Polytene chromosomes of an archipelagic subgenus, <Emphasis Type="Italic">Inseliellum</Emphasis> (Diptera: Simuliidae)

The black fly subgenus Inseliellum is present on a series of archipelagos in the South Pacific. In this study, larval polytene chromosome maps of six Inseliellum species are presented. Chromosomal relationships among taxa were determined through shared fixed inversions or chromosomal landmark positioning. Three fixed inversions (IL-2, IIS-1, and IIIS-1) were shared among species, as was the position of the nucleolar organizer (NO) (IL or IIL). Comparisons to two previously studied species of Inseliellum are included to produce a cytological transformation series among eight taxa. The NO position defines two clades in the phylogeny of Inseliellum, herein named the NO-IL and NO-IIL clades. The utility of this cytological data set is discussed.     

ANOLIS SEX CHROMOSOMES ARE DERIVED FROM A SINGLE ANCESTRAL PAIR

To explain the frequency and distribution of heteromorphic sex chromosomes in the lizard genus Anolis, we compared the relative roles of sex chromosome conservation versus turnover of sex‐determining mechanisms. We used model‐based comparative methods to reconstruct karyotype evolution and the presence of heteromorphic sex chromosomes onto a newly generated Anolis phylogeny. We found that heteromorphic sex chromosomes evolved multiple times in the genus. Fluorescent in situ hybridization (FISH) of repetitive DNA showed variable rates of Y chromosome degeneration among Anolis species and identified previously undetected, homomorphic sex chromosomes in two species. We confirmed homology of sex chromosomes in the genus by performing FISH of an X‐linked bacterial artificial chromosome (BAC) and quantitative PCR of X‐linked genes in multiple Anolis species sampled across the phylogeny. Taken together, these results are consistent with long‐term conservation of sex chromosomes in the group. Our results pave the way to address additional questions related to Anolis sex chromosome evolution and describe a conceptual framework that can be used to evaluate the origins and evolution of heteromorphic sex chromosomes in other clades.     

Meiotic drive shapes rates of karyotype evolution in mammals

Chromosome number is perhaps the most basic characteristic of a genome, yet generalizations that can explain the evolution of this trait across large clades have remained elusive. Using karyotype data from over 1000 mammals, we developed and applied a phylogenetic model of chromosome evolution that links chromosome number changes with karyotype morphology. Using our model, we infer that rates of chromosome number evolution are significantly lower in species with karyotypes that consist of either all bibrachial or all monobrachial chromosomes than in species with a mix of both types of morphologies. We suggest that species with homogeneous karyotypes may represent cases where meiotic drive acts to stabilize the karyotype, favoring the chromosome morphologies already present in the genome. In contrast, rapid bouts of chromosome number evolution in taxa with mixed karyotypes may indicate that a switch in the polarity of female meiotic drive favors changes in chromosome number. We do not find any evidence that karyotype morphology affects rates of speciation or extinction. Furthermore, we document that switches in meiotic drive polarity are likely common and have occurred in most major clades of mammals, and that rapid remodeling of karyotypes may be more common than once thought.     

Chromosomal analysis of twelve species of Microhylidae (Anura) from Madagascar

The karyotypes of four species of Dyscophinae and eight species of Cophylinae were analyzed. The chromosome number was 2n=26 in all cases. Between the two subfamilies a difference in the form of the karyotype was observed; the chromosomes show a gradual decrease in length in the Dyscophinae, whereas in the Cophylinae the karyotype demonstrates a clear discontinuity of size between pairs 5 and 6.Chromosomal polymorphism was found in Plethodontohyla tuberata, the chromosomes of pair 4 were subtelocentric in the homozygous specimens, whereas this pair showed a subtelocentric and a submetacentric chromosome of equal length in the heterozygous one, suggesting a pericentric inversion. Although in the Cophylinae the chromosome number is constant, the number of chromosome arms is variable. Pericentric inversions seem to play an important role in the chromosomal evolution of the Cophylinae.     

Inversions are bigger on the X chromosome

In many insects, X‐linked inversions fix at a higher rate and are much less polymorphic than autosomal inversions. Here, we report that in Drosophila, X‐linked inversions also capture 67% more genes. We estimated the number of genes captured through an approximate Bayesian computational analysis of gene orders in nine species of Drosophila. X‐linked inversions fixed with a significantly larger gene content. Further, X‐linked inversions of intermediate size enjoy highest fixation rate, while the fixation rate of autosomal inversions decreases with size. A less detailed analysis in Anopheles suggests a similar pattern holds in mosquitoes. We develop a population genetic model that assumes the fitness effects of inversions scale with the number of genes captured. We show that the same conditions that lead to a higher fixation rate also produce a larger size for inversions on the X.     

Cytosystematics of the Simulium tuberosum group (Diptera: Simuliidae) in Thailand

The polytene chromosomes of 3347 larvae of the Simulium tuberosum group in Asia were analysed, representing the largest ever cytogenetic study of black flies in the Oriental Region. Band‐by‐band comparisons, relative to the established standard chromosome map for the subgenus Simulium, revealed 17 cytogenetically distinct taxa in Thailand, plus an 18th in China. Six of these taxa correspond to morphologically described species (S. doipuiense, S. rufibasis, S. setsukoae, S. tani, S. yuphae and S. weji). Recognition of the 18 taxa is based largely on unique inversions, either fixed or sex linked, primarily in the long arm of chromosome III. The greatest cytological diversity was discovered in the S. tani lineage, with ten cytoforms. This marked chromosomal diversification within S. tani is based largely on two inversions that have assumed different roles over evolutionary time, variously functioning in different combinations as fixed inversions, sex‐linked inversions and autosomal polymorphisms. Shared unique chromosomal features, relative to the subgeneric standard chromosome map, allowed evolutionary relationships among the cytotaxa to be inferred. Fluctuations in climate during the Pleistocene might have promoted differentiation of the Southeast Asian S. tuberosum group in isolated refugia such as mountains. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155 , 289–315.     

Chromosomal evolution within the family Estrildidae (Aves) III. The Estrildae (waxbill finches)

The C- and G-banded karyotypes of five species of waxbill finches belonging to the Estrildidae were examined. Extensive chromosomal variation including inversions and fissions was found to differentiate the species, showing the waxbills to be the most chromosomally diverse group of estrildid finches. None of the variation, however, matched that recorded in related species of Pytilia (Christidis, 1983). By comparing the G-banded karyotypes of species belonging to all three estrildid lineages it was possible to reconstruct the presumed ancestral karyotype for the Estrildidae as a whole. This was found to approximate the karyotype of the Australian peophilid species, Poephila guttata most closely. From it chromosomal relationships within the three estrildid lineages, Poephilae, Lonchurae, Estrildae, can be determined.     

Increased differentiation and reduced gene flow in sex chromosomes relative to autosomes between lineages of the brown creeper Certhia americana

The properties of sex chromosomes, including patterns of inheritance, reduced levels of recombination, and hemizygosity in one of the sexes may result in the faster fixation of new mutations via drift and natural selection. Due to these patterns and processes, the two rules of speciation to describe the genetics of postzygotic isolation, Haldane's rule and the large‐X effect, both explicitly include quicker evolution on sex chromosomes relative to autosomes. Because sex‐linked mutations may be the first to become fixed in the speciation process, and appear to be due to stronger genetic drift (in birds), we may identify pronounced genetic differentiation in sex chromosomes in taxa experiencing recent speciation and diverging mainly via genetic drift. Here, we use nine sex‐linked and 21 autosomal genetic markers to investigate differential divergence and introgression between marker types in Certhia americana. We identified increased levels of genetic differentiation and reduced levels of gene flow on sex chromosomes relative to autosomes. This pattern is similar to those observed in other recently‐divergent avian species, providing another case study of the earlier role of sex chromosomes in divergence, relative to autosomes. Additionally, we identify three markers that may be under selection between Certhia americana lineages.     

Evolution of karyotype,sex chromosomes,and meiosis in mygalomorph spiders (Araneae: Mygalomorphae)

Spider diversity is partitioned into three primary clades, namely Mesothelae, Mygalomorphae, and Araneomorphae. Mygalomorph cytogenetics is largely unknown. Our study revealed a remarkable karyotype diversity of mygalomorphs. Unlike araneomorphs, they show no general trend towards a decrease of 2n, as the chromosome number was reduced in some lineages and increased in others. A biarmed karyotype is a symplesiomorphy of mygalomorphs and araneomorphs. Male meiosis of some mygalomorphs is achiasmatic, or includes the diffuse stage. The sex chromosome system X₁X₂0, which is supposedly ancestral in spiders, is uncommon in mygalomorphs. Many mygalomorphs exhibit more than two (and up to 13) X chromosomes in males. The evolution of X chromosomes proceeded via the duplication of chromosomes, fissions, X–X, and X‐autosome fusions. Spiders also exhibit a homomorphic sex chromosome pair. In the germline of mygalomorph males these chromosomes are often deactivated; their deactivation and pairing is initiated already at spermatogonia. Remarkably, pairing of sex chromosomes in mygalomorph females is also initiated at gonial cells. Some mygalomorphs have two sex chromosome pairs. The second pair presumably arose in early‐diverging mygalomorphs, probably via genome duplication. The unique behaviour of spider sex chromosomes in the germline may promote meiotic pairing of homologous sex chromosomes and structural differentiation of their duplicates, as well as the establishment of polyploid genomes. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 377–408.     

INVERSION LENGTH AND BREAKPOINT DISTRIBUTION IN THE DROSOPHILA BUZZATII SPECIES COMPLEX: IS INVERSION LENGTH A SELECTED TRAIT?

Length and position of breakpoints are characteristics of inversions that can be precisely determined on the polytene chromosomes of Drosophila species, and they provide crucial information about the processes that govern the origin and evolution of inversions. Eighty-six paracentric inversions described in the Drosophila buzzatii species complex and 18 inversions induced by introgressive hybridization in D. buzzatii were analyzed. In contrast to previous studies, inversion length and breakpoint distribution have been considered simultaneously. We conclude that: (1) inversion length is a selected trait; rare inversions are predominantly small while evolutionarily successful inversions, polymorphic and fixed, are predominantly intermediate in length; a nearly continuous variation in length, from small to medium sized, is found between less and more successful inversions; (2) there exists a significant negative correlation between length and number of polymorphic inversions per species which explains 39% of the inversion length variance; (3) natural selection on inversion length seems the main factor determining the relative position of breakpoints along the chromosomes; (4) the distribution of breakpoints according to their band location is non-random, with chromosomal segments that accumulate up to eight breakpoints.     

Chromosomal evolution of the Hawaiian Telmatogeton (Chironomidae,Diptera)

It is only in the Hawaiian Islands that species of the otherwise marine genus Telmatogeton have evolved into freshwater. An analysis of polytene chromosomes and karyotypes of two marine species and five freshwater species revealed that paracentric inversions and centric fusions were important in chromosomal evolution. The sequence of polytene chromosome bands common to most species, established as the Telmatogeton standard sequence, is found in a population of T. torrenticola from West Maui. Most species and other populations of T. torrenticola may be derived from the standard sequence by paracentric inversions. Similarities with the standard band sequence places T. japonicus (n=7) rather than T. pacificus (n=4) in the proposed phylogeny as the species closest to the marine ancestor of the freshwater species. One of three species (T. fluviatilis from Oahu, T. torrenticola from West Maui, or an undescribed species from East Maui), each with seven pairs of chromosomes is considered to be closest to the original freshwater species. T. torrenticola is a complex species in which there is an accumulation of fixed inversions and centric fusions in stepwise fashion in populations from west to east (West Maui n=7; East Maui n=6; Kohala Mountains n=5 and Mauna Kea n=4 both from the island of Hawaii). The population of T. torrenticola from Molokai has a reduced chromosome number (n=4) and fixed inversions. T. abnormis and T. hirtus, the only species which exhibit differentiated sex chromosomes, may be derived from the standard sequency by paracentric inversions. T. abnormis (n=4) has a simple XY system and T. hirtus (n=3/4) has a complex XY₁Y₂ system. Unique sequences of bands, differences in staining intensity of puffs and bands, and an inversion form the basis for the differentiation of the various Y-chromosomes in these species.     

Korrelationen zwischen der sexualdimorphen Gehirndifferenzierung und der Verhaltensauspr?gung bei Prachtfinken (Estrildidae)

Summary Sex dimorphism in song controlling brain areas is studied inUraeginthus bengalus, an Estrildid species, in which male and female sing regularly. Compared to Zebra finches the vocal brain centers such as RA, HVc and Area X are well developed in females ofUraeginthus.     

Chromosomal evolution within the family Estrildidae (Aves) II. The Lonchurae

Thirteen species of estrildid finches belonging to the Lonchurae were examined cytogenetically by G- and C-banding. The major forms of karyotypic change, both within and between species, were pericentric inversions and changes in the amount of heterochromatin. It appears that the direction of chromosome change in this lineage is towards an entirely telocentric karyotype because inversions converting a biarmed chromosome into a telocentric one only occur when all the macrochromosomes of smaller size are also telocentric. A comparison of hybrid fertility data and karyotypic differences indicates that genic factors affecting gonadal development, and not chromosomal rearrangements, are the primary influence in determining hybrid fertility. The chromosomal data was also used to clarify systematic relationships within the Lonchurae and demonstrate that the genus Lonchura as presently construed is polyphyletic.     

Genome structure and evolution in the cruciferous tribe Thlaspideae (Brassicaceae)

Whole-genome duplications (WGDs) and chromosome rearrangements (CRs) play the key role in driving the diversification and evolution of plant lineages. Although the direct link between WGDs and plant diversification is well documented, relatively few studies focus on the evolutionary significance of CRs. The cruciferous tribe Thlaspideae represents an ideal model system to address the role of large-scale chromosome alterations in genome evolution, as most Thlaspideae species share the same diploid chromosome number (2n = 2x = 14). Here we constructed the genome structure in 12 Thlaspideae species, including field pennycress (Thlaspi arvense) and garlic mustard (Alliaria petiolata). We detected and precisely characterized genus- and species-specific CRs, mostly pericentric inversions, as the main genome-diversifying drivers in the tribe. We reconstructed the structure of seven chromosomes of an ancestral Thlaspideae genome, identified evolutionary stable chromosomes versus chromosomes prone to CRs, estimated the rate of CRs, and uncovered an allohexaploid origin of garlic mustard from diploid taxa closely related to A. petiolata and Parlatoria cakiloidea. Furthermore, we performed detailed bioinformatic analysis of the Thlaspideae repeatomes, and identified repetitive elements applicable as unique species- and genus-specific barcodes and chromosome landmarks. This study deepens our general understanding of the evolutionary role of CRs, particularly pericentric inversions, in plant genome diversification, and provides a robust base for follow-up whole-genome sequencing efforts.     

Fissions,fusions, and translocations shaped the karyotype and multiple sex chromosome constitution of the northeast‐Asian wood white butterfly,Leptidea amurensis

Previous studies have shown a dynamic karyotype evolution and the presence of complex sex chromosome systems in three cryptic Leptidea species from the Western Palearctic. To further explore the chromosomal particularities of Leptidea butterflies, we examined the karyotype of an Eastern Palearctic species, Leptidea amurensis. We found a high number of chromosomes that differed between the sexes and slightly varied in females (i.e. 2n = 118–119 in females and 2n = 122 in males). The analysis of female meiotic chromosomes revealed multiple sex chromosomes with three W and six Z chromosomes. The curious sex chromosome constitution [i.e. W_1–3/Z_1–6 (females) and Z_1–6/Z_1–6 (males)] and the observed heterozygotes for a chromosomal fusion are together responsible for the sex‐specific and intraspecific variability in chromosome numbers. However, in contrast to the Western Palearctic Leptidea species, the single chromosomal fusion and static distribution of cytogenetic markers (18S rDNA and H3 histone genes) suggest that the karyotype of L. amurensis is stable. The data obtained for four Leptidea species suggest that the multiple sex chromosome system, although different among species, is a common feature of the genus Leptidea. Furthermore, inter‐ and intraspecific variations in chromosome numbers and the complex meiotic pairing of these multiple sex chromosomes indicate the role of chromosomal fissions, fusions, and translocations in the karyotype evolution of Leptidea butterflies.     

How chromosomal rearrangements shape adaptation and speciation: Case studies in Drosophila pseudoobscura and its sibling species Drosophila persimilis

The gene arrangements of Drosophila have played a prominent role in the history of evolutionary biology from the original quantification of genetic diversity to current studies of the mechanisms for the origin and establishment of new inversion mutations within populations and their subsequent fixation between species supporting reproductive barriers. This review examines the genetic causes and consequences of inversions as recombination suppressors and the role that recombination suppression plays in establishing inversions in populations as they are involved in adaptation within heterogeneous environments. This often results in the formation of clines of gene arrangement frequencies among populations. Recombination suppression leads to the differentiation of the gene arrangements which may accelerate the accumulation of fixed genetic differences among populations. If these fixed mutations cause incompatibilities, then inversions pose important reproductive barriers between species. This review uses the evolution of inversions in Drosophila pseudoobscura and D. persimilis as a case study for how inversions originate, establish and contribute to the evolution of reproductive isolation.     

Conserved ZZ/ZW sex chromosomes in Caribbean croaking geckos (Aristelliger: Sphaerodactylidae)

Current understanding of sex chromosome evolution is largely dependent on species with highly degenerated, heteromorphic sex chromosomes, but by studying species with recently evolved or morphologically indistinct sex chromosomes we can greatly increase our understanding of sex chromosome origins, degeneration and turnover. Here, we examine sex chromosome evolution and stability in the gecko genus Aristelliger. We used RADseq to identify sex‐specific markers and show that four Aristelliger species, spanning the phylogenetic breadth of the genus, share a conserved ZZ/ZW system syntenic with avian chromosome 2. These conserved sex chromosomes contrast with many other gecko sex chromosome systems by showing a degree of stability among a group known for its dynamic sex‐determining mechanisms. Cytogenetic data from A. expectatus revealed homomorphic sex chromosomes with an accumulation of repetitive elements on the W chromosome. Taken together, the large number of female‐specific A. praesignis RAD markers and the accumulation of repetitive DNA on the A. expectatus W karyotype suggest that the Z and W chromosomes are highly differentiated despite their overall morphological similarity. We discuss this paradoxical situation and suggest that it may, in fact, be common in many animal species.     

基于淫羊藿属(Epimedium L.)核型似近系数的聚类分析及其系统演化意义

为揭示淫羊藿属(Epimedium L.)植物染色体组遗传与进化,阐明该属植物系统亲缘关系和现代地理分布格局形成。该文对淫羊藿属植物51个分类群(43种、1亚种、6变种和1个栽培品种)和2种温哥华属(Vancouveria Morren et Decne.)植物的根尖进行了有丝分裂中期染色体核型分析,并运用核型似近系数聚类分析方法对这53个分类群植物的核型进行了聚类研究。结果表明:所有种类的染色体数均为12,二倍体(2n=2x=12),第1对同源染色体均为随体染色体,核型均为Stebbins的2A或1A型。可见,淫羊藿属植物染色体组在遗传进化中确实较为保守,种间核型非常相似。核型似近系数聚类分析为淫羊藿属植物系统进化研究提供了一些明显的线索。分析结果完全支持该属属下两个亚属(Subgen.Rhizophyllum和Subgen.Epimedium)的划分。亚属Epimedium的核型似近系数聚类结果显示,该类群物种间的系统亲缘关系与地理分布密切相关。核型似近系数分析结果还发现,来自东亚地区的淫羊藿属植物染色体组具有明显的变异,显示了更高的遗传多样性。基于上述研究结果,推断了淫羊藿属植物现代地理分布格局的形成过程。该研究结果可为淫羊藿属植物的资源利用、系统分类和遗传演化等领域的研究提供参考。     

Cytogenetics and evolution of Simulium ornatipes skuse (Diptera:Simuliidae) I. Sibling speciation

Larval populations of the morphologically distinct species Simulium ornatipes and S. neornatipes and a subspecies of the former, S. ornatipes norfolkense, were analysed using polytene chromosome cytology. Fixed inversion differences, distinct differentiated sex chromosomes and unique arrays of chromosomal polymorphisms reveal two sibling species, S. ornatipes A and B, within S. ornatipes, and two, S. neornatipes 1 and 2, within S. neornatipes. Chromosomally S. ornatipes norfolkense is similar to S. ornatipes A. An unprecedented level of shared inversion polymorphism is shown by S. ornatipes A and B indicating a very close relationship between them. While it may be postulated that such a situation has arisen by independent origin of the shared inversions within each species, or by introgression of these rearrangements between the two species, it is concluded that the shared polymorphisms in fact originate from a common ancestor. Within the four chromosomally defined species, differentiated sex chromosome systems occur which may involve any of the three chromosome pairs. Simulium ornatipes A is particularly interesting because there are indications that it may be polymorphic for differentiated sex chromosomes on two different elements and that it may also share a sex chromosome marker with S. ornatipes B, a unique situation in the Simuliidae. The evolution of sex chromosome systems in the four species is compared with that of other Simulids and its intimate connection with speciation emphasised. Finally the data on fixed inversions, differentiated sex chromosomes and shared polymorphisms is used to construct a phylogeny of the four species outlining their patterns of speciation.