The evolution of eutherian chromosomes

The gross organization of the genome of Eutheria (placental mammals) into chromosomes follows a simple architecture that, with some minor changes, is almost completely conserved for more than 100 million years in various species of almost all extant mammalian orders. Recent molecular cytogenetic results--especially those from the assumed oldest clade, the Afrotheria--suggest an ancestral karyotype that would calculate the "default" frequency of gross rearrangements to less than two changes within 10 million years of mammalian evolution. The main changes are the fission, movement and subsequent fusion of large chromosome segments or of chromosome arms. Reciprocal translocations are the exception. Chromosome numbers may have increased or decreased significantly in this fusion/fission process but, in most instances, the main architecture still remains evident. There are, however, some exceptions in mammals with extremely derived karyotypes.     

Integrative study on chromosome evolution of mammals, ants and wasps based on the minimum interaction theory

There is well-known evidence that in many eukaryotes, different species have different karyotypes (e.g. n=1-47 in ants and n=3-51 in mammals). Alternative (fusion and fission) hypotheses have been proposed to interpret this chromosomal diversity. Although the former has long been accepted, accumulating molecular genetics evidence seems to support the latter. We investigated this problem from a stochastic viewpoint using the Monte Carlo simulation method under the minimum interaction theory. We found that the results of simulations consistently interpreted the chromosomal diversity observed in mammals, ants and wasps, and concluded that chromosome evolution tends to evolve as a whole toward increasing chromosome numbers by centric fission. Accordingly, our results support the fission hypothesis. We discussed the process of chromosome evolution based on the latest theory of the molecular structure of chromosomes, and reconfirmed that the fission burst is the prime motive force in long-term chromosome evolution, and is effective in minimizing the genetic risks due to deleterious reciprocal translocations and in increasing the potential of genetic divergence. Centric fusion plays a biological role in eliminating heterochromatin (C-bands), but is only a local reverse flow in contrast to the previously held views.     

Two new karyotypes in the Peruvian owl monkey (Aotus trivirgatus)

The observation of remarkable karyotypic variation in owl monkeys (Aotus trivirgatus) stimulated us to study the chromosomal evolution of this New World genus. As an extension of this project, we examined the chromosome complement of a “phenotype-B” Aotus population from Peru. In addition to karyotype V(2n = 46), two new karyotypes with diploid numbers of 47 and 48 were identified. A G-band comparison of these karyotypes indicated that the chromosome number polymorphism in these Peruvian owl monkeys resulted from a single fusion or fission event involving a single metacentric and two acrocentric chromosome pairs. This mechanism is also known to be responsible for the chromosome number polymorphism in at least two other populations of phenotype B Aotus, one from Colombia and the other from Panama.     

Chromosome numbers and karyotypic evolution of Caraboidea

The chromosome numbers of 136 species of the Spanish caraboid fauna were studied. The most frequent karyotypes are 2n=37 (54 species) and 2n=24 (23 species), and the chromosome number ranges from 2n=21 to 2n=69, of which 2n=69 is the highest diploid number hitherto found among the Coleoptera. It is proposed that 2n=37 is the ancestral karyotype of the division Caraboidea and the suborder Adephaga as opposed to that of the suborder Polyphaga, 2n=20. Karyotypic evolution has led to increases and decreases of this number, both tendencies having taken place in four genera. Species of ten genera show a neo-XY bivalent due to an X-autosome fusion. The thirty-three chromosome numbers of Caraboidea reveal that these Coleoptera have a remarkable karyotypical heterogeneity.     

A test of the karyotypic fissioning theory of primate evolution

Karyotypic fissioning theory has been put forward by a number of researchers as a possible driving force of mammalian evolution. Most recently, Giusto and Margulis (BioSystems, 13 (1981) 267–302) hypothesized that karyotypic fissioning best explains the evolution of Old World monkeys, apes, and humans. According to their hypothesis, hominoid karyotypes were derived from the monkey chromosome complement by just such such a fissioning event. That hypothesis is tested here by comparing the G-banded chromosomes of humans and great apes with eight species of Old World monkeys. Five submetacentric chromosomes between apes and monkeys have identical banding patterns and nine chromosomes share the same pericentric inversion. Such extensive karyological similarities are not in accodance with, or predicted by karyotypic fissioning. Apparently, karyotypic fissioning is an extremely uneconomical model of chromosomal evolution. The strong conservation of banding patterns sometimes involving the retention of identical chromosomes indicates that ancient linkages of genes have probably been maintained through many speciation events.     

Microevolution, low clonal diversity and genetic affinities of parthenogenetic sitobion aphids in new zealand

In sharp contrast to their southeast Asian and European counterparts, Sitobion miscanthi and S. near fragariae aphids in Australia exhibit a complete absence of sexual reproduction. This demands an explanation within the context of the evolution and maintenance of sex and parthenogenesis. Accordingly, we executed a genetic analysis of the two species in neighbouring New Zealand. Microsatellites and single-stranded conformation polymorphism/sequence analysis of the nuclear gene elongation factor 1alpha were used to identify aphid clones and confirm species identification, respectively. Karyotypic variation was also investigated. The New Zealand fauna showed few (nonrecombining) genotypes and appears to have received migrants from both Australia and Asia. Other genotypes have apparently arisen in situ in New Zealand, exhibiting stepwise mutation of microsatellite alleles and also karyotypic change. Thus, these data represent rare evidence of evolution within wild-living parthenogenetic lineages. Karyotypic changes appear to occur at a rate even greater than that of microsatellite evolution. Strong geographical partitioning of genotypes/karyotypes was found, with certain ones predominating over large areas. These data suggest that clonal selection could be important in the distribution and patterning of genetic variation. We present a model to explain the genetic patterns, with particular reference to the absence of sexual reproduction in Sitobion aphids in New Zealand and Australia.     

Karyotype evolution in South American species of Hypochaeris (Asteraceae, Lactuceae)

The genus Hypochaeris (Asteraceae, Lactuceae) contains ten species in Europe, three in Asia, and approximately 50 in South America. Previous cytotaxonomic studies have shown two groups of taxa: (1) European species with different basic chromosome numbers and differentiated karyotypes, and (2) South American species with x=4 and uniform asymmetric and bimodal karyotypes. Karyotypic data are synthesized for South American species of Hypochaeris with new information for six Chilean species: H. acaulis, H. apargioides, H. palustris, H. spathulata, H. tenuifolia and H. thrincioides. Four main groups can be distinguished based on presence and localization of secondary constrictions – SCs (bearing Nucleolar Organizer Regions – NORs) on chromosomes 2 and 3, and 18S–25S and 5S rDNA loci number, localization, and activity. We propose karyotypic evolution of South American Hypochaeris (x=4) from H. maculata-like (x=5) European ancestors. The original South American karyotype would have possessed two SCs, one on the long arm of chromosome 2, and the other on the short arm of chromosome 3 (in terminal position). Further evolution would have involved inversion within the short arm of chromosome 3 and inactivation/loss of the SC on chromosome 2.     

Extensive genomic reshuffling involved in the karyotype evolution of genus Cerradomys (Rodentia: Sigmodontinae: Oryzomyini)

Rodents of the genus Cerradomys belong to the tribe Oryzomyini and present high chromosome variability with diploid numbers ranging from 2n=46 to 60. Classical cytogenetics and fluorescence in situ hybridization (FISH) with telomeric and whole chromosome-specific probes of another Oryzomyini, Oligoryzomys moojeni (OMO), were used to assess the karyotype evolution of the genus. Results were integrated into a molecular phylogeny to infer the hypothetical direction of chromosome changes. The telomeric FISH showed signals in telomeres in species that diverged early in the phylogeny, plus interstitial telomeric signals (ITS) in some species from the most derived clades (C. langguthi, C. vivoi, C. goytaca, and C. subflavus). Chromosome painting revealed homology from 23 segments of C. maracajuensis and C. marinhus to 32 of C. vivoi. Extensive chromosome reorganization was responsible for karyotypic differences in closely related species. Major drivers for genomic reshuffling were in tandem and centric fusion, fission, paracentric and pericentric inversions or centromere repositioning. Chromosome evolution was associated with an increase and decrease in diploid number in different lineages and ITS indicate remnants of ancient telomeres. Cytogenetics results corroborates that C. goytaca is not a junior synonym of C. subflavus since the karyotypic differences found may lead to reproductive isolation.     

Karyotypic diversity and speciation in Agrodiaetus butterflies

That chromosomal rearrangements may play an important role in maintaining postzygotic isolation between well-established species is part of the standard theory of speciation. However, little evidence exists on the role of karyotypic change in speciation itself--in the establishment of reproductive barriers between previously interbreeding populations. The large genus Agrodiaetus (Lepidoptera: Lycaenidae) provides a model system to study this question. Agrodiaetus butterflies exhibit unusual interspecific diversity in chromosome number, from n= 10 to n= 134; in contrast, the majority of lycaenid butterflies have n= 23/24. We analyzed the evolution of karyotypic diversity by mapping chromosome numbers on a thoroughly sampled mitochondrial phylogeny of the genus. Karyotypic differences accumulate gradually between allopatric sister taxa, but more rapidly between sympatric sister taxa. Overall, sympatric sister taxa have a higher average karyotypic diversity than allopatric sister taxa. Differential fusion of diverged populations may account for this pattern because the degree of karyotypic difference acquired between allopatric populations may determine whether they will persist as nascent biological species in secondary sympatry. This study therefore finds evidence of a direct role for chromosomal rearrangements in the final stages of animal speciation. Rapid karyotypic diversification is likely to have contributed to the explosive speciation rate observed in Agrodiaetus, 1.6 species per million years.     

基于比较核型分析方法追溯百合族 (百合科) 二型性核型的起源

百合族具有非常一致的二型性核型,由4条长染色体以及20条短染色体组成。目前有两个假说解释其二型性核型的来源,着丝粒横裂和多倍化。但是,具体是哪一种机制起主要作用仍然不清楚。根据文献以及自己的实验结果,我们整理并重新分析了百合亚科和美德兰亚科所有属的核型资料。比较核型分析结果表明,来自单条染色体的特征、染色体臂数、核型不对称性以及染色体的相对长度诸方面的证据都支持着丝粒横列是百合族核型进化的主要机制,但不能排除其它的机制也在起着作用,如臂间倒位和易位。臂间倒位和易位可能在郁金香族的核型进化中起着主要的作用。另外,本研究还报道了三个种的核型,粗茎贝母 (Fritillaria crassicaulis)、准格尔郁金香 (Tulipa suaveolens) 和尖果洼瓣花 (Gagea oxycarpa)。     

Robertsonian fusion and centric fission in karyotype evolution of higher plants

Robertsonian fusion and centric fission are uniquely detectable in comparative studies of karyotype patterns. They are the most important types of karyotype change in animals but seem to be relatively uncommon in higher plants. Both modify intra- and interchromosomal recombination and linkage relationships and consequently patterns of genetic variation. When differentiating populations or species they can produce postulating barriers to gene flow. The number of reported cases of fusion or fission in higher plants has increased over the years but remains low, and most of these are casual comparisons of karyotypes without any follow-up investigation. This review focuses on more adequate studies made in a few groups. Studies in the Tradescantieae produce the strongest evidence for fusion as a type of orthoselection in the subfamily. Some species ofLycoris are also considered to have evolved their karyotypes in that way. Some genera of slipper orchids and the cycad genusZamia have populations where atypical chromosome number increase can be attributed to fission probably as a result of stressful influences. It is suggested that fusion may have been involved in the evolution of many stable karyotypes and that fission is generally a secondary destabilizing mechanism which may lead to refusion in the long term. Their proven incidence remains making it unwise to suggest that they have been major influences in karyotype evolution in higher plants.     

Karyotypic conservatism in the suborder Feliformia (Order Carnivora)

Multidirectional comparative chromosome painting was used to investigate the karyotypic relationships among representative species from three Feliformia families of the order Carnivora (Viverridae, Hyaenidae and Felidae). Complete sets of painting probes derived from flow-sorted chromosomes of the domestic dog, American mink, and human were hybridized onto metaphases of the spotted hyena (Crocuta crocuta, 2n = 40) and masked palm civet (Paguma larvata, 2n = 44). Extensive chromosomal conservation is evident in these two species when compared with the cat karyotype, and only a few events of chromosome fusion, fission and inversion differentiate the karyotypes of these Feliformia species. The comparative chromosome painting data have enabled the integration of the hyena and palm civet chromosomes into the previously established comparative map among the domestic cat, domestic dog, American mink and human and improved our understanding on the karyotype phylogeny of Feliformia species.     

Tempo and mode in karyotype evolution revealed by a probabilistic model incorporating both chromosome number and morphology

Karyotype, including the chromosome and arm numbers, is a fundamental genetic characteristic of all organisms and has long been used as a species-diagnostic character. Additionally, karyotype evolution plays an important role in divergent adaptation and speciation. Centric fusion and fission change chromosome numbers, whereas the intra-chromosomal movement of the centromere, such as pericentric inversion, changes arm numbers. A probabilistic model simultaneously incorporating both chromosome and arm numbers has not been established. Here, we built a probabilistic model of karyotype evolution based on the “karyograph”, which treats karyotype evolution as a walk on the two-dimensional space representing the chromosome and arm numbers. This model enables analysis of the stationary distribution with a stable karyotype for any given parameter. After evaluating their performance using simulated data, we applied our model to two large taxonomic groups of fish, Eurypterygii and series Otophysi, to perform maximum likelihood estimation of the transition rates and reconstruct the evolutionary history of karyotypes. The two taxa significantly differed in the evolution of arm number. The inclusion of speciation and extinction rates demonstrated possibly high extinction rates in species with karyotypes other than the most typical karyotype in both groups. Finally, we made a model including polyploidization rates and applied it to a small plant group. Thus, the use of this probabilistic model can contribute to a better understanding of tempo and mode in karyotype evolution and its possible role in speciation and extinction.     

Karyotype polymorphism in the cycad Zamia loddigesii (Zamiaceae) of the Yucatan Peninsula, Mexico

The cycad %amia loddigesii Miq. forms a morphologically variable complex on the Yucatan peninsula, Mexico. Several diploid chromosome numbers have been found in the species: In = 17, 24, 25, 26 and 27. Differing karyotypes and chromosome numbers were found in individuals of the same population and die karyotypes differ widely in numbers of metacentric and telocentric chromosomes present. Centrometric fission as well as pericentric inversions and unequal translocations are suggested to be the probable mechanisms for this karyotype variation. There appears to be a correlation between high chromosome number and increasing dryness of the habitats. Coupled with the strongly asymmetrical karyotypes, this suggests that karyotype evolution in Z- loddigesii is recent.     

Using chromosomal data in the phylogenetic and molecular dating framework: karyotype evolution and diversification in Nierembergia (Solanaceae) influenced by historical changes in sea level

Karyotype data within a phylogenetic framework and molecular dating were used to examine chromosome evolution in Nierembergia and to infer how geological or climatic processes have influenced in the diversification of this solanaceous genus native to South America and Mexico. Despite the numerous studies comparing karyotype features across species, including the use of molecular phylogenies, to date relatively few studies have used formal comparative methods to elucidate chromosomal evolution, especially to reconstruct the whole ancestral karyotypes. Here, we mapped on the Nierembergia phylogeny one complete set of chromosomal data obtained by conventional staining, AgNOR‐, C‐ and fluorescent chromosome banding, and fluorescent in situ hybridisation. In addition, we used a Bayesian molecular relaxed clock to estimate divergence times between species. Nierembergia showed two major divergent clades: a mountainous species group with symmetrical karyotypes, large chromosomes, only one nucleolar organising region (NOR) and without centromeric heterochromatin, and a lowland species group with asymmetrical karyotypes, small chromosomes, two chromosomes pairs with NORs and centromeric heterochromatin bands. Molecular dating on the DNA phylogeny revealed that both groups diverged during Late Miocene, when Atlantic marine ingressions, called the ‘Paranense Sea’, probably forced the ancestors of these species to find refuge in unflooded areas for about 2 Myr. This split agrees with an increased asymmetry and heterochromatin amount, and decrease in karyotype length and chromosome size. Thus, when the two Nierembergia ancestral lineages were isolated, major divergences occurred in chromosomal evolution, and then each lineage underwent speciation separately, with relatively minor changes in chromosomal characteristics.     

Chromosomal fission accounts for small‐scale radiations in Zamia (Zamiaceae; Cycadales)

Zamia is unique among Cycadales in its diversity of morphology, ecology and chromosome numbers. The chromosome numbers in Zamia range from 16 to 28, excluding 20, manifest as both interspecific and intraspecific series. It has long been recognized that Robertsonian transformations (chromosomal fission or fusion) probably dominate karyotype evolution in Zamiaceae, although it has been debated whether chromosome numbers are increasing or decreasing. We re‐analyse published karyotypes of Zamia spp., relating both chromosome forms and sizes to recent phylogenetic data. We show that karyotype evolution is most probably moving towards increased asymmetry, with higher numbers of smaller chromosomes, thus supporting chromosomal fission. We also address additional hypotheses for increasing chromosome numbers, namely pericentric inversions and unequal translocations. Finally, we discuss the role of these chromosomal changes in evolutionary radiations. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2011, 165 , 168–185.     

Cytology and morphometry study of Alhagi (Leguminosae) species in Iran

Karyotypic and morphometric studies were performed on Alhagi species and populations. Morphometric analyses revealed the occurrence of two species in Iran. Both species possessed 2n = 16 chromosome number. Inter-population and inter-specific variation was observed in karyotypic features indicating that such data are not useful in taxonomy of the Alhagi species.     

Cytological study of Tibetia (Fabaceae) in the Hengduan Mountains region,China

The Hengduan Mountains comprise one of the world's most important hot spots of biodiversity. Tibetia (Ali) H.P. Tsui (Fabaceae), which has four or five species in two sections, is one of the genera endemic to the region. This paper describes for the first time the karyotype of three of those species. The chromosome counts of all three are 2 n = 16. The karyotypes of the species examined contain chromosomes of variable karyotypic symmetry with centromeres at median and submedian positions that correlate with the morphological characteristics of the species. Karyotypic variation at the diploid level appears to be the predominant feature of chromosome evolution in the genus and may provide a clue to the study of evolutionary patterns of plants in this region.     

Chromosome Numbers and Karyotypes of Six Oxytropis Species (Fabaceae) from the Qinghai Tibetan Plateau,China

Chromosome numbers and karyotypes of 13 populations of six Oxytropis species (Fabaceae) from the Qinghai Tibetan Plateau, China, were presented. The chromosome numbers and karyotypes in O.ochrocephala, O.tatarica, O.kansuensis and O.humifusa (2n=16) were reported for the first time. B chromosomes were found from O.stracheyana (2n=48). The basic chromosome number of x=8 is confirmed for the genus. The available chromosomal data indicate that polyploidy may have played an important role in the evolution of the genus, with the incidence of polyploidy in the genus reaching 58%. However, our results indicated that among the populations here examined only one was a hexaploid with 2n=48. Such a chromosomal pattern indicates that the karyotypic repatterning at the diploid level seems to be the predominant feature of chromosomal evolution in the Oxytropis species from the Qinghai Tibetan Plateau, and that sympatric speciation via hybridization and polyploidization has played a minor role in the species diversification of the genus from this area.     

Diagrammatic representation for chromosomal mutagenesis studies. I. Karyotypes most similar to that of man

In an attempt to discover which primate species have the karyotypes most similar to that of man, the chromosome numbers, the Nombre Fondamental and the numbers of chromosomal rearrangements separating the karyotypes of many species were considered. In addition to man, 6 species were selected for their karyotypic similarities and 1 for its dissimilarity. A model representing the karyotypes, by 2 diagrams, is proposed for these 8 species. These diagrams characterize each karyotype, as well as the theoretical random distribution of break-points, after the effect of a clastogenic agent. This allows one to make quantitative analyses of any type of chromosomal rearrangement, with objective criteria, and should improve the study of chromosomal mutagenesis, and particularly interspecies comparisons.