Preleptotene chromosome contraction in Lilium longiflorum “Croft”

A period of chromosome contraction between premeiotic interphase and leptotene was regularly observed in three samples of Lilium longiflorum Croft. Extensive preleptotene chromosome contraction was also observed in L. longiflorum Ace and in the Lilium hybrid Enchantment. Although the stage resembles late mitotic prophase, microsporocytes never develop to metaphase, but despiralize to leptotene, and regular alignment, pairing and chiasma formation follow. As preleptotene chromosome contraction is discovered in an increasing number of organisms it becomes less likely that it represents a true reversion to mitosis. However its absence in many organisms and its extreme variability in others do not support the concept of preleptotene chromosome contraction as a regular meiotic stage. It is suggested that the line of demarcation between mitosis and meiosis is often imprecise, and meiocytes may fluctuate to some extent between these states before a final transition to meiosis is made. The occurrence and extent of this fluctuation may possibly be related to some externally produced substances required for the orderly development of meiotic prophase.     

Evidence on the time of chromosome pairing from the preleptotene spiral stage in Lilium longiflorum “Croft”

A period of chromosome spiralization and contraction was observed between premeiotic interphase and leptotene in Lilium longiflorum Croft. There was variation in the extent of preleptotene spiralization and contraction of chromosomes among microsporocytes, anthers and buds. The chromosomes sometimes contracted sufficiently to be visible as separate entities. It could then be determined that the chromosomes were single and entirely separate; synapsis and crossing-over had not yet occurred. Furthermore there was no evidence of alignment or association of homologues during the preleptotene contraction period; the chromosomes appeared to be distributed at random. The chromosomes subsequently elongated into the leptotene stage. Wherever they were visible separately the chromosomes were single in early leptotene. These observations support the classical view that synapsis of homologous chromosomes takes place during zygotene, followed by crossing-over at pachytene.It is a pleasure to dedicate this paper to Dr. Sally Hughes-Schrader on the occasion of her seventy-fifth birthday.     

Role of Wall-bound β-Glucanases in Regulating Tip-growth of Lilium longiflorum Pollen Tubes

β-Glucanases were found in the cell wall of Lilium longiflorum Thunb. pollen tubes grown in vitro . The activity of β-glucanases was, in a certain extent, decreased by nojirimycin, an inhibitor of glucosidase. Pollen germination percentage reduced dramatically when nojirimycin was applied in the culture medium. In case that nojirimycin was added at 0 or 1 h after the onset of incubation, the inhibition rate was 99.6% and 91.4%, respectively. When 3 mmol/L of nojirimycin was applied in the liquid medium at 0, 1, 1.5 and 2 h after the onset of incubation, the growth of pollen tubes was interrupted, which resulted in the morphological change of the pollen tubes such as the newly grown portion of pollen tubes being bent, curved and swollen. Tracing the growth pattern of the individual pollen tube grown in semi-solid medium by video microscopy, the authors demonstrated that pollen tube growth rate was strongly inhibited by nojirimycin at concentrations ranged from 0.003 to 3 mmol/L. Moreover, the cytoplasmic arrangement and the morphology of the pollen tubes were also affected by nojirimycin. The growth inhibition brought about by nojirimycin was reversible. These results indicated that β-glucanases, which degrade 1,3-β-glucan and/or 1,4-β-glucan or 1,3:1,4-β-glucan constructed in the cell wall, are involved in pollen germination and pollen tube growth. It provides new insight into an understanding of the contribution of β-glucanases to the cell wall extensibility and the crucial role of cell wall in regards to the regulation of pollen tube growth.     

The rôle of membrane-bound cytoplasmic inclusions during gametogenesis in Lilium longiflorum thunb.

In the prophase of both mega- and microsporogenesis, a sizeable proportion of the meiocyte cytoplasm becomes invested in double or multiple membrane-bround inclusions. This cytoplasm remains thus isolated from the rest of the cell until the completion of meiosis II in the female cells, or the young spore stage in those of the male. Significantly this encapsulation proceeds immediately the elimination of the major part of the ribosome population from the cytoplasm and, further, the electron microscope reveals that those ribosomes contained in these membranous inclusions remain unaffected by the lytic enzymes active elsewhere in the cytoplasm at this time. This encapsulated cytoplasm is proposed to fulfill two rôles; one, that it carries reserves necessary for postmeiotic development through from the diplophase to the haplophase environment and, two, that it permits the continuity of protein synthesis throughout meiosis I and II, a period when the major part of the protein synthetic apparatus is absent.Abbreviations MI membrane-bound inclusions - MMI multimembraned inclusions     

Premeiosis and meiosis in Lilium “Enchantment”

Various aspects of premeiosis and meiosis in Lilium Enchantment are described. There was evidence of a premeiotic slow-down but no cells in premeiotic despiralization were observed. A relationship was found between sequence of bud development, or reproductive age of the individual, and degree of preleptotene chromosome contraction. The sequence of development of microsporocytes in the anther differed from the apex-to-base order previously reported in Lilium and, in contrast to observations in L. longiflorum cultivars, the maximum degree of preleptotene contraction was found in basal, last developing microsporocytes. Delayed despiralization of telophase nuclei was observed. There were extremely rare cells in meiotic division in anthers in which all other archesporial cells had not yet reached premeiotic interphase. Extreme variation was observed among anthers in proportions of microsporocytes in mid-stages of meiosis as well as in preleptotene contraction. These observations are discussed in relation to meiosis readiness, meiotic behavior in early and late developing regions of reproductive organs and in aging individuals, synchrony of meiotic development and rates of meiotic division.     

Characterization and function of wall-bound exo-β-glucanases of Lilium longiflorum pollen tubes

Two exo-β-glucanases (LP-ExoI, 83 kDa and LP-ExoII, 71 kDa) were extracted and partially purified from the cell wall of Lilium longiflorum pollen tubes. Both LP-ExoI and LP-ExoII hydrolyzed laminarin (1,3-β-glucan). These enzymes also exhibited some activity toward 1,3:1,4-β-glucans of Hordeum vulgare and Cetraria islandica and the 1,6-β-glucan of Umbilicaria papullosa. The pH for optimum activity for both exo-β-glucanases was 5.5. Methylation analysis of the reaction products revealed that purified LP-ExoI decreased both 1,3- and 1,4-glucosyl linkages in hemicellulosic polysaccharides isolated from the cell wall of lily pollen tubes. D-gluconolactone and nojirimycin, inhibitors of glucosidase, inhibited activities of both exo-β-glucanases, as well as growth of the lily pollen tubes. These results disclosed that the wall-bound exo-β-glucanases play an important role in the regulation of lily pollen tube growth. Received: 3 January 2000 / Revision accepted: 8 March 2000     

Are Mitotic Functions Required in Meiosis?

Sporulation of diploid yeasts (Saccharomyces cerevisiae), homozygous or heterozygous for temperature-sensitive mitotic cell-cycle mutations, was examined at the restrictive and permissive temperatures. Twenty genes, represented by 32 heterozygotes and 60 homozygotes, were divided into three groups, showing (i) normal sporulation, (ii) no sporulation at the restrictive temperature but normal sporulation at the permissive temperature, (iii) no sporulation at both temperatures. Group (i) as well as several other strains were tested for their meiotic behavior with regard to intragenic recombination and haploidization. The conclusion reached was that all the mitotic nuclear-division and DNA-synthesis functions were required in meiosis. The only cell-division mutations not to affect meiosis were in three cytokinesis loci and in one budemergence locus.     

The effect of colchicine on meiosis in Lilium speciosum cv. “Rosemede”

Chromosome pairing and chiasma frequency were studied in meiocytes at diakinesis of Lilium speciosum cv. Rosemede fixed up to 21 days after the start of either continuous or 3 day pulse colchicine treatment. The two treatments gave similar results. In pulse treated pollen mother cells (PMCs) the mean chiasma frequency per cell fell from 26.4 in controls to 8.5 after fourteen days while the mean number of univalents per cell increased from 0.05 to 17.58. There was a negative correlation between mean chiasma frequency per bivalent and per PMC in colchicine treated buds; univalents were preferentially induced in bivalents with one chiasma, and preferentially excluded in bivalents with 4 chiasmata. Some chiasmata were redistributed to surviving bivalents despite the concurrent reduction in chiasma frequency per meiocyte. — Colchicine sensitivity began in premeiotic interphase and extended to mid or late zygotene in PMCs; ongoing synapsis was unaffected. However, susceptibility to univalency was asynchronous between bivalents occurring at zygotene in short chromosomes but at late premeiotic interphase in the longest chromosomes. The number of chiasmata per bivalent could be altered by colchicine without inducing univalents, but the ultimate effect was to reduce the number of chiasmata per bivalent (or per chromosome arm) directly to zero. The major factors determining the order and extent of reduced pairing and chiasma number were total chromosome length and arm length. Pairing and chiasma formation in embryo sac mother cells were less sensitive to colchicine than in PMCs, but their behavior was otherwise similar.     

A Switch in Microtubule Orientation during C. elegans Meiosis

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Meiosis and sporulation induced in sporophytes of Ulva mutabilis (“slender”) with synchronous mitosis

Summary Sporulation preceded by meiosis was induced in young, diploid plants of Ulva mutabilis, mutant slender (Föyn). The induction was brought about either by replacement of the old culture medium (sea water plus soil extract) with new, or by the addition of a mixture of seven vitamins to old medium. The new chemical environment seems to induce a shift from presumptive mitosis to presumptive meiosis.Supported by a Danish State scholarship '63/'64 from the Danish Ministry of Education.     

Meiosis in haploid barley — An interpretation of non-homologous chromosome associations

Detailed meiotic studies were conducted on ten haploid plants representing six different genotypes of barley (Hordeum vulgare, 2n=14). At pachytene stages the non-homologous chromosomes were observed to pair as intimately as homologous chromosomes in many cells. Foldback pairing, involving single chromosomes, and multivalent associations were common. At diplotene, up to 4 chiasmatalike structures were observed in paired chromosomes but it is not likely that they resulted from crossing over. At diakinesis the bivalent frequency mean was from 1 to 1.3 per cell whereas by metaphase I the paired associations were rare with a single rod bivalent being observed in 3 to 5% of the cells. The frequencies of various types of secondary associations at metaphase were also recorded. — The origin and significance of bivalents and secondary associations in haploids is reviewed and discussed. Caution is urged in the interpretation that low levels of chromosome pairing in haploids is evidence of homology. It is concluded that very little chromosome duplication is likely to be found within the haploid set of barley chromosomes and that the basic chromosome number is seven.     

Comments on “a framework for assessing readiness for tiger reintroductions”

We comment on five aspects of Gray et al.’s (Biodivers Conserv,  https://doi.org/10.1007/s10531-017-1365-1, 2017a) framework for assessing the readiness of sites for tiger reintroductions. Clarifications in numbers of tigers, available habitat and prey requirements indicate that more land and additional recovery of preferred prey species will be critical for a successful reintroduction. A focus on threat assessments and mitigations will be more important than a focus on site management tools. Local attitudes and levels of poaching require more attention than in the current assessment, as they are likely to be major obstacles to successful large carnivore reintroductions in Asia. Given the limited resources for rangewide tiger conservation, the value of such long-term initiatives must be weighed relative to the urgent need to recover and secure existing tiger populations before they also become extinct.     

Cytological investigations of the effect of colchicine on meiosis in Lilium hybrid cv. “Black Beauty” microsporocytes

Using colchicine, two methods have been successfully applied for the induction of tetraploid meiocytes at premeiotic mitosis in the near achiasmate diploid hybrid cultivar Black Beauty of Lilium. The experiments were aimed at understanding whether the achiasmate condition is attributable to insufficient homology for effective pairing or to some genie defect in order to interpret the data obtained from molecular studies. Cytological observations demonstrate a strong correlation between the induction of tetraploidy and subsequent chiasma formation. The chiasma frequency per cell in untreated diploid control meiocytes ranged from 0 to 8 (mean 2.25), while in colchicine treated undoubled cells sampled 14±2 days after the start of treatment it ranged from 0 to 2 (mean 0.148). By comparison, the chiasma frequency per cell in colchicine-doubled tetraploid cells ranged from 29 to 51 (mean 42.24), such cells showing complete or near complete bivalent pairing. These results, similar to those reported previously for other higher plant species, demonstrate that the achiasmatic condition in Black Beauty is not due to a genie defect. The methods developed have made possible hitherto inaccessible biochemical analyses of meiocytes that had been treated with colchicine or other chemicals at premeiotic stages.     

Meiosis in polyhaploid Hordeum: hemizygous ineffective control of diploid-like behaviour in a hexaploid?

Meiotic chromosome behaviour was studied in the hexaploid Hordeum parodii (2n=6x=42) and in six haploids (2n=3x=21) obtained from a cross between H. parodii and H. bulbosum (2n=2x=14) whereby all bulbosum chromosomes were selectively eliminated. The alloploid nature of H. parodii was evident from the exclusive bivalent formation at the hexaploid level and the low and variable number of bivalents in its haploid derivatives. In haploids, both nonhomologous (intragenomic) and homoeologous (intergenomic) chromosomes paired at prophase. Foldbacks in single chromosomes, bivalents and trivalents were observed at prophase and metaphase I. At diakinesis, the associations involved a maximum of 20 chromosomes which decreased to 12 by metaphase I. This decrease was attributed to the failure of the non-homologous associations to persist until metaphase I. A hemizygous-ineffective control for the diploid-like behaviour of the hexaploid parodii is proposed to explain the homeologous chromosome pairing in its haploid derivatives.     

Die Ausl?sung von Chromosomenmutationen in der Meiosis durch tiefe Temperatur

Zusammenfassung 1. In 3 aufeinanderfolgenden Jahren beträgt die spontane Chromosomenmutationsrate in der Diakinese der Meiosis vonOenothera Hookeri mit nur unwesentlichen Schwankungen 0,67%.2. In 2 aufeinanderfolgenden Jahren beträgt die Chromosomenmutationsrate abgeschnittener, in destilliertem Wasser und 6 Tage bei konstant 10° C gehaltener Infloreszenzen 2,3 bzw. 3,5%.3. Die Rate von Topfpflanzen nach 6tägigem Aufenthalt in 10° C unterscheidet sich mit 2,5% nicht von derjenigen abgeschnittener Infloreszenzen.4. Die durchschnittliche Zahl der Endbindungen in der Diakinese beträgt in allen 3 Untersuchungsjahren im Freiland 13,9%.5. Abgeschnittene Infloreszenzen zeigen unter den oben genannten Versuchsbedingungen in 2 aufeinanderfolgenden Jahren mit 11,1 bzw. 12,9 Endbindungen statistisch verschiedene Werte.6. Topfpflanzen haben mit 11,1 weniger Endbindungen als der entsprechende Parallelversuch mit abgeschnittenen Infloreszenzen (12,9).7. Aus den Befunden wird geschlossen, daß die Meiosis gegenüber Temperatur hinsichtlich der Bereitschaft zu Chromosomenmutationen empfindlicher als die Mitose reagiert.8. Im Gegensatz zu den bisherigen Deutungen auf Grund von Mitoseuntersuchungen wird die Wirkungsweise der Temperatur in einer Erhöhung der Zahl potentieller Brüche gesehen; es wird die Hypothese begründet, daß die tiefe Temperatur über eine Erhöhung der O₂-Löslichkeit im Wasser des Protoplasten wirkt.9. Bei den Temperaturversuchen besteht eine deutliche Korrelation zwischen Chiasmafrequenz und Mutationsrate. Die zahlreichen anderen, in dieser Beziehung geprüften mutagenen Agenzien lassen keine strenge Regelmäßigkeit in dem Verhältnis von Chiasmahäufigkeit und Mutationsrate erkennen.     

Die Ausl?sung von Chromosomenmutationen in der Meiosis Durch Einwirkung von Chemikalien

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