Primitive Chromosome Research Articles

Emergence of linkage between cooperative RNA replicators encoding replication and metabolic enzymes through experimental evolution.

The integration of individually replicating genes into a primitive chromosome is a key evolutionary transition in the development of life, allowing the simultaneous inheritance of genes. However, how this transition occurred is unclear because the extended size of primitive chromosomes replicate slower than unlinked genes. Theoretical studies have suggested that a primitive chromosome can evolve in the presence of cell-like compartments, as the physical linkage prevents the stochastic loss of essential genes upon division, but experimental support for this is lacking. Here, we demonstrate the evolution of a chromosome-like RNA from two cooperative RNA replicators encoding replication and metabolic enzymes. Through their long-term replication in cell-like compartments, linked RNAs emerged with the two cooperative RNAs connected end-to-end. The linked RNAs had different mutation patterns than the two unlinked RNAs, suggesting that they were maintained as partially distinct lineages in the population. Our results provide experimental evidence supporting the plausibility of the evolution of a primitive chromosome from unlinked gene fragments, an important step in the emergence of complex biological systems.

The chromosomal evolution and analyses of Turkish Cousinia species

In this study, Turkish Cousinia species were examined in point of chromosome number and morphology. To our knowledge, this work is the first comprehensive chromosomal study regarding Turkish Cousinia as a whole, especially for endemics. As consistent with previous reports, the chromosome numbers are mostly 2n = 26 and have been determined also as 2n = 24 for only the members of Cynaroideae and Sphaerocephalae sections. An interesting result of this paper is that there is not any polyploid species in this taxonomic group and all of them are diploids. Concerning the chromosome evolution of the Cousinia species, we concluded that merely dysploidy is effective within the genus and evolutionally a unipolar reduction goes on in the basic chromosome number in speciation (from 13 to 12). In terms of general chromosomal information, the chromosomes of the genus consist of metacentric and submetacentric types and they could be categorized in small category according to Lima De Faria classification (0.67?3.43). According to chromosomal indices used in this paper, while most species have symmetrical chromosomes, fewer have asymmetrical ones. From this point, it could be said that Cousinia stapfiana could be assessed as the most evolved taxa in karyomorphology according to the average of all indexes used in here. Besides, Cousinia bicolor, Cousinia birandiana, and Cousinia sintenisii are species having the most primitive chromosome within the genus.

CMA staining analysis of chromosomes in several species of Aurantioideae

Fluorochrome staining with chromomycin A3 (CMA) was used to characterize and compare the CMA banding patterns of chromosomes of 17 species from 13 genera of Aurantioideae, which is one of the seven subfamilies of Rutaceae. All species used in this study had 2n = 18 chromosomes. These chromosomes were classified into five types based on the number and position of CMA-positive bands; B: one telomeric and one proximal band, C: two telomeric bands, D: one telomeric band, E: without bands and Dst: type D with a satellite chromosome. Each species possessed two or four types of chromosomes and unique CMA banding patterns. The CMA banding patterns were 6D + 12E in Clausena anisata (Willd.) Hook. f.,1C + 3D + 14E in Murraya koenigii (L.) Spreng., 1C + 11D + 6E in Murraya paniculata (L.) Jack, 1B + 1C + 10D + 6E in Merrillia caloxylon (Ridl.) Swing., 1C + 9D + 7E + 1Dst in Paramignya lobata Burkill, 2C + 5D + 11E in Severinia buxifolia (Poir.) Tenore, 5D + 13E in Hesperethusa crenulata (Roxb.) Roem., 16D + 2E in Citropsis gabunensis (Engl.) Swing. et M. Kell, 14D + 4E in Citropsis schweinfurthii (Engl.) Swing. et M. Kell, 8D + 10E in Atalantia monophylla DC., 1C + 3D + 14E in Atalantia ceylanica (Arn.) Oliv., 2D + 16E in Atalantia roxburghiana Hook. f., 18E in Swinglea glutinosa (Blanco) Merr., 2D + 16E in Aegle marmelos (L.) Corr., 4D + 14E in Afraegle paniculata (Schum.) Engl., 2B + 2D + 13E + 2Dst in Feronia limonia (L.) Swing. and 3B + 9D + 6E in Feroniella oblata Swing. The advanced genera in each tribe and subtribe such as Merrillia and Feroniella showed heterochromatin (CMA-positive bands) rich chromosome configuration in general. Swinglea glutinosa without heterochromatin rich chromosome, which is considered to be the most primitive chromosome configuration, seems to be the key genus for clarifying the evolution of the tribe Citreae.

Chromosome numbers and chromosomal polymorphism in Finnish species of Galerucella Crotch (Chrysomelidae, Coleoptera)

Nokkala, S. and Nokkala, C. 1987. Chromosome numbers and chromosomal polymorphism in Finnish species of Galeruceh Crotch (Chrysomelidae, Coleoptera). - Hereditas 106: 51–58. Lund, Sweden. ISSN 0018–0661. Received February 27, 1986 Chromosome numbers and chromosomal polymorphism in Finnish species of Galerucella were studied. The chromosome number of G. nymphaeae (L.), G. lineoh (F.), and G. calmariensis (L.) was found to be Zn = 34 (16+Xyp); of G. tenella (L.) 2n = 26 (12+Xyp); and of G. pusilla (Duft.) 2 n = 16 (7+neoXY). The primitive chromosome number of the genus appears to be 2n = 34. Pericentric inversions, translocations and centric fusions were found to be the principal chromosomal mutations modifying the karyotype in the genus. In G. nymphaeae, sex chromosome polymorphism caused by an unusually large Y chromosome was found. The occurrence of the large Y chromosome was confined to a form of G. nymphaeae living on aquatic plants, allowing the conclusion that the forms of G. nymphaeae living on aquatic and terrestrial plants are reproductively isolated. Hence, they represent individual species, G. nymphaeae (L.) feeding on aquatic plants and G. sagittariae (Gyll.) feeding on terrestrial plants. All species studied were found to carry B-chromosomes of their own type, excluding G. nymphaeae and G. sagittariae, where B-chromosomes appeared to be similar. The size of B-chromosomes was found to vary from dot-like in G. nymphaeae group to a large metacentric in G. tenella, the latter comprising about 15 % of the total length of the karyotype. All B-chromosomes, except those in G. calmariensis, are isopycnotic during meiotic prophase and able to form a chiasmatic bivalent, when two B's are present.

Preapoptotic chromatin changes induced by ultraviolet B irradiation in human erythroleukemia K562 cells

Exponentially growing human erythroleukemia K562 cells were permeabilized and the dose dependent decrease of DNA synthesis rate was measured after ultraviolet (UV B, 290 nm) irradiation. Cells were able to overcome 2 and 5 J/m2 UV doses, partial recovery was observed at 15 J/m2, while at high (25 J/m2) UV dose replicative DNA synthesis remained suppressed. K562 cells were subjected to synchronization prior to and after UV irradiation (24 J/m2) and 18 fractions were collected by centrifugal elutriation. Cell cycle analysis by flow cytometry did not show early apoptotic cells after UV irradiation. The gradual increase in DNA content typical for non-irradiated cells was contrasted by an early S phase block between 2.2 and 2.4 C-values after UV irradiation. Cell cycle dependent chromatin changes after ultraviolet irradiation were seen as a fine fibrillary network covering the mainly fibrous chromatin structures and incompletely folded primitive chromosomes. Based on observations after UV irradiation and on earlier results with cadmium treatment and gamma irradiation, we confirm that typical chromatin changes characteristic to genotoxic agents can be recognized and classified.

Physical organization of the 1.709 satellite IV DNA family in Bovini and Tragelaphini tribes of the Bovidae: sequence and chromosomal evolution

Repetitive DNA in the mammalian genome is a valuable record and marker for evolution, providing information about the order and driving forces related to evolutionary events. The evolutionarily young 1.709 satellite IV DNA family is present near the centromeres of many chromosomes in the Bovidae. Here, we isolated 1.709 satellite DNA sequences from five Bovidae species belonging to Bovini: Bos taurus (BTA, cattle), Bos indicus (BIN, zebu), Bubalus bubalis (BBU, water buffalo) and Tragelaphini tribes: Taurotragus oryx (TOR, eland) and Tragelaphus euryceros (TEU, bongo). Its presence in both tribes shows the sequence predates the evolutionary separation of the two tribes (more than 10 million years ago), and primary sequence shows increasing divergence with evolutionary distance. Genome organization (Southern hybridization) and physical distribution (in situ hybridization) revealed differences in the molecular organization of these satellite DNA sequences. The data suggest that the sequences on the sex chromosomes and the autosomes evolve as relatively independent groups, with the repetitive sequences suggesting that Bovini autosomes and the Tragelaphini sex chromosomes represent the more primitive chromosome forms.

A scale invariant clustering of genes on human chromosome 7

BackgroundVertebrate genes often appear to cluster within the background of nontranscribed genomic DNA. Here an analysis of the physical distribution of gene structures on human chromosome 7 was performed to confirm the presence of clustering, and to elucidate possible underlying statistical and biological mechanisms.ResultsClustering of genes was confirmed by virtue of a variance of the number of genes per unit physical length that exceeded the respective mean. Further evidence for clustering came from a power function relationship between the variance and mean that possessed an exponent of 1.51. This power function implied that the spatial distribution of genes on chromosome 7 was scale invariant, and that the underlying statistical distribution had a Poisson-gamma (PG) form. A PG distribution for the spatial scattering of genes was validated by stringent comparisons of both the predicted variance to mean power function and its cumulative distribution function to data derived from chromosome 7.ConclusionThe PG distribution was consistent with at least two different biological models: In the microrearrangement model, the number of genes per unit length of chromosome represented the contribution of a random number of smaller chromosomal segments that had originated by random breakage and reconstruction of more primitive chromosomes. Each of these smaller segments would have necessarily contained (on average) a gamma distributed number of genes.In the gene cluster model, genes would be scattered randomly to begin with. Over evolutionary timescales, tandem duplication, mutation, insertion, deletion and rearrangement could act at these gene sites through a stochastic birth death and immigration process to yield a PG distribution.On the basis of the gene position data alone it was not possible to identify the biological model which best explained the observed clustering. However, the underlying PG statistical model implicated neutral evolutionary mechanisms as the basis for this clustering.

The Origin of Chromosomes I. Selection for Linkage

A model is analysed of cells containing independently replicating genes, which segregate randomly when the cell divides. We follow the fate of a primitive chromosome, in which two genes are linked, so that they replicate and segregate together. Such a chromosome increases in frequency in the population provided that (i) the two genes act synergistically, so that a cell containing at least one copy of each grows faster than a cell lacking one or other, and (ii) the number of genes per cell is small. The increase in frequency occurs even if the chromosome takes twice as long as an individual gene to replicate, giving a two-fold selective disadvantage within a cell. The increase occurs because a gene that is linked does not run the risk of finding itself, in the next generation, in a cell that lacks its synergistic partner.

The Relationship Between Dysploidy and Reproductive Capacity in Myrtaceae

Australian Myrtaceae show a positive correlation between base chromosome number and reproductive capacity. There is some evidence of a similar trend in other Australian plant families. Reduced chromosome numbers in the Myrtaceae arose in lineages with critically reduced reproductive capacities, occurring mainly in small shrub species with four or fewer ovules per flower and one seed per fruit or per fruit loculus. Dysploid reduction serves the short-term advantage of ensuring greater genetic uniformity through reduced recombination, therefore resulting in less seed wastage. However, lineages with reduced chromosome numbers (x=5-10) have reduced evolutionary flexibility, as evidenced by their failure to fully penetrate the arid zones, in contrast to the species with the primitive chromosome number or above (x=11 or 12). Trends for decreased ovule and seed numbers have apparently resulted partly from the selection for larger seed size. Among species with more than one ovule per flower but normally only one seed per fruit, there is evidence of physiological and genetic selection systems determining which of the fertilised ovules becomes the seed.

Karyology of some Indian annonaceae.

For better understanding of intrafamilial relations of Annonaceae, number and morphology of chromosomes were studied. Additional informations of 16 collections and previous reports were considered. An ascending series of n=7→8 was available in Annona spp., Malmea Tribe, Cleistopholis sp., Fusaea Tribe, Uvaria spp. Number n=9 was common in Polyalthia, Miliusa, Artabotrys (8→9) and Asimina. In n=7→8 lines, Uvaria and Desmos had primitive chromosome morphology. In n=8→9 lines Artabotrys showed primitive karyotypes, Miliusa and Asimina the most advanced ones.

Confirmation of primitive chromosome structure in the hexaploid wheats

Hybrids between Triticum aestivum cv Chinese Spring (CS) and T. dicoccoides of chromosome type E1a showed only a few or no trivalents at meiosis, but both trivalents and quadrivalents were shown by hybrids with six other types. Since there is strong evidence that the E1a type has the primitive chromosome structure of T. dicoccoides, Chinese Spring can be said to have the primitive chromosome structure of the hexaploid wheats in regard to reciprocal translocation.

Complex hybridity in Isotoma petraea. III. Lethal system in Θ12 Bencubbin

Analysis of selfed capsules of a ring of 12 (Θ12) complex heterozygote confirms the presence of a balanced lethal system. One of the complexes, S' is two interchanges removed from the primitive chromosome end sequence and when homozygous, causes the abortion of seeds after differentiation of the testa has begun. The other complex, Ne, is three interchanges removed from the primitive sequence and the lethals in it operate, when homozygous, before the testa differentiates. Twin hybrids from crosses between the Θ12 and an alethal structural homozygote exhibit epistatic segregation ratios in their selfed capsules, demonstrating that the lethals in the two complexes are not controlled by single loci. An hypothesis based on relatively translocated internal chromosome segments is developed as the basis for the evolutionary elaboration of the balanced lethal systems in I. Petraea complex hybrids.

CHROMOSOME NUMBERS IN COMPOSITAE. IV. AMBROSIEAE

New chromosome observations are reported for 30 species and varieties from 4 genera of Ambrosieae: Ambrosia (including Franseria), Diceria, Hymenoclea and Iva. Neither polyploidy nor aneuploidy is known in the genera Dicorea, Hymenoclea or Xanthium. Aneuplcid reduction appears to have played a role in the genome evolution of several species of Iva and Ambrosia. Polyploid species occur in both Iva and Ambrosia and polyploid series exist for at least 5 species or species aggregates of the latter. All available evidence indicates that the primitive chromosome number for the tribe is x = 18, differentiation and speciation having occurred at this level, which is here termed diploid. The group, however, must ultimately have been of polyploid origin from forms with x = 9.

Chromosome Numbers in Compositae. IV. Ambrosieae

New chromosome observations are reported for 30 species and varieties from 4 genera of Ambrosieae: Ambrosia (including Franseria), Diceria, Hymenoclea and Iva. Neither polyploidy nor aneuploidy is known in the genera Dicorea, Hymenoclea or Xanthium. Aneuplcid reduction appears to have played a role in the genome evolution of several species of Iva and Ambrosia. Polyploid species occur in both Iva and Ambrosia and polyploid series exist for at least 5 species or species aggregates of the latter. All available evidence indicates that the primitive chromosome number for the tribe is x = 18, differentiation and speciation having occurred at this level, which is here termed diploid. The group, however, must ultimately have been of polyploid origin from forms with x = 9.