Low Chromosome Number Research Articles

Plant Regeneration from Mesophyll Protoplasts of Several Nicotiana Species

AbstractIn a search for model systems in plant cell genetics studies mesophyll protoplasts from eleven species of Nicotiana with low chromosome number (N. acuminata, N. alata, N. glauca, N. glutinosa, N. langsdorffii, N. longiflora, N. otophora, N. paniculata, N. plumbaginifolia, N. suaveolens, N. sylvestris) were shown to divide in a liquid culture medium. Plants were recovered from calli originating from protoplasts of all these species except N. glutinosa.

Karyotypes and Meiosis of the Morabine Grasshoppers Ii. The Genera Culmacris and Stiletta.

The genus Culmacris contains no species having the primitive XO:XX sex chromosome mechanism, as found in the tribe Morabini. Two of its species have an XY:XX sex-chromosome mechanism, due to the acquisition of an X-autosome fusion (F18) early in the phylogeny of the genus. It also includes a number of taxa with X1X2Y:X1X1X2X2 sex-chromosome systems. It is proposed that this condition arose from the XY:XX one as a result of a single Y-autosome fusion (F19). The species and races of the X1X2Y section of the genus differ from one another karyotypically in respect of the size and shape of the X2 and Y chromosomes, which are especially variable; in some of them the CD autosome has undergone dissociation into separate C and D acrocentrics (D5, D7 and D8) and in one taxon the AB has also undergone dissociation (D6). This extensive karyotypic diversification contrasts with the relative morphological uniformity of this complex. The genus Stiletta contains two species with the primitive XO:XX mechanism. Both of these have a fusion between two small autosomes (F8), and one of them has an additional fusion between small autosomes (F9), giving rise to 2n = 13, the lowest chromosome number known in the Morabinae.

Mitotic spindles of Drosophila melanogaster: A phase-contrast and scanning electron-microscope study

Mitotic spindles have been isolated from the blastema stage of Drosophila melanogaster embryos using modified tubulin-polymerizing medium. 'Clean' spindles, relatively free of contaminating cytoplasmic material, are obtained. Under phase contrast, mitotic stages appear remarkably similar to those seen in situ, as reported in early literature. This preservation of morphological integrity, coupled with relative structural simplicity due to low chromosome number (2n = 8), makes these spindles ideal subjects for study. Use of the scanning electron microscope provides excellent visulization of their general structural organization, changes in whole spindle structure during the course of mitosis, and higher resolution viewing of surface detail than is permitted with light microscopy.

Irradiation studies on the karyology of charophyta. I. X-rays and gamma-rays.

The present study deals with the effects of X-rays, gamma-rays on the karyology of some selected taxa of charophyta, antheridia having being exposed to radiations. Species of Nitella viz., N. opaca (n=6), N. flagelliformis (n=9) and N. acuminata (n=18) and four of Chara viz., C. fibrosa (n=14), C. globularis var. virgata (n=14), C. setosa (n=28) and C. zeylanica var. diaphora f. oerstidiana (n=42) were employed as experimental materials in various irradiation experiments. X-ray and gamma-ray doses ranged from 100 to 2000 rads. The materials in each case, after exposure to required amount of radiation, were transferred to fresh culture medium and were examined cytologically at varying periods of time. In all experiments controls were maintained.The qualitative changes that were observed in all the taxa with both types of radiations to a greater or lesser extent were: stickiness and of clumping of chromosomes at metaphase and anaphase, chromosome erosion, chromatid gaps, chromosome and chromatid breaks at metaphase and anaphase, anaphase bridges, laggards, unequal grouping of chromosomes, unequal separation, ring chromosomes and rarely, formation of micronuclei.The quantitative estimates of affected cells, based on percentages of those showing chromosome breakage at metaphase and anaphase and cells showing anaphase bridges, showed that levels of radiosensitivity of different taxa differed. The chromosomes of N. opaca having the lowest chromosome number (n=6) and of N. flagelliformis (n=9) with longest chromosomes seem to be more sensitive in comparison to other taxa, while C. zeylanica, with highest chromosome number in the series (n=42) as also with shortest chromosomes, was found to be more radioresistant of all the taxa investigated. However, charophyte taxa in the present study were shown to be more sensitive to radiations as compared with many other algal taxa belonging to other groups investigated earlier. The irradiation studies lend further support to the now more widely held view that Charophyta constitute a very distinctive group amongst algae. The generally held view that algae in general are more resistant to radiations in comparison to higher plants does not seem to apply to the members of Charophyta. On the basis of the present study it has been shown that the chromosomes of N. opaca and N. flagelliformis are as sensitive as to radiations as those of higher plants. The results obtained in the study were adequately discussed.

CYTOTAXONOMIC STUDIES ON THE GENUS BOONACRIS: I. The "eastern" taxa and a comparison with the related genera Dendrotettix and Appalachia (Orthoptera: Catantopidae: Podismini)

Thirteen populations of four distinct taxa in the wingless grasshopper genus Boonacris with an eastern distribution in Canada and the United States had a chromosome complement of 2n ♀; ♂ = 20A + XO;XX. A detailed analysis of chromosome lengths and of the heterochromatin content and distribution in the karyotypes of the populations sampled revealed extreme endophenotypic stability and conservatism. Two populations were found to contain heterochromatically variant karyomorphs, one being polymorphic with respect to a complex pattern of autosome and sex-chromosome heterochromatin expression, the other for the occurrence of a large heterochromatic supernumerary chromosome. The cytogenetic system of Boonacris species was compared with those of the related, largely sympatric species Appalachia arcana and Dendrotettix quercus. The last two species have 2n ♀; ♂ = 22A + XO;XX but their complements show structural differences. The available evidence from a number of podismine species suggests that the forms with the lower chromosome number (i.e. 21 ♀; 22 ♂) may have originated from a common 'proto-Podisma' ancestor with 23 chromosomes (♀) via evolutionary loss or elimination of a small, unstable megameric pair. This dichotomy of major phyletic lineages and other cytogenetic evidence is discussed in relation to a phenetic classification of the tribe.

HAPLOIDY IN HAPLOPAPPUS GRACILIS (N = 2)

Analysis of meiosis in a haploid (monoploid) sporophyte of Haplopappus gracilis (n = 2) showed a nonrandom distribution of chromosomes at anaphase I. Chromosomes A and B were associated at various prophase I stages in 34 % of the microsporocytes. Presumably, this association persisted long enough to disrupt random distribution in a portion of those meiocytes showing such an association. Pollen stainability of 26.5 % in the haploid was in agreement with the normal expectation. However, this number resulted from a nonrandom chromosome distribution. The usual method of predicting fertility of haploids, 1/2n, is not accurate for monoploids with low chromosome numbers, and more functional methods are proposed.

MARGINAL POPULATIONS OF CEPAEA NEMORALIS (L.) ON THE BRENDON HILLS, ENGLAND. II. VARIATION IN CHIASMA FREQUENCY.

In different species genetic recombination and the release of variation may be regulated in different ways and at different levels of organization since the components of the control system do not necessarily act in the same direction (e.g., Grant, 1958). For example, in herbaceous plants short generation times promote the release of variability and are often associated with low chromosome numbers: a restrictive device (Grant, 1958). The recombination system within a population may similarly be adjusted so that there is a balance between immediate fitness and long-term adaptability (Carson, 1955a). In populations of an outbreeding organism an important level of genetic control may be through the regulation of chiasma frequency since, given sufficient heterozygosity, a raised chiasma frequency will lead to an increase in the range of gametic genotypes. Differences in chiasma frequencies between populations are therefore possibly adaptive, perhaps reflecting differing ecological circumstances under which these populations exist. In Drosophila the frequency of inversion heterozygotes, in which crossing over is restricted, is important in determining the level of recombination within a population. In certain species, particularly D. robusta and D. willistoni, the amount of inversion heterozygosity decreases as the margins of the species' ranges are reached. In a series of papers (da Cunha et al., 1950; Townsend, 1952; da Cunha et al., 1959) the view was advanced that the degree of chromosomal polymorphism is a function of the variety of ecological niches in the territory which a population inhabits. Carson (1955a, b, 1956, 1964), however, suggested that greater chromosomal homozygosity at the margins serves to allow more recombination which is necessary to provide sufficient flexibility for adjustment to suboptimal conditions. He has also shown (Carson, 1958) that the response to selection in strains originating from marginal populations is greater than that in strains originating from central ones, perhaps reflecting the occurrence of more recombination in the former populations. A completely different situation occurs in the grasshopper Myrmeleotettix maculatus. The presence of B chromosomes has been shown to raise both the frequency and the variance of chiasmata in those individuals which contain them (Barker, 1960; Hewitt and John, 1967). The frequency of B-containing individuals varies between populations and it has been shown that this variation is related to climatic factors such as temperature and rainfall (e.g., Barker, 1966, Hewitt, 1973). Central populations in favorable (warm, dry) environments possess more B chromosomes and have higher chiasma frequencies than more marginal populations in stringent (colder, wetter) localities which have no B chromosomes. A comparable situation may occur in the grouse locust Tettigidea lateralis where Fontana and Vickery (1973) found that the effect of B chromosomes in a marginal population was to change chiasma position with the net effect of depressing recombination. Cepaea nemoralis (L.) has a haploid number of 22 chromosomes (Perrot, 1938). There is a large metacentric pair forming the A group, a medium sized pair, the B

Selection for diploid cells in suspension cultures of Haplopappus gracilis

CHANGES in the number and structure of chromosomes of plant cells cultured in vitro are of common occurrence1, and cell populations devoid of diploid karyotype have been reported2–4. The chromosome number is known to become variable even in clones derived from single cells5,6. In the case of Picea glauca Voss. (n = 21) callus cultures, a positive correlation between growth rate and chromosome number was observed7, which indicates a selection for polyploid cells. Only diploid cells were, however, observed in mitosis in suspension cultures of Haplopappus gracilis (Nutt.) Gray (n = 2) (ref. 2), Medicago sativa L. (n = 16) (ref. 8) and Crepis capillaris L. Wallr. (n = 3) (ref. 9). This could result from polyploid cells dividing less frequently than diploid cells, as diploid and polyploid cells differ physiologically. A strong selection for diploid cells was observed10 in Vicia hajastana Grossh. (n = 5) suspension cultures initiated from mature seeds. It therefore seemed likely that cell cultures of other plant species with a low chromosome number, n = 7 or less, would show a selection for diploid cells.

Effect of Different Chromosome Numbers of Tobacco Pollen on Plant Formation by Anther Culture and Fertilization

The ability of pollens with various chromosome numbers in fertilization to diploid plants and androgenetic plant formation by anther culture were compared by using several kinds of polyploid and aneuploid plants. The totipotency of pollens differentiated into plants by the anther culture was not likely to be strikingly affected by the difference of chromosome numbers, while the pollens fertilized to the diploid plants had a conspicuous advantage in lower chromosome numbers than the others.

Comparative nuclear cytology of Coelastrum

The nuclear cytology of two varieties of Coelastrum proboscideum and two strains of C. microporum is described. All four Coelastrum organisms possess low chromosome numbers. Coelastrum proboscideum var. gracile Vischer has six chromosomes; C. proboscideum var. dilatatum Vischer has four; C. microporum Naegeli (strain 280) has six; and C. microporum Naegeli (strain 281) has seven. Chromatin granules are observed in C. proboscideum var. gracile Vischer, C. proboscideum var. dilatatum Vischer, and C. microporum Naegeli (281) and their condensation leads to the formation of organized chromosomes. Chromatin threads are only observed in C. microporum Naegeli (280). It is evident that the four Coelastrum organisms are quite different in their chromosomal morphology and mitotic behavior. However, at present, it is not certain whether the chromosome numbers are the haploid or diploid numbers since sexual reproduction has not been confirmed and meiotic division has not been observed.

Chromosomes of the chocolate gourami: a cytogenetic anomaly.

The chocolate gourami, Sphaerichthys osphromonoides, has the lowest chromosome number reported for fishes, with 2n = 16 and n = 8. Other members of the family Belontiidae have somatic numbers of 42 to 48. Chromosome instability is demonstrated by the presence of somatic abnormalities and meiotic multivalents.

Incorporation of exogenous DNA into mammalian chromosomes.

MANY reports1–4 have shown that exogenous DNA can be taken up and incorporated into the nuclei of cells in culture as well as in vivo; but no morphological evidence has been presented to suggest that such foreign DNA actually becomes associated with the chromosomes of inoculated cells. The Indian barking deer, Muntiacus muntjak, has the lowest number of chromosomes yet reported for mammals, ♀ = 6; ♂ = 7 (ref. 5). The combination of such a low chromosome number and the unique morphology of each pair of chromosomes in the diploid set make this animal particularly attractive for comparative cytogenetic studies. The C- and G-banding patterns6–8 have also been established for this animal which allow even more detailed comparisons to be made than those based solely on gross chromosome morphology.

Cytological Analysis of the Parasexual Cycle in Dictyostelium discoideum

SUMMARY: We have developed a rapid and simple technique for examining chromosomes of the cellular slime mould Dictyostelium discoideum, based on standard cytological techniques involving fixation in methanol/acetic acid and staining with Giemsa stain. The haploid chromosome set is analysed. Four diploid strains are examined cytologically in order to determine their ploidal stability and to determine the pathway of haploidization in the parasexual cycle. In the absence of cloning, diploid strains eventually revert to haploid. Haploidization proceeds via transient aneuploidy. Aneuploids of all classes, n+1 through to 2n+2, have been demonstrated cytologically. It is concluded that aneuploids can divide but that they are unstable and readily lose chromosomes to form aneuploids of lower chromosome numbers and eventually haploids. Hence, the process of haploidization is similar to that found in the parasexual cycle of Aspergillus nidulans.

Pollen-Derived Haploids ofNicotiana Knightiana, N. raimondii,andN. attenuata

Haploid plantlets were obtained in large numbers in three diploid, 24-chromosome species of Nicotiana by culture of anthers at or just past the first pollen mitosis. The three species were N. Knightiana, N. raimondii, and N. attentiata. Efficiency of haploid production varied from about 10 per cent in N. attenuata cultures to 30 and 38 per cent respectively in cultures of N. raimondii and N. Knightiana. H-medium without hormones and standard cultural conditions were used. N. Knightiana appeared to be especially suitable for haploid studies on account of its high plantlet productivity, low chromosome number, and distinctive karyotype.

Location, structure and behaviour of C-heterochromatin during meiosis in Dichroplus silveiraguidoi (Acrididae-Orthoptera).

The constitutive heterochromatin of Dichroplus silveiraguidoi, a species which shows an exceptionally low chromosome number (2n=8), was studied at meiosis with a staining technique on normal and hypotonically treated specimens. The results showed: 1) an unusual behaviour of the heterochromatic blocks located in the so-called synaptic region of the sex bivalent (Neo Y-Neo X), which remains paired from early prophase through metaphase I; 2) in normal or in hypotonically treated cells a heterogeneous configuration of the C-heterochromatic blocks was observed. This configuration is characterized by the existence of small positive granules interconnected by euchromatic filaments and is enhanced by treatment with a low ionic strength solution; 3) A weakly positive stained (intermediate) material was demonstrated in the Neo X chromosome; 4) A large amount of heterochromatin is distributed in the form of granular material along the length of the autosomes and as telomeric and centromeric blocks in all chromosomes. The possible evolutionary mechanisms involved and the significance of the C-band heterochromatin demonstrated in this species are discussed.

CYTOTAXONOMY OF BOMBYLIIDAE (DIPTERA)

The karyotypes of 34 species of Bombyliidae are described, some in greater detail than others. It is suggested that two subfamilies, the Bombyliinae (for Homoeophthalmae) and Anthracinae (for Tomophthalmae) be recognized. In the Bombyliinae, one species has 2n = 8, one 2n = 10, three 2n = 12 and one 2n = 14. In the Anthracinae, two species have 2n = 10, four 2n = 12, six 2n = 14, three 2n = 16 and thirteen 2n = 18. Seventeen complements of Bombyliinae averaged 41.4 μ in total complement length (TCL) and thirtyeight of Anthracinae averaged 46.6 μ; so the 55 complements of the Bombyliidae averaged 44.9 μ in TCL, the complements with lower chromosome numbers averaging less than those with higher numbers. Thus Bombyliinae species have lower chromosome numbers and shorter complements than Anthracinae species which tend to have longer metacentric sex chromosomes. Thus both morphologically and karyotypically the Bombyliinae appear to be more advanced whereas the Anthracinae are a more primitive but highly variable group of species.

Evolution of karyotypes and differentiation in 13 Rattus Species

Karyotypes of 13 Rattus species collected in Asia and Oceania were analysed with special interest to karyotype evolution and species differentiation. They were classified into three groups according to their karyotype similarity. Four species (R. annandalei, R. exulans, R. muelleri and R. norvegicus) with 2n=42 and a karyotype similar to some of the polymorphic karyotypes in the Asian black rats (R. rattus) are classified into the first group. Pericentric inversion of some acrocentrics seemed to have caused the differentiation of these species. The other four species (R. bowersii, R. fuscipes, R. leucopus and R. conatus) with similar karyotypes as the above group, but lower chromosome numbers than 2n=42 are classified into the second group. Robertsonian fusion in some acrocentrics observed in the first group are suggested to have caused the development of the species in this group. The remaining four species (R. sabanus, R. canus, R. huang and R. niviventer) with karyotypes markedly different from the above two in having a fewer number of small metacentrics are classified into the third group. They seemed to be more primitive karyotypes than the other Rattus species. By the comparison between the polymorphic karyotypes in the black rat, and karyotypes in its related species it was suggested that the former had occurred as primary events to the differentiation of the latter. Parallelism between the karyotype evolution and the species differentiation was discussed.

Association of Homologous Chromosomes in the Somatic Cells of <i>Ornithogalum virens</i>

Homologous chromosomes were found to associate in pairs to a varying ex-tent in the root tip cells of Ornithogalum virens (2n=6). In approximately 2 per cent cells, meiosis-like behaviour, viz; bivalent configurations and 'pseudo-chias-mata' were seen. It is proposed that somatic association of homologous chromo-somes may be more prevalent in species with low chromosome number and having vegetative propagation.

Chromosome Numbers in Compositae VIII: Heliantheae

One hundred and ninety-three new counts are reported for the tribe Heliantheae of Compositae, mostly based on determinations of meiotic material, including first counts for the genera Adenothamnus, Chrysogonum, Enceliopsis, Guardiola, Isocarpha, Lipochaeta, Otopappus, and Oyedaea, as well as first counts for 66 species. The original counts are discussed in relation to those previously reported for the tribe, by genera and subtribe. Two-thirds of the approximately 150 genera and more than a third of the roughly 1500 species have now been examined. The incomplete knowledge of generic relationships in the tribe often make the interpretation of these chromosome numbers difficult. Three observations are documented and discussed: (1) genera with low chromosome numbers are few; (2) genera with aneuploid series are abundant; and (3) the original basic chromosome number in the tribe is probably in the range of x = 8 to x = 12.

CHROMOSOME NUMBERS IN COMPOSITAE VIII: HELIANTHEAE

One hundred and ninety‐three new counts are reported for the tribe Heliantheae of Compositae, mostly based on determinations of meiotic material, including first counts for the genera Adenothamnus, Chrysogonum, Enceliopsis, Guardiola, Isocarpha, Lipochaeta, Otopappus, and Oyedaea, as well as first counts for 66 species. The original counts are discussed in relation to those previously reported for the tribe, by genera and subtribe. Two‐thirds of the approximately 150 genera and more than a third of the roughly 1500 species have now been examined. The incomplete knowledge of generic relationships in the tribe often make the interpretation of these chromosome numbers difficult. Three observations are documented and discussed: (1) genera with low chromosome numbers are few; (2) genera with aneuploid series are abundant; and (3) the original basic chromosome number in the tribe is probably in the range of x = 8 to x = 12.