Low Chromosome Number Research Articles

AmaraandZabrus, two different patterns of karyotypic evolution (Coleoptera, Carabidae)

SUMMARYThe chromosome number of nine species belonging to the genus Amara varies between 2n = 37 and 2n = 20, whereas six species of Zabrus have 2n = 57 —59. These genera show different patterns in karyotypic evolution: 37 or lower chromosome numbers, mediocentric chromosomes, and alternation of XO—XY systems are found in Amara, whereas in Zabrus the chromosome numbers are higher than 46, there are small subtelocentric pairs, and XO sex chromosomes predominate.

Distribution of sinapine and related compounds in seeds of Brassica and allied genera

Total contents of aromatic choline esters and sinapine have been investigated in seeds of cruciferous species belonging to Arabis, Brassica, Cakile, Diplotaxs, Eruca, Hesperis, Matthiola, Raphanobrassica, Raphanus and Sïnapis. Choline esters such as sinapine, isoferuloylcholine, feruloylcholine, hesperaline and 4-hydroxybenzoylcholine, have been found in many of the plants under study. Qualitative and quantitative distribution patterns of these compounds are genus-characteristic. This suggests them to be a useful tool in the chemotaxonomy of Crucifereae. In spite of their genus-specific distribution pattern, the capacity of plants to accumulate and store phenolic choline esters in seeds is highly variable within the genus Brassica. Besides a genetic control, the aromatic choline esters content is also influenced by environmental conditions. Brassica species with high and low chromosome numbers were compared according to their qualitative and quantitative aromatic choline ester content. Genomic divergence or affinity among the species under study are discussed.

Chromosome Number and Meiotic Behavior of Cultivated Chia, Salvia hispanica (Lamiaceae)

Edible chia seeds, purchased from the local markets in Guatemala, Mexico, and southern California, were used for species identification, chromosome counts, karyotype construction, and meiotic analyses. Plants raised from those seeds had ovate leaves, dense racemose inflorescences, pale-blue flowers, and were identified from herbarium specimens as Salvia hispanica L. Mitotic analyses of root tips from 50 plants showed 2n = 12—the lowest chromosome number in the genus. Chromosomes were small, ranging from 2 to 3.5 pm. One pair of chromosomes was metacentric (with the long arm: short arm ratio, r = 1.5), four pairs were submetacentric (r = 2.6 to 3.6), and one pair was telocentric (r = 12). Meiosis was regular and six bivalents were observed at metaphase I. Ring and rod bivalents averaged 1.53 ± 1.05 and 4.47 ± 1.05, respectively;

Radiation-Induced Cell Transformation: Transformation Efficiencies of Different Types of Ionizing Radiation and Molecular Changes in Radiation Transformants and Tumor Cell Lines

This study aims to compare the efficiencies of 5.4 keV soft X-rays, alpha-particles, and gamma-rays in transforming C3H 10T1/2 cells and to assess the sequence of cellular and molecular changes during the process of radiation-induced transformation of Syrian hamster embryo (SHE) cells. The somewhat more densely ionizing soft X-rays are more effective than gamma-rays both for cell inactivation and cell transformation. The relative biological effectiveness (RBE) appears to be independent of dose; it is approximately 1.3 for either end point. The RBE of alpha-particles versus gamma-rays, on the other hand, increases with decreasing dose; the dose dependence is somewhat more apparent for cell transformation than for cell inactivation. SHE cells transformed by different types of ionizing radiation and related tumor cell lines isolated from nude mice tumors were found to have a distinct growth advantage compared to primary SHE cells, documented by higher plating efficiencies, shorter doubling times, and higher cloning efficiencies in semisolid medium. Most transformed and tumor cell lines that were investigated have elevated mRNA levels for the H-ras gene, some of them show restriction fragment length polymorphisms of the H-ras gene, and some exhibit a substantially amplified c-myc gene. In a sequence analysis of the Syrian hamster H-ras gene of eight tumor cell lines from radiation transformants, we have not found any mutation in codons 12, 13, 59, 61, nor in the flanking regions of these codons. The transformed and tumor cell lines tend to have lower chromosome numbers than primary SHE cells.

Radiation-induced cell transformation: transformation efficiencies of different types of ionizing radiation and molecular changes in radiation transformants and tumor cell lines.

This study aims to compare the efficiencies of 5.4 keV soft X-rays, alpha-particles, and gamma-rays in transforming C3H 10T1/2 cells and to assess the sequence of cellular and molecular changes during the process of radiation-induced transformation of Syrian hamster embryo (SHE) cells. The somewhat more densely ionizing soft X-rays are more effective than gamma-rays both for cell inactivation and cell transformation. The relative biological effectiveness (RBE) appears to be independent of dose; it is approximately 1.3 for either end point. The RBE of alpha-particles versus gamma-rays, on the other hand, increases with decreasing dose; the dose dependence is somewhat more apparent for cell transformation than for cell inactivation. SHE cells transformed by different types of ionizing radiation and related tumor cell lines isolated from nude mice tumors were found to have a distinct growth advantage compared to primary SHE cells, documented by higher plating efficiencies, shorter doubling times, and higher cloning efficiencies in semisolid medium. Most transformed and tumor cell lines that were investigated have elevated mRNA levels for the H-ras gene, some of them show restriction fragment length polymorphisms of the H-ras gene, and some exhibit a substantially amplified c-myc gene. In a sequence analysis of the Syrian hamster H-ras gene of eight tumor cell lines from radiation transformants, we have not found any mutation in codons 12, 13, 59, 61, nor in the flanking regions of these codons. The transformed and tumor cell lines tend to have lower chromosome numbers than primary SHE cells.

Linkage arrangement of restriction fragment length polymorphism loci in Brassica oleracea

A detailed genetic linkage map of Brassica oleracea was constructed based on the segregation of 258 restriction fragment length polymorphism loci in a broccoli × cabbage F2 population. The genetic markers defined nine linkage groups, covering 820 recombination units. A majority of the informative genomic DNA probes hybridized to more than two restriction fragments in the F2 population. "Duplicate" sequences having restriction fragment length polymorphism were generally found to be unlinked for any given probe. Many of these duplicated loci were clustered non-randomly on certain pairs of linkage groups, and conservation of the relative linkage arrangement of the loci between linkage groups was observed. While these data support previous cytological evidence for the existence of duplicated regions and the evolution of B. oleracea from a lower chromosome number progenitor, no evidence was provided for the current existence of blocks of homoeology spanning entire pairs of linkage groups. The arrangement of the analyzed duplicated loci suggests that a fairly high degree of genetic rearrangement has occurred in the evolution of B. oleracea. Several probes used in this study were useful in detecting rearrangements between the B. oleracea accessions used as parents, indicating that genetic rearrangements have occurred in the relatively recent evolution of this species.

Paired arrangement of nonhomologous centromeres during vertebrate spermiogenesis

Indirect immunofluorescence staining with human anti-kinetochore antibodies was used to study the position of centromeres during vertebrate spermiogenesis. Many species of Amphibia have a low chromosome number and very large spermatids and spermatozoa. The number of kinetochore dots correlates exactly with the haploid chromosome number. This implies that kinetochore duplication occurs in the interval between meiosis I and meiosis II. The nonhomologous centromeres are arranged in tandem during the entire course of spermiogenesis and in mature spermatozoa. A higher order centromere arrangement was found in spermiogenic cells of Anura and Urodela. In mammals, immunofluorescence analysis is complicated by the extreme condensation of chromatin during spermiogenesis and the high chromosome numbers. Nevertheless, centromere-centromere associations were observed in mammalian round spermatids and sporadically in testicular spermatozoa. This indicates that pair-wise association of centromeres is a universal principle of centromere arrangement at the postmeiotic stage.

Defective chlorophyll a/b-binding protein genes in the genome of a homosporous fern.

The majority of homosporous ferns have a chromosome number that is severalfold greater than that of diploid seed plants. These fern species have therefore been generally considered to be of polyploid origin. Enzyme electrophoretic investigations have demonstrated, however, that within fern genera, species having the lowest chromosome numbers (n = 27-52) have the number of isozymes typical of diploid seed plants; there is no isozyme evidence for polyploidy of these plants. We have constructed a genomic DNA library from Polystichum munitum (n = 41), a homosporous fern, and have screened the library for sequences homologous to the chlorophyll a/b-binding (CAB) protein genes of higher plants. The majority of the sequences isolated and characterized by nucleotide sequence determination represent defective CAB genes. This result is in contrast to the situation in the genomes of diploid angiosperms, where most, and sometimes all, copies of the CAB gene family represent functional members. Several hypotheses could explain the existence of multiple defective CAB genes in P. munitum. (i) The defective CAB genes are the result of "gene silencing" following polyploidy. (ii) P. munitum has not gone through a polyploidization event, but several, and perhaps the majority, of its CAB genes have mutated to a nonfunctional state (a phenomenon not yet observed in any of the genomes of non-fern plants so far examined). (iii) Some defective CAB genes have been specifically amplified in the genome of P. munitum.

Chromosomal polymorphisms involving telomere fusion, centromeric inactivation and centromere shift in the ant Myrmecia (pilosula) n=1

Detailed karyological surveys of the ant Myrmecia pilosula species group, which is characterized by the lowest chromosome number in higher organisms (2n=2), were attempted. We revealed that this species has developed highly complicated chromosomal polymorphisms. Their chromosome numbers are in the range 2n=2, 3, and 4, and six polymorphic chromosomes are involved, i.e., two for chromosome 1 (denoted as SM1 and ST1), three for chromosome 2 (A2, A′2, and M2), and M(1+2) for the 2n=2 karyotype. We suggested that these chromosomes were induced from a pseudo-acrocentric (A1M) and A2 as follows: (1) A1M→SM1 or ST1 by two independent pericentric inversions; (2) A2→A′2→M2 by chromosomal gap insertion and centromere shift; and (3) ST1+A2→M(1+2) by telomere fusion, where (3) means that the 2n=2 karyotype was derived secondarily from a 2n=4 karyotype. It is a noteworthy finding that active nucleolus organizer (NOR) sites, in terms of silver staining, are tightly linked with the centromere in this species, and that both the centromere and NOR of A2 were inactivated after the telomere fusion.

Phylogenetic Relationships of the Sorex araneus–arcticus Species Complex (Insectivora, Soricidae) Based on High-Resolution Chromosome Analysis

High-resolution chromosome analysis of four species of the Sorex araneus–arcticus complex (Insectivora, Soricidae), namely S. granarius, S. araneus, S. coronatus, and S. arcticus, revealed that their karyotypes are composed of the same original chromosome material that may have been present in chromosomes of S. granarius. The karyotypes of the other three species could have been derived from that of S. granarius by means of three types of chromosomal rearrangements: Robertsonian translocations, telomere–centromere translocations, and centromeric shifts. These types of rearrangements imply that the direction of chromosomal evolution in these species has been from high to low chromosome numbers. It is supposed that this reconstructed scheme of chromosomal evolution reflects the phylogenetic relationships among the species of the araneus–arcticus complex that were studied.

Chromosome analysis of preovulatory human oocytes and oocytes failing to cleave following insemination in vitro.

These investigations were carried out within a clinical in vitro fertilization and egg replacement program. A gradual fixation method for chromosome preparations of animal oocytes, when modified and applied to human oocytes, gave numerical analyzability in about 80%. Of preovulatory oocytes recovered in stimulated cycles, 53% of oocytes with numbers of chromosomes in the haploid range had an apparently normal set of 23 chromosomes. Oocytes with a very low number of chromosomes or in first meiosis were also found. In a separate investigation the chromosome constitution of oocytes not showing clinical signs of fertilization 48 hours after insemination in vitro was also studied. Of the oocytes with numbers of chromosomes in the haploid range, 50% had a normal 23,X karyotype. A different distribution of abnormal chromosome preparations was found, compared with the preovulatory oocytes. Twelve oocytes (16%) carrying between 10 and 18 chromosomes were found among the inseminated oocytes, but none in the other study. Moreover, 15 preparations displayed no chromosomes in the inseminated oocytes, whereas in the uninseminated group, none lacked chromosomes completely. These results may indicate how unfertilized human oocytes in vitro undergo chromosome loss and degeneration and could represent different stages of degeneration. Among both preovulatory and unfertilized oocytes the distribution between hyper- and hypohaploid oocytes was of particular interest in that hypohaploid chromosome complements were in excess. This finding supports the hypothesis that aberration in the number of chromosomes in an oocyte is caused not only by nondisjunction but also by anaphase lag, since nondisjunction alone would result in a 1: 1 ratio between hyper- and hypohaploid complements. In conclusion, human oocytes recovered for in vitro fertilization seem to have an incidence of numerical chromosome abnormalities in the region of 40-50%. This is close to the figures estimated on the basis of observations made in spontaneous abortions and could be a major explanation for the low developmental potential of human cleavage stage ova.

Short communication: Favorable prognosis of solid tumors with very low chromosome numbers?

In contrast to near-haploid leukemias, the rare solid cancers with very low chromosome numbers (below 34) may be characterized by a particularly favorable outcome, as was shown by three such tumors studied in this laboratory.

ARE LYCOPODS WITH HIGH CHROMOSOME NUMBERS ANCIENT POLYPLOIDS?

It has been established that the number of isozymes (different forms of an enzyme encoded by different gene loci) is highly conserved in diploid angiosperms and gymnosperms. In contrast, allopolyploid angiosperms display an increase in isozyme number due to the addition of divergent genomes. Lycopods (Microphyllophyta) are an ancient lineage of vascular plants having very high chromosome numbers. It has been maintained that lycopods acquired these high chromosome numbers through repeated episodes of polyploidy. Despite high chromosome numbers, however, lycopod species having the lowest chromosome numbers within genera possess the number of isozymes typical of diploid seed plants for all enzymes examined except triosephosphate isomerase. There is, therefore, no genetic evidence from enzyme electrophoresis for polyploidy in these plants. These results are comparable to findings for other homosporous pteridophytes including the ferns (Pteridophyta) and horsetails (Arthrophyta). Alternative hypotheses for widespread genetic diploidy in homosporous pteridophytes are 1) repeated cycles of allopolyploidy followed by gene silencing; 2) repeated cycles of autopolyploidy, which would result in duplicated, but not divergent genes for isozymes; 3) initiation of these lineages with relatively high chromosome numbers.

Modes of spontaneous chromosomal mutation and karyotype evolution in ants with reference to the minimum interaction hypothesis.

Aspects of chromosomal mutation and karyotype evolution in ants are discussed with reference to recently accumulated karyological data, and to detailed karyotype analyses of several species or species complexes with low chromosome number and unusual chromosomal mutations (the complexes of Myrmecia pilosula (Smith) (n = 1, 5 or 9 to 16); M. piliventris Smith (n = 2, 3-4, 17 or 32), and Ponera scabra Wheeler (n = 3 or 4, 2n = 7 or 8). Translocations and Robertsonian polymorphisms are confirmed to be non-randomly distributed among ants -the former are found at high frequencies in species with low chromosome numbers (n less than or equal to 12), while the latter predominate in those with high numbers (n greater than 12). This situation is consistent with the minimum interaction hypothesis of Imai et al. (1986), under which translocations are expected to occur most frequently in low-numbered karyotypes, and that the resulting genetic risks are minimized by increases in chromosome and/or arm numbers through centric fission and pericentric inversion. Centric fusion is considered to be a transient event in karyotype evolution, resulting from telomere instability in acrocentric chromosomes.

Karyotype and meiosis studies in three South African Pyrgomorpha species (Orthoptera: Pyrgomorphidae)

Two South African Pyrgomorpha species have reduced chromosome numbers, due to centric fusions between the largest autosomes and the medium and small autosomes. P. rugosa has 2n♂=11(XO) (4 pairs of submetacentric and 1 pair of acrocentric autosomes) and P. granulata has 2n♂=13(XO) (3 pairs of submetacentric and 3 pairs of acrocentric autosomes). A third South African species has a typical Pyrgomorphidae number of 2n♂=19(XO) (acrocentrics). The mean chiasma frequency of the 2n♂=19 species is higher than that of the other two, although the frequencies of distal chiasmata in all three are similar. The recombination potential of the two species with lower chromosome numbers has been reduced, due to fewer crossovers in comparison to the 2n♂=19 species, as well as to independent assortment.

Transfer of resistance traits from carrot into tobacco by asymmetric somatic hybridization: Regeneration of fertile plants

Transfer of methotrexate and 5-methyltryptophan resistance from carrot (Daucus carota) to tobacco (Nicotiana tabacum) was achieved by fusion between leaf mesophyll protoplasts of tobacco and irradiated cell culture protoplasts of carrot. Some of the regenerated somatic hybrids exhibited normal tobacco morphology with coexpression and independent segregation of the transferred resistance markers. Chromosomal instability resulted in aneuploid somatic hybrids with significantly lower chromosome number than predicted by simple addition of parental chromosome number. The methotrexate resistance phenotype was correlated with the expression of carrot-specific dihydrofolate reductase as judged by isozyme and immunological characteristics of the enzyme. The genomic construct of these somatic hybrids made the transmission of the resistance character into the next sexual generation possible.

Modifying effect of reduced glutathione on X-ray-induced chromosome aberration and cell-cycle delay in muntjac lymphocytes in vitro

Reduced glutathione treatment to unstimulated lymphocytes of Muntiacus muntjak 30 min prior to X-irradiation (2, 3 and 4 Gy) reduced the frequency of production of deletions in all the doses of X-ray and that of dicentrics and rings in 3- and 4-Gy exposed cultures. The consistency and degree of protection was more in the case of deletions than dicentrics and rings, and this may be correlated with the dominance in the production of deletions in the muntjac system which might probably be due to either the low chromosome number or a less efficient DNA repair. GSH pretreatment also reduced the radiation-induced cell-cycle delay in 2-Gy exposed cultures almost to the normal level of kinetics, although such a treatment to unirradiated cells caused a remarkable delay in cell progression. It is interpreted that GSH by binding to chromatin may be interfering with the replication unwinding and thus causing delay. But when the treatment is followed by X-irradiation, the free radicals which otherwise would have caused delay are scavenged by GSH and thereby get modified. GSH posttreatment did not reduce the radiation-induced delay but it yielded protection to terminal deletions. Lag periods were given to X-irradiated cultures which also reduced the radiation-caused cell-cycle delay effectively; however, this was not associated with any reduction in aberration frequencies. This suggests that the protection obtained for deletions and exchanges was independent of the reduction of cell-cycle delay obtained with GSH treatment.

The karyotype of the grass Zingeria biebersteiniana (2n = 4) by light and electron microscopy

The grass Zingeria biebersteiniana is one of only four angiosperm and two monocotyledonous species known with 2n = 4. Its 4C nuclear DNA amount, 7.4 pg, is similar to that of Haplopappus gracilis (4C = 8.2 pg), a dicotyledonous species with 2n = 4. Studies of the karyotype of Z. biebersteiniana using the electron microscope three-dimensional serial thin section reconstruction method showed it to be a highly suitable material in which to examine chromosome placement. Although the two chromosome types have very similar relative volumes (53 and 47%, respectively) and arm volume ratios (1.08 and 1.13, respectively), nevertheless, each chromosome are each arm could be reliably distinguished in single reconstructed cells at each stage of mitosis. Thus, the short arm of chromosome 1 was identified by its unique lighter stained nucleolus organizer region, while the short arm of chromosome 2 was distinguished from the long arm by its relatively smaller arm volume. The two chromosomes and their arms were also easily distinguished at mitotic metaphase in the light microscope by two diagnostic C-bands, one at the telomere of the short arm of chromosome 1 and the other near to, but not at, the short arm telomere of chromosome 2.Key words: Zingeria biebersteiniana (2n = 4), low chromosome number.

Genetic evidence suggests that homosporous ferns with high chromosome numbers are diploid

Homosporous ferns have usually been considered highly polyploid because they have high chromosome numbers (average n = 57.05). In angiosperms, species with chromosome numbers higher than n = 14 generally have more isozymes than those with lower numbers, consistent with their polyploidy. By extrapolation, homosporous ferns would be expected to have many isozymes. However, ongoing surveys indicate that within fern genera, species having the lowest chromosome numbers have the number of isozymes considered typical of diploid seed plants. Only species above these lowest numbers have additional isozymes. Therefore, homosporous ferns either have gone through repeated cycles of polyploidy and gene silencing or were initiated with relatively high chromosome numbers. The latter possibility represents a radical departure from currently advocated hypotheses of fern evolution and suggests that there may be fundamental differences between the genomes of homosporous ferns and those of higher plants. These hypotheses can be tested by genetic, karyological, and molecular techniques.

A cytological study ofPelargonium sect.Eumorpha (Geraniaceae)

The chromosome numbers of seven species ofPelargonium sect.Eumorpha have been determined from material of known wild origin, and karyotypic comparisons have been made. Within the section there is variation in basic chromosome number (x = 4, 8, 9, 11), variation in chromosome size, and two species have polyploid races. The three species with chromosome numbers based on x = 11 have the smallest chromosomes (1.0–1.5 µm); chromosomes are larger (1.0–3.0 µm) in the other species.P. elongatum has the lowest chromosome number in the genus (2n = 8).P. alchemilloides is exceptional in that it has four cytotypes, 2n = 16, 18, 34 and 36, and the form with 2n = 36 has large chromosomes (2.0–5.0 µm). Evidence from a synthesized hybrid suggests thatP. alchemilloides with 2n = 16 may be of polyploid origin. The three species based on x = 11 appear to be more closely related to species from other sections ofPelargonium that have the same basic chromosome number and small chromosome size, rather than to other species of sect.Eumorpha.