Chromosomal Banding Data Research Articles

Distribution of GC-rich heterochromatin and ribosomal genes in three fungus-farming ants (Myrmicinae, Attini, Attina): insights on chromosomal evolution.

Cytogenetic studies on fungus-farming ants have shown remarkable karyotype diversity, suggesting different chromosomal rearrangements involved in karyotype evolution in some genera. A notable cytogenetic characteristic in this ant group is the presence of GC-rich heterochromatin in the karyotypes of some ancient and derivative species. It was hypothesized that this GC-rich heterochromatin may have a common origin in fungus-farming ants, and the increase in species studied is important for understanding this question. In addition, many genera within the subtribe Attina have few or no cytogenetically studied species; therefore, the processes that shaped their chromosomal evolution remain obscure. Thus, in this study, we karyotyped, through classical and molecular cytogenetic techniques, the fungus-farming ants Cyphomyrmextransversus Emery, 1894, Sericomyrmexmaravalhas Ješovnik et Schultz, 2017, and Mycetomoelleriusrelictus (Borgmeier, 1934), to provide insights into the chromosomal evolution in these genera and to investigate the presence the GC-rich heterochromatin in these species. Cyphomyrmextransversus (2n = 18, 10m + 2sm + 6a) and S.maravalhas (2n = 48, 28m + 20sm) showed karyotypes distinct from other species from their genera. Mycetomoelleriusrelictus (2n = 20, 20m) presented the same karyotype as the colonies previously studied. Notably, C.transversus presented the lowest chromosomal number for the genus and a distinct karyotype from the other two previously observed for this species, showing the existence of a possible species complex and the need for its taxonomic revision. Chromosomal banding data revealed GC-rich heterochromatin in all three species, which increased the number of genera with this characteristic, supporting the hypothesis of a common origin of GC-rich heterochromatin in Attina. Although a single chromosomal pair carries rDNA genes in all studied species, the positions of these rDNA clusters varied. The rDNA genes were located in the intrachromosomal region in C.transversus and M.relictus, and in the terminal region of S.maravalhas. The combination of our molecular cytogenetic data and observations from previous studies corroborates that a single rDNA site located in the intrachromosomal region is a plesiomorphic condition in Attina. In addition, cytogenetic data obtained suggest centric fission events in Sericomyrmex Mayr, 1865, and the occurrence of inversions as the origin of the location of the ribosomal genes in M.relictus and S.maravalhas. This study provides new insights into the chromosomal evolution of fungus-farming ants.

Karyotype of the rare Johnston’s genetGenetta johnstoni (Viverridae) and a reassessment of chromosomal characterization among congeneric species

We present herein new data on karyotypes of members of the genusGenetta. G- and C-banded chromosomes of the Johnston’s genetGenetta johnstoni Pocock, 1908 (2n = 50 / FNa = 92) are described for the first time, and compared with those ofG. genetta (2n = 54 / FNa = 92). In addition, the standard karyotype ofG. maculata (2n = 52 / FNa = 96) was studied. A reassessment of taxonomic attribution of previously published material allowed us to characterize (2n, FNa, and chromosome morphology) the karyotypes of three genets, previously unknown (G. pardina, G. letabae andG. tigrina). Our results show that despite a rather low interspecific variability in 2n and FNa, all the species of genets (exceptG. pardina andG. maculata) appear differentiated when chromosomal morphology is taken into account. Although chromosomal banding data are limited, confrontation of G-band karyotypes with preliminary molecular phylogenetic results reveals that karyotypic evolution within the genusGenetta might involve various rearrangements like Robertsonian and tandem translocations, pericentric inversions, and centromere fissions; thus providing at least for some taxa a solid postzygotic isolation. Finally, our study suggests that cytogenetic analyses might constitute a useful tool for questioning interspecific boundaries, especially within the taxonomically debated complex of large-spotted genets.

Karyotypic Variation in Laboratory Lines of Amblyomma cajennense: Aneuploidy and Evidence of an Autosomal Translocation

Karyotypic data from 67 individuals of Amblyomma cajennense (F.) representing three inbred lines (M3, M6, and M7) and a laboratory colony indicate the presence of considerable chromosomal variation in two of the lines. The majority of the specimens possessed a diploid number of 21 (20 autosomes and a single X chromosome); however, one of the inbred lines (M6) had diploid numbers ranging from 21 to 24. Chromosome banding data indicate that the additional chromosomes are not supernumerary chromosomes and probably represent duplicated autosomes. Additionally, two of the inbred lines (M6 and M7) had individuals heterozygous for a translocation between the largest autosome and the small autosome bearing the nucleolus organizer region. The potential origin of the observed variation is discussed in light of previous accounts of karyotypic variation in ticks.

FLOW-CYTOMETRIC ANALYSES OF NUCLEAR DNA CONTENT IN FOUR FAMILIES OF NEOTROPICAL BATS.

Flow-cytometric analyses of 29 species of microchiropteran bats representing four families and 20 genera revealed that bats possess only 79% (5.43 pg) of the DNA content of a "typical" mammal (e.g., Mus musculus strain C57BL; 7 pg). Chiroptera, the second largest order of mammals, is thus an exception to the prevailing view that mammals possess a minimum nuclear DNA content of 7 pg. Limitations on cell size resulting from a high metabolic rate may have constrained evolution of DNA content and could explain why the extensive heterochromatic additions that are common in some groups of mammals are absent in bats. Chromosomes of bats have been well studied; detailed chromosomal banding data are available for nearly all the species used in this investigation. However, no significant correlations were found between DNA content and karyotypic characteristics such as 2n, fundamental number, and rate or pattern of chromosomal evolution.

A REEVALUATION OF CHROMOSOMAL VARIATION IN POPULATIONS OF PEROMYSCUS MANICULATUS ALONG AN ELEVATIONAL GRADIENT.

In a recent report, Macey and Dixon (1987) presented data suggesting a relationship between chromosomal variation and environmental differences. Individuals representing three subspecies of the deer mouse Peromyscus maniculatus were collected along a latitudinal transect which comprised an elevational gradient ranging from 1,070 m to 4,110 m. Karyotypic analysis revealed an apparent positive correlation between increasing altitude and decreasing proportion of animals homozygous for the acrocentric form of chromosome 6. The occurrence of individuals heterozygous at chromosome 6 (acrocentric/submetacentric) was interpreted as evidence of gene flow between the subspecies sampled. Although the data in Macey and Dixon (1987) do seem to show a general correlation between the relative number of acrocentric-6 homozygotes and elevation, the conclusions made by these authors on the basis of chromosome 6 in P. maniculatus are in need of reevaluation. First, these authors focused their attention on an autosome that is highly polymorphic for pericentric inversions within deer mouse populations, and yet they assumed a hybrid origin for chromosomally heterozygous individuals. Additionally, the different inversion morphs of chromosome 6 occur in a wide variety of habitats throughout North America. These authors concentrated only on the pattern along their transect and ignored evidence from other studies that contradicts their conclusions on the relationship between chromosomal morphology and environmental conditions. Finally, and most importantly, Macey and Dixon (1987) observed an apparent correlation between altitude and chromosomal phenotype and assumed a cause-and-effect relationship; it is invalid to make such assumptions from circumstantial evidence. Chromosomal banding data reveal extensive pericentric-inversion polymorphism within P. maniculatus and two closely related species P. sitkensis and P. oreas (Greenbaum et al., 1978; Dixon et al., 1980; Clark, 1983; Pengilly et al., 1983; Greenbaum and Reed, 1984; Van Fleet, 1985; Gunn, 1986, 1988; Gunn and Greenbaum, 1986; Hale, 1986; Macey and Dixon, 1987; Hale and Greenbaum, 1988). The considerable chromosomal variability observed both among and within populations of P. maniculatus is largely attributable to pericentric-inversion polymorphisms identified in chromosomes 6, 7, 10, 14, 15, and 19. Karyotypic data from various subspecies across the continental United States indicate that populations are frequently polymorphic for pericentric inversions of chromosome 6

Chromosome Studies of The Common Carp,Cyprinus Carpio.I. Karyotype of Amurian Carp,C. Carpio Haematopterus

SUMMARYKaryotype of native carp from the Amur river, Cyprinus carpio haematopterus, was studied. Diploid chromosome number 2n = 100 was found. Because of the absence of any chromosome banding data, the karyotype was divided as precisely as possible into eight groups according to the morphology of particular chromosomes: group A composed of 6 pairs of mediocentrics, group B with 8 pairs of metacentrics with non-median centromere position, group C with 3 pairs of large submetacentrics, group D with 15 pairs of submetacentrics, group E with single pair of achromatic-region-carrying submetacentrics, group F with 2 pairs of subtelocentrics, group G with 9 pairs of subtelocentrics, and group H with 6 pairs of clearly acrocentric chromosomes. These results were compared with remaining few studies on the chromosomes of wild carp populations, and the differences between the results of various authors and the present study were briefly discussed.

Chromosomal variation and differentiation among insular populations of Peromyscus from the Pacific Northwest

Karyotypic data from 561 deer mice (Peromyscus spp.) from populations on islands off British Columbia and Washington indicate the presence of two distinct karyotypic groups, a low fundamental number (FN) group (FN = 74–78) and a high FN group (FN = 85–92). The absence of karyotypic intermediates at nine localities of sympatry is interpreted as demonstrating that the two FN groups represent distinct species. Based on data from comparable mainland forms, the low FN form is considered to represent Peromyscus maniculatus and the high FN form is suggested to be conspecific with Peromyscus areas. Chromosomal banding data indicate extensive chromosomal variation within each of these species and provide discrete karyotypic characters unique to each species. The inclusion of the high FN insular forms with P. oreas does not affect the chromosomally determined cladistic placement of P. oreas as a laxon derived from P. maniculatus.

Chromosomal Studies and Evolutionary Relationships of an Endangered Species, Reithrodontomys raviventris

G- and C-banded karyotypes of Reithrodontomys raviventris were examined to identify the amount and types of chromosomal change that have occurred in this species. The evolutionary origin and phylogenetic relationships of R. raviventris were evaluated by comparing G- and C-band data with six additional species of Reithrodontomys ( fulvescens, creper, mexicanus, megalotis, montanus , and sumichrasti ). Banding homologies indicate that the low diploid number forms ( raviventris, montanus, megalotis , and sumichrasti ) shared a common ancestry and that a major dichotomy exists among these taxa. R. raviventris and R. montanus share a phylogenetic relationship as indicated by 10 shared-derived chromosomal rearrangement events. R. megalotis and R. sumichrasti represent a second well-defined clade. These chromosomal banding data suggest an alternative hypothesis to the traditionally accepted view that R. raviventris is most closely related to R. megalotis .

Cytosystematic Value of Chromosomal Inversion Data in the Genus Peromyscus (Rodentia: Cricetidae)

G-band chromosomal data are presented for seven species of subtropical and tropical deer mice: Peromyscus banderanus, P. guatemalensis, P. gymnotis, P. mexicanus, P. yucatanicus, P. zarhynchus, and P. lepturus. P. banderanus retains the primitive karyotype for the genus with only chromosomes 1, 22, and 23 biarmed. Species examined from the mexicanus group (P. guatemalensis, P. gymnotis, P. mexicanus, P. yucatanicus, and P. zarhynchus) are characterized by a single karyotype with chromosomes 1, 2, 3, 9, 22, and 23 biarmed. P. lepturus has chromosomes 1, 2, 3, 5, 6, 7, 9, 10, 15, 22, and 23 biarmed. Autosomal heterochromatin (C-band positive material) was restricted to the centromeric regions in each of the seven species examined. A most parsimonious cladogram was constructed from the chromosome banding data of these and 19 other species. Due to the high frequency of apparently convergent inversions of chromosome 6, this chromosome was eliminated as a character in the cladistic analysis. Additionally, a synthetic phylogenetic analysis incorporating chromosomal and subgeneric taxonomic inferences for the 26 species is presented. Within the genus Peromyscus, chromosome banding data are viewed as most reliable at the species group level (in certain cases) and as indicators of potential areas of discrepancy in intrageneric taxonomy.

Determination of the Primitive Karyotype for Peromyscus

In order to determine the most probable primitive karyotype for the Peromyscus maniculatus complex and for the genus Peromyscus, G- and C-band data for P. floridanus and P. gossypinus are analyzed and compared to data available for other species. It is concluded that the primitive karyotype for the genus is characterized by chromosomes with heterochromatin restricted to centromeric regions. Heterochro-matic short arms appear to have been added independently to several Peromyscus lineages, suggesting that heterochromatin provides some selective advantage in this genus. When autosomes are identified according to the standardized karyotype for Peromyscus (Committee, 1977), the primitive karyotype is biarmed for pairs 1, 22, and 23; pairs 2, 3, 6, and 9 may or may not be biarmed, and all other pairs are acrocentric. Based on the trend in the P. maniculatus group for orthoselection, which increases the number of biarmed elements, it is hypothesized that chromosomes 2, 3, 6, and possibly 9 are acrocentric in the primitive karyotype and that P. crinitus, with only chromosomes 1, 22, and 23 biarmed, has retained an essentially primitive karyotype for the genus. The euchromatic long arm of the X chromosome has remained stable throughout all species examined, although heterochromatic short arms on the X are quite variable. Y chromosomes are biarmed and C-band positive in all species examined, but position of the centromere varies. Chromosome banding data support the hypothesis of Bowers et al. (1973) that the primitive karyotype for the P. maniculatus group is like that of P. melanotis with 30 acrocentric elements. C-bands reveal that heterochromatin in the primitive karyotype of the P. maniculatus group was confined to the centromeric regions. The primitive karyotype for this group has pairs 1, 2, 3, 6, 9, 20, 22, and 23 biarmed and all other pairs acrocentric. Peromyscus floridanus (subgenus Podomys) shares what are believed to be derived chromosomal inversions with members of the subgenus Peromyscus, suggesting evolutionary affinities between these subgenera. Chromosomal banding data appear to have considerable promise as a tool for determining clades within Peromyscus.