BC1 Generations Research Articles

Anther culture and chromosome reduction in wheat × Thinopyrum wide crosses

There is little information on the anther culture response and cytogenetic nature of pollen-derived calli and regenerants of wide crosses in wheat. The anther culture response of three Thinopyrum species (wheatgrasses), their hybrids and backcrosses with wheat, and the chromosome composition of calli and regenerants were studied to determine the feasibility of efficient chromosome elimination and production of alien addition lines from wide crosses by anther culture in comparison with conventional backcrossing and selfing. Wide hybrids between wheat and wheatgrasses had up to a 2% callus induction response. Pollen sterility of wide hybrids and recalcitrance of wheatgrass parents may largely be responsible for their low response, in addition to culture stage, media and environmental factors. Anther culturability improved in backcrosses as the proportion of wheatgrass chromosomes decreased and fertility increased. Overall the wheat × Th. trichophorum cross had the best response; that is up to 37% callus induction from some BC2 plants. A large variation was found in chromosome numbers in callus roots examined in the BC2 generation. Mixoploidy was frequent. Data indicated that the chromosome numbers in the pollen-derived organs/plants can be reduced to produce alien addition lines.

Inheritance of Arabidopsis DNA in offspring from Brassica napus and A. thaliana somatic hybrids

Chromosome counts and RFLP markers mapped to Arabidopsis thaliana were used to determine the proportion of eliminated chromosomes and retained A. thaliana DNA in the back-crossed (BC) progeny derived from symmetric and asymmetric somatic hybrids between Brassica napus and A. thaliana. All plants were analysed for the presence of two RFLP markers per chromosome, preferably with one located on each chromosome arm. A reduction in both A. thaliana RFLP markers and chromosome numbers was found in the BC1 and BC2 generations of the symmetric hybrids as well as in the BC1 generation of the asymmetric hybrids. In the symmetric hybrids, two back-crosses to B. napus were required to reduce the frequency of retained A. thaliana loci to 42.4% and mean chromosome number to 39.4. In comparison, the BC1 progeny of the asymmetric hybrids had 16% of the analysed A. thaliana loci present and an average of 38.4 chromosomes maintained. When the frequency of A. thaliana chromosomes with both analysed loci maintained was compared with the frequency of chromosomes with one locus lost and one kept, a reduction in the number of complete chromosomes between BC1 and BC2 derived from the symmetric hybrids was observed. Among the BC1 plants in the asymmetric group the situation was different, with higher amounts of incomplete donor chromosomes compared to whole chromosomes. The results indicate that A. thaliana chromosome fragments are more often found in the progeny of irradiated hybrids, while back-crossed symmetric hybrids have more complete chromosomes.

Expression and mode of inheritance of resistance in coffee to leaf miner Perileucoptera coffeella

The coffee leaf miner (Perileucoptera coffeella, Guerin-Meneville), is the most serious pest on coffee in Brazil. Coffee varieties with resistance to this pest have been developed in Brazil using genes from the Coffea racemosa Lour. species. Resistance to P. coffeella was studied in C. racemosa and in advanced breeding populations derived from BC3 and BC4 generations through artificial infestation of coffee leaves (laboratory) and natural infestation (greenhouse). Based on an analysis of variance carried out with notes attributed to lesions, two weeks after egg hatching, C. racemosa plants and BC4 progenies were classified in 3 groups according to their degree of resistance to the insect. The age of the leaves seemed to play a role in the expression of resistance of C. racemosa and of BC4 progenies to P. coffeella. The data seemed to show that resistance to P. coffeella derived from C. racemosa can be best explained by two complementary dominant genes. Segregations for resistance assessed in families obtained by selfing or open pollination or by back-crossing with a susceptible variety were similar to the expected segregation of 9 resistant: 7 susceptible or 1 resistant: 3 susceptible, respectively. The symbols Lm1, lm1, Lm2 and lm2 are proposed for the alleles. Consequences for breeding coffee varieties with resistance are discussed.

INHERITANCE OF METRIC TRIATS IN PEARL MILLET

The P1 P2 F2 F2 BC1 and BC2 generations of five pearl millet crosses were studied for seven metric traits. The additive dominance model was adequate for three crosses for number of tillers and earhcad length, and two crosses in each of leaf length and leaf breadth. An epistatic digenic model was assumed for other crosses. Heterosis breeding is suggested for improvement of all traits. Duplicate epistasis played a relatively greater role than complementary epistasis. Among the interactions, dominance X dominance played a major role. Therefore, reciprocal recurrent selection is suggested for development of a superior variety.

Morphological characteristics of hybrids between white clover, Trifolium repens L., and Caucasian clover, Trifolium ambiguum M. Bieb

AbstractHybrids between the stoloniferous Trifolium repens (2n = 4x = 32) and the rhizomatous Trifolium ambiguum (2n = 4x = 32) have been produced using T. repens as the recurrent parent. Morphological characteristics of the parent species and the F1, BC1, BC2 and BC3 hybrids were examined in a glasshouse experiment. Leaflet ratios and general plant shape indicated that the BC3 hybrid was similar to T. repens. Separation of plants into components of above‐ and below‐ground growth showed that T. repens had a greater total plant dry weight than T. ambiguum but 24% of its dry weight was in roots compared with 22% in roots and 44% in rhizomes in T. ambiguum. The BC3 generation contained plants that were predominantly T. repens‐like with stolons, but also had a small proportion of their total dry weight (3%) as rhizome, confirming the potential to produce plants that combine stoloniferous and rhizomatous growth. The proportions of rhizome and stolon varied within the BC3 generation, enabling farther selection and plant breeding. The BC3 hybrids produced similar numbers of inflorescences to T. repens however, fertility was lower, although there was some variation within the BC3 for both characters.

Inheritance of resistance to alfalfa mosaic virus in Lycopersicon hirsutum f. glabratum PI 134417

Alfalfa mosaic virus (AMV) is of sporadic occurrence in tomato crops. More recently, important epidemics caused by AMV have been reported in fieldgrown canning tomatoes in southern Italy. Since no lines or commercial varieties resistant to AMV are available, we have studied the inheritance of resistance to this virus, derived from the accession of Lycopersicon hirsutum f. glabratum PI 134417. A necrogenic tomato isolate of AMV was used as source of inoculum. Visual observations of symptoms and enzyme-linked immunosorbent assay (ELISA) were used to screen plants after mechanical inoculation. All the F1 tested were resistant while the segregation ratios of F2 and BC1 generations were consistent with a simple dominant inheritance. We propose tentatively to name this gene Am.

Inheritance of Tannin in Relation to Shootfly Resistance in Sorghum

The inheritance of tannin content as source of shootfly resistance in sorghum was studied deploying parental, F1, RF1, F2, BC1, and BC2 generations of six crosses involving two resistant parent, IS 4664 and IS 5566 and two susceptible SPV 422 and SPV 475. Estimation of tannin content was based on ‘Catechin equivalent’. The result indicated digenic interaction along with additive dominance model to explain the inheritance of tannin content. Greater dominance effects and duplicate type of epistasis showed the heterosis as main method to exploit tannin content in sorghum for shootfly resistance.

Inheritance of ffs, a Gene Conditioning Fasciated Flower Stem in Red Beet

A fasciated flower stem character arose spontaneously during development of the red beet (Beta vulgaris L.) inbred line W411. The fasciated character is manifest by a flattened flower stem with petioles coalesced into a twisted, ribbonlike appearance. No fasciation is present in the vegetative stem or petioles. An inheritance study was conducted to determine the genetic control of flower stem fasciation. The inbred line W411 was used both as a male and female parent in crosses with four red beet inbred lines. Segregating progenies in both the BC1 and F2 generations were developed and scored for the fasciated flower stem character. Variable expression of the fasciated flower stem phenotype was observed in these progenies; however, the presence of flattened flower stems at the stem-hypocotyl junction was unequivocal. Chisquare goodness-of-fit tests in the BC1 and F2 generations did not deviate significantly from expected ratios for a monogenic recessive character for each genetic background. No reciprocal differences were detected for any cross in this group of four inbred lines, which suggests the lack of maternal effect for the fasciated character. The symbol ffs is proposed to describe the genetic control of the fasciated flower stem phenotype.

The ffs Allele Conditions Flower Stem Fasciation in Red Beet

A fasciated flower stem character arose spontaneously during development of the red beet (Beta vulgaris L.) inbred line W411. The fasciated character is manifest by a flattened flower stem with petioles coalesced into a twisted, ribbonlike appearance. No fasciation is present in the vegetative stem or petioles. An inheritance study was conducted to determine the genetic control of flower stem fasciation. The inbred line W411 was used both as a male and female parent in crosses with four red beet inbred lines. Segregating progenies in both the BC1 and F2 generations were developed and scored for the fasciated flower stem character. Variable expression of the fasciated flower stem phenotype was observed in these progenies; however, the presence of flattened flower stems at the stem/hypocotyl junction was unequivocal. Chi-square goodness-of-fit tests in both the BC1 and F2 generations did not deviate significantly from expected ratios for a monogenic recessive character for each genetic background. No reciprocal differences were detected for any cross in this group of four inbred lines, which suggests the lack of maternal effect for the fasciated character. The symbol ffs is proposed to describe the genetic control of the fasciated flower stem phenotype.

Genetic Analysis of Resistance to Low Temperatures in Wheat

Tolerance to low temperature is one of the most important component of winter hardiness in winter wheat. In the process of creation new winter wheat varieties resistant to low temperatures it is important to know the genetics of this character. Seven wheat varieties Noa, Amigo, Sava, Rana Niska, Saitama, UPI 301, NS-5260 and their F1, F2, BC1, and BC2 generations were used to determine the genetic base of low temperature resistance. The mode of inheritance observed in most of the F1 hybrids was partial dominance, although some cases where the parent with the higher or lower mean value dominated were also recorded. In the F2 generation, intermediacy was also the predominant mode of inheritance. In Noa x Sava and Noa x Rana Niska both additive and dominant gene effects were found. Frost tolerance was under control of additive genes in cross Noa x Amigo, Saitama x Sava and NS-5260 x UPI 301 and dominant genes in Saitama x Rana Niska. In the process of winter wheat breeding the most importance have combinations in which additive gene effects predominate, since those crosses have greater chance to fix the favorable genes responsible for cold tolerance. The selection of progeny tolerant to low temperatures from those combinations should be more successfully.

Hybrid vigour and gene action for two quantitative traits of castor plant (<i>Ricinus communis</i> L.)

Five homozygous lines of castor plant, namely RS1-Om, RN1-Om, RTl-2m, RSl- Owm, and RNl- Omb, were crossed to raise F1, F2, BC1, and BC2 generations. The hybrids were tested for hybrid vigour for two metric traits, viz; number of pods per plant and seed yield per plant. Highly significant hybrid vigour was detected for both traits in all the crosses. The contributions of additive and non-additive gene effects to both traits were also investigated. Additive gene effects were more relevant in the inheritance of number of pods per plant. The effects of the digenic interactions, coupled with the highly significant additive gene effects on number of pods per plant in most of the crosses, tend to suggest that improvement of the character can be enhanced by the use of breeding methods such as the reciprocal recurrent selection that would encourage the concentration of genes involved in the inheritance of the trait. The contributions of dominance gene effects to the total genetic variance were positive and highly significant in both pod and seed yield per plant in all the hybrids. This would tend to favour hybridization and the use of first generation hybrids as an effective means of improving both traits in castor plant. Thus, reciprocal recurrent selection and the use of hybrid programmes are the breeding strategies suggested for the improvement of pod and seed yield in castor plant. Ghana Jnl agric. Sci. Vol.31(1) 1998: 81-86

The impact on biosafety of the phosphinothricin-tolerance transgene in inter-specific B. rapa×B. napus hybrids and their successive backcrosses

There is strong evidence indicating that gene flow from transgenic B. napus into weedy wild relatives is inevitable following commercial release. Research should now focus on the transmission, stability, and impact of transgene expression after the initial hybridization event. The present study investigated the transfer of a phosphinothricin-tolerance transgene by inter-specific hybridization between B. rapa and two transgenic B. napus lines. The expression of the transgene was monitored in the F1 hybrids and in subsequent backcross generations. The transgene was transmitted relatively easily into the F1 hybrids and retained activity. Large differences in the transmission frequency of the transgene were noted between offspring of the two transgenic lines during backcrossing. The most plausible explanation of these results is that the line showing least transmission during backcrossing contains a transgene integrated into a C-genome chromosome. Approximately 10% of offspring retained the tolerant trait in the BC3 and BC4 generations. The implications of these findings for the stable introgression of transgenes carried on one of the chromosomes of the C-genome from B. napus and into B. rapa are briefly discussed.

Homoeologous pairing and recombination in backcross derivatives of tomato somatic hybrids [Lycopersicon esculentum (+) L. peruvianum

Genomic in situ hybridization (GISH) was used to examine genome interactions in two allohexa ploid (2n=6x=72) Lycopersicon esculentum (+) L. peruvianum somatic hybrids and their seed progenies originated from subsequent backcrosses to L. esculentum. The ability of GISH to distinguish between chromatin derived from two closely related species, L. esculentum and L. peruvianum (both 2n=2x=24), allowed the precise chromosomal constitution of somatic hybrids and their backcross progenies to be unequivocally established. This enabled the interaction of species genomes to be observed at meiosis, providing clear evidence of strictly regular homoeologous pairing and the high degree of homoeologous recombination in allodiploid plants (2n=2x=24) of the BC1 generation. In segmental allodiploids of the BC2 and BC3 generations, the recombinant chromosomes continued to pair with a homoeologous partner (in the absence of a homologous one), and therefore could be stably incorporated into gametes. Chiasmata were found almost exclusively in more distal, rather subterminal, chromosome segments. A considerable proportion of meiotic recombination was detected in subterminal heterochromatic regions, often involving distal euchromatin, located in close proximity. GISH also supplied information on the extent of the overall sequence homology between the genomes of L. esculentum and L. peruvianum, indicating that despite their different breeding systems, these species may not be differentiated to a high degree genetically. The present study has demonstrated that somatic hybridization between two such closely related, but sexually incompatible or difficult to cross species, provides a way of transferring genes, via homoloeogous crossing-over and recombination, across the incompatibility barriers. Indeed, such hybrids may offer the preferred route for gene transfer, which subsequently results in more stable gene introgression than other methods.

Assignment of DNA markers to Nicotiana sylvestris chromosomes using monosomic alien addition lines

A sesquidiploid hybrid (PPS, 2n=32) between Nicotiana plumbaginifolia (PP, 2n=20) and N. sylvestris (SS, 2n=24) was backcrossed to N. plumbaginifolia to produce monosomic alien addition lines. A total of 89 2n=21 plants, each containing two sets of N. plumbaginifolia chromosomes and a single N. sylvestris chromosome, were obtained in the BC1 and BC2 generations. These plants were classified into 12 groups based on morphological characteristics. The N. sylvestris chromosomes in these plants were identified by RFLP and karyotype analyses. Among the 84 probes tested, 20 could not detect N. sylvestris-specific DNA bands, and the remaining 64 were assigned to 9 normal and 6 aberrant synteny groups. The 9 normal synteny groups corresponded to chromosomes 2, 4, 5, 6, 7, 8, 9, 10 and 12, respectively. Four aberrant synteny groups were the result of chromosome translocations, and 2 were deletions.

Identification of male sterility maintainer lines for Ogura radish (Raphanus sativus L.)

‘Ogura radish’, a cytoplasmic genetic male sterile line, was crossed with four local and three Japanese cultivars to identify maintainer lines. Out of seven F1 families, one cross involving a local cultivar, Aushi, produced 100% male sterile (MS) progeny. The crosses involving the other two local cultivars, Tangail Local and Kuni, produced about 90% MS progeny, indicating the presence of maintainer gene(s) for male sterility. The fourth local cultivar, Tasaki, produced 100% male fertile (MF) progeny. All three exotic cultivars appeared to possess the chromosomal gene(s) for controlling the male sterility. In BC1, BC2 and BC3 generations, segregation of MS plants were more frequent when ‘Aushi’ was used as recurrent parent. The expression of male sterility was not affected by seasonal influences. Thus the local cultivar ‘Aushi’ may be used as maintainer line for ‘Ogura radish’. To produce hybrid seed, ‘Tasaki’ can be used as pollinator line as it exhibit high heterosis with ‘Aushi’.

Resistance to cowpea severe mosaic virus, determined by three dosage dependent genes in Vigna unguiculata L. Walp

P1, P2, F1, F2, BC1 and BC2 generations of four resistant × susceptible crosses and three resistant × resistant crosses of cowpea (Vigna unguiculata L. Walp) were screened for resistance to cowpea severe mosaic virus (CPSMV), in an insect protected screen house. The segregation ratio, at maturity, showed a 63 susceptible : 1 resistant ratio in the F2 generation indicating that resistance is governed by three major genes. The backcross tests and the F3 test confirmed this. The intermediate levels of symptoms observed in the F1 generation and the progression of symptom expression in the F2 generation suggested that resistance is gene dosage dependent. The genetic basis of immunity, tolerance, high level of resistance is discussed based on the three gene model. Delayed symptom expression was evident in genotypes with one or two susceptible alleles. The implications of delayed expression of symptoms in selection and breeding for resistance to CPSMV are discussed.

Genetic analysis of the cause of gastroschisis in the HLG mouse strain

An inbred mouse strain HLG shows a high incidence of gastroschisis after X-ray exposure to the zygotes. About 11 % of the fetuses display this malformation after irradiation with 1 Gy. The C57BL-strain does not show the increased frequency of gastroschisis after radiation-exposure to the zygotes. The genetic background of this malformation was investigated in a backcross of HLG × C57BL females to HLG males. The pregnant HLG × C57BL females were irradiated in a stage in which the (HLG × C57BL) × HLG [BC1] embryos were in the 1-cell stage. The frequency of gastroschisis in the BC1 generation was compared with a genetic model of a single recessive mutation with 11% penetrance. This frequency does not fit a single-locus inheritance. The number of loci involved was estimated to be about two or three. HLG mouse strain may be a valuable animal model in the study of polygenic traits.

Impact of low linolenic acid content on seed yield of winter oilseed rape (Brassica napus L.)

AbstractAn essential quality improvement of rapeseed oil can be obtained by reduction of its linolenic acid (C18:3) content from about 10% to less than 3% of the total fatty acids. Genotypes low in C18:3 have been developed by mutagenesis. The initial summer rapeseed mutant had been low yielding and highly susceptible to various diseases. It has been debated whether the low C18:3 character can be successfully combined with high seed yield for physiological reasons. Therefore, the low linolenic character of mutant M48 was transferred into high‐yielding genotypes by repeated backcrossing to well‐adapted low erucic acid, low glucosinolate (00‐) winter rapeseed cultivars. Lines with low C18:3 content were selected from BC3 and BC4 generations and examined in 1990–95. Positive selection response for seed yield was shown to continue over the years. Presently, the best lines are yielding as well as the control cultivars being equivalent also in oil and glucosinolate contents.In order to test the effect of a low C18:3 content on seed yield, plants with low and with high C18:3 content, respectively, were selected from 16 segregating BC5‐F2 populations and bulked to form 32 F3 populations. These ‘isogenic’ bulk populations were tested for field performance at four locations in 1995. The results show that C18:3 content of the seed oil is not associated with seed yield, oil content, beginning of flowering, plant height and disease resistance. Means of relative seed yield for the high and the low linolenic F3 bulk populations were not significantly different with 88.0% and 86.9% of the control cultivars, respectively. There was a significant interaction between genotypes with high or low C18:3 content and location. This shows that under specific environmental conditions a low C18:3 content may be either favourable or unfavourable. The results indicate that the low C18:3 character of the original mutants per se does not cause a decrease in seed yield, oil content or general field performance.

Introduction of a gene from fertility restored radish (Raphanus sativus) into Brassica napus by fusion of X-irradiated protoplasts from a radish restorer line and iodacetoamide-treated protoplasts from a cytoplasmic male-sterile cybrid of B. napus

To establish a cytoplasmic male-sterile/restored fertility (cms-Rf) system for F1 seed production in Brassica napus, we transferred a gene from fertillity restored radish to B. napus by protoplast fusion. X-irradiated protoplasts, isolated from shoots of Raphanus sativus cv Kosena (Rf line), were fused with iodoacetamide-treated protoplasts of a B. napus cms cybrid. Among 300 regenerated plants, six were male-fertile. The fertile plants were characterized for petal color, chromosome number and the percentage of viable pollen grains. Three fertile plants had aneuploid chromosome numbers and white or cream petals, which is a dominant marker in radish. Of these three plants, one which had 2n = 47 chromosomes and white petals was used for further backcrosses. After two backcrosses, chromosome number and petal color became identical to that of B. napus. No female sterility was observed in the BC3 generations.

Inheritance and Biochemical Analysis of a Carrot Carotenoid Mutant

Pigments in orange carrot tissue, such alpha and beta carotene, are important vitamins in the human diet. Previously identified white or nonpigmented carrot roots, such as those from wild carrot and white derivatives of yellow or orange types, are dominant to the production of pigment, which is recessive. A nonpigmented carrot root was discovered during routine propagation of the inbred line W266 in 1992. Subsequent segregation analysis in the F2 and BC1 generations in three genetic backgrounds demonstrated the lack of pigmentation is due to a single recessive gene (reduced-pigment: rp). Total carotenoid content was reduced 92% in the roots of rprp genotypes compared to RPRP genotypes, however there were no differences in carotenoid content in leaves. Plants carrying rprp also exhibit white-speckled leaves during early stages of development, suggesting rp has an effect on leaf chlorophyll content. This character may prove useful in dissecting the complex inheritance of carotenoids in carrot.