Chromosome variation in the plethodontid salamander, Aneides ferreus

Sex Chromosome Evolution: So Many Exceptions to the Rules

Genomic analysis of many nonmodel species has uncovered an incredible diversity of sex chromosome systems, making it possible to empirically test the rich body of evolutionary theory that describes each stage of sex chromosome evolution. Classic theory predicts that sex chromosomes originate from a pair of homologous autosomes and recombination between them is suppressed via inversions to resolve sexual conflict. The resulting degradation of the Y chromosome gene content creates the need for dosage compensation in the heterogametic sex. Sex chromosome theory also implies a linear process, starting from sex chromosome origin and progressing to heteromorphism. Despite many convergent genomic patterns exhibited by independently evolved sex chromosome systems, and many case studies supporting these theoretical predictions, emerging data provide numerous interesting exceptions to these long-standing theories, and suggest that the remarkable diversity of sex chromosomes is matched by a similar diversity in their evolution. For example, it is clear that sex chromosome pairs are not always derived from homologous autosomes. In addition, both the cause and the mechanism of recombination suppression between sex chromosome pairs remain unclear, and it may be that the spread of recombination suppression is a more gradual process than previously thought. It is also clear that dosage compensation can be achieved in many ways, and displays a range of efficacy in different systems. Finally, the remarkable turnover of sex chromosomes in many systems, as well as variation in the rate of sex chromosome divergence, suggest that assumptions about the inevitable linearity of sex chromosome evolution are not always empirically supported, and the drivers of the birth-death cycle of sex chromosome evolution remain to be elucidated. Here, we concentrate on how the diversity in sex chromosomes across taxa highlights an equal diversity in each stage of sex chromosome evolution.

Evidence for Sex Chromosome Turnover in Proteid Salamanders

A major goal of genomic and reproductive biology is to understand the evolution of sex determination and sex chromosomes. Species of the 2 genera of the Salamander family Proteidae - Necturus of eastern North America, and Proteus of Southern Europe - have similar-looking karyotypes with the same chromosome number (2n = 38), which differentiates them from all other salamanders. However, Necturus possesses strongly heteromorphic X and Y sex chromosomes that Proteus lacks. Since the heteromorphic sex chromosomes of Necturus were detectable only with C-banding, we hypothesized that we could use C-banding to find sex chromosomes in Proteus. We examined mitotic material from colchicine-treated intestinal epithelium, and meiotic material from testes in specimens of Proteus, representing 3 genetically distinct populations in Slovenia. We compared these results with those from Necturus. We performed FISH to visualize telomeric sequences in meiotic bivalents. Our results provide evidence that Proteus represents an example of sex chromosome turnover in which a Necturus-like Y-chromosome has become permanently translocated to another chromosome converting heteromorphic sex chromosomes to homomorphic sex chromosomes. These results may be key to understanding some unusual aspects of demographics and reproductive biology of Proteus, and are discussed in the context of models of the evolution of sex chromosomes in amphibians.

Karyotype of a Japanese salamander Hynobius katoi and its implication on breeding ecology (Amphibia: Caudata)

Karyotype of a Japanese small salamander, Hynobius katoi, was first described. All individuals examined had 2n=58 chromosomes, consisting of nine pairs of biarmed macrochromosomes, four pairs of biarmed medium-sized chromosomes, six pairs of biarmed microchromosomes, and 10 pairs of uniarmed microchromosomes, although distinction of the second and the third groups of chromosome pairs was not clear. All pairs appeared homologous and no sexual dimorphism was found. Possession of 2n=58 chromosomes in H. katoi strongly suggests its lotic-breeding habits as was expected from the number and size of eggs and adult morphology. When compared morphology of chromosomes among lotic-breeders with 2n=58 chromosomes, metacentric nature of No. 10 seems to characterize the karyotype of H. katoi.

Molecular and historical evidence for the introduction of clouded salamanders (genus Aneides) to Vancouver Island, British Columbia, Canada, from California

Genetic evidence shows that Vancouver Island populations of clouded salamanders (genus Aneides) are nearly identical with some California populations, in both allozymes and mitochondrial DNA. Historical evidence suggests that salamanders were introduced to Vancouver Island during the nineteenth century. They were probably included in shipments of tan oak bark from California. Tan oak bark was used extensively in the tanning of leather at that time. The introduction to Vancouver Island provides an opportunity to study environmental effects in a species that can not be easily studied on a short-term basis. The California and Canadian populations differ strikingly from Oregon populations of clouded salamanders and are described as a new species, Aneides vagrans.

B-chromosome origin in the endemic New Zealand frog Leiopelma hochstetteri through sex chromosome devolution

The endemic New Zealand frog Leiopelma hochstetteri has variable numbers of mitotically stable B chromosomes. To assess whether the B chromosomes were derived from the autosome complement, they were isolated by micromanipulation and their DNA amplified by degenerate oligonucleotide primed PCR. Southern hybridizations of B chromosome DNA probes to genomic DNA from males and females characterized by differing numbers of B chromosomes demonstrated that the B chromosomes were derived from the univalent W sex chromosome characteristic of North Island populations. The presence of homologous B chromosome specific sequences from geographically distinct populations indicates a single origin of the B chromosomes. Furthermore, a primitive homology shared by B chromosomes and the W sex chromosome from an ancestral WZ/ZZ karyotype, which is still present in frogs from Great Barrier Island, shows that the B chromosomes originated soon after the univalent W sex chromosome had originated. Sequence analysis revealed that B chromosome DNA is composed of repeat sequences and has the potential to form stable hairpin structures. The molecular dynamics of these structures may reflect an inherent propensity to undergo rapid change in nucleotide sequence and chromosome structure.

Chromosomal variation in the northwestern American newts of the genus Taricha (Caudata: Salamandridae)

Chromosomal variation was analysed in 22 populations of newts of the salmandrid genus Taricha of western North America, and compared with that of the eastern North American newts of the genus Notophthalmus. The karyotypes of the species Taricha and Notophthalmus were very similar. However, there was considerable variation in the distribution patterns of heterochromatins (revealed by C-banding) and in the sites of the nucleolar organizing region (NOR) (revealed by fluorochrome chromomycin A3 banding) within and between species of these two genera. Chromosomal variation patterns were interpreted in relation to a phylogenetic hypothesis derived from data on mitochondrial DNA sequences and allozyme variation. This study suggests that the pattern distributions of heterochromatins in chromosomes of Taricha are more derived than those of its sister taxon Notophthalmus. Furthermore, the chromosomal NOR types found in the southernmost and northernmost populations of T. granulosa, in the northernmost populations of T. t. sierrae, and in the southern populations of T. t. torosa are recently derived. The implications of this chromosomal variation for phylogeny and biogeography are discussed.

HETEROMORPHISM FOR A HIGHLY REPEATED SEQUENCE IN THE NEW ZEALAND FROG LEIOPELMA HOCHSTETTERI

A satellite DNA sequence, Lhl, was cloned from the New Zealand endemic frog Leiopelma hochstetteri. Large tandem arrays of Lh1 were localized by in situ hybridization to the long arm of a small telocentric autosome in some individuals, but these arrays were absent from other individuals. Lh1 is also present in varying amounts on some supernumerary chromosomes in some individuals. Heteromorphism for the presence of Lh1 exists in two populations that have been separated by a sea channel since the end of the Pleistocene, indicating that the heteromorphism either has arisen repeatedly or has persisted for at least 10,000 years. Individuals lacking Lh1 thus appear to be at no significant selective disadvantage. The variation in Lh1 copy number probably results from its interstitial chromosomal location, which exposes it to more frequent unequal crossovers than the pericentromeric or telocentric locations of most satellite DNA. Lh1 may be parasitic or simply inert junk, but in either case it may be deleted or dispersed throughout the rest of the genome through unequal crossing over.

Supernumerary chromosome variation and heterochromatin distribution in the endemic New Zealand frog Leiopelma hochstetteri

Specimens of the endemic New Zealand frog Leiopelma hochstetteri from Tapu on North Island were found to have six, nine or ten supernumerary chromosomes in their karyotypes. In comparison with previously published data, these results further indicate probable geographic variation in supernumerary chromosome number between populations. Increased numbers of supernumeraries in these frogs is correlated with apparent decrease of centromeric heterochromatin in the five large metacentric chromosomes of the karyotype, as detected by C-banding. Meiosis was abnormal in a male with a high number of supernumeraries. In lampbrush preparations from a single female with one supernumerary univalent, the supernumerary often had a denser, beaded appearance in comparison with the regular bivalents. Evidence is consistent with the notion that these supernumerary chromosomes may have arisen from centromeric fragments.

Heterochromatic DNA in Triturus (Amphibia, Urodela). I. A satellite DNA component of the pericentric C-bands

We have studied the structure, genome organization, chromosomal location, conservation across species and transcription on lampbrush chromosomes, of an AT-rich satellite DNA component of the newt, Triturus vulgaris meridionalis. The satellite (Sat G), originally isolated by gradient centrifugation, represents about 2% of the vulgaris genome and comprises a highly repetitive sequence family (HindIII family), whose monomers have been cloned. The repeat units are about 330 bp long, as measured on gels, and a cloned unit (pTvm1) is 310 bp long, as shown by sequencing. Abundant clusters of the HindIII family sequences are located within the pericentric heterochromatin (i.e. the C-bands placed at both sides of, and at a certain distance from, the centromeres) in most chromosomes. Both the sequence family and its overall pattern of chromosomal distribution are conserved within the genus Triturus, despite a few species-specific differences. The great majority of the HindIII family sequences are unexpressed on lampbrush chromosomes; they reside within pericentric, condensed segments of the chromosome axis ("loopless bars"). Only a few sequences are transcribed on some loops, suggesting that transcription promotion does not depend on the satellite sequences themselves.

Cytophotometric evidence of variation in genome size of desmognathine salamanders

The amount of DNA per haploid genome, the C-value, is often directly correlated with nuclear and cell volume, but inversely correlated with cell replication rate. Also, rates of cellular growth sometimes appear to be correlated with organismal developmental rates and life history patterns. Among vertebrates, salamanders exhibit the greatest variation in genome size. In the present study we have examined interspecific and intraspecific variation in blood cell DNA levels in the genus Desmognathus, which shows greater variation in life history traits than any other salamander genus. Specimens of Desmognathus quadramaculatus, D. monticola, D. ochrophaeus and D. wrighti were collected from nature at two localities in the southern Appalachian Mountains. Estimates of genome size in pg of DNA were obtained from blood smears by DNA-Feulgen cytophotometry, using erythrocyte nuclei of Xenopus laevis as an internal reference standard of 6.35 pg DNA per cell. C-values of Desmognathus are the smallest in the order Caudata. Although significant variation in DNA levels was found among the four species, the differences were small, and do not support previously proposed relationships between C-value and life-history variation.

Synergistic effect of TPA and T-cell mitogens in nonmammalian vertebrates

The phorbol ester, 12-0-tetradecanoyl-phorbol-13-acetate (TPA) was used as a comitogen with the plant lectins phytohemagglutinin (PHA) and concanavalin A (ConA) in short-term cultures of whole blood from nonmammalian vertebrates. Stimulation with TPA in addition to standard mitogens resulted in a synergistic effect, consistently yielding more metaphases than cultures stimulated with either PHA, ConA, or TPA alone and is successful with blood samples as small as 0.1 ml. The increased mitotic index makes it possible to use different banding procedures for systematic studies. Also, because the amount of blood needed is so small, this procedure, unlike other published techniques, does not require the destruction of smaller animals to do chromosome studies.

Chromosome and C-heterochromatin polymorphisms in the Italian newt, Triturus italicus

A combined chromosome and C-heterochromatin polymorphism in pair 12 in the complement of the newt species, T. italicus is described. The C-heterochromatin polymorphism is presumably due to a loss in the proximal C-band, whereas the chromosomal polymorphism has its origin in two different independent pericentric inversions both including the centromere and the proximal C-band of chromosome 12. The double-inversion polymorphism has a wide distribution over the range and follows a clear bipolarity between a northern area where the karyotype is homomorphic for the standard type of pair 12 (ST/ST) and an opposite area where the ST type is completely replaced by variant M1 and M2 metacentric chromosomes 12. Various karyophylogenies are possible, but the simplest and the most probable presumes an ancestral karyotype of ST/ST and a mechanism of gradual replacement of the heterobrachial chromosome ST by two independent pericentric inversions. The present data are discussed in relation to existing theories on karyological evolution of Urodeles and the functional significance of telocentric chromosomes suggested by Sessions et al. (1982).

Evidence for chromosome number reduction and chromosomal homosequentiality in the 24-chromosome Korean frog Rana dybowskii and related species

The karyotype of the Korean frog Rana dybowskii with its pattern of C-band heterochromatin distribution was numerically analyzed. There are 2n = 24 chromosomes in the karyotype representing a reduction in number from the typical 2n = 26 chromosome karyotype of Rana. The karyotype shows other evidence of reorganization relative to 26-chromosome species. The chromosomes grade smoothly in size from largest to smallest without the two size classes that are characteristic for 26-chromosome species. In contrast to many 26-chromosome species, there are few centromeric C-bands but many interstitial ones. C-bands for each homologous chromosome pair are distinctive. A prominent secondary constriction is located on one of the smallest chromosomes, chromosome 11, in a position similar to that seen in most 26-chromosome species. The karyotype of R. dybowskii is compared to those of other species of Rana known to have 2n = 24 chromosomes; it is most similar to that of R. chensinensis, less so that of R. ornativentris and less still to that of R. arvalis in terms of the positions of centromeres and secondary constrictions. C-bands as well as secondary constrictions in the karyotypes of these frogs show evidence of chromosomal homosequentiality. The process and possible consequences of chromosome number reduction from an ancestral 26-chromosome karyotype is also evident in the karyotypes of these closely allied palearctic frogs. Pericentric inversions followed by fusion of two small elements apparently produced a new chromosome, chromosome 6, occurring originally among northeast Asian populations.

Multiple ribosomal gene sites revealed by in situ hybridization of Xenopus rDNA to Triturus lampbrush chromosomes

A variety of 3H-labelled ribosomal gene probes were hybridized in situ to the nascent transcripts of lampbrush chromosomes from the crested newt, Triturus cristatus carnifex. The probes were from Xenopus laevis and included rDNA isolated by CsCl gradient centrifugation, recombinant plasmids and purified restriction fragments of rDNA. All the probes gave essentially the same result. About 10-15 loop pairs were distinctly labelled in each preparation, almost all of them located on the heteromorphic arms (HTAs) of chromosome 1. Ribosomal gene probes were also hybridized in situ to the DNA of denatured mitotic chromosomes from some of the individuals used to provide lampbrush preparations. Minor, scattered sites of hybridization were found in the HTAs, but the main clusters of ribosomal genes were found on chromosomes 6 and/or 9, in agreement with previous determinations of nucleolus organizer position in this species. However, the nucleolus organizers were not sites of labelled loops in lampbrush transcript hybridizations.--We have incubated isolated lampbrush-stage nuclei in media containing alpha-amanitin and labelled RNA precursors. Although extrachromosomal nucleolar genes incorporated label, supposedly due to transcription by RNA polymerase I, no lampbrush loops were labelled.--It appears that in T. c. carnifex there are ribosomal gene sequences at the main nucleolus organizers and at a number of sites scattered along the HTAs. The ribosomal genes at the nucleolus organizers are not extended in the form of actively transcribing loops unlike the ribosomal sequences on the HTAs, which are heavily labelled in transcript hybridization. The ribosomal sequences on the HTAs appear not to be transcribed by the same RNA polymerase that transcribes the ribosomal genes of extrachromosomal nucleoli.

Chromosome polymorphism in the Italian newt, Triturus italicus

A chromosomal variation, changing shape and C-banding pattern of chromosome XII of Triturus italicus was detected among the offspring of two F1 hybrid families of T. italicus female x T. vulgaris meridionalis male. In both families a number of individuals appeared to have a metacentric instead of the expected subtelocentric chromosome XII of T. italicus.--Investigations in three well separated localities in the range of the species showed the polymorphism to have a wide distribution and to be part of a complex pattern involving at least two inversions and (presumably) deficiencies of large C-bands. At meiosis, the shape of bivalent XII, and the location and frequency of chiasmata in the bivalent varied with the karyomorph involved. It is suggested that large rearrangements may still play an important role in the karyological evolution of Triturus.

The nucleolus organizers of Plethodon and Aneides located by in situ nucleic acid hybridization with Xenopus 3H-ribosomal RNA

The main clusters of ribosomal genes, or nucleolus organizers, have been located by in situ nucleic acid hybridization of Xenopus laevis 3H labelled ribosomal RNA to mitotic chromosomes in squash preparations of intestinal epithelium from 7 species of Plethodon and 3 species of Aneides. The species used were chosen on account of having well known karyotypes and genome sizes. The Plethodon species covered a range of genome size of 20--69.4 pg. The locations of those nucleolus organizers that could be detected by autoradiography after in situ hybridization varied from species to species, and in Aneides there were differences between two populations of the same species. On the other hand, some distantly related species of Plethodon, with widely different genome sizes, had nucleolus organizers at corresponding positions. The results are discussed in relation to ideas on karyotype stability, homosequentiality and chromosome repatterning.