Inheritance of Carbon Isotope Discrimination in Bread Wheat (Triticum Aestivum L.)

Summary Reliable selection of families with increased grain yield is difficult in breeding programs targeting water-limited environments. Carbon isotope discrimination (Δ) is negatively correlated with transpiration efficiency, and low Δ is being used for indirect selection of high wheat yield in rainfed environments. Yet little is known of genetic control and opportunities for improving selection efficiency of Δ in wheat. Half-diallel and generation means mating designs were undertaken to provide estimates of the size and nature of gene action for Δ in a range of wheat genotypes varying for this trait. Significant (P < 0.01) differences were observed for leaf tissue Δ among parents (19.3 to 20.7‰) and F₁ progeny (19.4 to 20.9‰) in the half-diallel. General (GCA) and specific combining ability (SCA) effects were significant (P < 0.05), while Baker's GCA/SCA variance ratio of 0.89 was close to unity, indicating largely additive gene effects. GCA effects varied from −0.38 to + 0.34‰ for low and high Δ genotypes `Quarrion' and `Gutha', respectively. GCA effects and parental means were strongly correlated (r = 0.95, P < 0.01) while directional dominance and epistasis contributed to small, non-additive gene action for Δ. Smaller Δ in F₁ progeny was associated with accumulation of recessive alleles from the low Δ parent. Narrow-sense heritability was high (0.86) on a single-plant basis. Generation means analysis was undertaken on crosses between low Δ genotype Quarrion and two higher Δ genotypes `Genaro M81' and `Hartog'. The F₁, F₂ and midparent means were not statistically (P > 0.05) different, whereas backcrossing significantly changed Δ toward the mean of the recurrent parent. Gene action was largely additive with evidence for additive × additive epistasis in one cross. Narrow-sense heritabilities were moderate in size (0.29 to 0.43) on a single-plant basis. Genetic gain for Δ in wheat should be readily achieved in selection among inbred or partially inbred families during the later stages of population development.     

Marker-assisted backcross selection in an interspecific <Emphasis Type="Italic">Cucumis</Emphasis> population broadens the genetic base of cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.)

Cucumber (Cucumis sativus L.) is a major cucurbit vegetable species whose genetic base has been drastically reduced during its domestication. The crop’s narrow genetic base (3–12% DNA polymorphism) has resulted from the use of limited genetic material and intense selection during plant improvement. Recently, however, interspecific hybridization has been successful in Cucumis via mating of C. hystrix Chakr. and C. sativus, which resulted in the amphidiploid C. hytivus. We report herein a marker-assisted strategy for increasing genetic diversity in cucumber through introgression backcrossing employing C. hytivus. The comparatively late-flowering but high-yielding, indeterminate, monoecious line WI 7012A (P₁; donor parent) derived from a C. hytivus × C. sativus-derived line (long-fruited Chinese C. sativus cv. Beijingjietou) was initially crossed to the determinate, gynoecious C. sativus line WI 7023A (P₂; recurrent parent 1), and then advanced backcross generation progeny (BC₂) were crossed with the gynoecious indeterminate line WI 9-6A (P₃; recurrent parent 2). More specifically, a single F₁ individual (P₁ × P₂) was backcrossed to P₂, and then BC progeny were crossed to P₂ and P₃, where marker-assisted selection (MAS) for genetic diversity (8 mapped and 16 unmapped markers; designated Sel) or no selection (designated NSel) was applied to produce BC₃P₂ (Sel) and BC₃P₃ (Sel), and BC₂P₂ (NSel) and BC₂P₂S₁ (NSel) progeny. Relative vegetative growth, number of lateral branches (LB), days to flowering (DF), yield (fruit number), and fruit quality [as measured by length:diameter (L:D) and endocarp:total diameter (E:T) ratios] were assessed in parents and cross-progeny. DF varied from ~20 (BC₃P₂Sel) to ~25 days (BC₂P₃Sel) among the populations examined, where progeny derived from P₂ possessed the shortest DF. Differences in cumulative yield among the populations over six harvests were detected, varying from ~8 fruits per plant in BC₃P₂ (Sel) to ~39 fruits per plant in BC₂P₃ (Sel). Although the vigorous vegetative growth of line P₁ was observed in its backcross progeny, highly heterozygous and polymorphic backcross progeny derived from P₃ were comparatively more vigorous and bore many high-quality fruit. Response to selection was detected for LB, DF, L:D, and E:T, but the effectiveness of MAS depended upon the parental lines used. Data indicate that the genetic diversity of commercial cucumber can be increased by introgression of the C. hystrix genome through backcrossing.     

Means and variances for Fusarium head blight resistance of F2-derived bulks from winter triticale and winter wheat crosses

Fusarium head blight (FHB), caused by Fusarium graminearum and Fusarium culmorum, is a devastating disease in cereals. This study was undertaken to estimate progeny means and variances in each of five winter triticale and winter wheat crosses using unselected F₂−derived lines in F₄ or F₅ generation bulked at harvest of the previous generation. Fifty (triticale) and 95 (wheat) progeny per cross were inoculated in two (triticale) or three (wheat) field environments. FHB rating was assessed on a whole-plot basis. Mean disease severities of the parents ranged from 2.3 to 6.4 in triticale and from 3.1 to 6.5 in wheat on a 1-to-9 scale (1 = symptomless, 9 = 100% infected). The midparent values generally resembled the means of their derived progeny. Significant (P < 0.01) genotypic variance was detected within each cross, but genotype × environment interaction and error variances were also high for both crops. Medium to high entry-mean heritabilities (0.6–0.8) underline the feasibility of selecting F₂-derived bulks on a plot basis in several environments. Phenotypic correlation of FHB resistance between generation F_2:4 and F_2:5 was r = 0.87 (P < 0.01) tested across 150 wheat bulks at two locations. Our estimates of selection gain are encouraging for breeders to improve FHB resistance in triticale and wheat by recurrent selection within adapted materials.     

Effects of high- vs. low-yield environments on selection for increased biomass yield in switchgrass

No information is available on the effects of different biomass yield environments on selection efficiency in switchgrass (Panicum virgatum L.) breeding improvement. This study was conducted to assess the effects of high- and low-biomass yield environments (HYE and LYE, respectively) on recurrent selection for general combining ability (RSGCA) in a lowland population of switchgrass (NL-94). The top 14 of 65 NL-94 C₀ parent plants were selected based on biomass yield of half-sib (HS) progeny tested for one post-establishment year under HYE and LYE conditions. Nine of the 14 C₀ parent plants were the same based on HS performance under HYE and LYE. Selected plants were intercrossed to produce NL-94 HYE and NL-94 LYE C₁ populations. One hundred and twenty-five HS C₁ progeny families (60 NL-94 HYE and 65 NL-94 LYE) were evaluated for biomass yield for 3 years (2002–2004) under HYE and LYE conditions. The HYE produced about 2.5 times higher biomass yields than the LYE in both C₀ and C₁ HS progeny tests. Estimated additive genetic variance and predicted gains from selection (ΔG) were high in the C₁ populations indicating that RSGCA should achieve higher biomass yields. Mean biomass yields of C₁ HS families originating from the LYE protocol were significantly higher than those of families originating from the HYE protocol in both HYE and LYE performance tests, suggesting greater selection response under LYE in the C₀ population. The estimates of narrow-sense heritability ( ) and ΔG from the C₁ populations indicate that positive response to selection for biomass yield is possible in subsequent cycles of selection under either HYE or LYE, with a possible small advantage for HYE.     

Inheritance of beta-carotene-associated flesh color in cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.) fruit

The nutritional value of cucumber (Cucumis sativus L.) can be improved by the introgression of β-carotene (i.e., provitamin A and/or orange flesh) genes from “Xishuangbanna gourd” (XIS; Cucumis sativus var. xishuangbannanesis Qi et Yuan) into US pickling cucumber. However, the genetics of β-carotene content has not been clearly defined in this US market type. Thus, three previous populations derived from a US pickling cucumber (‘Addis’) × XIS mating were evaluated for β-carotene content, from which the high β-carotene inbred line (S₄), ‘EOM 402-10’, was developed. A cross was then made between the US pickling cucumber inbred line ‘Gy7’ [gynoecious, no β-carotene, white flesh; P₁] and ‘EOM 402-10’ [monoecious, possessing β-carotene, orange flesh; P₂] to determine the inheritance of β-carotene in fruit mesocarp and endocarp tissue. Parents and derived cross-progenies (F₁, F₂, BC₁P₁, and BC₁P₂) were evaluated for β-carotene content in a greenhouse in Madison, Wisconsin. While F₁ and BC₁P₁ progeny produced mature fruits possessing white, light-green, and green (0.01–0.02 μg g⁻¹ β-carotene) mesocarp, the F₂ and BC₁P₂ progeny mesocarp segregated in various hues of white, green, yellow (0.01–0.34 μg g⁻¹ β-carotene), and orange (1.90–2.72 μg g⁻¹ β-carotene). Mesocarp and endocarp F₂ segregation adequately fit a 15:1 [low-β-carotene (0.01–0.34 μg g⁻¹): high-β-carotene (1.90–2.72 μg g⁻¹)] and 3:1 (low-β-carotene: high-β-carotene) ratio, respectively. Likewise, segregation of carotene concentration in mesocarp and endocarp tissues in BC₁P₂ progeny adequately fit a 3:1 (low-β-carotene: high-β-carotene) and 1:1 (low-β-carotene: high-β-carotene) ratio, respectively. Progeny segregations indicate that two recessive genes control the β-carotene content in the mesocarp, while one recessive gene controls β-carotene content in the endocarp. Single marker analysis of F₂ progeny using the carotenoid biosynthesis gene Phytoene synthase determined that there was no association between this gene and the observed β-carotene variation in either fruit mesocarp or endocarp.     

Improvement of resistance to Fusarium head blight by recurrent selection in an intermating breeding spring wheat population using the dominant male-sterile gene ms 2

Four cycles of recurrent selection for FHB resistance were conducted in an intermating wheat breeding population using the dominant male-sterile gene ms ₂ during 1987–1991.Five cycles of phenotypic mass selection for male-sterile plants were evaluated using the soil-surface inoculation method in Experiment I. Experiment II evaluated changes in FHB scores during five cycles of progeny selection for fertile plants using the single-floret inoculation method. In Experiment I, the average level of FHB response increased to MR level in C₄, compared to MS level in C₀. The numbers of infected spikelets and diseased kernels decreased 0.32 and 2.68 per cycle, respectively. In Experiment II, the average level of FHB response increased to R level in C₄F₁. The numbers of infected spikelets and diseased kernels decreased 0.93 and 4.58 per cycle, respectively. In both experiments, the largest selection gains were realized in the first cycle. The frequencies of R and MR individuals were increased significantly. The frequencies of individuals with FHB response equal and/or superior to Sumai 3 were increased to 5–8% in C₄ and 25% in C₄F₁after the fourth cycle. Agronomic traits tended to be slightly improved in selected populations. Compared to 2% in C₀, about 34% of lines superior in both FHB resistance and agronomic traits in C₄F₁ were selected to enter the conventional breeding program for further evaluation. Sixty three semidwarf lines superior in both FHB resistance and yield potential were selected from the F₅ generations derived from C₁F₁ to C₄F₁. From them, two resistant cultivars with high-yielding potential were developed and commercialized in the Lower Yangtze Valley. Recurrent selection appears to be highly effective and feasible in shifting the average FHB response of the intermating population in the desirable direction, thereby enhancing the frequency of resistant individuals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Polymorphic microsatellite marker set for <Emphasis Type="Italic">Carica papaya</Emphasis> L. and its use in molecular-assisted selection

Conventional methods for the selection of papaya pure lines are time-consuming. Procedures involving the use of molecular markers and the indirect selection of homozygous plants can reduce this time considerably. The objective of this study was to evaluate the informativeness of a microsatellite marker set when used in marker-assisted selection (MAS) for the development of new papaya lines. Eighty-three lines originating from two segregating F₃ populations and from papaya germplasm were used for the molecular analysis of 27 microsatellite primers. Twenty polymorphic microsatellite primers were identified, allowing the identification of 86 alleles, with an average of 3.18 alleles per primer. The observed heterozygosity values were low for both the markers (0.00–0.29) and the individual lines (0.00–0.35). The inbreeding coefficient (f) ranged from 0.634 to 1.00. Eleven lines with f = 1.00 and 18 lines with f varying from 0.953 to 0.961 were identified. In addition, papaya lines showed high genetic diversity, which will certainly contribute to the development of new varieties. Our results show that the use of microsatellites in MAS is a quick and effective procedure for the development of papaya lines.     

SSR markers for marker assisted selection of root-knot nematode (<Emphasis Type="Italic">Meloidogyne incognita</Emphasis>) resistant plants in cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L)

Cotton (Gossypium hirsutum L) cultivars highly resistant to the southern root-knot nematode (RKN) [Meloidogyne incognita (Kofoid and White) Chitwood] are not available. Resistant germplasm lines are available; however, the difficulty of selecting true breeding lines has hindered applied breeding and no highly resistant cultivars are available to growers. Recently, molecular markers on chromosomes 11 and 14 have been associated with RKN resistance, thus opening the way for marker assisted selection (MAS) in applied breeding. Our study aimed to determine the utility of these markers for MAS. Cross one was RKN resistant germplasm M240 RNR × the susceptible cultivar, FM966 and is representative of the initial cross a breeder would make to develop a RKN resistant cultivar. Cross two consists of Clevewilt 6 × Mexico Wild (PI563649), which are the two lines originally used to develop the first highly RKN resistant germplasm. Mexico Wild is photoperiodic. We phenotyped the F₂ of cross one for gall index and number of RKN eggs per plant and genotyped each plant for CIR 316 (chromosome 11) and BNL 3661 (chromosome 14). From this, we verified that MAS was effective, and the QTL on chromosome 14 was primarily associated with a dominant RKN resistance gene affecting reproduction. In the first F₂ population of cross two, we used MAS to identify 11 plants homozygous for the markers on chromosomes 11 and 14, and which also flowered in long days. Progeny of these 11 plants were phenotyped for RKN gall index and egg number and confirmed as RKN highly resistant plants. Generally about 7–10 generations of RKN phenotyping and progeny testing were required to develop the original RKN highly resistant germplasms. Our results show that commercial breeders should be able to use the markers in MAS to rapidly develop RKN resistant cultivars.     

Participatory selection assisted by DNA markers for enhanced drought resistance and productivity in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

In this study, a F₂ population derived from the cross between deep-rooted variety “Moroberekan” with shallow-rooted variety “IR20” were used to identify and validate of SSR markers associated with root morphological traits. The F₂ lines were divided into two groups. In the first group, 152 seedling having minimum of four tillers were chosen and separated into four plantlets to plant them in polyvinyl chloride pipes for root study under well-watered (WW) condition at maturity stage. The lines were genotyped using SSR markers. QTLs for maximum root length (MRL) and root dry weight showed co-segregation with RM472, RM7 and RM201. The same material was forwarded to next generation (F₃) to validate the linked markers under both WW and low-moisture stress (LMS) conditions. These three markers were associated consistently with MRL across generations. In the second group, 1240 F₂ plants were forwarded to F₅ using SSD breeding method to test the effectiveness of the marker-assisted selection (MAS) method for drought resistant. The high performing genotypic group was significantly superior to low performing genotypic group for MRL, grain yield, root volume, root dry weight and root number, indicating the efficiency of MAS for root-related traits under LMS. Comparing MAS with farmer selection in F₆, the results showed that MAS group means were significantly different from farmer group means for MRL, root volume, root dry weight and root number. Thus, MAS was combined with participatory selection to select five high-yielding and deep rooted promising lines. Identification of stable QTL for root morphological traits under WW and LMS conditions can aid in MAS and to introduce them into varieties with good yield potential and accepted by farmer.     

Hybrids of <Emphasis Type="Italic">Avena sativa</Emphasis> with two diploid wild oats (CIav6956) and (CIav7233) resistant to crown rust

Two diploid accessions of wild oat, CIav6956 and CIav7233, were identified as carrying seedling resistance to oat crown rust (caused by Puccinia coronata f. sp. avenae; Pca). Two vigorous interploidy F₁ hybrids were generated from crosses involving the hexaploid oat cultivar Wintaroo and the diploid oat Avena strigosa Schreb. accession CIav6956. An additional interploidy F₁ hybrid, designated “F1-Aa1”, was produced from a cross of Wintaroo and the diploid oat accession CIav7233. All three hybrids were more vigorous and taller than the cultivated parent Wintaroo. The three F₁ hybrids contained full chromosome complements from both parents (2n = 4x = 28), but no seeds were obtained when the three F₁ hybrids were selfed. Meiotic analyses of the hybrids indicated that they exhibited a high degree of inter-genome and intra-genome pairing. Trivalent configurations were detected in 95–96% of meiotic cells and a minimum of three bivalents was present in all cells. An average chiasma frequency of 7.2–7.9 per cell was observed for the three F₁ hybrids. A fourth F₁ hybrid was subsequently generated from a cross between the diploid oat accession CIav7233 and Wintaroo. One octaploid (2n = 8x = 56) was generated from this hybrid and progeny were resistant to two Pca races. The chromosome number of the octaploid progeny varied between 51 and 54 chromosomes. Development of a chromosome addition line(s) with the crown rust resistance should be possible from these partial-octaploids.     

Inheritance of beta-carotene-associated mesocarp color and fruit maturity of melon (<Emphasis Type="Italic">Cucumis melo</Emphasis> L.)

Melon (Cucumis melo L.) fruit production in U.S. can be improved through the introgression of early fruit maturity (FM) and the enhancement of fruit color [i.e., quantity of β-carotene (QβC); orange mesocarp]. However, the genetics of FM and QβC have not been clearly defined in U.S. Western Shipping market class melons (USWS). Thus, a cross was made between the monoecious, early FM Chinese line ‘Q 3-2-2’ (non-carotene accumulating, white mesocarp) and the andromonecious, comparatively late FM USWS line ‘Top Mark’ (carotene accumulating; orange mesocarp) to determine the inheritance of FM and QβC in melon. Parents and derived cross-progenies (F₁, F₂, F₃, BC₁P₁, and BC₁P₂) were evaluated for FM and QβC at Hancock, Wisconsin over 2 years. Estimates of narrow-sense heritability (h _N²) for QβC and FM as defined by F₁, F₂, and BC (by individuals) were 0.55 and 0.62, respectively, while estimates based on F₃ families were 0.68 and 0.57, respectively for these traits. Mesocarp color segregation (F₂ and BC₁P₂) fit a two gene recessive epistatic model, which in turn, interacts with other minor genes. Although the inheritance of QβC and FM is complex, introgression (e.g., by backcrossing) of early FM genes resident in Chinese germplasm into USWS market types is possible. Such introgression may lead to increased yield potential in USWS market types while retaining relatively high β-carotene fruit content (i.e., orange mesocarp), if stringent, multiple location and early generation family selection (F_3–4) is practiced for FM with concomitant selection for QβC.     

Gamete selection for improving physiological resistance to white mold in common bean

White mold (WM), caused by Sclerotinia sclerotiorum (Lib.) de Bary, is a widespread disease of dry and green bean (Phaseolus vulgaris L.) in North America. Gamete selection (GS) was effective to combine and pyramide resistant genes and quantitative trait loci (QTL) for common bacterial blight. Our objective was to determine the effectiveness of GS to introgress physiological resistance to white mold. Two inter-gene-pool double-cross populations were developed. Selection for WM resistance was practiced from F₁ to F₄. Thirteen selected F_1:5 breeding lines of each population and their four parents were evaluated. Two separate inoculations were made on each plant 1 week apart using a cut-stem method. The WM reaction was scored at 16, 23, and 33 days post inoculation (DPI) using a scale from 1 (no disease) to 9 (severely diseased or dead). In F₁, 52% of Pop I (USPT-WM-1/CORN 601//USPT-CBB-1/92BG-7) and 67% of Pop II (Chase/I9365-25//ABL 15/A 195) susceptible plants were discarded. In F_4, only 1.2% of families from Pop I, and 0.9% for Pop II, survived the selection process. An average of 20.5% gain in WM resistance was obtained for both populations in F₄. Four breeding lines of Pop I had significantly (P = 0.05) lower WM score (4.1–4.6) and four were equal (4.7–4.9) to the best WM-resistant parent 92BG-7 (4.9), while ten breeding lines of Pop II were equal (4.5–4.8) to the best WM-resistant parent A 195 (4.6). Thus, GS was effective for improving WM resistance in common bean.     

Identification of molecular markers associated with resistance to squash silverleaf disorder in summer squash (<Emphasis Type="Italic">Cucurbita</Emphasis> <Emphasis Type="Italic">pepo</Emphasis>)

Squash silverleaf (SSL), caused by the silverleaf whitefly [Bemisia argentifolii (formerly known as Bemisia tabaci Gennadius, B strain)], is an important physiological disorder that affects squash (Cucurbita spp.) by reducing yield potential. Breeding squash with resistance to SSL disorder can be facilitated by using marker-assisted selection (MAS). Resistance to SSL disorder, in Cucurbita pepo, is conferred by a single recessive gene (sl). The objective of this study was to identify molecular markers associated with resistance. A zucchini squash, SSL disorder resistant breeding line, ‘Zuc76’ (sl/sl) and a SSL disorder susceptible zucchini cultivar ‘Black Beauty’ (Sl/Sl) were screened with 1,152 randomly amplified polymorphic DNA (RAPD) primers and 432 simple sequence repeat (SSR) markers to identify polymorphisms. Using F₂ and BC₁ progeny segregating for SSL disorder resistance, three RAPD (OPC07, OPL07 and OPBC16) primers and one SSR (M121) marker were found associated with sl. Fragments amplified by RAPD primer OPC07 was linked in coupling phase to sl, whereas RAPD primer OPL07 was linked in repulsion phase. RAPD primer OPBC16 and SSR marker M121 were co-dominant. The allelic order of these loci was found to be M121–sl–OPC07–OPL07–OPBC16. The closest marker to sl is M121 with an estimated genetic distance of 3.3 cM. The markers identified in this study will be useful for breeding summer squash (C. pepo) for SSL disorder resistance derived from zucchini squash breeding line ‘Zuc76’.     

A genetic map of macadamia based on randomly amplified DNA fingerprinting (RAF) markers

The first genetic linkage map of macadamia (Macadamia integrifolia and M. tetraphylla) is presented. The map is based on 56 F₁ progeny of cultivars ‘Keauhou’ and ‘A16’. Eighty-four percent of the 382 markers analysed segregated as Mendelian loci. The two-way pseudo-testcross mapping strategy allowed construction of separate parental cultivar maps. Ninety bridging loci enabled merging of these maps to produce a detailed genetic map of macadamia, 1100 cm in length and spanning 70–80% of the genome. The combined map comprised 24 linkage groups with 265 framework markers: 259 markers from randomly amplified DNA fingerprinting (RAF), five random amplified polymorphic DNA (RAPD), and one sequence-tagged microsatellite site (STMS). The RAF marker system unexpectedly revealed 16 codominant markers, one of them a putative microsatellite locus and exhibiting four distinct alleles in the cross. This molecular study is the most comprehensive examination to date of genetic loci of macadamia, and is a major step towards developing marker-assisted selection for this crop. This revised version was published online in July 2006 with corrections to the Cover Date.     

Using three selected overlapping RILs to fine-map the yield component QTL on Chro.D8 in Upland cotton

Cotton yield improvement is vital to fulfill rising global demands. The identification of major quantitative trait loci (QTL) for yield components was helpful in molecular marker-assisted selection (MAS) to improve cotton yield. We previously identified a densely populated QTL region for fiber qualities and yield components on chromosome D8 (Chro.D8) of Upland cotton from a (7235 × TM-1)RIL. In the present study, to fine-map yield component QTLs, we chose three overlapped recombinant inbred lines (RILs) with different intervals included the yield component QTLs, and backcrossed each line with TM-1 to develop three large sized mapping populations. Phenotypic data for yield components were collected in Nanjing (JES/NAU) and Xinjiang (BES/XJ) in 2006 and 2007. Three simple sequence repeat (SSR) genetic linkage maps on chro.D8 were constructed using 907 individuals in (7TR-133 × TM-1)F₂ (Pop A), 670 in (7TR-132 × TM-1)F₂ (Pop B), and 940 in (7TR-214 × TM-1)F₂ (Pop C). Three stable QTLs for boll size, two for lint percentage and one for boll number per plant,were detected on chro.D8 following analysis of three RIL backcrossed F₂/F_2:3 progeny at JES/NAU and BES/XJ although their cultivation practices differ greatly between these two cotton-growing regions. One QTL for boll number per plant exhibited a phenotypic variance (PV) of 5.6–10.1%, three QTLs for boll size exhibited 15.0–35.5% PV and two lint percentage QTLs exhibited 10.9–19.3% PV. Negative correlation between lint yield and fiber strength was confirmed.     

Stay green trait: variation, inheritance and its association with spot blotch resistance in spring wheat (Triticum aestivum L.)

One thousand four hundred and seven spring wheat germplasm lines belonging to Indian and CIMMYT wheat programs were evaluated for stay green (SG) trait and resistance to spot blotch caused by Bipolaris sorokiniana during three consecutive crop seasons, 1999–2000, 2000–2001 and 2001–2002. Disease severity was recorded at six different growth stages beginning from tillering to late milk stage. SG trait was measured by following two approaches: difference for 0–9 scoring of green coloration (chlorophyll) of flag leaf and spike at the late dough stage (GS 87) and a new approach of leaf area under greenness (LAUG). Germplasm lines showed a wide range (7–89) for LAUG and were grouped into four viz., SG, moderately stay green, moderately non-stay green and non-stay green (NSG). However, very few (2.2%) lines showed high expression of SG trait, i.e., LAUG >60. LAUG appeared to be a better measure of SG trait than a 0–9 scale. Mean spot blotch ratings of SG genotypes were significantly lower than those of NSG genotypes at all growth stages. Two spot blotch resistant genotypes (Chirya 3 and Chirya 7) having strong expressions of SG trait were crossed with NSG, spot blotch susceptible cv. Sonalika. Individually threshed F₂ plants were used to advance the generations. SG trait and spot blotch severity were recorded in the parents and F₁, F₃, F₄, F₅, F₆ and F_6–7 generations under disease-protected and inoculated conditions. SG trait in the F₁ generation was intermediate and showed absence of dominance. Evaluation of progenies (202–207) in the segregating generations revealed that SG trait was under the control of around four additive genes. Lines homozygous for SG trait in F₄, F₅, F₆ and F_6–7 generations showed significantly lower mean area under disease progress curve (AUDPC) for spot blotch than those with NSG expression. A positive correlation (0.73) between SG trait and AUDPC further indicated a positive influence of SG on severity of spot blotch. The study established that variation for SG trait exists in spring wheat; around four additive genes control its inheritance in the crosses studied and there is positive association between SG trait and resistance to spot blotch.     

Recurrent selection using male sterility and hydroponic tiller culture in pedigree breeding of wheat

A procedure for recurrent selection based on the male sterility gene, Ms3, was implemented. To facilitate the production of large numbers of hybrid progeny, a simple hydroponic system was developed in which male‐sterile tillers cut at the flowering stage can be pollinated and maintained for about 8 weeks‐long enough to produce a large quantity of viable hybrid seeds. The recurrent selection steps were integrated with a pedigree breeding programme employing different selection cycles for male and female plants. F₁ female plants are subjected to a single screening for seedling resistance. In addition to F₁ seedling screening, F₂‐F₄ male families are field‐selected for disease resistance, agrotype and quality in a pedigree programme before being used in crosses.     

Simulations and practical problems of applying multiple marker assisted selection and doubled haploids to wheat breeding programs

Marker assisted selection (MAS) and wheat doubled haploids (DH) are relatively new technologies, recently applied to wheat breeding programs. Simulations demonstrate that DHs increase the efficiency of MAS, and offer faster strategies for combining large numbers of genes with a minimum number of marker tests. When small numbers of marked loci (1-3) are selected simultaneously, selection of DH progeny is 5-6 times more efficient than selecting F₄ derived families. Combining 4-8 marked loci, screening of F₂ plants and using only those plants homozygous or segregating for all of the marked loci as parents for DH production (10-31% of F₂ plants) is 3-10 times as efficient as using F₁ plants. A number of protocols have been proposed involving sib-matings and selection to fix some genes, with further selection in the second generation to improve the proportion of useful DH lines. In one scheme (recombinant F₂ selection) all F₂ plants, either homozygous or heterozygous for the marked alleles, are intercrossed at random and the recurrent F₁ plants still having these alleles are used for DH production. An alternative strategy (recurrent DH selection) is to select from an initial DH population and intercross those lines having most favourable marked loci with a second cycle of DHs to fix all favourable marked loci. Combining more than 12 marked gene loci does not seem feasible, due to the very large numbers of F₂s (>2000) required. This has implications when using MAS for quantitative trait loci, where many minor gene loci would have to be combined. Direct selection for some multi-genic quantitative traits amongst the DH lines may be more efficient than using MAS where recurrent selection is used. At the Cereal Research Centre, the practical problems of using these protocols as part of the spring wheat breeding program are being evaluated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Marker selection for <Emphasis Type="Italic">Fusarium</Emphasis> head blight resistance based on quantitative trait loci (QTL) from two European sources compared to phenotypic selection in winter wheat

Fusarium head blight (FHB) infects all cereals including maize and is considered a major wheat disease, causing yield losses and mycotoxin contamination. This study aimed to compare the realized selection gain from marker and phenotypic selection in European winter wheat. A double cross (DC) combined three FHB resistance donor-QTL alleles (Qfhs.lfl-6AL and Qfhs.lfl-7BS from ‘Dream’, and one QTL on chromosome 2BL from ‘G16-92’) with two high yielding, susceptible winter wheats, ‘Brando’ and ‘LP235.1’. The base population of 600 DC derived F₁ lines was on one hand selected for the respective QTLs by SSR markers (marker-selected cycle, CM), resulting in 35 progeny possessing different combinations of beneficial donor-QTL alleles. On the other hand it was selected phenotypically, only by FHB rating, and the best 20 lines were recombined and selfed (phenotypically selected cycle, CP). The variants CP, CM, and an unselected variant (C0) were tested at four locations by inoculation of Fusarium culmorum. Resistance was measured as the mean of multiple FHB ratings (0–100%). FHB severity was reduced through both phenotypic and marker selection by 6.2 vs. 5.0%, respectively. On a per-year basis, marker selection by 2.5% was slightly superior to phenotypic selection with 2.1%, because the first variant saved 1 year. Marker-selected lines were on average 8.6 cm taller than phenotypically selected lines. A high genetic variation within the marker-selected variant for FHB resistance and the high effect of a resistance-QTL allele on straw length indicate that additional phenotypic selection will further enhance selection gain.     

Experimental evaluation of several cycles of marker-assisted selection in maize

A program was initiated in 1994 to compare the efficiency of marker-assisted selection (MAS) and conventional phenotypic selection. A population of 300 F_3:4 families was generated from the cross between two maize inbred lines F2 and F252 and selected on an index combining grain yield and grain moisture at harvest. This population was characterised for 93 RFLP markers and evaluated as testcrosses in a large range of environments. Three methods of selection were applied (i) two cycles of conventional phenotypic selection; (ii) two cycles of MAS based on an index combining phenotypic values and QTL genetic values and (iii) one cycle of combined MAS followed by two cycles of selection based only on the QTL effects estimated in the first generation. The different populations were characterised for RFLP markers. The evolution of allele frequencies showed that selection on only-markers was very efficient for fixing QTL alleles found favourable in the initial population. This evolution was quite different from that observed for phenotypic selection or combined MAS. Genetic gain was evaluated and found significant for each method of selection. Nevertheless, the difference between phenotypic selection and combined MAS was not significant. The two additional cycles of MAS on only-markers did not improve significantly the genetic value of the population. Moreover, the genetic variance of this population remained high, despite most of the QTL initially detected were almost fixed. The results suggest that the QTL effects estimated in the initial population were not stable due to epistasis and/or QTL by environment interactions. This revised version was published online in July 2006 with corrections to the Cover Date.