Chromosomal rearrangements,genome reorganization,and speciation

Historical analysis of studying chromosome changes in evolution allows better understanding of the current level of research in this area. Reorganizations of the genetic system due to chromosomal rearrangements have important evolutionary consequences and may lead to speciation. Despite the complexity of evaluating the primacy of chromosome changes in speciation events, such phenomena are possible and occur in nature, as recent studies have demonstrated.     

Chromosomal rearrangements and speciation of sportive lemurs (Lepilemur species).

Theoretical configurations of meiotic chromosomes of potential hybrids between the different Lepilemur species were examined, and the classification of this genus was reviewed in the light of this information. Among the chromosomal rearrangements that occurred during the chromosomal evolution of the sportive lemurs, only those which would generate a pronounced reproductive barrier were considered in relation to the geographic distribution of this genus. The analysis showed that the pattern of geographic distribution is compatible with the inferred chronological occurrence of these chromosomal rearrangements in the phylogenetic tree of the genus Lepilemur.     

Chromosomal rearrangements in Plantago insularis Eastw.

Seeds of Plantago insularisEastw. which were irradiated with gamma rays yielded 37–67% semi-sterile plants. Twenty-four out of sixty-four of these plants were heterozygous for one or more chromosomal rearrangements. Twothirds of these were translocations, and one-third were inversions. Homozygous lines for four translocations were established. The karyotypes of these provide chromosome markers either at pachynema or in mitotic divisions, or both.Breakage positions were usually located within hetrochromatic segments or at the ends of heterochromatic regions (72.6% of all breaks), and half of all breaks occurred at the juncture of the centromere with the proximal heterochromatin. The consequences of proximal breakage were non-random, in that 93% of such breaks resulted in translocations and only 7% in inversions, whereas more than half of breaks in non-centromeric regions became involved in inversions.The individual chromosomes differed in the types of breakage and of aberrations produced, and these differences appeared correlated with length ratios of heterochromatic segments flanking the centromeres.The research for this paper was supported by National Science Foundation Grant Number GB 5713X.     

Chromosomal rearrangements in wheat: their types and distribution.

Four hundred and sixty polyploid wheat accessions and 39 triticale forms from 37 countries of Europe, Asia, and USA were scored by C-banding for the presence of translocations. Chromosomal rearrangements were detected in 70 of 208 accessions of tetraploid wheat, 69 of 252 accessions of hexaploid wheat, and 3 of 39 triticale forms. Altogether, 58 types of major chromosomal rearrangements were identified in the studied material; they are discussed relative to 11 additional translocation types described by other authors. Six chromosome modifications of unknown origin were also observed. Among all chromosomal aberrations identified in wheat, single translocations were the most frequent type (39), followed by multiple rearrangements (9 types), pericentric inversions (9 types), and paracentric inversions (3 types). According to C-banding analyses, the breakpoints were located at or near the centromere in 60 rearranged chromosomes, while in 52 cases they were in interstitial chromosome regions. In the latter case, translocation breakpoints were often located at the border of C-bands and the euchromatin region or between two adjacent C-bands; some of these regions seem to be translocation "hotspots". Our results and data published by other authors indicate that the B-genome chromosomes are involved in translocations most frequently, followed by the A- and D-genome chromosomes; individual chromosomes also differ in the frequencies of translocations. Most translocations were detected in 1 or 2 accessions, and only 11 variants showed relatively high frequencies or were detected in wheat varieties of different origins or from different species. High frequencies of some translocations with a very restricted distribution could be due to a "bottleneck effect". Other types seem to occur independently and their broad distribution can result from selective advantages of rearranged genotypes in diverse environmental conditions. We found significant geographic variation in the spectra and frequencies of translocation in wheat: the highest proportions of rearranged genotypes were found in Central Asia, the Middle East, Northern Africa, and France. A low proportion of aberrant genotypes was characteristic of tetraploid wheat from Transcaucasia and hexaploid wheat from Middle Asia and Eastern Europe.     

Chromosomal rearrangements induced by temperate bacteriophage D108.

As previously shown for mutator phage Mu-1, to which it is closely related, temperate bacteriophage D108 induces chromosomal rearrangements (replicon fusion and transposition of chromosomal segments) in its host genome.     

Chromosomal rearrangements and karyotype evolution in carnivores revealed by chromosome painting

Chromosomal evolution in carnivores has been revisited extensively using cross-species chromosome painting. Painting probes derived from flow-sorted chromosomes of the domestic dog, which has one of the most rearranged karyotypes in mammals and the highest dipoid number (2n=78) in carnivores, are a powerful tool in detecting both evolutionary intra- and inter-chromosomal rearrangements. However, only a few comparative maps have been established between dog and other non-Canidae species. Here, we extended cross-species painting with dog probes to seven more species representing six carnivore families: Eurasian lynx (Lynx lynx), the stone marten (Martes foina), the small Indian civet (Viverricula indica), the Asian palm civet (Paradoxurus hermaphrodites), Javan mongoose (Hepestes javanicas), the raccoon (Procyon lotor) and the giant panda (Ailuropoda melanoleuca). The numbers and positions of intra-chromosomal rearrangements were found to differ among these carnivore species. A comparative map between human and stone marten, and a map among the Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis), stone marten and human were also established to facilitate outgroup comparison and to integrate comparative maps between stone marten and other carnivores with such maps between human and other species. These comparative maps give further insight into genome evolution and karyotype phylogenetic relationships among carnivores, and will facilitate the transfer of gene mapping data from human, domestic dog and cat to other species.     

Chromosomal rearrangements breakpoints clusterization: is the clonal selection involved

Certain types of cancer are often correlate with certain chromosomal rearrangements. The chimaeric genes are formed as a result of this rearrangements, and the chimaeric proteins are the products of their expression. The breakpoints of such translocation are often clustered in the genome. Moreover, such breakpoint clusters often contain specific genomic elements like topoisomerase II consensus sites, nuclear matrix attachment regions and DNA sequences, which can make up secondary non-canonical structure. In this review we discuss whether breakpoints may be induced by chromatin structure. Furthermore, we bring up not touched upon literature question about the relation between the breakpoint clusters and the domain organization of corresponding proteins. We also consider possible mechanisms of chromosomal rearrangements.     

Chromosomal rearrangements induced in mouse spermatogonia by 14.5-MeV neutrons

Inbred CBA male mice were irradiated with 14.5-MeV neutrons. Three acute doses, 75, 150 and 250 rad, and one chronic dose, 250 rad, were given. The percentages of affected spermatocytes as counted from reciprocal translocations which had been induced in spermatogonia were 0.7, 0.8 and 1.6 respectively for the acute series and 2.2 after chronic exposure. The data could be fitted to a linear or concave curvilinear regression line. There seemed to be a slight increase of damage with dose, even if the percentages were generally lower than those reported earlier for fast neutrons with energies around 1 MeV. The existence of dose-rate effects is discussed, and the conclusion drawn so far is that there seems to be no such effect either for 1-MeV fast neutrons or 14.5-MeV high energy neutrons. The term “reversed dose-rate effect”, as used earlier, relates to another phenomenon. The difference between the point estimates for the chronic and acute 250 rad series is not significant. The effectiveness of neutrons with energies around 14 MeV versus neutrons with energies around 1 MeV is discussed.     

Chromosomal polymorphism and speciation in the genusEunemobius (Orthoptera: Grylloidea)

Karyotypes are described for the three species of ground crickets belonging to the North American genusEunemobius. All species possess three pairs of metacentric autosomes and a submetacentric X-chromosome. The sex-determination mechanism is XX-XO with 2n=7 in males and 2n=8 in females. There is no evidence that chromosomal rearrangements have been involved in speciation in the group.     

Chromosomal rearrangements and the genomic distribution of gene-expression divergence in humans and chimpanzees

The genomic DNA sequences of humans and chimpanzees differ by only 1.24%. Recently, however, substantial differences in gene-expression patterns between the two species have been revealed. In this article, we investigate the genomic distribution of such differences. Besides confirming previous findings about the evolution of sex chromosomes and duplications, we show that chromosomal rearrangements are associated with increased gene-expression differences in the brain and that rearrangements can have both direct and indirect effects on the expression of linked genes. In addition, our results are consistent with a role for some rearrangements in the original speciation events that separated the human and chimpanzee lineages.     

Chromosomal rearrangements associated with LINE elements in the mouse genome.

Two segments of DNA that have apparently inserted in the interval between the two adult beta-globin genes in BALB/c (Hbbd haplotype) but not in C57B1/10 (Hbbs haplotype) mouse strains have been described (1). These putative insertions, each about 1000 bp in length, mapped near a repetitive element. To determine the precise position of these alleged insertions, their target sites, and the nature of their boundaries, we cloned and sequenced the appropriate regions of both chromosomes. One of the two segments is not an insertion but rather a region between two independently integrated L1 repetitive elements (LINEs) (2), one in Hbbd and the other in the Hbbs chromosome. The other segment is an insertion of 940 bp which is located within the L1 element in the Hbbd chromosome. This insert is unusual in that it exists in only one copy in the BALB/c genome.     

Chromosomal rearrangements in the rye genome relative to that of wheat

Summary An RFLP-based genetic map of Secale Cereale has provided evidence for multiple evolutionary translocations in the rye genome relative to that of hexaploid wheat. DNA clones which have previously been mapped in wheat indicated that chromosome arms 2RS, 3RL, 4RL, 5RL, 6RS, 6RL, 7RS and 7RL have all been involved in at least one translocation. A possible evolutionary pathway, which accounts for the present day R genome relative to the A, B and D genomes of wheat, is presented. The relevance of these results for strategies designed to transfer useful genes from rye, and probably other related species, to wheat is discussed.     

Chromosomal rearrangements by an IS2 insertion in phage Mu-1

We have isolated and characterized a mutant of temperate phage Mu-1 carrying an IS2 insertion in the middle of its β region. This mutant gives rise spontaneously to secondary mutants which have deletions of different sizes adjacent to IS2. One particular derivative however, was found to have acquired an additional insertion sequence adjacent to IS2. This derivative gave rise to tertiary mutants carryinh a deletion next to the tandem insertion. The tandem insertion was located at the same place in the Mu β region as another 2.6 kb insertion independently isolated by Chow et al. (1977) and was found to be homologous to that insertion. The properties of this particular secondary mutant show that Mu phage particles lacking their S end are defective for growth and lysogenisation.     

Chromosomal and bionomic heterogeneities suggest incipient speciation in Anopheles funestus from Burkina Faso

Sampling of day-resting Anopheles funestus was carried out in September-November 1991, October-December 1992, and November 1994 at two sites near Ouagadougou, Burkina Faso: the small village of Noungou where humans outnumber cattle, and the nearby Fulani settlement of Loumbila where cattle outnumber humans. Collections made inside human dwellings were supplemented in 1992 by outdoor-resting samples from artificial pit-shelters. Indoor-resting An. funestus were also collected in November 1992 and November 1994 in four villages of the Banfora area (southern Burkina Faso) and in a sudanese-sahelian village in northern Burkina Faso (Tougouri). Half-gravid female sub-samples were preserved in carnoy's fixative and processed for polytene chromosome analysis. The material from the two villages near Ouagadougou was analysed by ELISA to know (i) the human/animal origin of the blood meal; (ii) the infectivity for Plasmodium falciparum malaria; and (iii) the possible correlation of these parameters with chromosomal variants. A total of 1416 An. funestus could be scored for the whole polytenic complement, while the origin of the blood meal and circumsporozoite protein (CSP) positivity were asserted from 1076 and 1154 specimens, respectively. With a few exceptions, four polymorphic paracentric inversions (3a, 3b, 2a and 5a in decreasing order of frequency) were observed in all populations. Inversion 2s, whose breaking points include those of inversion 2a, was found only as the heterokaryotype 2s/+ floating at an overall frequency of 3.7% in two villages of the Banfora area and in the two sites near Ouagadougou. Two heterokaryotypes 2a/t out of 186 scored specimens were observed in different years from one village of the Banfora area. Wide variations in inversion frequencies were observed among the samples without consistent geographical or temporal clines. Highly significant departures from Hardy-Weinberg equilibrium were recorded for inversions 3a and 3b in most samples, with the alternative homokaryotypes (standard and inverted) significantly more frequent than expected. Conversely, inversion 5a was in Hardy-Weinberg equilibrium in most samples, whereas the 2a-s inversion system was intermediate between these extremes. However, a deficit of heterokaryotypes was apparent practically in all samples. Significantly higher frequencies of the standard homokaryotypes were recorded (i) in the exophilic samples collected in Loumbila for arrangement 3a; (ii) in the animal-fed sub-sample collected outdoors in Noungou vs. the parallel human-fed sub-sample for arrangements 2a-s, 3a, and 3b, or vs. the samples obtained from indoor catches in both Loumbila and Noungou in the case of inversion 3a; (iii) in the December CSP-negative sub-sample from Loumbila vs. the parallel CSP-positive sub-sample for arrangement 2a. A plausible working hypothesis is that An. funestus in Burkina Faso includes two taxonomic units, one of which is mainly monomorphic standard with most inverted arrangements floating at very low frequencies, and probably uniquely characterised by arrangement 2s, while the other taxon is nearly fixed for arrangement 3ab and polymorphic for all the other inversions at intermediate to high frequencies. The latter would be characterised by a higher vectorial capacity and would probably correspond to An. funestus s.s. from East Africa. About the former hypothetical taxon, its endophily and anthropophily appear less marked and its relationship with other members of the An. funestus subgroup will require specific investigations.     

Chromosomal rearrangements associated with pelagic larval duration in Labridae (Perciformes)

The family Labridae is one of the largest and most important groups of reef fishes in the Southern Atlantic. There is a remarkable ecologic interest in this family because of their complex interactions in the reef environment. Predictions of genetic variability in fish based on biological patterns have often been contradictory. The present work aimed to increase the cytogenetic data about the family and verify the possible correlation between larval pelagic phase and chromosomal rearrangements based on the putative basal Perciformes karyotype (2n = 48a). Therefore, cytogenetic analyses were performed in the species Halichoeres brasiliensis (2n = 48, 48a, FN = 48); Halichoeres radiatus (2n = 48, 48a, FN = 48) and in three populations of Halichoeres poeyi (2n = 48, 4m + 44st-a, FN = 52) from Brazilian coastline. A conserved diploid number was observed in all species and populations. Single NORs were identified in H. brasiliensis and in two populations of H. poeyi (BA and RJ), while multiple NORs were observed in H. radiatus and in H. poeyi from Rio Grande do Norte. The constitutive heterochromatin is reduced and distributed over centromeric and pericentromeric regions. The ribosomal sites allowed differentiating two groups of H. poeyi along the Brazilian coast; one of them comprising the population from RN, bearing multiple NORs, and another representing the populations from BA and RJ, bearing single NORs. The recently separated species, H. brasiliensis and H. radiatus, although presenting similar diploid numbers and chromosomal formulae, could be distinguished by the number of NOR-bearing chromosomes. The results revealed an evolutionary pattern chiefly derived from pericentric inversions. The correlation between larval pelagic phase and cytogenetic data on Labridae indicates that the degree of karyotypic diversification reported within this family, ranging from a highly conserved to a derived pattern, is probably influenced by the species-specific duration of larval pelagic phase.     

Chromosomal rearrangements and genetic structure at different evolutionary levels of the Sorex araneus group

Robertsonian (Rb) fusions received large theoretical support for their role in speciation, but empirical evidence is often lacking. Here, we address the role of Rb rearrangements on the genetic differentiation of the karyotypically diversified group of shrews, Sorex araneus. We compared genetic structure between 'rearranged' and 'common' chromosomes in pairwise comparisons of five karyotypic taxa of the group. Considering all possible comparisons, we found a significantly greater differentiation at rearranged chromosomes, supporting the role of chromosomal rearrangements in the general genetic diversification of this group. Intertaxa structure and distance were larger across rearranged chromosomes for most of the comparisons, although these differences were not significant. This last result could be explained by the large variance observed among microsatellite-based estimates. The differences observed among the pairs of taxa analysed support the role of both the hybrid karyotypic complexity and the level of evolutionary divergence.     

Chromosomal and molecular rearrangements in somatic hybrids between tetraploid Medicago sativa and diploid Medicago falcata

The aim of this study was to produce somatic hybrids between tetraploid (2n=4x=32) M. sativa and diploid (2n=2x=16) M. ?falcata and analyse their genomic structure. Protoplasts from genotypes selected for regeneration ability from the cultivar Rangelander of M. sativa and Wisfal-1 of M. falcata were electrofused. Seven somatic hybrid calli were produced and one of them regenerated plants. The hybrid nature of these plants and their genetic composition were assessed with morphological, cytological, and molecular analyses. The resulting plants were hyper-aneuploid (2n=33) and contained one extra long chromosome, indicating that a translocation had taken place. The presence of both types of parental sequences in the RAPDs analysis confirmed the true hybrid nature of the plants. Rearrangements within the parental genomes and the presence of somaclonal variation among hybrid plants were observed through an RFLP analysis of the nucleolar organizing region (NOR). The possible causes for the gross genomic alterations, and the suitability of this method for transferring useful agronomic traits from wild species to cultivated alfalfa, are discussed.