Multivalent Formation Research Articles

Gamma Ray Induced Meiotic Abnormalities inCapsicum AnnuumL.

SUMMARYDry dormant seeds of CA 960 (red) and CA 1968 (yellow) strains of Capsicum annuum were irradiated with gamma rays at 10, 20, 30 and 40 kR dose levels. The plants grown from treated and control seeds were investigated for meiotic anomalies and pollen sterility. The meiotic abnormalities included stickiness of chromosomes, clumping, formation of multivalents and univalents, breakage, non-orientation of chromosomes on metaphase plate, unequal grouping of chromosomes, laggards and abnormal microspores. The frequency of occurrence of these abnormalities is proportional to the dosage of the r-rays. Pollen sterility seems to be the cumulative result of various aberrant meiotic stages as well as physiological and genetic damages induced probably by the breakage of chromosomes through the formation of antimetabolic agents in the cells. The differences observed in the frequency of meiotic abnormalities in the two strains can be traced to differences in the radiosensitivity of the two strains.

Comparison of ethyl methanesulfonate and radiation induced meiotic abnormalities in Turnera ulmifolia Linn. var. angustifolia Willd.

Meiosis is normal in control plants. As a result of gamma rays or EMS treatment, the plants showed varying degree of meiotic irregularity. This irregularity increased with the higher exposure to gamma rays and higher concentration and duration of EMS treatment. The various types of abnormalities observed in diffrent stages of R1 and M1 generations include stickiness of chromosomes, chromosome fragments, disturbed bivalent associations resulting in multivalent formation, precocious movement, laggards, bridge with or without fragment and formation of micronuclei at meiosis I and II.The percentage of PMC's showing meiotic abnormalities was less in plants raised from EMS-treated seeds than that of gamma irradiation.Meiotic irregularities were less in the second generation whereas in the third generation they were noticed occasionally.

The cytogenetics of Phaseolus vulgaris L., Phaseolus coccineus L, their interspecific hybrids, derived amphidiploid and backcross progeny in relation to their potential exploitation in breeding.

Chromosome number in Phaseolus vulgaris and Ph. coccineus is 2n=22. In F1 hybrids pairing is predominantly bivalent, the formation of occasional multiple figures indicating some structural differentiation. Multivalent formation is observed in the derived amphidiploid; frequency declines from C1-C5 with corresponding improvement in seed fertility and pollen stainability. Factors favouring homologous pairing are evidently selected. Meiosis and seed fertility in F2 and most F1 backcrosses are more or less normal except for the BC1 F1 (Ph. vulgaris×Ph. coccineus)×Ph. coccineus which had very low fertility.Introgression is possible reciprocally between the two species because the F1 interspecific hybrid, initially made on Ph. vulgaris can be backcrossed as pollen parent to both original parents used as seed parent. It is however very difficult to transfer two characters ‘cotyledon’ and ‘stigma’ positions between species which may be related to chromosomes structural differences. The fertility of the amphidiploid may be raised to the diploid level by recurrent selection.

Unusual chromosome pairing and B chromosomes inBriza spicata (Poaceae)

One plant from a population ofBriza spicata (Poaceae) was found to have highly irregular meiotic behaviour. It is characterized by having a reduced chiasma frequency, a large between cell variance in chiasma frequency and the formation of multivalents involving pairs of A chromosomes. The B chromosome present in this plant also forms multivalents with a pair of A chromosomes. It is suggested that the normal control of strict bivalent pairing has broken down and homoeologous chromosomes are associating as multivalents. Furthermore, the partial homology of the B chromosome with a pair of A chromosomes is revealed.

B Chromosomes and multivalent formation in tetraploid hybrids between Briza media and Briza elatior

In colchicine induced tetraploid hybrids between Briza media and Briza elatior, B chromosomes are shown to reduce homoeologous chromosome pairing. Large numbers of Bs have a considerably greater effect than small numbers on the frequency of multivalents and bivalents. A possible mechanism to explain the effects of B chromosomes on chromosome pairing in species hybrids is suggested which is related to the mechanism that prevents pairing between A and B chromosomes.

Pachytene Chromosome Pairing and Multivalent Formation in an Autotetraploid Tomato

Pachytene Chromosome Pairing and Multivalent Formation in an Autotetraploid Tomato

Cytogenetics of Areca catechu L., A. triandra Roxb. and their F1 hybrids (palmae)

Meiotic observations on two cultivars of A. catechu? three ecotypes of A. triarzdra and their hybrids are presented. Jntracultivar variation for meiotic behaviour was very high in A. catechu. In spite of the high degree of multivalent associations observed in A. catechu, pollen fertihty was high. The possibility of the frequency of multivalent formation being under genotypic control and being subjected to selection has been indicated. Chromosome pairing in A. trkzndra and A. catechu-A. triandra hybrids revealed that partiaJ desynapsis occurs to varying degrees in both. The higher number of univalents in the hybrids is inferred to be the result of non-homology of certain of the parental chromosomes. The increase in pairing at metaphase I in A. triandra and A. catechu x A. triandra has been attributed to ‘distributive pairing’. The pollen sterility in A. triandra is believed to be caused by meiotic abnormalities at various stages. The higher percentage of meiotic abnormalities of one of the parental species, the disharmonious interaction between the genotype of one and the cytoplasm of the other and unbaJanced gene combinations appeared to be the possible cause of the high sterility in the hybrids. It is concluded that there is considerable homology between the chromosomes of A. catechu and A. tnizndra. The possibilities of transferring mite resistance and larger number of femaJe flowers from A. triandra to A. catechu through interspecific hybridisation and back-crossing have been indicated.

Studies on Libyan Grasses I. Cytogenetics and Distribution of AneuploidHyparrhenia Hirta(L.) Stapf

SUMMARYMeiotic studies of Hyparrhenia hirta (L.) Stapf revealed aneuploidy with 2 n=36, 54 and 56 chromosomes. The plants with 2 n=54 chromosomes had much wider distribution than the other two types of plants. Diakinesis and metaphase I revealed a very high multivalent ratio. It is argued that the formation of multivalents is mainly due to the loss of chromosomes carrying genes which suppress multivalent formation. Furthermore, scarcity of autopolyploidy in nature is attributed to the gene-controlled pairing of chromosomes blurring the difference between auto-, allo-, and segmental allopolyploidy. Some of the multivalent associations might be partly due to translocation and other kinds of chromosomal rearrangements.

Pairing restriction and non-homologous pairing in autoploids of Adiantum capillus-veneris.

The high basic chromosome numbers in homosporous ferns are polyploid in origin, and their haploid chromosome complements may contain two or more homoeologous sets of chromosomes (Klekowski 1973). Therefore, chromosome associations in three generations of an autotetraploid of the Venus-hair fern, Adiantum capillus-veneris, have been analysed to test the possibility of a fully penetrant genecontrolled mechanism in the so-called diploid homosporous ferns which limit chromosome associations to only bivalents. Multivalent formation occurs in the autotetraploids, which, though not unexpected, is against a simple gene control to form only bivalents. In the autotetraploid, there are relatively fewer quadrivalents and more bivalents. In the autotriploid, the frequency of trivalents compares closely with those of quadrivalents in the autotetraploids. The occurrence of large number of bivalents in the autotetraploids have been accommodated partly in the model of John and Henderson (1962) which assumes random association of chromosomes within each homologous set. Further accentuation in the number of bivalents occurs partly owing to proximal chiasmata, and partly due to a pairing restriction upon multivalent formation. Following Timmits and Rees (1971), it has been estimated that at least five of the thirty homologous sets in the autotetraploids and autotriploid of A. capillus-veneris associate themselves exclusively in pairs. Chromosome analysis in the autotriploid has also revealed evidence of non-homologous pairing, which seemingly supports the concept of “homoeologous” genomes comprising the haploid complements of extant diploid species in ferns, and also raises the probability of occasional homoeologous pairing and recombination coupled with the pairing restriction.

FUSION OF CHROMOCENTERS IN PREMEIOTIC INTERPHASE OFSECALE CEREALEAND ITS POSSIBLE RELATIONSHIP TO CHROMOSOME PAIRING

The premeiotic interphase of rye has been found to exhibit a synchronized fusion of chromocenters. This results in a solid knot of heterochromatin localized to one side of the nucleus. Colchicine injected into the immature inflorescence of diploid rye has been found to induce univalent and multivalent formation at metaphase I. The early premeiotic interphase was the phase of development most sensitive to colchicine. Colchicine was also found to have a similar disruptive effect on fusion of chromocenters in the premeiotic interphase. The relationship of these observations to chromosome pairing is discussed.

THE CONTROL OF DIPLOID-LIKE MEIOSIS IN POLYPLOID TAXA OF <i>CHRYSANTHEMUM</i> (COMPOSITAE)

Genetic stabilisation of diploid-like meiosis occurs in hexaploid, octoploid and decaploid taxa of Chrysanthemum. This stabilisation is ineffective in F1-hybrids between diploids and the several polyploids which have been obtained by ovary culture. The suppression of multivalent formation is effective in even-ploid hybrids although they are amphihaploid. A significant frequency of trivalents occurs in odd-ploid hybrids.

CYTOLOGICAL DIPLOIDIZATION IN THE CULTIVATED OCTOPLOID STRAWBERRYFRAGARIA×ANANASSA

Chromosome pairing during meiosis was studied in 9 cultivated genotypes and 32 seedlings produced by selfing several different genotypes of the octoploid strawberry Fragaria × ananassa Duch. (2n = 8x = 56). In all genotypes studied the chromosomes paired exclusively as bivalents indicating cytological diploidization. Meiotic studies of one of the S1plants, which was completely sterile, revealed that the chromosomes were paired at pachytene but desynapsed during diplotene, so that very little bivalent association was observed in late prophase of meiosis. The pairing in the F. × ananassa × F. nubicola pentaploid hybrids showed an average of 11.6 bivalents per PMC and frequent multivalent formation. There is thus an indication of residual homology between the ancestral genomes of the octoploid strawberry.

The increase of seed fertility of Brassicoraphanus through cytological irregularity

Amphidiploids (Brassicoraphanus) were produced by means of colchicine treatment of F1 hybrids between Brassica japonica Sieb. and Raphanus sativus L. The cytology of the amphidiploids was studied from F1 to F3 generations. Some plants had the euploid chromosome number 2n=38, whereas others had the aneuploid number 2n=37. One or two of either quadrivalents or trivalents, as well as some univalents, were seen in most of the plants examined. All the plants showed a low seed fertility. In F3 generation there arose some yellow-flowered plants, all of which showed a higher seed fertility than normal white-flowered plants. It is postulated that the change of flower colour might originate in the segmental exchange of only partially homologous chromosomes following multivalent formation. A gene causing white flower colour was perhaps closely linked to a gene causing sterility, and both genes were probably excluded together through the segmental exchange of the chromosomes. Therefore, it can be said that the increase of fertility was induced by cytological irregularity.

The mechanism of increased seed fertility accompanied with the change of flower colour in Brassicoraphanus

In Brassicoraphanus (amphidiploids between Brassica japonica Sieb. and Raphanus sativus L.), yellow-flowered plants that occurred among originally white-flowered plants showed an increased seed fertility. It is assumed that the gene Y (yellow-flower gene) from Brassica and the gene W (white-flower gene) from Raphanus are located at corresponding loci of only partially homologous chromosomes. W is dominant (epistatic) over Y. The normal white-flowered plants have the genotype YYWW. A YYYW-plant was found, which is assumed to have arisen through crossing-over following multivalent formation. In the progeny of this plant, yellow-flowered plants (YYYY) as well as white-flowered plants (YYWW, YYYW) appeared. The gene for flower colour is closely linked to a gene which controls the development of embryos (or endosperm). This gene promotes the development of embryos in homozygous condition. Therefore, the embryo having only the yellow-flower gene can develop more easily into viable seed than the embryo having the white-flower gene. It is also possible that the sterility of white-flowered plants is caused by a discordance between the cytoplasm of Brassica and W (or genes linked to W) of Raphanus.

A CYTOGEOGRAPHIC STUDY OF ERIOPHYLLUM LANATUM (COMPOSITAE, HELENIEAE)

Analysis of 368 plants derived from 239 natural populations showed that this taxonomically perplexing and wide‐ranging species‐complex consists of diploids (n = 8), tetraploids, hexaploids and octoploids. Microsporocytes were the source of most of the chromosome counts. Meiosis was basically regular. Multivalent formation was uncommon, but 11 % of all the plants examined had one or more full‐sized extra chromosomes. The frequency of plants with extra chromosomes varied significantly among the taxa, from 0 (five varieties) to over 20 % (two varieties). Except in one instance, where one population yielded a diploid and a triploid, different ploidy levels were not found in the same population. The frequency of diploid, tetraploid, hexaploid and octoploid populations was, respectively, 71, 22, 4 and 2%. Variety obovatum appears to be exclusively diploid, and var. aphanactis exclusively tetraploid. Diploids and one or more polyploid levels occurred in the other taxa. No correlation was found between polyploidy and geological history, soils, topography or climate, nor were the polyploids more widely distributed than the diploids. Some of the polyploid populations seem to have been derived from inter‐varietal hybridizations, but others do not. The complex has a “pillar” structure in which 10 diploid taxa support a three‐level polyploid superstructure. The available evidence suggests that the major role of polyploidy here has been to stabilize the products of intra‐ and inter‐varietal hybridizations.

A Cytogeographic Study of Eriophyllum lanatum (Compositae, Helenieae)

Analysis of 368 plants derived from 239 natural populations showed that this taxonomically perplexing and wide-ranging species-complex consists of diploids (n = 8), tetraploids, hexaploids and octoploids. Microsporocytes were the source of most of the chromosome counts. Meiosis was basically regular. Multivalent formation was uncommon, but 11 % of all the plants examined had one or more full-sized extra chromosomes. The frequency of plants with extra chromosomes varied significantly among the taxa, from 0 (five varieties) to over 20 % (two varieties). Except in one instance, where one population yielded a diploid and a triploid, different ploidy levels were not found in the same population. The frequency of diploid, tetraploid, hexaploid and octoploid populations was, respectively, 71, 22, 4 and 2%. Variety obovatum appears to be exclusively diploid, and var. aphanactis exclusively tetraploid. Diploids and one or more polyploid levels occurred in the other taxa. No correlation was found between polyploidy and geological history, soils, topography or climate, nor were the polyploids more widely distributed than the diploids. Some of the polyploid populations seem to have been derived from inter-varietal hybridizations, but others do not. The complex has a “pillar” structure in which 10 diploid taxa support a three-level polyploid superstructure. The available evidence suggests that the major role of polyploidy here has been to stabilize the products of intra- and inter-varietal hybridizations.

Cytogenetical Studies on Ecological Races of <i>Mecardonia dianthera</i> (SW) Pennel

Mecardonia dianthera, a Scrophulariaceous weed of Tropical America comprises of two edaphic ecotypes viz. erect and repent. While the former is a facultative calcicole, the latter is an obligate calcifuge. The present paper gives an account of plant performance and cytological behaviour of colchicine induced tetraploids that were produced by treating the inbred lines of these plants with 0.3% colchicine for 12 hours. Majority of the surviving plants were tetraploids exhibiting a slower growth delayed phenological events, deep green thick green rough foliage, increased dry matter production, enlarged epidermal and mesophyll cells, chloroplast, stomatal apparatus and frequency, pollen grains and a reduced seed viability than that of the diploids. While repent ecotype tetraploids had increased flower number per plant than that of the diploids, reverse was observed in the erect ecotype tetraploids.At metaphase I, uni, bi, tri, tetra and hexa-valents in a varying frequency were observed in the colchiploids and the total number of multivalent associations and univalents per PMC on an average was equal in the tetraploids of each ecotype. This indicates a competition for pairing between homologous chromosomes which is supported by the fact that a significant positive correlation between chiasma frequency and multivalent formation was also observed. However, the colchiploids of the erect ecotype possessed greater number of multivalents and univalents than those of the repent ecotype colchiploids. Further stages of meiosis in the colchi-ploids were abnormal and resembled the typical autoploid meiois. Multiple spindles, complement fractionation and secondary association were also observed. The contribution of univalents and multivalents in inducing pollen sterility in the tetraploid material is significant. Attempts to cross these colchiploids with diploids produced non-viable abortive seeds. Artificial crossings amongst these colchiploids resulted in an early degeneration of the ovary and a failure of the seed formation. Natural self pollination yielded 5-9% of seeds that died after an extrusion of radicle when germinated.The study indicated that the two edaphic ecological races (ecotypes) exhibit a differential susceptibility to colchicine in relation to frequency of induced polyploids, morphological and reproductive features, number and frequrncy of chromosomal associations and univalents, pollen and seed fertility. However, a general agreement in having a morphological gigas, slowed growth rate, delayed phenological events and reduced reproductive capacity occurs in the tetraploids in comparison to the diploid progenitors, of both the ecotypes. A tendency to change the plant form (repent erect), which is governed by a single gene, was not observed, indicating thereby that the gene remains uneffected by the drug, in both the ecological races of M. dianthera.

Cytological Studies in <i>Lens nigricans</i>: A Case of Translocation Heterozygote

Lens nigricans, a rare species of lentil possess a somatic chromosome number of 14 (2n). The somatic complement comprises of 4 pairs of median chromosome, out of which one pair bear satellite. The secondary constriction is very near to the primary constriction. This type of SATELLITE has been termed as “Intercallary trabant” by Darlington (1965). The meiotic system is highly irregular in this species. Formation of univalent, multivalents, chains, rings, bridges and laggards has been recorded. The meiotic chromosomes shows asynaptic behaviour, which may be the outcome of various types of exchanges between chromosomes or it may be due to change in the behaviour of the gene concerned. From the present investigation it appears that L. nigricans has not acquired stability as yet and translocation has played an important role in its evolution.

Facultative Apomixis and Polyploidy in Matteuccia orientalis

Life-cycles in leptosporangiate homosporous ferns which normally occur in nature have been reported to be either obligately sexual, with a haploid-diploid alternation of generations, or obligately apogamous. Apogamous reproduction is a process which requires two basic phenomena: production of viable spores either by meiosis (Dopp, 1932; Manton, 1950; Mehra & Singh, 1957) or by mitosis (Evans, 1964) and production of a diploid or haploid sporophyte from a diploid or haploid gametophyte without an intervening act of syngamy. In many sexual species apogamous outgrowths from the gametophyte can be induced by abnormally prolonged cultural conditions. This type of reproduction in sexual species has not been observed in nature, and sporophytes produced in this manner in cultivation have not produced viable spores (Manton, 1950, p. 201). Further elaborations of the fern life-cycle include vegetative reproduction, which is found in a large number of species, and apospory, which has been produced in the laboratory but has yet to be shown conclusively to occur in nature (see DeBenedictis, 1969, for a review of literature on apogamy and apospory in ferns). In addition to the obligate nature of the sexual or apogamous life-cycle, some species of homosporous ferns can be characterized as being high polyploids, a condition apparently evolved in response to the primarily homothallic nature of their gametophytic reproductive biology (Klekowski & Baker, 1966). As a result of this general dominant polyploid theme, base numbers of extant genera are themselves composed of several sets of duplicate chromosomes, with multivalent formation during meiosis restricted or eliminated by genetic or physiological control of bivalent pairing. Evidence is accumulating that the polyploid condition in ferns has selective advantage in that it provides storage of genetic information which is later released by homoeologous recombination (Klekowski, 1973). It is the object of this paper to provide evidence from the life-cycle of a single plant of Matteuccia orientalis (Hooker) Trev. which circumstantially indicates the presence of facultative apomixis (the production of sporophytes from gametophytes by both sexual and apogamous pathways) and the production of both haploid spores and diploid spores with their subsequent germination and growth into gametophytes of two ploidy levels. Matteuccia orientalis is an Asian species distributed from India throughout China to Siberia, Korea, and Japan. It consists of two cytotypes, a diploid (n = 40) found mostly on the Asian mainland and a tetraploid (n = 80) found in Japan. Lloyd (1971) shows that the tetraploid cytotype is of autoploid origin.

The effect of reciprocal translocations on segregation and multivalent formation in autotetraploids of rye, Secale cereale

Chromosomal segregation, and the frequency of large multivalents in Secale cereale were studied in autotetraploid duplex translocation heterozygotes. Models for estimating expected segregations and frequencies of multivalents were developed incorporating the probabilities of different chromosomal segments being bound by chiasmata. It appeared that the segregation of the two translocations tested fitted quite well the expected corrected segregation ratio of approximately 1: 11.5: 1, suggesting that induced preferential pairing was not strong enough to enhance preferential segregation resulting from random translocation segregation. Interspecific hybrids with S. montanum carrying the same translocations showed strong preferential pairing, i.e. significant deviation from the expected ratios.