Evaluation of the Effect of Pollen Irradiation on Karyotype Variability in M1 Cotton Plants

The effect of pollen irradiation at dose rates of 10, 15, 20, and 25 Gy on variability in cotton plants Gossypium hirsutum L. was studied. The modified plants showed a reduced fertility, mainly caused by chromosomal rearrangements and genomic mutations during meiosis. The genomic mutations involved primary and tertiary monosomics, monotelodisomics, and a haploid plant. The decrease in meiotic index and pollen fertility in the cotton aneuploids was related not only to aberrations in chromosome pairing but also to genetic features of the original plants. It was found that heterozygosity for interchromosomal exchanges found in M₁ plants resulted in the formation of multivalent associations of chromosomes of various forms and types of segregation from translocation complexes. Another result was high variability in pollen fertility. An increase in irradiation dose rate caused an increase in the number of translocants with a high frequency of quadrivalents. The results suggest that the great diversity of forms observed in M₁ after pollination with irradiated pollen is determined, first, by elimination of some chromosomes or their arms or the whole paternal genotype and second, by interchromosomal rearrangements. The high variability in pollen fertility of translocants hampers using this trait as a marker of heterozygosity for exchanges in cotton.     

Evaluation of the effect of pollen irradiation on karyotype variability in M2 cotton plants

The karyotypes of biomorphologically abnormal cotton (Gossypium hirsutum L.) plants obtained in M2 after pollination with pollen irradiated at dose rates 10, 15, 20, and 25 Gy were studied. Various genomic and chromosomal mutations were detected in 57 M2 families. The primary monosomics isolated in M2 were found to be cytologically more stable and more viable, since they had higher meiotic indices, pollen fertility, and seed formation. In M2, a decrease in the number of plants with multiple karyotype aberrations and interchromosomal exchanges with high frequency of multivalent formation was observed. The multivalents had diverse patterns and types of chromosome segregation and translocation complexes. Their pollen fertility was higher than in translocants found in M1. Desynapsis often occurred in M2, including plants with chromosome deficiency or rearrangements. The variation in the number of univalents in various cells was found to result from different expression of synaptic genes. The results indicate stabilization of karyotypes, increase in cytologic stability and viability, and the absence of sterility in aberrant plants.     

Estimation of the efficiency of seed irradiation by thermal neutrons for inducing chromosomal aberrations in M1 of cotton Gossypium hirsutum l

Exposure of cotton seeds to thermal neutrons at doses of 15, 25, and 35 Gy was shown to induce many altered plants, including sterile and chimeric ones. Most of these phenotypic changes were shown to result from novel genomic, chromosomal, and desynaptic mutations. The presence of these mutations in the karyotype of M, plants often decreased meiotic index and pollen fertility. In translocation forms, the decrease in pollen fertility was caused by the prevalence of quadrivalents in form of rings and chains with adjacent segregation of chromosomes from the translocation complexes. Based on the shapes and sizes of multivalent associations, we performed preliminary localization of translocation breakpoints. A specific feature of the effect of thermal neuron irradiation in M1 was induction of numerous unique chromosomal aberrations, consisting in the appearance in the same plant of several types of mutations (genomic and chromosomal), exchange complexes in the same nucleus, and multiple exchanges involving three nonhomologous chromosomes.     

Evaluation of the Effect of Pollen Irradiation on Karyotype Variability in M2 Cotton Plants

The karyotypes of biomorphologically abnormal cotton (Gossypium hirsutum L.) plants obtained in M₂ after pollination with pollen irradiated at dose rates 10, 15, 20, and 25 Gy were studied. Various genomic and chromosomal mutations were detected in 57 M₂ families. The primary monosomics isolated in M₂ were found to be cytologically more stable and more viable, since they had higher meiotic index, pollen fertility, and seed formation. In M₂, a decrease in the number of plants with multiple karyotype aberrations and interchromosomal exchanges with high frequency of multivalent formation was observed. The multivalent configurations had different types and chromosome disjunctions. Their pollen fertility was higher than in translocants found in M₁. Desynapsis often occurred in M₂, including plants with chromosome deficiency or rearrangements. The variation in the number of univalents in various cells was found to result from different expression of synaptic genes. The results indicate stabilization of karyotypes, increase in cytologic stability and viability, and the absence of sterility in aberrant plants.     

Estimation of the efficiency of seed irradiation by thermal neutrons for inducing chromosomal aberrations in M<Subscript>1</Subscript> of cotton <Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.

Exposure of cotton seeds to thermal neutrons at doses of 15, 25, and 35 Gy was shown to induce many biomorphologically abnormal plants, including sterile and chimeric ones. Most of these phenotypic changes were shown to result from novel genomic, chromosomal, and desynaptic mutations. The presence of these mutations in the karyotype of M₁ plants often decreased meiotic index and pollen fertility. In translocation forms, the decrease in pollen fertility was caused by the prevalence of quadrivalents in form of rings and chains with adjacent segregation of chromosomes from the translocation complexes. Based on the shapes and sizes of multivalent associations, we performed preliminary localization of translocation breakpoints. A specific feature of the effect of thermal neuron irradiation in M₁ was induction of numerous unique chromosomal aberrations, consisting in the appearance in the same plant of several types of mutations (genomic and chromosomal), interchange complexes in the same nucleus, and multiple interchanges involving three nonhomologous chromosomes.     

Cytogenetic effects of the treatment of cotton seeds with thermal neutrons

After irradiation of cotton seeds with thermal neutrons, primary and tertiary monosomics and also plants with translocations were detected among offsprings M1 and M2. The interchanges had high frequency of multivalents at metaphase-I of meiosis. Genetic analysis of chlorophyll deficient cotton mutants has shown that recessive monogenic factors caused new mutations of the types xantha and viridis. New viridis mutant was also characterized by the heterozygosity of interchromosome translocations.     

Estimation of efficiency of seed irradiation by thermal neutrons for inducing chromosomal aberration in M2 of cotton Gossipium hirsutum L

Cytogenetic analysis of M2 plants after irradiation of cotton by thermal neutrons was performed in 56 families. In 40 plants of 27 M2 families, different abnormalities of chromosome pairing were found. These abnormalities were caused by primary monosomy, chromosomal interchange, and desynapsis. The presence of chromosome aberrations in some cases decreased meiotic index and pollen fertility. Comparison of the results of cytogenetics analysis, performed in M1 and M2 after irradiation, showed a nearly two-fold decrease in the number of plants with chromosomal aberrations in M2, as well as narrowing of the spectrum of these aberrations. The latter result is explained by the fact that some mutations are impossible to detect in subsequent generations because of complete or partial sterility of aberrant M1 plants. It was established that the most efficient radiation doses for inducing chromosomal aberrations in the present study were 15 and 25 Gy, since they affected survival and fertility of altered plant to a lesser extent.     

Molecular cytogenetic study on the plants of Elymus nutans with varying fertility on the Qinghai-Tibet Plateau

A molecular cytogenetic investigation was conducted on plants of the allohexaploid species Elymus nutans with varying fertility on the Qinghai-Tibet Plateau. Molecular karyotyping revealed that chromosome variants were distributed unevenly among genomes and among different homologue chromosomes in each genome. The plants with varying fertility exhibited significantly higher numbers of chromosome variants than did the normal fertility samples, although both kinds of plants showed the same pattern of high-to-low polymorphism from the Y to St and H genomes. Heterozygosis and karyotype heterozygosity in the plants with varying fertility were 3- and 13-fold higher than those in normal samples, respectively. Significant negative correlations were found not only between seed setting rates and total genome heterozygosity but also between seed setting rates and heterozygosity of each genome in the plants of varying fertility. Chromosome pairing analysis was performed using genomic in situ hybridization in selected plants of different fertility levels. The pairing of chromosomes at meiotic metaphase I was mostly bivalent, although univalent, trivalent, quadrivalent, and other polyvalents also occurred; in addition, chromosome configuration forms and frequencies varied among the studied samples. ANOVA results showed that the average number of ring bivalents in the Y genome was significantly higher than those in the St and H genomes. Significant positive correlations between pollen grain fertility and ring bivalent number were found in the St and H genomes but not in the Y genome. Furthermore, chromosome configuration parameters (total bivalents, numbers of ring and rod bivalents) were found to be significantly correlated with heterozygosity and seed setting rates in the St and H genomes, respectively, but not in the Y genome. It was inferred that the seed setting rate and pollen grain fertility in E. nutans are strongly influenced by the heterozygosity of each genome, but the Y genome differs from the St and H genomes due to chromosome pair alterations. The St and H genomes may contain more chromosome structural variations than the Y genome in E. nutans.     

The Development and a Cytogenetic Study of Monosomics of Gossypium Hirsutum L.

Monosomics of cotton (Gossypium hirsutum L.) were obtained by irradiation of pollen by -rays and by irradiation of seeds by thermal neutrons. Many monosomics were derived directly from irradiation, but a number of monosomics were also recovered in the progeny of plants with translocations and of desynaptic plants. Only 28 primary monosomics showed normal pairing at metaphase-1 of meiosis. The others formec rare trivalents or additional univalents. Partial desynapsis was detected in some monosomics. The pollen fertility levels of monosomics are presented. New morphological characters were detected among the monosome plants of cotton.     

Possible mechanisms for occurrence of chromosomal restructuring. Interchromosomal contacts during metaphase and their role in chromosomal restructuring

The influence of colchicine-hypotonic treatment on interchromosomal aberrations at metaphase was studied in bone marrow cells of BALB mice irradiated by X-rays within the dose range from 0.25 to 1.50 Gy. In was found that after 30 min treatment with 0.002% colchicine of cells dividing 10 h following irradiation, the frequency both of chromosomal exchanges and interchromosomal contacts decrease about 3.5 times, the amount of chromosomal breaks increasing. It is calculated from the data of this experiment that two breaks induced by irradiation, which were scored at the same K metaphase as independent ones, appeared to be associated with each other at high frequency through exchange in the absence of colchicine or hypotonic treatment. It is assumed that regions of interchromosomal contacts at native metaphase are the most radiation-sensitive zones of the genome preferentially involved in chromosomal aberrations of X-irradiated cells.     

Heterochromatic regions in corn A- and B- chromosomes

A collection of maize forms from the Vavilov All-Russia Institute of Plant Breeding (VIR) was studied. We compared variation in the following traits: the number and size of heterochromatic knob regions (HKRs) of chromosomes of mother pollen cells at pachytene depending on the presence (B+) or absence (B0) of B-chromosomes; size of B-chromosomes (general, of heterochromatic part); and the frequency of B+ plants in groups of forms contrasting in the HKR number (10-15, 4-10, 2-7, 1-4). It was shown that B chromosomes had statistically significantly different effects on HKR polymorphism, relative heterochromatin "content" in the cell of multiknob and knobless forms, selection of plants for early ear flowering in these forms. The combination of maximum (and minimum) sizes of heterochromatic regions of A- and B-chromosomes was established; the genomic level of control of the trait is suggested. The role of the relationship of polymorphism at heterochromatic regions of A- and B-chromosomes between them and with the systems of maize reproduction is considered as a mechanism of maintenance of optimum plant heterozygosity via adaptive ontogenetic redistribution of heterochromatin among loci, chromosomes, and gametes.     

Estimation of efficiency of seed irradiation by thermal neutrons for inducing chromosomal aberration in M<Subscript>2</Subscript> of cotton <Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.

Cytogenetic analysis of M₂ plants after irradiation of cotton by thermal neutrons was performed in 56 families. In 40 plants of 27 M₂ families, different abnormalities of chromosome pairing were found. These abnormalities were caused by primary monosomy, chromosomal interchange, and desynapsis. The presence of chromosome aberrations in some cases decreased meiotic index and pollen fertility. Comparison of the results of cytogenetics analysis, performed in M₁ and M₂ after irradiation, showed a nearly twofold decrease in the number of plants with chromosomal aberrations in M₂, as well as narrowing of the spectrum of these aberrations. The latter result is explained by the fact that some mutations are impossible to detect in subsequent generations because of complete or partial sterility of aberrant M₁ plants. It was established that the most efficient radiation doses for inducing chromosomal aberrations in the present study were 15 and 25 Gy, since they affected survival and fertility of altered plant to a lesser extent.     

Cytogenetic characterization and fae1 gene variation in progenies from asymmetric somatic hybrids between Brassica napus and Crambe abyssinica.

Sexual progenies of asymmetric somatic hybrids between Brassica napus and Crambe abyssinica were analyzed with respect to chromosomal behavior, fae1 gene introgression, fertility, and fatty-acid composition of the seed. Among 24 progeny plants investigated, 11 plants had 38 chromosomes and were characterized by the occurrence of normal meiosis with 19 bivalents. The other 13 plants had more than 38 chromosomes, constituting a complete chromosomal set from B. napus plus different numbers of additional chromosomes from C. abyssinica. The chromosomes of B. napus and C. abyssinica origin could be clearly discriminated by genomic in situ hybridization (GISH) in mitotic and meiotic cells. Furthermore, meiotic GISH enabled identification of intergenomic chromatin bridges and of asynchrony between the B. napus and C. abyssinca meiotic cycles. Lagging, bridging and late disjunction of univalents derived from C. abyssinica were observed. Analysis of cleaved amplified polymorphic sequence (CAPS) markers derived from the fae1 gene showed novel patterns different from the B. napus recipient in some hybrid offspring. Most of the progeny plants had a high pollen fertility and seed set, and some contained significantly greater amounts of seed erucic acid than the B. napus parent. This study demonstrates that a part of the C. abyssinica genome can be transferred into B. napus via asymmetric hybridization and maintained in sexual progenies of the hybrids. Furthermore, it confirms that UV irradiation improves the fertility of the hybrid and of its sexual progeny via chromosomal elimination and facilitates the introgression of exotic genetic material into crop species.     

An Analysis of Regional Constraints on Exchange in DROSOPHILA MELANOGASTER Using Recombination-Defective Meiotic Mutants

The frequency of crossing over per unit of physical distance varies systematically along the chromosomes of Drosophila melanogaster . The regional distribution of crossovers in a series of X chromosomes of the same genetic constitution, but having different sequences, was compared in the presence and absence of normal genetically mediated regional constraints on exchange. Recombination was examined in Drosophila melanogaster females homozygous for either normal sequence X chromosomes or any of a series of X chromosome inversions. Autosomally, these females were either (1) wild type, (2) homozygous for one of several recombination-defective meiotic mutations that attenuate the normal regional constraints on exchange or (3) heterozygous for the multiply inverted chromosome TM2. The results show that the centromere, the telomeres, the heterochromatin and the euchromatic-heterochromatic junction do not serve as elements that respond to genic determinants of the regional distribution of exchanges. Instead, the results suggest that there are several elements sparsely distributed in the X chromosome euchromatin. Together with the controlling system affected by recombination-defective meiotic mutations, these elements specify the regional distribution of exchanges. The results also demonstrate that the alteration in the distribution of crossovers caused by inversion heterozygosity (the interchromosomal effect) results from the response of a normal controlling system to an overall increase in the frequency of crossing over, rather than from a disruption of the system of regional constraints on exchange that is disrupted by meiotic mutations. The mechanisms by which regional constraints on exchange might be established are discussed, as is the possible evolutionary significance of this system.     

A comparison of mutagenic effectiveness and efficiency of NMU and MNG in Sorghum

Summary A comparative study of the effectiveness and efficiency of NMU and MNG in relation to the effects in M1 plants and induction of mutations in M2 was made in a cultivated variety ofSorghum. There was a decrease in the values obtained in each of the biological criteria in the M1, namely germination, survival, seedling growth and seed fertility, with an increase in the concentration of NMU and MNG. Of the characters, survival following MNG treatments and seed fertility following NMU treatments showed the maximum reduction. NMU was not only effective in decreasing the mean of various characters in the M1, but also efficient in inducing a high frequency and wide spectrum of chlorophyll mutations in the M2 compared with MNG. Low concentrations were found to be more efficient than higher concentrations of NMU. The treated M2 population showed not only a decrease but also an increase in the mean height of plant and length of ear compared with the untreated control population. NMU caused a greater decrease in the mean of both characters and induced greater variability in the length of ear than did MNG treatment.     

Genetics of gamma-irradiation-induced mutations in Arabidopsis thaliana: large chromosomal deletions can be rescued through the fertilization of diploid eggs.

Despite the demonstrated value of chromosomal deletions and deficiencies as tools in plant and animal genome research, in the genetic model plant species Arabidopsis thaliana, such mutations have not been extensively studied. For example, it is not known whether large deletions in different regions of the genome can be tolerated in diploid plants that are heterozygous for such mutations. Similarly the viability or inviability of monosomics has not been examined in detail. To investigate these questions, we have used gamma-irradiated haploid wild-type pollen to pollinate diploid and tetraploid multimarker lines of Arabidopsis. Examination of M1 progenies revealed that chromosome loss mutations and large deletions were induced in the irradiated pollen. Such mutations were eliminated in diploid M1 plants due to dominant lethality but could be rescued in triploid M1 progeny. The use of irradiated pollen and tetraploid marker lines of Arabidopsis is a convenient way of generating deletions and modified chromosomes and provides a genetic tool for deletion mapping and for analysis of chromosomal regions essential for chromosome maintenance.     

Short-term storage of cotton pollen

Several methods of storing cotton pollen (Gossypium hirsutum L.) were evaluated. A successful pollen storage method that maintains fertility would enhance the crossing of breeding materials. Storing pollen at ultra-low temperatures in liquid nitrogen or at 5°C was not successful. No storage method maintained pollen fertility for more than 72 h. Cotton pollen did maintain adequate fertility up to 24 h at 10 and 15°C, at both low and high humidity when the pollen was stored in the detached flowers. Minimally acceptable pollen fertility was maintained in flowers stored at 15°C at 100% R.H. for 72 h. Use of these methods will allow for better utilization of parental plants when both parents do not flower on the same days.USDA-ARS, in cooperation with the New Mexico Agricultural Experimental Station, Journal Article No. 1161, Las Cruces, NM 88003, USA     

Greater ability of pollen tube growth in rye plants with 2B chromosomes

Summary To contribute to knowledge on the significance of B chromosomes in rye populations, a component of fertility has been estimated by a comparative study of germination and growth of pollen tubes in plants with and without B chromosomes. The highest percentage germination of pollen and mean speed of pollen tube growth was shown by 14 + 2B plants, followed by normal plants.Observations of first pollen mitosis indicated that in most cases the chromatid constitution of the tube nucleus was the same in 14, 14 + 1 and 14 + 2B plants. In contrast, the mean growth rate of pollen tubes differed significantly among the three cases. From these data it seems that the determination of germination ability and pollen tube growth depends on the sporophyte and is related to the number of Bs carried by the plant.     

Induced interchange heterozygosity in diploid Chrysanthemum

Summary Following a programme of recurrent irradiation and planned hybridization, it has been possible in Chrysanthemum carinatum to synthesise an interchange stock in which 12 of the 18 chromosomes are involved in rearrangements and form a single multiple association during meiosis. Induced interchange heterozygosity of this type has been made use of for studying the relationship between fertility and meiotic behaviour. Relatively high fertility of the interchange heterozygotes was found to be mainly due to a pronounced tendency of the interchange multiples to show preferentially a particular type of orientation at metaphase. Factors governing the orientation of multiple association resulting from interchange heterozygosity have been discussed.     

Cytogenetic studies on the effect of chronic gamma irradiation onVicia faba

Vicia faba seeds (cv. Giza 1) were planted in the Inshas gamma radiation field where they were chronically irradiated during the whole life of the plant. The percentage of the induced abnormal P.M.Cs, as well as the frequency of abnormal P.M.Cs in the different meiotic stages were proportional with the given doses. The main types of chromosome aberrations were anaphase and telophase bridges, fragmentation and lagging chromosomes. The nearest plants to the source showed an inhibition of shoot growth, flower and seed sterility and irregular branching. At the dosage levels used irradiation had no effect on pollen fertility. Seeds of the 1st filial generation were used for both mitotic and meiotic studies. The percentage of the mitotic abnormalities was proportional with the doses. The most dominant type of anomaly was the presence of micronuclei in the different stages of mitosis and in the resting cells. Irradiation affected also other types of anomaliese.g. lagging chromosomes, fragments, bridges...etc. Meiosis, and pollen fertility (2nd generation) were normal.