Differences In Diploid Number Research Articles

Karyotype reexamination of Corydoras paleatus (Siluriformes) and a review of the cytogenetics of genus with a focus on the ribosomal genes

The subfamily Corydoradinae is the second largest among the callictids, comprising three genera, with Corydoras Lacépède, 1803 being the largest in the number of species. The genus Corydoras has been divided into five groups of species based on differences in diploid number, chromosomal morphology, and DNA content. Chromosome studies can provide crucial details on population differentiation. To better understand the diversification that occurs in this fish group, the current study based on such markers, presents data from a population of Corydoras paleatus from the first plateau of Paraná, putting them in a comparative scenario. The specimens studied has 2n=44 chromosomes with a karyotypic formula of 18m+26sm. Although conserved, this karyotypic structure shows variation in other populations of C. paleatus already studied, a consequence of chromosomal rearrangements that modify the morphology of chromosomes without modifying the 2n, such as centromeric repositioning. C-banding revealed conspicuous pericentromeric markings in metacentric pairs 4 and 8 and in submetacentric pairs 10 and 14, whose bands have been considered a chromosomal marker not only for C. paleatus but also for the genus. Fluorescent in situ hybridization (FISH) showed the major rDNA (45S) in the terminal region of the long arm of metacentric pair 5. Such location has already been described in other populations of C. paleatus, as they have also been mapped in short arms and with multiple patterns. The present study also provides a review of the genus Corydoras regarding the 2n, karyotypic formula and mainly regarding the number and location of the 45S and 5S rDNA, confirming a scenario in which chromosomal rearrangements have been modeling karyotypes of different populations and settling in the absence of gene flow, a consequence of vicariant events that occurred in the different basins.

Cytogenetic, morphological and molecular characterization of two cryptic species of the genus Omophoita Chevrolat, 1837 (Coleoptera: Chrysomelidae: Galerucinae)

Genus Omophoita is the most cytogenetically studied among the genera within Oedionychina subtribe. Most of the analyzed specimens have an uniform karyotype, with 2n = 22 while a few species show variations of the diploid number or sex chromosomes. The genera belonging to the tribe Alticini are very similar morphologically to each other, this similarity leads to inconsistency in the identification of species and divergences in the taxonomic position of groups, such as, Alticini’s monophilia in relation to Galerucinae. Thus, this work aims to differentiate two cryptic species of Omophoita by conventional cytogenetic, molecular and morphological analyzes. The cytogenetic data showed a great difference in diploid number and chromosome morphology, dividing them into two cytotypes. Cytotype I shows a diploid number 2n = 22 and cytotype II shows 2n = 12. This great variation in the diploid number is the first described in Omophoita. A comparative analysis showed that the average size of cytotype II chromosomes is 2.5 times bigger than cytotype I chromosomes. In addition, a morphology study of the aedeagus showed that the male genitalia present differences, indicating a probable differentiation between the two species. Phylogenetic reconstruction by means of the mitochondrial cytochrome oxidase I gene also showed differences between the cytotypes sequences. Considering that the cytotypes are two cryptic species and the cytogenetic data the karyotypic derivation was based on chromosomal rearrangements of fusion type.

Two new cytotypes reinforce that Micronycteris hirsuta Peters, 1869 does not represent a monotypic taxon

BackgroundThe genus Micronycteris is a diverse group of phyllostomid bats currently comprising 11 species, with diploid number (2n) ranging from 26 to 40 chromosomes. The karyotypic relationships within Micronycteris and between Micronycteris and other phyllostomids remain poorly understood. The karyotype of Micronycteris hirsuta is of particular interest: three different diploid numbers were reported for this species in South and Central Americas with 2n = 26, 28 and 30 chromosomes. Although current evidence suggests some geographic differentiation among populations of M. hirsuta based on chromosomal, morphological, and nuclear and mitochondrial DNA markers, the recognition of new species or subspecies has been avoided due to the need for additional data, mainly chromosomal data.ResultsWe describe two new cytotypes for Micronycteris hirsuta (MHI) (2n = 26 and 25, NF = 32), whose differences in diploid number are interpreted as the products of Robertsonian rearrangements. C-banding revealed a small amount of constitutive heterochromatin at the centromere and the NOR was located in the interstitial portion of the short arm of a second pair, confirmed by FISH. Telomeric probes hybridized to the centromeric regions and weakly to telomeric regions of most chromosomes. The G-banding analysis and chromosome painting with whole chromosome probes from Carollia brevicauda (CBR) and Phyllostomus hastatus (PHA) enabled the establishment of genome-wide homologies between MHI, CBR and PHA.ConclusionsThe karyotypes of Brazilian specimens of Micronycteris hirsuta described here are new to Micronycteris and reinforce that M. hirsuta does not represent a monotypic taxon. Our results corroborate the hypothesis of karyotypic megaevolution within Micronycteris, and strong evidence for this is that the entire chromosome complement of M. hirsuta was shown to be derivative with respect to species compared in this study.

Mapping five repetitive DNA classes in sympatric species of Hypostomus (Teleostei: Siluriformes: Loricariidae): analysis of chromosomal variability

Fish belonging to the genus Hypostomus are known for exhibiting a striking diversity in its karyotype structure, however the knowledge concerning the distribution patterns of heterochromatin and location of repetitive DNA sequences in the karyotypes is still limited. Aiming a better understanding of the chromosomal organization in this group, we analyzed three sympatric species of Hypostomus collected in the Hortela stream, a component of the Paranapanema River basin, Botucatu/SP/Brazil. The analyses involved the cytogenetic characterization and chromosomal mapping of repetitive sequences and intra/interspecific comparisons using sequences of the cytochrome C oxidase subunit I. The results revealed that H. ancistroides presents a karyotype with 2n = 68 chromosomes, H. strigaticeps 2n = 72 chromosomes, and H. nigromaculatus 2n = 76 chromosomes. In addition to differences found in the diploid number, it was also observed variations in karyotypic formulae, amount of constitutive heterochromatin, and location of nucleolus organizer regions. The cytogenetic mapping of 5S and 18S rDNA, as well as of the H3 histone gene, disclosed a differential dispersion process among the three species. In some cases the Rex1 transposable element showed to be co-located with 5S rDNA sites. The molecular analyses support the cytogenetic data and represent an additional tool for the characterization of the analyzed species. The results evidenced that chromosomal variations are not restricted to differences in diploid number or karyotypic macrostructure in the genus Hypostomus, indicating that events such as transposition of heterochromatin and rDNA segments may participate in the differentiation process occurred in these species.

A new karyotype for Cavia magna (Rodentia: Caviidae) from an estuarine island and C. aperea from adjacent mainland

We describe a new karyotype for Cavia magna Ximenez, 1980 from an estuarine island and the karyotype of Cavia aperea Erxleben, 1777 from an adjacent mainland. The species have differences in diploid number (2n), autosomal fundamental number, quantity, and distribution of heterochromatin as dissimilar distributions of the nucleolus-organizing regions (Ag-NORs). The C. aperea karyotype has a diploid number of 64 as previously reported for C. aperea and most other Cavia species. In contrast, this new C. magna karyotype exhibits a variant diploid number of 2n = 62, considering that previous work reported a karyotype of 2n = 64 for C. magna. The discovery of a distinct diploid number within C. magna represents the first record of intra-specific chromosomal variation in a species of Caviidae. The diploid number of 2n = 62, heterochromatin quantity, Ag-NOR distributions, and inversed X chromosome from this population of C. magna are as seen in the geographically proximate (Cavia intermedia Cherem Olimpio and Ximenez; intermediate Cavy). These data provide further evidence supporting C. magna as the sister species of C. intermedia.

CRYPTIC SPECIES IN THE MEXICAN POCKET GOPHER CRATOGEOMYS MERRIAMI

A study of chromosomal variation in the Mexican pocket gopher Cratogeomys merriami revealed differences in diploid number that correspond to 3 major genetic and morphological clades within the species. Each of these 3 clades is diagnosable based on multiple characters, including chromosomal diploid number, quantitative and qualitative morphological characters, and mitochondrial DNA. Accordingly, we restrict the name C. merriami (Thomas) to include only pocket gophers of this genus from the states of Mexico, Mexico D.F., northern Morelos, and west-central Puebla. We resurrect the species name C. fulvescens Merriam to represent members of this genus from southern Tlaxcala, east-central Puebla, and parts of west-central Veracruz. Finally, we resurrect the species name C. perotensis Merriam to represent members of this genus from southern Hidalgo, northern Tlaxcala, north-central Puebla, and parts of west-central Veracruz. Based on the observation that differences in diploid number usually signal reproductive isolation between populations of pocket gophers, we hypothesize that C. merriami, C. fulvescens, and C. perotensis are reproductively incompatible. We provide synonymies and descriptions for these 3 species, along with a key to the C. castanops species group to which these species belong.

Reversal and convergence in marsupial chromosome evolution

The karyotypes of marsupial species are characterized by their relatively low number of chromosomes, and their conservation. Most species have diploid numbers lying between the two modes, 2n = 14 and 2n = 22, but the karyotype of Aepyprymnus rufescens is exceptional in containing 2n = 32 chromosomes. Many differences in diploid number between marsupial species can be accounted for by particular fissions and fusions, which are easy to detect because of the low numbers of chromosomes in each karyotype. This should be a system in which it is possible to detect reversals and repeated chromosome rearrangements. We have used chromosome-specific paints derived from A. rufescens to compare the karyotypes of eight marsupial species, representing closely and distantly related taxa, to trace chromosome change during evolution, and especially to detect reversals and convergence. From these and other painting comparisons, we conclude that there have been at least three reversals of fusions by fissions, and at least three fusions or fissions that have occurred independently in different lineages.

Sympatric occurrence of two cytotypes of Astyanax scabripinnis (Characiformes, Characidae)

Among four specimens of Astyanax scabripinnis sympatrically collected from the Tamanduá stream (State of São Paulo, Brazil), three, named cytotype I, had 2n = 50 chromosomes (6M + 26SM + 4ST + 14A) and a fundamental number (FN) of 86, and one specimen, named cytotype II, had 2n = 48 chromosomes (6M + 28SM + 4ST + 10A) and FN = 86. Besides the difference in diploid number, the cytotypes showed a clear difference in the distribution of constitutive heterochromatin. One NOR-bearing chromosome pair was detected in both cytotypes and one specimen of cytotype I had multiple NORs (tree chromosome pairs). Robertsonian translocations and an increase or loss of heterochromatin are proposed to explain the karyotypic divergence observed. Some aspects related to the chromosome evolution of Astyanax scabripinnis are discussed.

Reciprocal chromosome painting shows that the great difference in diploid number between human and African green monkey is mostly due to non-Robertsonian fissions.

We used reciprocal chromosome painting with both African green monkey (C. aethiops) and human chromosome specific DNA probes to delineate homologous regions in the two species. Probes were derived by fluorescence-activated chromosome flow sorting and then were reciprocally hybridized to metaphase spreads of each species. Segments in the size range of a single chromosome band were identified, demonstrating the sensitivity of the approach when comparing species that diverged more than 20 million years ago. Outgroup analysis shows that the great difference in diploid numbers between the African green monkey (2n = 60) and humans (2n = 46) is mainly owing to fissions, and the direction of change is towards increasing diploid numbers. However, most break points apparently lie outside of the centromere regions, suggesting that the changes were not solely Robertsonian as has been previously assumed. No reciprocal translocations have occurred in the phylogenetic lines leading to humans or African green monkeys. The primate paints established here are a valuable tool to establish interspecies homology, to define rearrangements, and to determine the mechanisms of chromosomal evolution in primate species.

Comparative chromosome analysis (R- and C-bands) of two South African murid species, Lemniscomys rosalia and Rhabdomys pumilio (Rodentia, Murinae)

We carried out a comparative chromosome analysis (R- and C-bands) on two South African murid rodent species, Lemniscomys rosalia and Rhabdomys pumilio, whose banded karyotypes are reported here for the first time. The study revealed that, in spite of minor differences in diploid number and the number of autosomal arms (48 vs. 46 and 58 vs. 60, respectively), these species differ by at least 10 structural rearrangements, comprising seven Robertsonian translocations (two of which share monobrachial homology), two tandem translocations, and one pericentric inversion. Despite the high level of chromosomal differentiation observed, almost complete homologous banding patterns were detected between the two species. The present study therefore strengthens the hypothesis about the close phylogenetic affinities between L. rosalia and R. pumilio and their belonging to a set of genera referred to as “arvicanthine” rodents.

Cytogenetic Properties of Nine Species of Golden Moles (Insectivora: Chrysochloridae)

Standard karyotypes and the location of nucleolar organizer regions are reported for nine species of chrysochlorids representing five genera ( Amblysomus, Calcochloris, Chlorotalpa, Chrysochloris , and Chrysospalax ). Intraspecific karyotypic variation is negligible except in Amblysomus hottentotus , which displays three allopatric cytotypes characterized by differences in diploid number (30, 34, 36) and fundamental number (56, 60, 62, 68) as well as the number and location of nucleolar organizer regions. With the exception of Calcochloris , a monotypic genus with 2n = 28, most genera display 2n = 30, but differ in fundamental number and locations of nucleolar organizer regions. Evidence that the three cytotypes in A. hottentotus represent distinct biological species is presented. Cladisdc reladonships based on chromosomal properties support the recognition of Chlorotalpa and Calcochloris as discrete genera and the subgeneric recognition of the taxon Neamblysomus for the species A . ( N .) gunningi and A . ( N .) julianae . Limited banding studies indicate that centric fissions, tandem fusions, pericentric inversions, and heterochromatic additions may have played a role in the chromosomal evolution of chrysochlorids. Karyotypic evolution in the Chry-sochloridae appears to be similar in mode and tempo to that in other fossorial Insectivora, in contrast to subterranean rodents, which are characterized by greater chromosomal diversity and karyotypic orthoselection.

Complex sex chromosome system in Eigenmannia sp. (Pisces, Gymnotiformes)

Chromosomes of Eigenmannia sp. (7 males and 15 females) collected from the Tiete River in Botucatu (SP, Brazil) were examined from gill, kidney and testicular cells. The diploid chromosome number in males was 2n=31 and in females, 2n=32. In both sexes the number of chromosomal arms was 40. The difference in diploid number was due to the fusion of two acrocentrics. Mitotic and meiotic studies suggested that one of the fused acrocentrics was the Y chromosome. The sex-determining mechanism in Eigenmannia sp. could therefore be XX, AA in the female and X, \-YA A in the males. One of the males presented 2n=30 chromosomes due to the occurrence of another fusion of acrocentrics. C-banding analysis of the mitotic chromosomes revealed constitutive heterochromatin in the centromeric regions of all acrocentrics. However, small metacentrics were C-band negative. The YA chromosome is C-band negative except for a small amount of heterochromatin in the centromeric region. The nucleolar organizer region as identified by Ag-staining is present in the interstitial region of chromosome pair No. 10.

Karyotypic Uniformity in the Red-Winged Blackbird

-Karyotypes of 52 Red-winged Blackbirds (Agelaius phoeniceus) from four widely separated populations were analyzed for possible regional differences in diploid numbers (2n) and in the sizes and shapes of the seven largest chromosomes. G-banded karyotypes for a subset of these individuals were compared to assess the banding homology between two of the populations. Results thus far indicate that Red-winged Blackbirds are karyotypically invariant across much of their geographic range and have an estimated 2n of 76. The recent discoveries of intraspecific chromosome polymorphisms in several avian species (Thorneycroft 1966, 1976; Shields 1973, 1982; Ansari and Kaul 1979a, b; Bass 1979; Kaul and Ansari 1979) have generated considerable new interest in avian cytogenetics. Therefore, the once common notion that birds are karyotypically conservative organisms (Ohno et al. 1964, Takagi and Sasaki 1974) needs to be reevaluated, especially in relation to recent speculation on the role that chromosomal polymorphisms may play in speciation processes (White 1978, but see Shields 1982). Intraspecific chromosomal variation may be more common in birds than is presently recognized. In order to detect such variation, one must sample the karyotypes of many individuals. Most avian karyotypes, however, have been determined from too few individuals to yield information about intraspecific variation. Consider a hypothetical example in which an avian species possesses a pair of chromosomes with two different morphologies because of a pericentric inversion. If the polymorphism exists in Hardy-Weinberg equilibrium with relative frequencies of p = 0.8 and q = 0.2 for the two morphs, then one may use a binomial distribution to calculate the probability of not detecting the lower(or higher-) frequency morph at given sample sizes. In this example, most of the individuals (p2 or 64%) are homozygous for the higher-frequency morph and thus yield no information about the presence or absence of any intraspecific polymorphism. The remaining 36% (2pq or 32% + q2 or 4%) exhibit the polymorphism in either the heteroor homozygous condition. If repeated samples of five individuals were karyotyped in this species, then the chromosomal polymorphism would be undetected in slightly over 10% of the samples (the exact probability is [0.64]5 = 0.1074). Because even five individuals are an unusually large sample in many avian cytoge etic studies, many of the 556 avian species so far karyotyped (de Boer 1983) may harbor undetected chromosomal polymorphisms. In addition, banded chromosomes can show differences that are not evident in standard karyotypes (Mateescu et al. 1974, Pollock and Fechheimer 1981, Shields 1983). In a recent review of avian cytogenetics, Shields (1982) contended that karyotypic variability has not been satisfactorily assessed in any avian species studied to date, regardless of the number of individuals karyotyped. He argued that detailed Cand G-banded karyotypes are seldom used, and in no instance have current techniques been used to karyotype several individuals from different populations within the geographic range of a species. We present here the results of an extensive karyological study of the mitotic chromosomes of the Red-winged Blackbird (Agelaius phoeniceus). Between 9 and 20 individual birds from each of four widely separated breeding populations were analyzed for differences in the sizes and shapes of the seven largest chromosomes (hereafter referred to as macrochromosomes) and also for differences in diploid chromosome numbers (2n). A subset of these karyotypes was G-banded for comparisons of banding homologies between two populations. The Red-winged Blackbird has considerable morphological size and shape variation across its North American range (Power 1970, James 1983), with 14 subspecies recognized by the American Ornithologists' Union (1957). The extent to which this phenotypic variation covaries with genetic variation (either chromosomal or electromorphic) is unknown, but such information is crucial to understanding evolutionary processes in this species, as well as other avian species. Although chromosomal morphology is just one spect of the total genetic constituency of any

Robertsonian translocation, pericentric inversion and heterochromatin block in the evolution of the Tailless Fruit Bat

The karyotype ofMegaerops ecaudatus from Peninsular Malaysia consists of 24 chromosomes as compared to 2n=26 for specimens from Thailand. The difference in diploid number is the result of Robertsonian translocation. The Peninsular Malaysian specimens also exhibit pericentric inversion in the smallest pair of autosomes, and the presence of a totally heterochromatic short arm in the second longest metacentric pair. There is 1 pair of Ag-NOR, located on the secondary constriction of the longest metacentric autosome.

Chromosome Studies inGymnotus CarapoandGymnotus Sp.(Pisces, Gymnotidae)

SUMMARYKaryotype studies were carried out on two fish species of the family Gymnotidae. In five samples of Gymnotus carapo from different localities chromosome number varied from 2n = 42 to 2n = 54. Specimens from the Amazon Basin had 2n = 42 (Belém—State of Pará) and 2n = 48 (Humaitá - State of Amazonas); from the Paraná Basin, 2n = 52 (Brotas—State of São Paulo) and 2n = 54 (Botucatu—State of São Paulo); from the East Basins, 2n = 54 (Miracatu—State of São Paulo). These differences in diploid number were accompanied by differences in the number of acrocentric chromosomes present in the complement. Only one sample of Gymnotus sp., from Miracatu, was analyzed. Here the diploid number was 2n = 52 and the complement was similar to that of G. carapo from Brotas.