Chromosomal Evolution in Aspredinidae (Teleostei, Siluriformes): Insights on Intra- and Interspecific Relationships with Related Groups.

The family Aspredinidae comprises a clade of complex systematic relationships, both from molecular and morphological approaches. In this study, conventional and molecular cytogenetic studies coupled with nucleotide sequencing were performed in 6 Aspredininae species (Amaralia hypsiura, Bunocephalus cf. aloikae, Bunocephalus amaurus, Bunocephalus aff. coracoideus, Bunocephalus verrucosus, and Platystacus cotylephorus) from different locations of the Amazon hydrographic basin. Our results showed highly divergent diploid numbers (2n) among the species, ranging from 49 to 74, including the occurrence of an XX/X0 sex chromosome system. A neighbor-joining phylogram based on the cytochrome c oxidase I (COI) showed that Bunocephalus coracoideus is not a monophyletic clade, but closely related to B. verrucosus. The karyotypic data associated with COI suggest an ancestral karyotype for Aspredinidae with a reduced 2n, composed of bi-armed chromosomes and a trend toward chromosomal fissions resulting in higher diploid number karyotypes, mainly composed of acrocentric chromosomes. Evolutionary relationships were discussed under a phylogenetic context with related species from different Siluriformes families. The karyotype features and chromosomal diversity of Aspredinidae show an amazing differentiation, making this family a remarkable model for investigating the evolutionary dynamics in siluriforms as well as in fish as a whole.

Venomous Loxosceles Species (Araneae, Haplogynae, Sicariidae) from Brazil: 2n♂ = 23 and X1X2Y Sex Chromosome System as Shared Characteristics.

The family Sicariidae comprises the genera Hexophthalma, Sicarius and Loxosceles. This latter is subdivided in eight monophyletic groups based on genitalia morphology and molecular analyses: amazonica, gaucho, laeta, and spadicea (South America); reclusa (North America); rufescens (Mediterranean); spinulosa and vonwredei (Africa). In Brazil, the genus Loxosceles is represented by 50 species. The mitotic and meiotic characteristics of eight Loxosceles species were analyzed in order to discuss the chromosome evolution, as well as the correspondence between cytogenetic data and morphological/molecular data for the delimitation of the South American groups of species belonging to this genus. All species studied in this work showed 2n♂ = 23, including a X1X2Y sex chromosome system (SCS). Despite the similarity of diploid number and SCS, the species studied here differed regarding the chromosome morphology of some autosomal pairs, presence of secondary constrictions, size of X chromosomes and localization of Ag-NOR/rDNA sites. Based on all these chromosomal data, we verified a close relationship between Loxosceles species belonging to the amazonica and gaucho groups. Transmission electron microscopy (TEM) analysis of spread pachytene cells of L. gaucho showed regular synapsis between homologous autosomal chromosomes, but asynaptic behavior of the sex chromosomes. The axial elements of the sex chromosomes undergo conspicuous morphological modifications resulting in shortening of their length.

Extraordinary Diversity in the Origins of Sex Chromosomes in Anurans Inferred from Comparative Gene Mapping.

Sex determination in frogs (anurans) is genetic and includes both male and female heterogamety. However, the origins of the sex chromosomes and their differentiation processes are poorly known. To investigate diversity in the origins of anuran sex chromosomes, we compared the chromosomal locations of sex-linked genes in 4 species: the African clawed frog (Xenopus laevis), the Western clawed frog (Silurana/X. tropicalis), the Japanese bell-ring frog (Buergeria buergeri), and the Japanese wrinkled frog (Rana rugosa). Comparative mapping data revealed that the sex chromosomes of X. laevis, X. tropicalis and R. rugosa are different chromosome pairs; however, the sex chromosomes of X. tropicalis and B. buergeri are homologous, although this may represent distinct evolutionary origins. We also examined the status of sex chromosomal differentiation in B. buergeri, which possesses heteromorphic ZW sex chromosomes, using comparative genomic hybridization and chromosome painting with DNA probes from the microdissected W chromosome. At least 3 rearrangement events have occurred in the proto-W chromosome: deletion of the nucleolus organizer region and a paracentric inversion followed by amplification of non-W-specific repetitive sequences.

Both cell-autonomous mechanisms and hormones contribute to sexual development in vertebrates and insects.

The differentiation of male and female characteristics in vertebrates and insects has long been thought to proceed via different mechanisms. Traditionally, vertebrate sexual development was thought to occur in two phases: a primary and a secondary phase, the primary phase involving the differentiation of the gonads, and the secondary phase involving the differentiation of other sexual traits via the influence of sex hormones secreted by the gonads. In contrast, insect sexual development was thought to depend exclusively on cell-autonomous expression of sex-specific genes. Recently, however, new evidence indicates that both vertebrates and insects rely on sex hormones as well as cell-autonomous mechanisms to develop sexual traits. Collectively, these new data challenge the traditional vertebrate definitions of primary and secondary sexual development, call for a redefinition of these terms, and indicate the need for research aimed at explaining the relative dependence on cell-autonomous versus hormonally guided sexual development in animals.

Comparative chromosomal mapping of microsatellites in Leporinus species (Characiformes, Anostomidae): unequal accumulation on the W chromosomes.

Approximately 90 species in the genus Leporinus (Characiformes, Anostomidae) are known, and most of them do not present differentiated sex chromosomes. However, there is a group of 7 species that share a heteromorphic ZW sex system. In all of these species, the W chromosome is the largest one in the karyotype and is mostly heterochromatic. We investigated the distribution of several microsatellites in the genome of 4 Leporinus species that possess ZW chromosomes. Our results showed a very large accumulation of mostly microsatellites on the W chromosomes. This finding supports the presence of an interconnection between heterochromatinization and the accumulation of repetitive sequences, which has been proposed for sex chromosome evolution, and suggests that heterochromatinization is the earlier of the 2 processes. In spite of the common origin that has been proposed for W chromosomes in all of the studied species, the microsatellites followed different evolutionary trajectories in each species, which indicates a high plasticity for sex chromosome differentiation.

Karyological characterization of the endemic Iberian rock lizard, Iberolacerta monticola (Squamata, Lacertidae): insights into sex chromosome evolution.

Rock lizards of the genus Iberolacerta constitute a promising model to examine the process of sex chromosome evolution, as these closely related taxa exhibit remarkable diversity in the degree of sex chromosome differentiation with no clear phylogenetic segregation, ranging from cryptic to highly heteromorphic ZW chromosomes and even multiple chromosome systems (Z1Z1Z2Z2/Z1Z2W). To gain a deeper insight into the patterns of karyotype and sex chromosome evolution, we performed a cytogenetic analysis based on conventional staining, banding techniques and fluorescence in situ hybridization in the species I. monticola, for which previous cytogenetic investigations did not detect differentiated sex chromosomes. The karyotype is composed of 2n = 36 acrocentric chromosomes. NORs and the major ribosomal genes were located in the subtelomeric region of chromosome pair 6. Hybridization signals of the telomeric sequences (TTAGGG)n were visualized at the telomeres of all chromosomes and interstitially in 5 chromosome pairs. C-banding showed constitutive heterochromatin at the centromeres of all chromosomes, as well as clear pericentromeric and light telomeric C-bands in several chromosome pairs. These results highlight some chromosomal markers which can be useful to identify species-specific diagnostic characters, although they may not accurately reflect the phylogenetic relationships among the taxa. In addition, C-banding revealed the presence of a heteromorphic ZW sex chromosome pair, where W is smaller than Z and almost completely heterochromatic. This finding sheds light on sex chromosome evolution in the genus Iberolacerta and suggests that further comparative cytogenetic analyses are needed to understand the processes underlying the origin, differentiation and plasticity of sex chromosome systems in lacertid lizards.

Insights into the meiotic behavior and evolution of multiple sex chromosome systems in spiders.

A characteristic feature of spider karyotypes is the predominance of unusual multiple X chromosomes. To elucidate the evolution of spider sex chromosomes, their meiotic behavior was analyzed in 2 major clades of opisthothele spiders, namely, the entelegyne araneomorphs and the mygalomorphs. Our data support the predominance of X(1)X(2)0 systems in entelegynes, while rare X(1)X(2)X(3)X(4)0 systems were revealed in the tuberculote mygalomorphs. The spider species studied exhibited a considerable diversity of achiasmate sex chromosome pairing in male meiosis. The end-to-end pairing of sex chromosomes found in mygalomorphs was gradually replaced by the parallel attachment of sex chromosomes in entelegynes. The observed association of male X univalents with a centrosome at the first meiotic division may ensure the univalents' segregation. Spider meiotic sex chromosomes also showed other unique traits, namely, association with a chromosome pair in males and inactivation in females. Analysis of these traits supports the hypothesis that the multiple X chromosomes of spiders originated by duplications. In contrast to the homogametic sex of other animals, the homologous sex chromosomes of spider females were already paired at premeiotic interphase and were inactivated until prophase I. Furthermore, the sex chromosome pairs exhibited an end-to-end association during these stages. We suggest that the specific behavior of the female sex chromosomes may have evolved to avoid the negative effects of duplicated X chromosomes on female meiosis. The chromosome ends that ensure the association of sex chromosome pairs during meiosis may contain information for discriminating between homologous and homeologous X chromosomes and thus act to promote homologous pairing. The meiotic behavior of 4 X chromosome pairs in mygalomorph females, namely, the formation of 2 associations, each composed of 2 pairs with similar structure, suggests that the mygalomorph X(1)X(2)X(3)X(4)0 system originated by the duplication of the X(1)X(2)0 system via nondisjunctions or polyploidization.

A W-linked palindrome and gene conversion in New World sparrows and blackbirds.

A hallmark feature of the male-specific region of the human Y chromosome is the presence of large and near-identical palindromes. These palindromes are maintained in a state of near identity via gene conversion between the arms of the palindrome, and both neutral and selection-based theories have been proposed to explain their enrichment on the human Y and X chromosomes. While those proposed theories would be applicable to sex chromosomes in other species, it has not been established whether near-identical palindromes are a common feature of sex chromosomes in a broader range of taxa, including other tetrapods. Here, we report the genomic sequencing and features of a 279-kb region of the non-recombining portion of the W chromosome spanning the CHD1W locus in a New World sparrow, the white-throated sparrow (Zonotrichia albicollis), and the corresponding region on the Z chromosome. As has been observed for other Y and W chromosomes, we detected a high repetitive element content (51%) and low gene content on the white-throated sparrow W chromosome. In addition, we identified a 22-kb near-identical (>99%) palindrome on the W chromosome that flanks the 5' end of the CHD1W gene. Signatures of gene conversion were readily detected between the arms of this palindrome, as was the presence of this palindrome in other New World sparrows and blackbirds. Near-identical palindromes are therefore present on the avian W chromosome and may persist due to the same forces proposed for the enrichment of these elements on the human sex chromosomes.

Differentiation of sex chromosomes and karyotypic evolution in the eye-lid geckos (Squamata: Gekkota: Eublepharidae), a group with different modes of sex determination.

The eyelid geckos (family Eublepharidae) include both species with temperature-dependent sex determination and species where genotypic sex determination (GSD) was suggested based on the observation of equal sex ratios at several incubation temperatures. In this study, we present data on karyotypes and chromosomal characteristics in 12 species (Aeluroscalabotes felinus, Coleonyx brevis, Coleonyx elegans, Coleonyx variegatus, Eublepharis angramainyu, Eublepharis macularius, Goniurosaurus araneus, Goniurosaurus lichtenfelderi, Goniurosaurus luii, Goniurosaurus splendens, Hemitheconyx caudicinctus, and Holodactylus africanus) covering all genera of the family, and search for the presence of heteromorphic sex chromosomes. Phylogenetic mapping of chromosomal changes showed a long evolutionary stasis of karyotypes with all acrocentric chromosomes followed by numerous chromosomal rearrangements in the ancestors of two lineages. We have found heteromorphic sex chromosomes in only one species, which suggests that sex chromosomes in most GSD species of the eyelid geckos are not morphologically differentiated. The sexual difference in karyotype was detected only in C. elegans which has a multiple sex chromosome system (X(1)X(2)Y). The metacentric Y chromosome evolved most likely via centric fusion of two acrocentric chromosomes involving loss of interstitial telomeric sequences. We conclude that the eyelid geckos exhibit diversity in sex determination ranging from the absence of any sexual differences to heteromorphic sex chromosomes, which makes them an interesting system for exploring the evolutionary origin of sexually dimorphic genomes.

Nonrandom chromosome segregation in Neocurtilla (Gryllotalpa) hexadactyla: an ultrastructural study.

During meiosis I in males of the mole cricket Neocurtilla (Gryllotalpa) hexadactyla, the univalent X1 chromosome and the heteromorphic X2Y chromosome pair segregate nonrandomly; the X1 and X2 chromosomes move to the same pole in anaphase. By means of ultrastructural analysis of serial sections of cells in several stages of meiosis I, metaphase of meiosis II, and mitosis, we found that the kinetochore region of two of the three nonrandomly segregating chromosomes differ from autosomal kinetochores only during meiosis I. The distinction is most pronounced at metaphase I when massive aggregates of electron-dense substance mark the kinetochores of X1 and Y chromosomes. The lateral position of the kinetochores of X1 and Y chromosomes and the association of these chromosomes with microtubules running toward both poles are also characteristic of meiosis I and further distinguish X1 and Y from the autosomes. Nonrandomly segregating chromosomes are typically positioned within the spindle so that the kinetochoric sides of the X2Y pair and the X1 chromosome are both turned toward the same interpolar spindle axis. This spatial relationship may be a result of a linkage of X1 and Y chromosomes lying in opposite half spindles via a small bundle of microtubules that runs between their unusual kinetochores. Thus, nonrandom segregation in Neocurtilla hexadactyla involves a unique modification at the kinetochores of particular chromosomes, which presumably affects the manner in which these chromosomes are integrated within the spindle.

Chromosomal basis of dosage compensation in Drosophila. IX. Cellular autonomy of the faster replication of the X chromosome in haplo-X cells of Drosophila melanogaster and synchronous initiation.

[(3)H]Thymidine labeling patterns have been examined in gynandric mosaic salivary glands of drosophila melanogaster. The Ring-X stock, R(1) w(ve)/In(1)dl 49, l (1) J1 y w lz(s), was used for this purpose. 365 labeled XX2A and 40 labeled XO2A nuclei were obtained from a total of 624 nuclei in nine pairs of mosaic salivary glands. It was observed that in all but those nuclei which had DD, 1C, and 2C patterns, the X chromosome of the XO2A nuclei always had fewer sites labeled than the X chromosomes of the XX2A nuclei, for a given pattern of the autosomes in either sex. Such asynchronous labeling of the X chromosome in the XO2A (male) nuclei was observed regardless of the proportion of the XO2A cells (2.0-73.7 percent), in the mosaic glands. Moreover, while the frequency of [(3)H]thymidine labeling for all of the 39 replicating units except the two late replicating sites (3C and 11A) in the X chromosome of the XO2A nuclei, was consistently lower than in the X chromosome of the XX2A nuclei, the mean number of grains on the X chromosome was relatively (to autosomes) similar in both XX2A and XO2A cells. The results, therefore, suggest that, as in XY2A larval glands, the X chromosome in the XO2A cells also completes the replication earlier than autosomes and that the XO2A nuclei show cellular autonomy with respect to the early replication of the X chromosome, like its counterpart, RNA transcription. Absence of the asynchrony during the initial phase (DD-2C) further completes the replication earlier but that the rate of replication of its DNA is possibly faster, and (b) that there might be a common regulation with respect to the initiation of replication of different chromosomes in a genome.

Transcription sites in spread meiotic prophase chromosomes from mouse spermatocytes.

Mouse spermatocytes at pachytene stage have been examined by whole-mount electron microscope techniques complemented with autoradiography as an approach for visualizing their transcriptive activity. Structural elements of meiotic bivalents, such as synaptonemal complexes and chromatin fibers, have been satisfactorily displayed in the total set of autosomal and sexual bivalents in single spermatocytes. Adequate preservation of the entire set of bivalents has provided a basis for recognition of sites where presumptive preribosomal RNA and heterogeneous nuclear RNA species are being transcribed at different segments of autosomal bivalents. Nucleoli attached to the basal knob region where nucleolar organizer cistrons are assumed to be located and ribonucleoprotein fibrils associated with distinct chromatin loops have been recognized. These structural findings have been correlated with display of [(3)H]uridine incorporation sites in thin-section and whole-mount electron microscopy autoradiographic preparations. A low transcriptive activity of the sexual bivalent contrasted with extensive gene expression in autosomal bivalents. Each sex chromosome shows a double axial core. A short region of pairing with a synaptonemal complex joins the two chromosomes at one end. We conclude that variations in the rate of RNA synthesis throughout meiotic prophase stages in the mouse are expressed as fluctuations in the amount and distribution of distinct RNA species at specific segments of the bivalents.

Heteromorphic sex chromosomes in male rainbow trout.

A pair of subtelocentric chromosomes differs in the size of the short arm in male, but not female, rainbow trout (Salmo gairdneri). The morphological similarity of the X and Y chromosomes, and the observation of Y chromosomes intermediate between the X and normal Y, suggest that the sex chromosomes are at an early stage of differentiation in this species.

Derived X chromosome in the turtle genus Staurotypus.

C-banding, G-banding, and silver (Ag-AS) staining techniques reveal a distinctive sex chromosome system in the turtle Staurotypus salvinii. Unlike previously described systems in most other vertebrate groups in which the Y or W is derived and the homogametic sex represents the primitive condition, the reverse is true for S. salvinii. The X chromosome is derived; thus the homogametic sex (female) is more derived than the heterogametic sex. The male is intermediate between the female and the ancestral condition observed in other turtle species. Staurotypus does not confirm to the general model of sex chromosome evolution for diploid dioecious organisms.

Temperature-dependent sex determination: current practices threaten conservation of sea turtles.

Temperature determines the sex of hatchling green turtles (Chelonia mydas) produced from eggs incubated in a beach hatchery under different temperature regimens. Cola and cool nests (less than 28 degrees C) produced almost no females (0 to 10 percent) and warm, thermostable nests (greater than 29.5 degrees C) produced almost all females (95 to 100 percent). A few intersex hatchlings were produced at lower temperatures. Since little concern is given to temperatures at which sea turtle eggs are incubated in artificial hatcheries, present conservation practices may be producing all male, all female, or even intersex hatchlings.

Mouse chromosome translocations: visualization and analysis by electron microscopy of the synaptonemal complex.

Pachytene chromosomes of mice heterozygous for known translocations are clearly depicted by configurations of the synaptonemal complexes in spread (whole mount) preparations. In one autosomal and two X-autosome translocations analyzed, breakpoints are identifiable; localization by measurement agrees with mitotic data and shows the translocations to be reciprocal. Synapsis with the Y is inhibited in one translocation in which the breakpoint is the pairing region of the X.

Synaptonemal complex complement of man in spreads of spermatocytes, with details of the sex chromosome pair.

Human pachytene chromosome pairs have been characterized electron microscopically in spread preparations on the basis of synaptonemal complex length, kinetochore position and attached nucleoli when present. The X and Y chromosomes can be followed by their filamentous axial cores from partial synapsis, through precocious disjuction and end-to-end attachment, to differentiation of a network in the sex chromosome pair.

Evolution of primate chromosomes.

Human and higher primate chromosomes have been compared by general and regional banding methods, including hybridization in situ. The general banding patterns of the chromosomes of gorilla, chimpanzee, and orangutan, but not gibbon, are similar to those of the human. Preliminary results show that chromosomes with similar banding patterns in different species often carry the same genes. Repetitious DNA's have undergone changes in structure and distribution which are reflected in changes in the regional banding patterns. These studies confirm that the evolutionary distance between the gibbon and the orangutan is relatively great compared to the distance between the orangutan and the other great apes, and suggest that man is more closely related to the gorilla than to the chimpanzee.

Sex preselection in mammals? Separation of sperm bearing Y and "O" chromosomes in the vole Microtus oregoni.

The two sex determining sperm populations of the vole Microtus oregoni were separated according to DNA content by use of flow sorting instrumentation. Although the sperm were not viable, they should be useful for addressing the question of haploid expression of genes linked to sex chromosomes and for efficiently searching for biochemical markers that differentiate the two populations.

Physical chromosome mapping of repetitive DNA sequences in Nile tilapia Oreochromis niloticus: evidences for a differential distribution of repetitive elements in the sex chromosomes.

Repetitive DNAs have been extensively applied as physical chromosome markers on comparative studies, identification of chromosome rearrangements and sex chromosomes, chromosome evolution analysis, and applied genetics. Here we report the characterization of repetitive DNA sequences from the Nile tilapia (Oreochromis niloticus) genome by construction and screening of plasmid library enriched with repetitive DNAs, analysis of a BAC-based physical map, and hybridization to chromosomes. The physical mapping of BACs enriched with repetitive sequences and C(o)t-1 DNA (DNA enriched for highly and moderately repetitive DNA sequences) to chromosomes using FISH showed a predominant distribution of repetitive elements in the centromeric and telomeric regions and along the entire length of the largest chromosome pair (X and Y sex chromosomes) of the species. The distribution of repetitive DNAs differed significantly between the p arm of X and Y chromosomes. These findings suggest that repetitive DNAs have had an important role in the differentiation of sex chromosomes.