturity in F₂ generation, while the broad sense heritability estimates were relatively higher. Seasonal and environmental effects were also found to be significant. In view of the complex nature of gene action for earliness, it is suggested that breeders should look for transgressive recombinants of earliness and other desirable attributes in later segregating generations to gain higher genetic advance. The variety NM92 was found to be the best source of earliness in mungbean breeding.     

The Inheritance of Earliness and Fruit Weight in Crosses between Cultivated Tomatoes and two Wild Species of Lycopersicon

The inheritance of earliness and fruit Weight in tomato was studied in tour interspecific crosses. Two cultivated varieties, i.e.‘HS 101’ and “HS 102′, were hybridized with two wild species. L. birsutum f. glabratum‘B 6013’ and L. pimpinellifolium‘A 1921′. Six generations of these crosses were evaluated for these traits and the estimates of gene effects were derived from the generation mean using an epistatic (six parameters) model. There were very wide differences between cultivated and wild species for earliness and fruit weight and in the segregating populations, plants with delayed maturity and smaller fruit size were recorded with high frequency. It was found that the inheritance pattern was mainly governed by additive gene action. Epistatic effects also contributed towards the inheritance of both traits.     

Inheritance of drought resistance related traits in two crosses of groundnut (<Emphasis Type="Italic">Arachis hypogaea</Emphasis> L.)

Groundnut (Arachis hypogaea L.) is an important oilseed crop grown in more than 100 countries across wide range of environments. Frequent occurrence of drought is one of the limiting factors adversely affecting groundnut productivity, especially in rainfed areas, and hence genotypes having high water use efficiency (WUE) under limited available water need to be developed. In groundnut, WUE is correlated with SPAD chlorophyll meter reading (SCMR) and specific leaf area (SLA). These two traits, SCMR and SLA, can be used as surrogate traits for selecting for WUE. In order to improve SCMR and SLA, and in turn WUE in groundnut, a good knowledge of the genetic system controlling the expressions of these traits is essential for the selection of the most appropriate and efficient breeding procedure. The present investigation was conducted to determine the gene action controlling the inheritance of SCMR and SLA in two crosses, ICG 7243 × ICG 9418 and ICG 6766 × Chico, and their reciprocals. Six generations of each cross (P₁, P₂, F₁, F₂, BC₁P₁, and BC₁P₂) were evaluated for SCMR and SLA at two stages of the crop growth viz., 60 and 80 days after sowing (DAS). For SCMR at 80 DAS, additive effects were important in both the crosses whereas predominance of dominance effects with duplicate epistasis was observed for SCMR at 60 DAS and SLA at both stages in both the crosses. Predominance of additive effect for SCMR at 80 DAS suggested effective selection could be practiced even in early generations whereas for SCMR at 60 DAS and SLA at both stages in both crosses, it would be better to defer selection to later generations. Further, recording of SCMR and SLA should be done between 60 and 80 DAS for screening the germplasm lines for drought tolerance.     

Recessive genes controlling resistance to Helminthosporium leaf blight in synthetic hexaploid wheat

A study was conducted under controlled environment conditions in a phytotron to determine the nature of the inheritance of resistance Helminthosporium leaf blight (HLB) in a synthetic hexaploid wheat line, ‘Chirya‐3’, against the isolate KL‐8 of Bipolaris sorokiniana from the major wheat growing region of India. Crosses were made between two susceptible lines ‘WH 147’ and ‘Chinese Spring’. Analyses of F₁ and F₂ populations of these two crosses (‘WH 147’בChirya‐3’ and ‘Chinese Spring’בChirya‐3’) showed that resistance against the isolate in ‘Chirya‐3’ was governed by two recessive genes functioning in a complementary interaction giving an F₂ segregation pattern of 1 : 15 (resistant : susceptible). The segregation pattern of the resistant F₂ progenies in F₃ families from both crosses confirmed that two homozygous recessive genes were responsible for resistance to the isolate of Bipolaris sorokiniana in the synthetic line ‘Chirya‐3’. It is proposed that the genes be designated as hlbr1 and hlbr2.     

Inheritance of resistance to the 'Kanpur' race of Phytophthora drechsleri in pigeonpea

Phytophthora drechsleri causes stem blight, which is one of the most serious diseases of pigeonpea. Eight races of this fungus have been identified, but the inheritance of resistance to all these races is not clear except for race P2. This study examined the inheritance of resistance to race ‘Kanpur’ (KPR) of P. drechsleri in eight crosses involving four resistant parents, viz.‘KPBR 80‐2‐1′, ‘KPBR 80‐2‐2′, ‘Hy 3C and ‘BDN 1′, and two susceptible parents, viz.‘Bahar’ and ‘PDA 10′. The reactions of the parental lines, and their F₁, F₂ and backcross generations were studied in an infected plot. In the F₁ generation of all crosses, a susceptible reaction was observed that indicated dominance of susceptibility over resistance. The segregation pattern in F₂ indicated that two homozygous recessive genes (pdr₁pdr₁pdr₂pdr₂) were responsible for imparting resistance in the parents, ‘KPBR 80‐2‐1’ and ‘KPBR 80‐2‐2′, and that a single homozygous recessive gene (pdrpdr) was responsible for resistance in the parents ‘Hy 3C and ‘BDN 1′. Therefore, ‘KPBR 80‐2‐1’ and ‘KPBR 80‐2‐2’ with two genes for resistance are better donors because the resistance transferred from them will be more durable compared with ‘Hy3C and ‘BDN1’ with only one gene for resistance.     

Heritability and correlations among traits in four-way soybean crosses

Recurrent selection programs can use several crossing arrangements to synthesize basic populations for breeding purposes. This study evaluated heritability and correlations among traits in forty-five populations of F_2[4] and F_2:3[4] generations obtained throughout four-way crosses [4] between two dialells among ten semi-late and ten late maturity soybean (Glycine max L. Merrill) genotypes. The generation advance was made by the TSHD (Thinned Single Hill Descent) method and all individual plants were evaluated for the following traits: number of days to flowering (NDF) and maturity (NDM), plant height at flowering (PHF) and maturity (PHM), agronomic value (AV) and seed yield (YLD). Four-way crosses associated with TSHD method provided high genetic variability in the populations with little or no reduction from F_2[4] to F_2:3[4]. The ‘narrow sense’ heritability estimated using correlations between F_2[4] and F_2:3[4] plants was lower than but closely related to broad-sense estimates. The populations 21(IAC-4 × IAC-9) × (GO79-1039 × Paranagoiana) and 23 (IAC-4 × Santa Rosa) × (GO79-1039 × Tropical) were the most productive with high variability in F_2[4] and/or F_2:3[4] generations. Genotypic correlations measured were generally high, positive and consistent for the two generations except for those related to AV and YLD. These results indicate that the use of four-way crosses can be an efficient method to increase the genetic base of populations for recurrent selection or cultivar development. Significant AV × YLD correlations indicated visual selection can be used as additional selection criteria for improving populations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of insensitivity to culture filtrate of Pyrenophora tritici-repentis, race 2, in wheat (Triticum aestivum L.)

Tan spot of wheat is caused by the fungus Pyrenophora tritici‐repentis. On susceptible hosts, P. tritici‐repentis induces two phenotypically distinct symptoms, tan necrosis and chlorosis. This fungus produces several toxins that induce tan necrosis and chlorosis symptoms in susceptible cultivars. The objectives of this study were to determine the inheritance of insensitivity to necrosis‐inducing culture filtrate of P. tritici‐repentis, race 2, and to establish the relationship between the host reaction to culture filtrate and spore inoculation with respect to the necrosis component. The F₁, F₂, and BC₁F₁ plants and F_2:8 lines of five crosses involving resistant wheat genotypes ‘Erik’, ‘Red Chief’, and line 86ISMN 2137 with susceptible cultivars ‘Glenlea’ and ‘Kenyon’ were studied. Plants were spore‐inoculated at the two‐leaf stage. Four days later, the newly emerged uninoculated third leaf was infiltrated with a culture filtrate of isolate Ptr 92–164 (race 2). Reactions to the spore inoculation and the culture filtrate were recorded 8 days after spore inoculation. The segregation observed in the F₂ and BC₁F₁ generations and the F_2:8 lines of all crosses indicated that a single recessive gene controlled insensitivity to necrosis caused by culture filtrate. This gene also controlled resistance to necrosis induced by spore inoculation.     

Inheritance of plant height in the dwarf mutant 'Enana' of safflower

The dwarf safflower mutant ‘Enana’ has been developed from the cultivar ‘Rancho’ by chemical mutagenesis. The objective of the present research was to study the inheritance of plant height in crosses between ‘Enana’ and ‘Rancho’. Plants of both lines were reciprocally crossed and the F₁, F₂ and F₃ generations were obtained. The evaluation of plant height in the F₂ generation suggested the presence of a single locus controlling this trait. This was confirmed after the evaluation of 164 F_2:3 lines, which revealed an F₂ segregation fitting a 1:2:1 (dwarf: intermediate: standard) ratio. The locus was designated D^w. As the mutated allele was partly dominant over the wild‐type allele, the proposed genotype for ‘Rancho’ was d^w d^w, whereas that for ‘Enana’ was D^w D^w. Furthermore, a partial cytoplasmic effect on plant height was detected, with the heterozygote D^w d^w being about 6 cm taller when ‘Rancho’ was used as female.     

Genetic Analysis of Morpho-Physiological Traits in Chickpea

The present study was conducted to investigate the genetic inheritance of morpho-physiological leaf traits in chickpea (Cicer arietinum L.). The experimental material comprised six generations, viz., two inbred parents, ‘T88’ and ‘Bold Seeded’, having contrasting leaf traits, and their derived F₁, F₂ and backcross of F₁ to either parent (B₁ and B₂). The experiment was randomized complete block design with three replications. Genetic parameters were estimated by generation mean analysis using all the six generations. Data were collected on individual plants within each family just before flowering on leaflet area (LA), number of leaflets per leaf (LL), rachis length (RL), and leaflet density (LD), which was calculated as number of leaflets per unit length of rachis. A simple additive-dominance model was found to be adequate to describe the inheritance of LL and LA, while dominance × dominance (i.e. [1]) and additive × dominance (i.e. [i]) interactions were also significant for RL and LD, respectively. Improvement or seed yield per plant may result from selection for LA by improving both RL and LL. Leaflet area may be included in the ongoing selection schemes, as a supplementary trait to increase the speed of improvement in seed yield per plant. Lanceolate leaflet shape was observed to be monogenically dominant over obovate leaflet shape, and segregated independently from purple/white flower color.     

Inheritance of partial rust resistance in pearl millet

Quantitative disease resistance should be exploited to complement the use of genes for qualitative or hypersensitive resistance. The expression and inheritance of partial rust resistance of pearl millet inbreds 700481-21-8 and ‘ICMP 501’ crossed to moderately susceptible Tift 383’ were evaluated in seedling assays in the greenhouse and in generation mean and single-seed descent populations in the field. Uredinium sizes on seedling leaves of hybrids were generally intermediate to those of the parental inbreds and consistent differences could be discerned in uredinium lengths. Area under the disease progress curves (AUDPCs) of individual plants of the parents, F₁, F₂, and backcross F₁S to each parent were determined from field trials. Broad-sense heritability estimates for both crosses were 43%. In generation mean analyses, additive genetic effects were significant in the cross of 700481–21–8 × Tift 383′, whereas additive, dominance, and dominance × dominance epistatic effects were significant for ‘ICMP 501’בTift 383’. The number of genes conferring partial resistance was estimated to be two for 700481–21–8 and 2.5 for ‘ICMP 501’. A hierarchical single-seed descent analysis revealed significant differences in AUDPC among F₃-derived F₄ progenies in the F₆ generation. Selection for progenies with greater resistance should be possible among F₄ families. Higher levels of resistance were observed in progeny derived from ‘ICMP 501’. Because segregation of resistance differed among progeny derived from 700481–21–8 and ‘ICMP 501’, the genetic basis for resistance probably differs between the two inbreds.     

Evidence of a major genetic factor conditioning freezing sensitivity in winter wheat

The inheritance of freezing tolerance in F₂‐derived F₄ populations from all possible crosses of winter wheat cultivars ‘Kestrel’, ‘Eltan’, ‘Tiber’, ‘Froid’ and germplasm line Oregon Feed Wheat #5 (ORFW) was investigated. When frozen to a temperature equal to the LT₅₀ of the least freezing tolerant parent (ORFW), survival frequency distributions were skewed to greater survival in six of the 10 crosses, however, very few of the progeny from the four crosses to ORFW survived. The inheritance of this freezing sensitivity was investigated with freezing of F_2:4 populations from the crosses of ORFW to ‘Eltan’ or ‘Tiber’ to the LT₅₀ of the hardier parent. Very few of the F_2:4 populations survived as well as ‘Eltan’ or ‘Tiber’, indicating a small number of strongly dominant genetic factors in ORFW that conditioned freezing sensitivity. Molecular analysis indicated these factors were not spring‐type vernalization alleles.     

Inheritance of threshability in synthetic hexaploid (Triticum turgidum×T. tauschii) by T. aestivum crosses

Triticum tauschii provides breeders with a valuable source of resistance and tolerance genes. Elucidation of the inheritance of traits in this species that hinder its use in breeding programmes is therefore of interest to wheat breeders. Inheritance of threshability was investigated in the crosses of four non-free-threshing (NFT) synthetic hexaploids (Triticum turgidum×T. tauschii) and two free-threshing (FT) T. aestivum cultivars during four crop seasons over 3 years at E1 Batan and Ciudad Obregon, Mexico. The parents, their F₁ Hybrids and individual F₂ plant-derived F₃ progenies of the crosses revealed that ‘Altar 84’/T. tauschii (219), ‘Chen’/T. tauschii (205), ‘Chen’/T. tauschii (224), and ‘Duergand’/T. tauschii (214) have independently segregating loci with two dominant alleles controlling threshability. Intercrosses among the synthetics, except ‘Altar 84’/T. tauschii (219), showed the genes to be allelic to each other. The cross between the FT cultivars showed no segregation in the F₃ generation, indicating common recessive genes. Based on these findings, population sizes of the synthetic-derived breeding materials should be increased to improve the chances of selecting FT desirable plants in the programme.     

Genetic control of crossability of triticale with rye

Limited genetic knowledge is available regarding crossability between hexaploid triticale (2n= 6x= 42, 21″, AABBRR, amphiploid Triticum turgidum L.‐Secale cereale L.) and rye (2n= 14, 7″, RR). Our objectives were to determine (1) the crossability between triticales and rye and (2) the inheritance of crossability between F₂ progeny from intertriticale crosses and rye. First, ‘8F/Corgo’, a hexaploid triticale, was crossed as a female with two landrace ryes, ‘Gimonde’ and, ‘Vila Pouca’ and two derived north European cultivars, ‘Pluto’ and ‘Breno’. These crosses produced 21.7, 20.9, 5.9, and 5.6%, seed‐set or crossability, respectively, showing that the landrace ryes produced higher seed‐set than the cultivars. Second, ‘Gimonde’ rye was crossed as a male with four triticales for 3 years. The control cross, ‘Chinese Spring’ wheat × rye, produced 80‐90% seed‐set. Of the four triticales, ‘Beagle’ produced 35.7‐56.8% seed‐set. The other three triticales produced less than 20% seed‐set, showing that the triticales differ in crossability with ‘Gimonde’ rye. Third, six FiS from intertriticale crosses (‘8F/Corgo’בBeagle’, ‘Beagle’בCachirulo’, ‘Lasko’בBeagle’, ‘8F/Corgo’בCachirulo’, ‘Lasko’בCachirulo’, ‘Lasko’ב8F/Corgo’) were crossed to ‘Gimonde’ rye. Results indicated that lower crossability trait was partially dominant in the two F₁S from crosses involving ‘Beagle’(high crossability) with‘8F/Corgo’ and ‘Cachirulo’(low crossability) and completely dominant in the ‘Beagle’בLasko’ cross, as it happens in wheat. Fourth, segregants in four F₂ populations (‘Lasko’בBeagle’, ‘8F/Corgo’בBeagle’, ‘Lasko’ב8F/Corgo’, and‘8F/Corgo’בCachirulo’) were crossed with rye. Segregation for crossability was observed, although distinct segregation classes were blurred by environmental and perhaps other factors, such as self‐incompatibility alleles in rye. Segregation patterns showed that ‘Beagle’, with high crossability to rye, carries either Kr1 or Kr2. The three triticales with low crossability with rye were most likely homozygous for Kr1 and Kr2. Therefore, it is likely that the Kr loci from A and B genomes acting in wheat also play a role in triticale × rye crosses.     

Inheritance of resistance to Pyrenophora teres f. teres in spring barley

The inheritance of seedling resistance to a Swedish isolate of Pyrenophora teres f. teres was investigated in four resistance sources of spring barley. Accessions CI 2330, CI 5791, CI 5822 and CI 9779 were used as resistance sources, and the cultivar ‘Alexis’ was used as a susceptible parent in different crosses. From the disease reaction in the F₁, F₂ and F₃ generations it was concluded that the resistance was governed by the same two complementary genes in CI 5791, CI 822 and CI 9776. One of these genes was present in CI 2330. The first three cultivars were highly resistant to the isolate used in this investigation. These results, when combined with earlier studies, suggest that CI 5791, CI 5822 and CI 9776 may be of great value as sources of resistance to barley net blotch. Spearman's rank correlation between the disease reaction of F₂ plants and their F₃ progeny was highly significant (r = 0.75; P ≥ 0.001) It is suggested that selection in the F₂ generation is effective. In a backcross breeding scheme, single plant reactions in F₁ or F₂ need to be confirmed in later generations.     

Inheritance of petal colour and its independent segregation from seed colour in Brassica rapa

The inheritance of petal (flower) colour and seed colour in Brassica rapa was investigated using two creamy‐white flowered, yellow‐seeded yellow sarson (an ecotype from Indian subcontinent) lines, two yellow‐flowered, partially yellow‐seeded Canadian cultivars and one yellow‐flowered, brown‐seeded rapid cycling accession, and their F₁, F₂, F₃ and backcross populations. A joint segregation of these two characters was examined in the F₂ population. Petal colour was found to be under monogenic control, where the yellow petal colour gene is dominant over the creamy‐white petal colour gene. The seed colour was found to be under digenic control and the yellow seed colour (due to a transparent coat) genes of yellow sarson are recessive to the brown/partially yellow seed colour genes of the Canadian B. rapa cvs.‘Candle’ and ‘Tobin’. The genes governing the petal colour and seed colour are inherited independently. A distorted segregation for petal colour was found in the backcross populations of yellow sarson × F₁ crosses, but not in the reciprocal backcrosses, i.e. F₁× yellow sarson. The possible reason is discussed in the light of genetic diversity of the parental genotypes.     

Correlated selection responses of fiber properties measured by high volume instrument and advanced fiber information system in Upland cotton

Fiber properties measured by high volume instrument (HVI) and advanced fiber information system (AFIS) are currently being used in Upland cotton (Gossypium hirsutum L.) breeding. It would be interesting to know if it is equally efficient in selection for parameters measured by the two instrumental systems and how are the correlated selection responses of the parameters between the two systems. A study was designed to determine the correlated selection responses between fiber properties measured by the two systems in F₃, F₄, and F₅ generations of two crosses, FM 832 × SP 205 and MD 52ne × JJ 1145ne. Selections for HVI-fiber length, HVI-short fiber content, and HVI-maturity were made in the F₃ generation and the correlated responses of AFIS-fiber properties to the selections were estimated in the subsequent F₄ and F₅ generations. Selections for AFIS-fiber length, AFIS-short fiber content, and AFIS-maturity were made in the F₄ generation and the correlated responses of HVI-fiber properties to the selections were estimated in the F₅ generation. Moderate to high realized heritability was identified for both HVI- and AFIS-fiber properties. Significant correlated selection responses of fiber length and maturity between the HVI and AFIS methods were consistently observed in both crosses, suggesting similar genetic basis for fiber length and maturity measured by HVI and AFIS. Therefore, equal effectiveness is expected of the selection for both traits by the two instrumental systems. Correlated selection responses of short fiber content parameters between the HVI and AFIS methods were not consistent, suggesting a possible different genetic basis or differential variations of measurement quality between the two systems.     

Indica-Japonica differentiation of paddy rice and its relationship with heterosis

The Indica‐Japonica differentiation of three photoperiod‐sensitive and/or thermosensitive genetic male‐sterile rice (PGMS or TGMS, respectively) lines and 47 male parental lines from seven ecotypes were studied for their restriction fragment length polymorphism marker data to determine which ecotype crosses with the three PGMS and/or TGMS lines could lead to higher yield potential, and to estimate the relationship between the Indica‐Japonica differentiation of parents and heterosis in grain yield and its components. The results indicated that hybrids derived from ‘N422s’ and the early‐middle ripening Indica varieties from southern China, and hybrids between ‘Pei'ai64s’ and three Japonica ecotypes, including North‐eastern Japonica varieties, restoring lines of Japonica hybrid rice and north China Japonica varieties, showed the highest grain yields. There was less variation of yield among the F1s between ‘108s’ and the seven ecotypes than among the other F₁s. Highly significant positive correlations between heterosis of the F₁ yield and genetic distance of the parents were detected, although the correlation between F₁ yield performance and genetic distance did not reach a significant level. Considerable variation of correlation between heterosis and genetic distance was also detected in the Indica × Indica crosses and Indica × Japonica crosses. There was much higher correlation (r = 0.63) between the F₁ yield performance and the genetic distance of parents in the Indica × Indica crosses than in the others. It is proposed that a genetic distance of 0.4‐0.8 between the two parents of hybrid rice might be appropriate not only for F₁ performance, but also for heterosis.