BC1 Families Research Articles

The sporophytic type of fertility restoration in the A3 CMS-inducing cytoplasm of sorghum and its modification by plant water availability conditions

The A3 type of CMS in sorghum is one of the most difficult to restore fertility because of the low frequency of fertilityrestoring genes among sorghum accessions, the complex mechanism of fertility restoration that occurs with the complementary interaction of two gametophytic genes Rf3 and Rf4, and the sensitivity of their expression to air and soil drought. In order to test the hypothesis of the sporophytic type of fertility restoration in CMS lines with A3 type cytoplasm developed in our laboratory, we analyzed segregation in the self-pollinated progeny of fertile F1hybrids grown under different water availability conditions (in a dryland plot, in plots with additional irrigation, in a growth chamber, and in an experimental field with a natural precipitation regime) and in their backcrosses to the maternal CMS-line. The presence of sterile plants in the F2 and BC1 families with the maternal CMS line grown in all tested water availability conditions argues for the sporophytic mechanism of fertility restoration. Cytological analysis of fertile F1 hybrids revealed a significant amount of degenerating pollen grains (PGs) with impaired starch accumulation and detachment of the PG contents from the cell wall. It is assumed that the expression of the fertility-restoring genes Rf3 and Rf4 in the hybrids with studied CMS lines starts already in the sporophyte tissues, normalizing the development of a certain part of the PGs carrying the recessive alleles of these genes (rf3 and rf4), which are involved in fertilization and give rise to sterile genotypes found in F2 and BC1 families. For the first time, the transgenerational effect of water availability conditions of growing a fertility-restoring line on male fertility of the F2 generation was detected: a pollinator grown in a plot with additional irrigation produced more fertile and less sterile individuals compared to the same pollinator grown under a rainfall shelter (p < 0.01), and the segregation pattern changed from digenic to monogenic, indicating heritable inhibition of the expression of one of the fertility-restoring genes (kind of “grandfather effect”). The possibility of selection for the stability of the fertility restoration system of the A3 cytoplasm to functioning under conditions of high vapor pressure deficit during the flowering period was shown. These data may contribute to the creation of effective fertility restoring lines for this type of CMS in sorghum.

Multiallelic digenic control of vernalization requirement in carrot (Daucus carota L.)

Carrots are typically classified as annual or biennial, depending on their vernalization requirement for flowering, a trait that is genetically-conditioned. Wild carrots are predominantly annual, requiring less hours of cold temperatures for flower induction, whereas most cultivated carrots are biennial (i.e., they have higher cold temperature requirements). After vernalization, if followed by long days, floral stem elongation and flowering take place. Previous studies using F2 and BC1 families derived from crosses between an early and a late-flowering line revealed segregation ratios consistent with a monogenic trait, with annual habit being dominant over biennial. In this work, we studied inheritance and segregation of the vernalization requirement in carrot F2 populations derived from crosses involving carrots of different genetic backgrounds and geographical origins. Nine crosses between biennial and annual phenotypes were analyzed, for 2 years, by means of percentage of flowering plants (parental lines, F1 and F2 families were sown in the fall for adequate discrimination between annual and biennial plants). Based on the obtained segregation ratios, a genetic model for this trait was proposed. The results are consistent with a model of two genes (Vrn-A and Vrn-B) with three alleles controlling the vernalization requirement. Dominance of annuality was clear for both genes, with A1 allele having an epistatic effect over Vrn-B. Vrn-A and Vrn-B interact generating different vernalization requirement levels.

Combinations of Independent Dominant Loci Conferring Clubroot Resistance in All Four Turnip Accessions (Brassica rapa) From the European Clubroot Differential Set

Clubroot disease is devastating to Brassica crop production when susceptible cultivars are planted in infected fields. European turnips are the most resistant sources and their resistance genes have been introduced into other crops such oilseed rape (Brassica napus L.), Chinese cabbage and other Brassica vegetables. The European clubroot differential (ECD) set contains four turnip accessions (ECD1–4). These ECD turnips exhibited high levels of resistance to clubroot when they were tested under controlled environmental conditions with Canadian field isolates. Gene mapping of the clubroot resistance genes in ECD1–4 were performed and three independent dominant resistance loci were identified. Two resistance loci were mapped on chromosome A03 and the third on chromosome A08. Each ECD turnip accession contained two of these three resistance loci. Some resistance loci were homozygous in ECD accessions while others showed heterozygosity based on the segregation of clubroot resistance in 20 BC1 families derived from ECD1 to 4. Molecular markers were developed linked to each clubroot resistance loci for the resistance gene introgression in different germplasm.

Studying the Genetics of Resistance to CyHV-3 Disease Using Introgression from Feral to Cultured Common Carp Strains

Sustainability and further development of aquaculture production are constantly challenged by outbreaks of fish diseases, which are difficult to prevent or control. Developing fish strains that are genetically resistant to a disease is a cost-effective and a sustainable solution to address this challenge. To do so, heritable genetic variation in disease resistance should be identified and combined together with other desirable production traits. Aquaculture of common carp has suffered substantial losses from the infectious disease caused by the cyprinid herpes virus type 3 (CyHV-3) virus and the global spread of outbreaks indicates that many cultured strains are susceptible. In this research, CyHV-3 resistance from the feral strain “Amur Sassan” was successfully introgressed into two susceptible cultured strains up to the first backcross (BC1) generation. Variation in resistance of families from F1 and BC1 generations was significantly greater compared to that among families of any of the susceptible parental lines, a good starting point for a family selection program. Considerable additive genetic variation was found for CyHV-3 resistance. This phenotype was transferable between generations with contributions to resistance from both the resistant feral and the susceptible cultured strains. Reduced scale coverage (mirror phenotype) is desirable and common in cultured strains, but so far, cultured mirror carp strains were found to be susceptible. Here, using BC1 families ranging from susceptible to resistant, no differences in resistance levels between fully scaled and mirror full-sib groups were found, indicating that CyHV-3 resistance was successfully combined with the desirable mirror phenotype. In addition, the CyHV-3 viral load in tissues throughout the infection of susceptible and resistant fish was followed. Although resistant fish get infected, viral loads in tissues of these fish are significantly lesser than in those of susceptible fish, allowing them to survive the disease. Taken together, in this study we have laid the foundation for breeding CyHV-3-resistant strains and started to address the mechanisms underlying the phenotypic differences in resistance to this disease.

Physiological drought resistance and accumulation of leaf phenolics in white clover interspecific hybrids

This study investigated drought responses in first (BC1) and second (BC2) backcross generation hybrids of Trifolium repens L.×T. uniflorum L., and compared these to high-yielding T. repens L. (white clover) cultivars. Plant attributes included accumulation of leaf phenolics as well as other biochemical and physiological water stress responses. Measurements were made across broad clover types as well as on a subset of related material. Under drought, net photosynthesis decreased by 44–48% and transpiration rates decreased by 60% in the BC2 family and in the white clover parent, but were unaltered in the BC1 family. Drought-induced decreases in leaf water potential were more pronounced in the BC1 family (−47%) than the BC2 (−31%) and parental (−28%) material. Quercetin glycoside accumulation generally increased 2–3 fold under drought. Compared with the BC2 family and with parent plants, the BC1 family had 2.5–4x higher constitutive kaempferol glycoside levels, and 1.5–2.5x higher kaempferol glycoside accumulation under drought. Constitutive kaempferol glycoside accumulation was related to reduced senescence and to less pronounced decreases in shoot DW under drought stress. Hydroxycinnamic acid accumulation increased by 56–73% under water stress, particularly for the BC1 generation. These results identify physiological and biochemical traits that help explain drought resistance of T. repens×T. uniflorum hybrids. The findings suggest merit for the selection of BC1 families with increased accumulation of protective phenolic compounds in breeding programmes for improved drought resistance.

Single Nucleotide Polymorphisms in Linkage Disequilibrium with the Male-fertility Restoration (Ms) Locus in Open-pollinated and Inbred Populations of Onion

Maintainer lines are used to seed propagate male-sterile lines for the development of hybrid onion (Allium cepa L.) cultivars. The identification of maintainer lines would be more efficient with molecular markers distinguishing genotypes at the nuclear male-fertility restoration (Ms) locus. Ms has been mapped to chromosome 2 of onion and linked genetic markers identified. However, linkages between these markers and Ms were detected using F2 or BC1 families at maximum linkage disequilibrium and, for many markers, their efficacy to predict genotypes at Ms in onion populations at or near linkage equilibrium remains unknown. In this research, near isogenic lines homozygous-dominant and -recessive at Ms were developed and screened for 930 single nucleotide polymorphisms (SNPs). Three SNPs tightly linked on chromosome 2 remained in linkage disequilibrium with genotypes at Ms among randomly selected plants from three open-pollinated populations of onion as well as among a collection of inbred lines. These SNPs should be useful for selection of the recessive ms allele to aid in the development of maintainer lines for hybrid onion development.

Genetic evaluation of seedling heat tolerance in sorghum

Surface temperatures of tropical soils at planting time, where sorghum (Sorghum bicolor) is a traditional crop, can exceed 50 oC for hours. Seedling heat tolerance is critical for adequate crop establishment in the semi-arid tropics. Improvement of seedlings heat tolerant genotypes would reduce crop losses due to sufficient plant populations. The objectives of this study were to estimate seedling tolerance to heat, determine individual parental contribution and estimate additive, dominance and epistatic effects for seedling tolerance. In our experiments, seedling heat tolerance termed heat tolerance index (HTI) was defined as a ratio of resumed coleoptile growth after a controlled heat shock, compared to normal growth. Genetic parameters of HTI were determined by crossing four lines with varying HTI, with three tester lines, and deriving F1, F2, F3, BC1 and BC11 families forgeneration means analysis. Line IS20969 from Egypt showed the highest HTI of 0.71, while 290R, an experimental line from the University of Nebraska was the lowest at 0.51. Additive and dominance effects contributed to coleoptile elongation under normal conditions, but only additive effects were significant in recovery growth. Epistatic effects were present in both conditions. General combining ability (GCA) effects for HTI were highly significant in both conditions, but specific combining ability effects were negligible. These results indicate that it is possible to improve seedling heat tolerance and, thus, improve sorghum variety and hybrid plant populations in tropical areas where hot soil temperatures occur.

Development of a breeding strategy for interspecific hybrids between Caucasian clover and white clover

A single 8× F1 hybrid between Caucasian clover (T. ambiguum) and white clover (T. repens) was backcrossed to seven white clover plants to generate fertile 6× BC1 families. To determine the most useful combination of desired traits of both parental species, and to develop a breeding strategy for this population, subsequent generations were produced in various ways. Self‐pollination and intercrossing among BC1F1 families produced F2, and later, F3 and F4 generations. Second, third and fourth backcrosses to white clover produced BC2, BC3 and BC4 generations. All generations were compared with the parental species in a single experiment grown in sand. Expression of the desired below‐ground traits of Caucasian clover was best in the BC1 generation. By contrast, the best expression of the desired above‐ground white clover traits was shown by the BC2F1 generation. However, the BC2F1 was pentaploid and did not stably transmit its desirable features to subsequent generations. On the other hand, advancement in the BC1 from F1 to F4 revealed significant positive variation in most of the important traits and suggested that selection would be successful. A breeding strategy was developed based on the genetically stable BC1 generation, advancing to BC1F6 by selecting for increases in shoot dry matter (DM), stolon numbers, stolon growth, improved nodal rooting radius, larger numbers of flowers and high fertility and seed‐set.

Inheritance of Vernalization Requirement in Carrot

Carrot (Daucus carota L.) is a cool-season vegetable normally classified as a biennial or winter annual species, requiring vernalization to induce flowering. Nevertheless, some cultivars adapted to warmer climates require less vernalization and can be classified as early flowering or annual. The genetic control of this phenotypic difference has not been previously studied in carrot. The authors evaluated F1, F2, and BC1 progenies of an intercross between the early-flowering carrot (‘Criolla INTA’) and a late-flowering biennial carrot. F1 progenies were completely annual. Observed segregation ratios in the F2 and BC1 families were not significantly different from expected segregation ratios under the hypothesis of a single dominant gene conditioning the early-flowering habit. The authors conclude that annual habit is dominant over biennial and is a monogenic character.

Effects of quantitative trait loci on chromosomes 1, 2, 4, and 7 on growth, carcass, and meat quality traits in backcross Meishan × Large White pigs1

The aim of this work was to estimate whether genetic dissection of QTL on chromosomes 1, 2, 4, and 7, detected in an F2 Meishan x Large White population, can be achieved with a recombinant back-cross progeny test approach. For this purpose, a first generation of backcross (BC1) was produced by using frozen semen of F1 Large White x Meishan boars with Large White females. Four BC1 boars were selected because of their heterozygosity for at least 1 of the 4 regions. The BC1 boars were crossed with Large White sows, and the resulting BC2 offspring were measured for several growth and body composition traits. Contrary to the F2 animals, BC2 animals were also measured for meat quality traits in adductor, gluteus superficialis (GS), longissimus dorsi, and biceps femoris (BF) muscles. Each BC1 boar was tested for a total of 39 traits and for the 4 regions with statistical interval mapping analyses. The QTL effects obtained in BC1 families showed some differences compared with those described in F1 families. However, we confirmed QTL effects for growth in the SW1301-SW2512 markers interval on chromosome 1 and also for body composition in the SW1828-SW2512 markers interval on chromosome 1, in the SW2443-SWR783 markers interval on chromosome 2, and in the SW1369-SW632 markers interval on chromosome 7. In addition, we detected new QTL for growth traits on chromosome 2 and for meat quality traits on chromosomes 1 and 2. Growth of animals from weaning to the end of the test was influenced by the IGF2 gene region on chromosome 2. Concerning meat quality, ultimate pH of adductor, longissimus dorsi, and BF were affected by the interval delimited by UMNP3000 and SW2512 markers on chromosome 1, and a* of GS, L* of BF, and water-holding capacity of GS were affected by QTL located between marker loci SW2443 and SWR783 on chromosome 2. Recombinant progeny testing appeared to be a suitable strategy for the genetic dissection of the QTL investigated.

Genetic analysis of grain mold resistance in white seed sorghum genotypes

Grain molds in rainy season sorghums can cause poor grain quality resulting in economic losses. Grain molds are a major constraint to the sorghum production and for adoption of the improved cultivars. A complex of fungi causes grain mold. Information on genetics of grain mold resistance and mechanisms is required to facilitate the breeding of durable resistant cultivars. A genetic study was conducted using one white susceptible, three white resistant/tolerant sources, and one colored resistant source in the crossing programme to obtain four crosses. P1, P2, F1, BC1, and BC2, and F2 families of each cross were evaluated for the field grade and threshed grade scores, under sprinkler irrigation. Generation mean analyses and frequency distribution studies were carried out. The frequency distribution studies showed that grain mold resistance in the white-grained resistance sources was polygenic. The additive gene action and additive × additive gene interaction were significant in all the crosses. Simple recurrent selection or backcrossing should accumulate the genes for resistance. Epistasis gene interactions were observed in colored resistance × white resistance cross. Gene interaction was influenced by pronounced G × E. Pooled analysis showed that environment × additive gene interaction and environment × dominant gene interaction were significant. The complex genetics of mold resistance is due to the presence of different mechanisms of inheritance from various sources. Evaluation of segregating population for resistance and selection for stable derivatives in advanced generations in different environments will be effective.

Heritability of early blight resistance in a Lycopersicon esculentum×Lycopersicon hirsutum cross estimated by correlation between parent and progeny

AbstractSixteen‐hundred BC1 plants of a cross between an early blight (EB) susceptible tomato (Lycopersicon esculentum Mill.) breeding line (‘NC84173’ maternal and recurrent parent) and a resistant accession (‘PI126445’) of the tomato wild species Lycopersicon hirsutum Humb. and Bonpl. were grown in a field in 1998. This population was segregating (among other traits) for growth habit, self‐incompatibility and earliness in maturity. To eliminate confounding effects of these factors on disease evaluation and h2 estimation, plants that were self‐incompatible, indeterminate and/or late‐maturing were eliminated. The remaining plants (146), which were self‐compatible and determinate (sp./sp.) in growth habit, with early‐ to mid‐season maturity, were evaluated for EB resistance and self‐pollinated to produce BC1S1 seed. The 146 BC1S1 progeny families, consisting of 30 plants per family, were grown in a replicated field trial in 1999 and evaluated for EB resistance and plant maturity. For each of the 146 BC1 plants and corresponding BC1 families, the area under the disease progress curve (AUDPC) and final disease severity (final percentage defoliation) were determined and used to measure disease resistance. The distributions of the AUDPC and final percentage defoliation values in the BC1 and BC1S1 generations indicated that resistance from ‘PI126445’ was quantitative in nature. Estimates of h2 for EB resistance, computed by correlation between BC1S1 progeny family means and BC1 individual plant values, ranged from 0.69 to 0.70, indicating that EB resistance of ‘P1126445’ was heritable. Across BC1S1 families, a small, but significant, negative correlation (r = ‐0.26, P < 0.01) was observed between disease resistance and earliness in maturity. However, several BC1S1 families were identified with considerable EB resistance and reasonably early maturity. These families should be useful for the development of commercially acceptable EB‐resistant tomato lines.

Genetic analysis of grain mould resistance in coloured sorghum genotypes

Grain moulds are a major constraint to sorghum production and to adoption of improved cultivars in many tropical areas. Information on the inheritance of grain mould reaction is required to facilitate breeding of resistant cultivars. The genetic control of grain mould reaction was studied in 7 crosses of 2 resistant sorghum genotypes. P1, P2, F1, F2, BC1 and BC2 families of each cross were evaluated under sprinkler irrigation for field grade and threshed grade scores and subjected to generation mean analysis. Frequency distributions for grain mould reaction were derived and F2 and BC1 segregation ratios were calculated. Grain mould reaction in crosses of coloured grain sorghum was generally controlled by two or three major genes. Resistance to grain moulds was dominant. Significant additive gene effects were also found in all cross/season combinations. Significant dominance effects of similar magnitude to additive effects were also observed in five out of ten cross/season combinations. Gene interactions varied according to the parents with both resistant and susceptible parents contributing major genes. Choice of parents with complementary resistance genes and mechanisms of resistance will be critical to the success of resistance breeding.

Cytological and molecular characterization of BC1 progeny from two somatic hybrids between dihaploid Solanum acaule and tetraploid S. tuberosum

Two hexaploid somatic hybrids, DA12-3 (2n = 68) and DA28 (2n = 74), derived from cell fusion between dihaploid Solanum acaule and tetraploid Solanum tuberosum, were crossed with S. tuberosum. Two BC1 families, DA12T and DA28T, were generated, each containing 25 plants. Although the average of somatic chromosome numbers fitted hypothetical numbers assuming equal separation, considerable variation was found: 2n = 54-62 in DA12T and 2n = 58-65 in DA28T. Fifty-two of 80 RFLP single-copy probes generated 108 marker bands specific for S. acaule, by which BC1 progeny were analyzed. The segregation of marker bands from at least 44 probes was explained by two alleles per locus, which had to be heterozygous in dihaploid S. acaule and maintained in homozygous condition in homoeologous chromosomes of tetraploid S. acaule. On average, 73.7% of the chromosomes derived from S. acaule were detected as recombinants, suggesting that frequent homoeologous recombination occurred between S. acaule and S. tuberosum chromosomes. Therefore, S. acaule germplasm can be effectively transferred to the cultivated potato gene pool through backcrossing hexaploid somatic hybrids with S. tuberosum. In both BC1 families, S. acaule chromosomes 3, 4, 6, and 11 were detected as recombinants with higher frequencies (84-98%) than those of chromosomes 9 and 10 (60 and 58%, respectively), suggesting that chromosomal differentiation exists between S. acaule genomes.Key words: hexaploid somatic hybrids, Solanum acaule, homoeologous recombination, RFLP markers.

A gene controlling biosynthesis of isoorientin, a compound in corn silks antibiotic to the corn earworm

AbstractThe corn earworm (Helicoverpa zea Boddie) is an important pest of corn (Zea mays L.), and its larvae sometimes cause severe ear damage to hybrids grown in the southeastern United States. The antibiotic compound isoorientin is present in silks of some corn inbreds at a concentration that is harmful to corn earworm larvae. The inbred T218, which produces biologically active levels of this compound (>2.0% dry weight), was evaluated in hybrid combination with two other non‐isoorientin producing inbreds to determine the mode of isoorientin inheritance in corn silks. Silk masses from individual ears of each parent, the F1, F2, first backcrosses, F3 families and selfed BC1 families were evaluated in 1994 and 1995 for isoorientin concentration. Reversed‐phase high performance liquid chromatography (HPLC) was used to make chemical determinations. Segregation ratios in the F2, first backcross to T218, F3 families and selfed BC1 families were tested. The tests were conclusive in the identification of a single recessive gene controlling high isoorientin concentration in the silks of inbred T218. Some evidence for modifiers exists, in that there was a statistically nonsignificant trend for more plants than expected to occur in the low isoorientin concentration classes. Development of inbreds with a high concentration of the simply inherited isoorientin in their silks will add to the arsenal of compounds available in corn silks to combat damage to corn by corn earworm larvae.

Genetic Studies in Peach: Inheritance of Sweet Kernel and Male Sterility

Inheritance of the sweet kernel trait was studied in F1 and F2 families generated by crossing `Summer Beaut' nectarine (sweet kernel) with `Ellerbe' and `Biscoe' peach. F1 plants showed bitter kernel. Segregation in the F2 fit a 3 bitter : 1 sweet phenotypic ratio, suggesting that sweet kernel is controlled by a single recessive gene, for which the symbol sk is proposed. Sweet kernel (sk) was linked to nectarine (g) at a map distance of 12 cM. Seed bitterness phenotype is controlled by the genotype of the maternal tree and not the genotype of the individual embryo. Inheritance of male sterility derived from plant introduction (PI) 240928 and allelism of male sterile genes found in `Chinese Cling' and `White Glory' were investigated. Analysis of F1, F1 open-pollinated, and BC1 families derived from crossing PI 240928 with six different wild-type cultivars showed that male sterility in PI 240928 is controlled by cytoplasmic factors. Allelism studies showed that the male-sterile gene found in `White Glory' is not allelic to ps found in `Chinese Cling', and hence is designated ps2.

Responses of Zinnia angustifolia × Z. violacea Backcross Hybrids to Three Pathogens

True-breeding lines of Zinnia marylandica Spooner, Stimart & Boyle [allotetraploids of Z. angustifolia H.B.K. and Z. violacea Cav. (2n = 4x = 46)] were backcrossed with autotetraploid Z. angustifolia (2n = 4x = 44) and Z. violacea (2n = 4x = 48). Seed-generated, backcross (BC1) families were screened for resistance to alternaria blight (Alternaria zinniae Pape), bacterial leaf and flower spot [Xanthomonas campestris pv. zinniae (Hopkins and Dowson) Dye], and powdery mildew (Erysiphe cichoracearum DC. ex Merat). All BC1 families exhibited high levels of resistance to alternaria blight and powdery mildew. BC1 families derived from crossing Z. marylandica with autotetraploid Z. angustifolia were highly resistant to bacterial leaf and flower spot, whereas BC1 families derived from crossing Z. marylandica with autotetraploid Z. violacea were susceptible to this disease. Our results suggest that one Z. angustifolia genome in BC1 allotetraploids is sufficient to confer resistance to A. zinniae and E. cichoracearum, but at least two Z. angustifolia genomes are required in BC1 allotetraploids to provide resistance to X. campestris pv. zinniae.

Linkage Relationships of Isozymes and Morphological Traits in Interspecific Chestnut Crosses

Linkage relationships among eight isozyme genes (Acp-3, Est-1, Est-5, Prx-1, Prx-2, Prx-3, Me, and Adh) and two morphological markers (Inh and Twh) were investigated in one F2 and two BC1 families of interspecific crosses between the American chestnut (Castanea dentata Borkh.) and the Chinese chestnut (C. mollissima Blume). Inh was consistently linked with Prx-1 and Est-5 in all families. In addition, four other gene pairs, Acp-3–Inh, Acp-3–Prx-1, Me–Inh, and Twh–Inh, were linked in one of the three families investigated. The two isozyme genes and two morphological marker genes were tentatively integrated into one linkage group with the gene order Twh–Inh–Prx-1–Est-5.

232 LINKAGE RELATIONSHIPS OF ISOZYME AND MORPHOLOGICAL MARKERS IN INTERSPECIFIC CROSSES OF CHESTNUT

Linkage relations among eight isozyme genes, Acp-3, Est-1, Est-5, Prx-1, Prx-2, Prx-3, Me and Adh, and two morphological markers, Inh, and Twh, were investigated in one F2 and two BC1 families of interspecific crosses between the American chestnut (Castanea dentata) and the Chinese chestnut (C. mollissima). Inh was found to be consistently linked with Prx-1 and Est-5 in all families. The order of these three genes was determined to be-Ihn--Prx1--Est5. In addition, four other gene pairs, Acp3--Inh, Acp3--Prx1, Me--Inh and Twh--Inh were found to be linked in one of the three families investigated. The four isozyme genes and two morphological marker genes were tentatively integrated into one linkage group with the following gene order Acp3--Me--Twh--Inh--Prx1-Est5. This study demonstrated that isozyme genes can be integrated with morphological marker genes into a single linkage map without the need for additional crosses.

Genetic Control of Reduced Palmitate Content in Soybean

Reduced palmitate content in soybean [Glycine max (L.) Merr.] is controlled by two major alleles, fap1 and fap3. This study was conducted to determine the role that modifying genes may have on the reduced palmitate trait in lines homozygous for the major alleles. Two lines homozygous for fap1 and fap3, AX5152‐34 and AX5152‐105, were each crossed as donor parents to a high‐yielding cultivar with normal palmitate, ‘Kenwood’ or ‘Marcus’. The F1 plants were backcrossed to the high‐yielding cultivar. Two experiments were grown in randomized complete‐block designs at three locations in Iowa with two replications per location. The Kenwood experiment consisted of 14 F4:6 lines from the cross AX5152‐105 × Kenwood, 83 BC1F3:5 lines from five BC1 families from the backcross Kenwood × (AX5152‐105 × Kenwood), Kenwood and AX5152‐105. The Marcus experiment contained 14 F4:6 lines from the cross AX5152‐34 × Marcus, 69 BC1F3:5 lines from the backcross Marcus × (AX5152‐34 × Marcus) among six backcross families, Marcus and AX5152‐34. All lines, except Kenwood and Marcus, were homozygous for both fap1 and fap3. The influence of modifying genes on palmitate content was evident in both experiments. There were significant differences in palmitate content among single‐cross lines, among backcross lines, and among lines within individual backcross families. The palmitate content of each donor parent was significantly different from the means of the corresponding single‐cross and backcross populations. Lines with reduced palmitate were obtained that had higher yield than the recurrent parents, which indicated that reduced palmitate is not detrimental to yield. Lines with reduced palmitate generally had reduced oil content.