Interchange quadrivalents and chromosome order at meiotic metaphase I in Briza L. (Gramineae)

In interchange heterozygotes of Briza humilis and B. media the interchange quadrivalent is shown to be preferentially positioned in flattened, lateral spreads at metaphase I. The positioning of the interchange quadrivalents is different in the two species but in both the frequency of alternate or adjacent orientation is different for different positions on the metaphase plate. B chromosomes in B. humilis are found to alter the positioning of the quadrivalent and the B chromosomes themselves are also found to show a nonrandom distribution on the metaphase plate.     

Cytogenetical studies onBougainvillea. I. a case of interchange heterozygosity

A case of interchange heterozygosity has been found inB. peruviana cv “Princess Margaret Rose” in which there is a regular formation of 15 bivalents and an interchange multiple of 4 chromosomes. The multiple is always associated with the nucleolus at diakinesis, indicating that one of the chromosome involved is nucleolar. The nucleolar pair of chromosomes shows a slight heteromorphicity which may be due to an unequal interchange. Although 80% of interchange multiples orientate non-disjunctionally, yet 65% pollen is stainable. The pollen is ineffective in self pollination, but highly effective in crosses with 2x and 3x cultivars ofB. spectabilis. The higher pollen stainability indicates that the deficiencies and duplications caused by non-disjunction do not have serious physiological offects on pollen grains and that its genome can withstand rearrangements.     

X-ray treatment of Oenothera chromosomes

Summary The fertilisation of untreated ovules ofOenothera blandina by pollen subjected to doses of X-rays resulted in F₁ generations consisting of normal plants with no pollen sterility, normal plants with about 50% or more pollen sterility and variant plants usually with 50% or more bad pollen.The pollen-sterile normal plants included seven with segmentally interchanged chromosomes, the remainder examined having only normal chromosomes. Five plants had one ring of four chromosomes, one had one ring of six chromosomes and one had two rings of four chromosomes. The inheritance of the interchanges was traced in several cases. Some of them were not transmitted because they were lethal to pollen carrying them and also failed to form embryo sacs.Most of the variant plants appeared cytologically normal, having seven ring pairs at metaphase 1. Three plants had interchanges, two of them having a chain of four chromosomes as the maximum catenation. The F₂'s of the variant plants consisted of normalblandina with the exception of the narrow leaved one that had a ring of four chromosomes.It is concluded that the variant plants, the pollen sterile normals and the interchange normals in which the interchange was not transmitted through the pollen were all defectives, resulting from deletions produced in the parental pollen by the action of the X-rays.Some indications of a quantitative relationship between X-ray dosage and percentage numbers of variants, pollen steriles and interchange heterozygotes respectively could be traced.     

Intraspecific hybridization and its bearing on chromosome evolution inVicia narbonensis (Fabaceae)

Nine accessions ofVicia narbonensis, considered to be the wild progenitor of faba bean (Vicia faba), were investigated to ascertain the nature and extent of intraspecific karyotypic polymorphism. The chromosome complements resolved into four distinct types (A, B, C, D), and the meiotic data of F₁ hybrids (A × B, B × C, A × C) revealed that alteration in chromosome morphology is the result of segmental interchanges. The interchange complexes indicate that the parents differ from each other by 1 to 2 interchanges. It is also evident that karyotype B, and not A as previously reported, is the normal karyotype of the species, and A and C are single homozygotes for unequal interchange. The comparative karyomorphology of the parents and the hybrids, and of two interchange heterozygotes of four chromosomes each in F₁ hybrids of A × C shows that the chromosomes involved in the single interchange homozygotes (A, C) are not common and the breaks in both interchanges occurred in short and long arms of the involved chromosomes. Identification of the interchanged chromosomes in the complements and the frequency of ring and chain quadrivalents in the heterozygotes enabled location of the breakpoints. The present results provide probably the first example indicating that interchange homozygosity (A) is not only firmly established but also has enabled the species to spread further by adapting to a wide range of habitats. — The genetic relationships between A and D are very different. All seven chromosome pairs in D could be distinguished from A, and for that matter, B and C as well. From the meiotic pairing properties it is also amply clear that genome D is well differentiated from A and possibly B, and C, and deserves special status.     

Multiple chromosomal interchanges in pearl millet

Summary Intercrossing and irradiation were successfully used in pearl millet (Pennisetum typhoides) to develop multiple interchanges involving up to the total complement of all the chromosomes in one complex. In interchange heterozygotes showing 12 + 1 II and 14, 90.9 and 87.8 per cent of the cells, respectively, had chromosome configurations other than that of 12 and 14 chromosomes. In general, the frequency of such cells resulting from breakdown of the expected complex configuration increased with the increase in the number of translocated chromosomes in the complex. The higher the number of chromosomes involved in the interchange ring, the higher were the pollen and ovule sterility. The results indicated that meiotic instability, deficiency-duplication gametes, and unequal distribution of chromosomes account for increased sterility of multiple interchange heterozygotes. Even though interchanges in pearl millet predominantly show the alternate type of segregation, sterility seems to be the major barrier for the exploitation of the multiple interchange method for gamete selection and the establishment of homozygous lines in this plant species.     

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.     

Study of a translocation in tetraploid rye

Colchicine induced tetraploids (4x=28) from diploidSecale cereale heterozygous for a translocation showed a strong tendency of non-preferential pairing for the interchanged chromosomes. The normal chromosomes associated in configurations up to quadrivalents, and the translocation complex formed multivalents up to octavalents. Most of the interchanged chromosome associations were characterized by their heteromorphic nature. The percentage of the chromosomes in the interchange complex forming multivalent associations was far higher than that of the remaining twenty chromosomes. Abnormalities were observed at anaphase I and II in the pollen mother cells. The tetraploids appeared to be completely sterile. It is suggested that the high frequency of multivalent formation may be explained on the basis that the interchange might have involved a region of localized chiasmata. The absence of polyploidy in the genusSecale as against its widespread occurrence in the related grass genera may be accounted for, in part, on the basis of non-preferential pairing.     

The effects of X-rays on the chromosomes of locust embryos

The structure and frequency of chromatid interchange types in C-metaphase cells of embryos of Schistocerca gregaria are described at the first mitosis following irradiation at the G ₂ stage of interphase. The relationship between the different interchange types and the organisation of the interphase nucleus is discussed. It is concluded that most chromatid interchanges are induced between polarised chromosomes though the occurrence of loops and to a lesser extent of terminal overlapping may subsequently modify the appearance of these aberrations at metaphase.     

Chromosomal interchange inEleutherine plicata Herb

Chromosome behaviour at metaphase I and anaphase I of meiosis inEleutherine plicata Herb. (2n=14) is studied. Cells with chromosome associations comprising an association of four long chromosomes, in addition to five bivalents were observed more frequently than those with seven bivalents. it is concluded that the ring of four is due to a segmental interchange between the two long non-homologous chromosome pairs. The ring of four at anaphase I showed delayed disjunction, bridge formation and irregular separation of chromosomes in a number of cells while the behaviour of the other bivalents was normal.     

Cytogenetics of pearl millet

Summary The somatic karyotype of pearl millet Pennisetum americanum (L.) Leeke. (2n = 14) has been studied in several cultivars, but few cytological markers have been discovered which could help in the easy identification of the chromosomes. Analysis of pachytene bivalents permits such identification but is feasible only in a few cultivars. Recently, several lines having telocentric chromosomes have been produced and classified but their potentialities as cytogenetic tools have yet to be explored. Some African populations of pearl millet carry B-chromosomes in their karyotype. Cytogenetics of B-chromosomes has been reported in great detail. Bs undergo spontaneous changes to produce deficient- and iso-chromosomes. The main effect of B-chromosomes is on chiasma frequency which is exerted by the relative amounts of chiasma promoting euchromatin and the chiasma depressing heterochromatin in the Bs. Haploid plants occur occasionally and sometimes show a low degree of seed set, offering a possibility of establishing homozygous inbred lines. Cytogenetics of several spontaneous and induced autotetraploids have been reported. In general quadrivalent formation between the seven sets of four homologues was random. Seed set of the autotetraploids could be improved by selection; improved seed fertility was found to be associated with increased chiasma frequency, increased quadrivalent frequency and regular distribution of chromosomes at anaphase I. Genes controlling morphological characters of plant phenotype segregate independent of those controlling fertility and in pearl millet polyploidy per se is not limiting to plant vigour. Primary trisomics represent the best studied among the aneuploids of pearl millet. All the seven primary trisomics have been identified and described. Some were used in assigning genes to specific chromosomes but in general trisomies have poor vigour and fertility, and show low frequency of transmission. Apart from B-chromosomes, cytogenetics of interchanges has been the best studied aspect of pearl millet. The frequency of co-orientation of an interchange complex at metaphase I, which determines the fertility or sterility of the interchange heterozygote, is influenced by the genetic background and thus is theoretically amenable for selection leading to improved fertility of the heterozygote. Interchange tester-stocks have been assembled which can be used to identify the chromosomes involved in any newly obtained interchange. A complex interchange line involving all the chromosomes of the complement has also been produced, but the ring-of-fourteen produces total male and female sterility.Genotypic control of mitosis and meiosis has been reported, with reference to chromosome numerical mosaicism, multiploid sporocytes, desynapsis and chromosome fragmentation, and male sterility. Pearl millet being a largely outbreeding species, forced inbreeding was mainly found to result in loss of morphological vigour and reduction in mean chiasma frequency per PMC. Interspecific hybrids between pearl millet and several related species have been cytologically investigated and homology of the seven chromosomes of pearl millet with seven of the fourteen chromosomes of P. purpureum has been demonstrated. Cytogenetic evidence from haploids, autopolyploids and interspecific hybrids has indications to suggest that the haploid number of x = 7 is derived from x = 5, but the evidence is inconclusive and needs critical evaluation.     

Genetic length and interference in telo-substituted interchange heterozygotes of rye

Substituting one of the (sub)metacentric chromosomes of a simple interchange complex by the two corresponding telocentrics marks one of the unchanged arms, one of the interchange segments and one of the interstitial segments. When the two telocentrics can be distinguished and when only infrequently MI configurations are formed for which more than a single origin is possible, rather exact estimates of the frequency of occurrence of chiasmata in the six segments can be obtained. When the two telocentrics can not be distinguished and in addition the frequency of MI configurations with more than one possible origin is not negligible, the analysis is more complex. With some previous knowledge of the interchange and some simplification an acceptable simple solution remains possible. — Of both cases an example for rye, Secale cereale, is given, together with an analysis of interference between these segments, and a conversion of probabilities of being bound at MI to genetic length.     

The fate of multivalents during meiotic prophase in the hybrid Gibasis consobrina x G. karwinskyana Rafin. (Commelinaceae)

The chromosomes of the two closely related diploid species, Gibasis consobrina and G. karwinskyana (Commelinaceae; 2n=2x=10), are morphologically alike, yet form few chiasmate associations at metaphase I in the f₁ hybrid. During meiotic prophase, however, synaptonemal complexes join the majority of the chromosomes of the complement in complex multiple pairing configurations. The F₁ hybrid between different tetraploid genotypes of the same two species similarly forms multivalents during meiotic prophase, which are subsequently eliminated in favour of strictly homologous bivalents before metaphase I. One quadrivalent comprising interchange chromosomes inherited from one of the parents, usually persists to first metaphase. Evidently the resolution of multivalents to bivalents at first metaphase, which accounts for diploidisation, is not attributable to the elimination of multivalents per se, but of multivalents comprising chromosomes of limited homology.     

Tertiary trisomics in pearl millet (Pennisetum typhoides Stapf & Hubb)

Four tertiary trisomic plants are reported here, two of them (Nos. Tr11 and Tr13) from selfed progeny of a triploid Pearl millet and the other two (Nos. 3/12 and 16/7) from the progenies of radiation induced interchange heterozygotes. The extra chromosome in Tr13 and 3/12 was the nucleolus organizing chromosome. In No. 16/7 an extra chromosome enters into an association chromosomes were also involved. Meiotic behaviour in these four trisomics indicates that Tr11 and 3/12 are tertiary trisomics. It is suggested that two reciprocal translocations have occurred between two sets of chromosomes in the triploid parent and that syngamy has taken place in such a way that four interchange chromosomes and one non-interchange nucleolus organizing chromosome have come together in the offspring. The extra chromosome in No. 16/7 is an interchange chromosome which is homologous to one of the chromosomes of an interchange complex of six chromosomes.     

Structural and numerical chromosomal polymorphism in natural populations ofAlopecurus (Poaceae)

Considerable structural and numerical chromosomal variation has been found in natural populations ofAlopecurus. Interchange heterozygotes, identified by multivalent formation during meiosis, have been recovered in four out of six species studied and are reported for the first time in the diploidsA. bulbosus, andA. myosuroides, and the tetraploidA. pratensis. B chromosomes have been found in two species,A. pratensis andA. myosuroides and also in inter-specific hybrids betweenA. pratensis andA. arundinaceus. The characteristics, distribution and meiotic behaviour of both interchange heterozygotes and B chromosomes are described.     

KARYOTYPES OF IRIS PUMILA AND RELATED SPECIES

Randolph , L. F. and Jyotirmay Mitra . (Cornell U., Ithaca.) Karyotypes of Iris pumila and related species. Amer. Jour. Bot. 46(2): 93-102. Illus. 1959.—The karyotypes of 30- and 32-chromosome geographical variants of the amphidiploid I. pumila from Russia and the Balkans were compared with the karyotype of the typical 32-chromosome Austrian forms of this species and with those of the diploid I. attica and I. pseudopumila, previously reported to be the basic species from which I. pumila originated. Plants from 3 collections of a Crimean form of I. pumila with 32 chromosomes had a pair of long chromosomes with submedian centromeres morphologically similar to chromosome 1 of the typical form of I. pumila. In addition, there was another heteromorphic pair of submedian chromosomes with one of the members having a shorter short arm. The manner in which this altered chromosome could have arisen as a result of a heterobrachial inversion is described. Five different collections of I. pumila with 30 chromosomes from Russia differ in several respects from the typical 32-chromosome I. pumila. They have an unusually long pair of chromosomes with a submedian centromere and a secondary constriction in the long arm. This chromosome is the original chromosome 2 which had been altered by the addition of a segment equivalent to the most of the long arm of one of the shorter chromosomes with subterminal centromere. The manner in which this could have occurred as the result of unequal reciprocal translocation is described. Loss of the remaining diminutive portion of the short chromosome with subterminal constriction assumed to have been involved in the unequal interchange of segments producing the modified, longer chromosome 2 would account for the reduction in chromosome number from 32 to 30 in the Russian form of I. pumila. Four pairs of chromosomes with satellites have been found in the 30-chromosome plants whereas 6 pairs of satellited chromosomes are present in the 32-chromosome I. pumila. The spontaneous occurrence of chromosomal alterations of the type here described are considered to be significant factors in the process of chromosomal repatterning resulting in the appearance of new geographical races, and eventually of species of iris, with altered chromosome numbers and modified karyotypes. More specifically it is concluded that amphidiploidy accompanied by chromosomal repatterning resulting from segmental interchange, heterobrachial inversion and related types of chromosomal alterations has played an important role in the evolution of I. pumila and karyological forms of this species occupying different geographical areas.     

An interchange heterozygote inThelepogon elegans roth ex roem et schult.

In a population of 231 plants of the diploidThelepogon elegans (n=5), seven were found to be interchange heterozygotes. A multiple of four, six or eight chromosomes was observed in 62.32% of the pollen mother cells at MI, the remainder having five bivalents. Chiasmata were mostly localised terminally. The orientation of the multiples was predominantly alternate. The subsequent course of meiosis was normal, and pollen fertility was high.     

Autosyndetic pairing in Gibasis (Commelinaceae) hybrids revealed by C-banding

Two closely related species of Gibasis, G. karwinskyana and G. consobrina, and their F₁ hybrids were studied cytologically at the diploid and tetraploid level. Despite similarity in their basic karyotype, pairing was extremely limited in the diploid hybrid and almost exclusively autosyndetic in the tetraploid, except for multivalent formation due to interchange heterozygosity. The analysis was considerably facilitated by the use of C-banding techniques at meiosis, by which the chromosomes of each species could be readily identified. In the parents, quadrivalents were formed between homologous but non-identical chromosomes, which also formed autosyndetic bivalents in the hybrids. Meiotic pairing in the hybrids was unaffected by polytypy for C-bands among different populations of the parental species.     

The nature and time of occurrence of radiation-induced nondisjunction of the acrocentric X and fourth chromosomes in Drosophila melanogaster females

Radiation-induced nondisjunction in Drosophila melanogaster females usually-possibly invariably-involves the participation of chromosomes other than the pair in which the numerical aberration is noted, with one of the two acrocentric pairs frequently being involved in the assortative error of the other. Nearly one-half of all diplo-X eggs produced following the irradiation of immature oocytes of females having free X's are found to be nullo-4, and, in agreement with earlier reports^9,11, about one-fourth of all nullo-X eggs are diplo-4. The incidence of structural alterations is markedly higher in chromosomes involved in nondisjunctions than in those recovered from normal segregations, with the structural changes being those expected from interchange between X and fourth chromosomes where only one of the two interchange products (a “half-translocation”) is recovered. X chromosomes may acquire an arm of chromosome 4, and fourth chromosomes may lose the marker from the left arm, as if the short, heterochromatic right arm of the X had been substituted. Homozygosis of markers near the centromere of the X chromosome shows that nearly all failures of segregation must occur at division I. While the data do not require that there be some division II nondisjunction, neither do they categorically deny the possiblility of its occurring at a very low level. The findings are as expected on the model of heterologous conjunction via chromatid interchange as the major and perhaps exclusive cause of radiation-induced nondisjunction.     

Chiasma formation in the short arm of the nucleolar chromosome of the tomato

Summary A translocation heterozygote in tomato (Lycopersicon esculentum) is shown to have a cyclical type of interchange between the long arms of chromosomes 1, 2 (nucleolar) and 3. A study of chromosome association in this plant at metaphase I has indicated that in 21% of the cells a ring of six chromosomes is present. Since an open ring hexavalent can occur only if there is chiasma formation in all the translocated segments and in all the short arms of the three chromosomes, it is concluded that there is considerable frequency of chiasma formation in the short arm of the nucleolar chromosome. This conclusion contradicts the previous observations that chiasma formation is either absent or very rare in the entirely dark staining chromatic, sometimes referred to as heterochromatic, short arm of the nucleolar chromosome.Part of this investigation was carried out at the Department of Genetics, Agricultural University, Wageningen, when the author was serving a contract between the EURATOM-I.T.A.L. and the Agricultural University.     

Cytogenetics of tropical bulbous ornamentals VIII: Pollen grain mitosis in Zephyranthes puertoricensis

The immediate result of recombination during male meiosis within the complex interchange heterozygote polyploids in Zephyranthes could be assessed from the chromosomal constitution of pollen grains in one species with 2n=25. The numbers found in the pollen grains were n=1–16, 20 and 24. Considering n=12 (22.3%) as the haploid number, nearly 57.2% were subhaploid, out of which 19.6% had n=11. Occurrence of such a high proportion of subhaploid grains may be explained on the basis of the polyploid constitution (4x+1) of the species (x=6). In this sense, grains with a basic set of 6 chromosomes may be able to function. Therefore, in the real sense only grains with numbers less than 6 are submonoploid which constitute only 10.7%. In the subhaploid grains there was a preference for large and medium sized chromosomes, while in grains with higher numbers there was noted a tendency for smaller chromosomes to be represented more than once, indicating their non-disjunction during meiosis. Totally new types of nucleolar chromosomes emanating after recombination were also seen.This investigation has shown the extent of potential cytological polymorphism possible in heterozygotes like Zephyranthes puertoricensis. How much of this polymorphism is meaningful in creating the aneuploid pattern found in the genus, depends on the competitive advantage different numbers have during fertilisation vis-a-vis possibly similar polymorphism created during the female meiosis.