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At least 19 recordsLinked to original sources
PubMed2003

Sex chromosomes, sex determination, and sex-linked sequences in Microtidae.

The Arvicolidae is a widely distributed rodent group with several interesting characteristics in their sex chromosomes. Here, we summarize the actual knowledge of some of these characteristics. This mammalian group has species with abnormal sex determination systems. In fact, some species present the same karyotype in both males and females, with total absence of a Y chromosome, and hence of SRY and ZFY genes. Other species present fertile, sex-reversed XY females, generally due to mutations affecting X chromosomes. Furthermore, in Microtus oregoni males and females are gonosomic mosaic (the females are XO in the soma and XX in the germ cells, while the males are XY in the soma and OY in the germ cells). Regarding sex chromosomes, some species present enlarged (giant) sex chromosomes because of the presence of large blocks of constitutive heterochromatin, which have been demonstrated to be highly heterogeneous. Furthermore, we also consider the alterations affecting composition and localization of sex-linked genes or repeated sequences. Finally, this rodent group includes species with synaptic and asynaptic sex chromosomes. In fact, several species with asynaptic sex chromosomes have been described. It is interesting to note that within the genus Microtus both types of sex chromosomes are present.

Animals↗
PubMed1979

Studies on metatherian sex chromosomes. IX. Sex chromosomes of the greater glider (Marsupialia: Petauridae).

The greater glider, currently but incorrectly known as Schoinobates volans, is widely distributed in forested regions in eastern Australia. All animals studied from six different localities had 20 autosomes but there were four chromosomally distinct populations. At Royal National Park, N.S.W., all female greater gliders studied had 22 chromosomes including two large submetacentric X chromosomes with subterminal secondary constrictions in their longer arms. This form of X chromosome occurred also at Bondo State Forest, Myall Lakes and Coff's Harbour, N.S.W., and at Eidsvold, Qld. At Coomooboolaroo, Qld, the X chromosome was also a large submetacentric but a secondary constriction occurred in the shorter arm. Two chromosomally distinct types apparently occur in Royal National Park, one with XY males as in all other populations, and one with XY1Y2 males. Y or Y1, but not Y2, chromosomes were eliminated from the bone marrow in all populations but were present in spermatogonia, primary spermatocytes and cultured fibroblasts. Animals from Bondo State Forest had three or more acrocentric or metacentric supernumerary chromosomes.

Animals↗
PubMed2002

Comparative mapping of Z-orthologous genes in vertebrates: implications for the evolution of avian sex chromosomes.

Sex chromosomes of birds and mammals are highly differentiated and share several cytological features. However, comparative gene mapping reveals extensive conserved synteny between the chicken Z sex chromosome and human chromosome 9 but not the human X sex chromosome, implying an independent origin of avian and mammalian sex chromosomes. To better understand the evolution of the avian Z chromosome we analysed the synteny of chicken Z-linked genes in zebrafish, which is the best-mapped teleost genome so far. Existing zebrafish maps do not support the existence of an ancestral Z linkage group in the zebrafish genome, whereas mammalian X-linked genes show at least some degree of synteny conservation. This is consistent with in situ hybridisation mapping data in the freshwater pufferfish, Tetraodon nigroviridis where mammalian X-linked genes show a much higher degree of conserved synteny than human chromosome 9 or the avian Z chromosome. Collectively, these data argue in favour of a more recent evolution of the avian Z chromosome, compared with the mammalian X.

Animals↗
PubMed2001

Contrasting levels of nucleotide diversity on the avian Z and W sex chromosomes.

Sex chromosomes may provide a context for studying the local effects of mutation rate on molecular evolution, since the two types of sex chromosomes are generally exposed to different mutational environments in male and female germ lines. Importantly, recent studies of some vertebrates have provided evidence for a higher mutation rate among males than among females. Thus, in birds, the Z chromosome, which spends two thirds of its time in the male germ line, is exposed to more mutations than the female-specific W chromosome. We show here that levels of nucleotide diversity are drastically higher on the avian Z chromosome than in paralogous sequences on the W chromosome. In fact, no intraspecific polymorphism whatsoever was seen in about 3.4 kb of CHD1W intron sequence from a total of >150 W chromosome copies of seven different bird species. In contrast, the amount of genetic variability in paralogous sequences on the Z chromosome was significant, with an average pairwise nucleotide diversity (d) of 0.0020 between CHD1Z introns and with 37 segregating sites in a total of 3.8 kb of Z sequence. The contrasting levels of genetic variability on the avian sex chromosomes are thus in a direction predicted from a male-biased mutation rate. However, although a low gene number, as well as some other factors, argues against background selection and/or selective sweeps shaping the genetic variability of the avian W chromosome, we cannot completely exclude selection as a contributor to the low levels of variation on the W chromosome.

Animals↗
PubMed2001

Sex from W to Z: evolution of vertebrate sex chromosomes and sex determining genes.

Sex determination in major vertebrate groups appears to be very variable, including systems of male heterogamety, female heterogamety and a variety of genetic and environmental sex determining systems. Yet comparative studies of sex chromosomes and sex determining genes now suggest that these differences are more apparent than real. The sex chromosomes of even widely divergent groups now appear to have changed very little over the last 300+ million years, and even independently derived sex chromosomes seem to have followed the same set of evolutionary rules. The sex determining pathway seems to be extremely conserved, although the control of the genes in this pathway is vested in different elements. We present a scenario for the independent evolution of XY male heterogamety in mammals and ZW female heterogamety in birds and some reptiles. We suggest that sex determining genes can be made redundant, and replaced by control at another step of a conserved sex determining pathway, and how choice of a gene as a sex switch has led to the evolution of new sex chromosome systems. J. Exp. Zool. 290:449-462, 2001.

Animals↗
PubMed2001

Sex chromosomes, sex-linked genes, and sex determination in the vertebrate class amphibia.

In this chapter the different categories of homomorphic and heteromorphic sex chromosomes, types of sex-determining mechanisms, known sex-linked genes, and data about sex-determining genes in the Amphibia have been compiled. Thorough cytogenetic analyses have shown that both XY/XX and ZW/ZZ sex chromosomes exist in the order Anura and Urodela. In some species quite unusual systems of sex determination have evolved (e.g. 0W-females/00-males or the co-existence of XY/XX and ZW/ZZ sex chromosomes within the same species). In the third order of the Amphibia, the Gymnophiona (or Apoda) there is still no information regarding any aspect of sex determination. Whereas most species of Anura and Urodela present undifferentiated, homomorphic sex chromosomes, there is also a considerable number of species in which an increasing structural complexity of the Y and W chromosomes exists. In various cases, the morphological differentiation of the sex chromosomes occurred as a result of quantitative and/or qualitative changes to the repetitive DNA sequences in the constitutive heterochromatin of the Y and W chromosomes. The greater the structural differences between the sex chromosomes, the lesser the extent of pairing in meiosis. No dosage compensation of the sex-linked genes in the somatic cells of the homogametic (XX or ZZ) individuals have been detected. The genes located to date on the amphibian sex chromosomes lead to the conclusion that there is no common ancestral or conserved sex-linkage group. In all amphibians, genetic sex determination (GSD) seems to operate, although environmental factors may influence sex determination and differentiation. Despite the accumulated evidence that GSD is operating in Anura and Urodela, there is little substantial information about how it functions. Although several DNA sequences homologous to the mammalian ZFY, SRY and SOX genes have been detected in the Anura or Urodela, none of these genes is an appropriate candidate to explain sex determination in these vertebrates.

Amphibians↗
PubMed2002

Plant sex determination and sex chromosomes.

Sex determination systems in plants have evolved many times from hermaphroditic ancestors (including monoecious plants with separate male and female flowers on the same individual), and sex chromosome systems have arisen several times in flowering plant evolution. Consistent with theoretical models for the evolutionary transition from hermaphroditism to monoecy, multiple sex determining genes are involved, including male-sterility and female-sterility factors. The requirement that recombination should be rare between these different loci is probably the chief reason for the genetic degeneration of Y chromosomes. Theories for Y chromosome degeneration are reviewed in the light of recent results from genes on plant sex chromosomes.

Evolution, Molecular↗
PubMed1995

Female preponderance for development of arthritis in rats is influenced by both sex chromosomes and sex steroids.

Autoimmune arthritis was induced after a single injection of the non-immunogenic adjuvant (avridine) or with autologous rat type II collagen. Females of two different rat strains, DA and LEW, were found to be more susceptible than males. To investigate further the mechanisms behind the female preponderance, we selected the avridine induced arthritis model. This is known to be a chronic joint-specific disease which is T-cell dependent and associated with MHC genes and, therefore, is an appropriate model for rheumatoid arthritis. To address the possibility of sex chromosome involvement, reciprocal F1 hybrids were produced. Female (DAxLEW)F1 rats were found to be more prone to arthritis than their male counterparts. This difference could be explained, at least partly, by the influence of sex chromosomes since reciprocal (LEWxDA)F1 rats showed no sex linkage. However, the sex linkage was more pronounced in normal rats when compared to castrated (DAxLEW)F1 rats indicating a role for sex hormones in conjunction with the sex chromosome-linked effect. Both oestrogen and testosterone had a suppressive effect on the development of arthritis. The findings presented here suggest the presence of a sex chromosome gene, which mediates its function only in the presence of sex hormones and is associated with a female preponderance for development of arthritis.

Animals↗
PubMed2001

[Origin and evolution of mammalian sex chromosomes].

Mammals present an XX/XY system of chromosomal sex determination, males being the heterogametic sex. Comparative studies of the gene content of sex chromosomes from the major groups of mammals reveal that most Y genes have X-linked homologues and that X and Y share homologous pseudoautosomal regions. These observations, together with the presence of the two homologous regions (pseudoautosomal regions) at the tips of the sex chromosomes, suggest that these chromosomes began as an ordinary pair of homologous autosomes. Birds present a ZW/ZZ system of chromosomal sex determination where females are the heterogametic sex. In this case, avian sex chromosomes are derived from different pairs of autosomes than mammals. The evolutionary pathway from the autosomal homomorphic departure to the present-day heteromorphic sex chromosomes in mammals includes suppression of X-Y recombination, differentiation of the nascent non-recombining regions, and progressive autosomal addition and attrition of the sex chromosomes. Recent results indicate that the event marking the beginning of the differentiation between the extant X and Y chromosomes occurred about 300 million years ago.

Animals↗
PubMed1997

Sex chromosomes and sex determining mechanisms in birds.

The sex chromosomes in birds are designated Z and W, and the male is the homomorphic sex (ZZ) and the female heteromorphic (ZW). In most avian species the Z chromosome is a large chromosome, usually the fourth or fifth largest, and it contains almost all the known sex-linked genes. The W chromosome is generally a much smaller microchromosome, containing a high proportion of repeat sequence DNA. Recently a gene encoding a protein involved in transcriptional activation of chromatin has been detected on the W chromosome. The weight of evidence suggests that sex determination in birds is by a genic balance mechanism, in which the ratio of autosomes to Z chromosomes is the crucial factor. DNA sequences homologous to the testis determining factor in humans have been detected in both male and female birds, but it is not clear that they have a sex-related function in birds. A number of different practical methods have been developed to distinguish the sex of birds, based on sex-linked genes, the amount of DNA per cell and using DNA probes for sex-linked sequences.

Animals↗
PubMed2002

The evolution of chromosomal sex determination.

There is a great diversity of sex determination mechanisms, with evidence for numerous evolutionary transitions between different systems. For example, environmental sex determination is widespread in lower vertebrates, and genetic sex determination has probably evolved from it several times. This requires the establishment of genes that override environmental cues. Close linkage between male and female determining loci is favoured by selection, and represents the first step towards the evolution of highly differentiated sex chromosomes. Once crossing over between primitive sex chromosomes has been suppressed, the primitive Y (W) chromosome is vulnerable to the operation of forces that lead to a reduction in its effective population size. This reduces the ability of natural selection to maintain the functionality of genes on the proto-Y, so that it gradually degenerates. Primitive sex chromosome systems, and systems of neo-X and neo-Y chromosomes formed by translocations involving autosomes and sex chromosomes, provide an opportunity to test evolutionary models of the degeneration of Y chromosomes and to determine the time-scales involved. Recent data confirm that newly-evolving Y or neo-Y chromosomes experience a sharp reduction in effective population size, and indicate that degeneration can occur over a few million generations.

Animals↗
PubMed2005

Sex chromosomes and sex determination in reptiles.

Reptiles occupy a crucial position with respect to vertebrate phylogeny, having roamed the earth for more than 300 million years and given rise to both birds and mammals. To date, this group has been largely ignored by contemporary genomics technologies, although the green anole lizard was recently recommended for whole genome sequencing. Future experiments using flow-sorted chromosome libraries and high-throughout genomic sequencing will help to discover important findings regarding sex chromosome evolution, early events in sex determination, and dosage compensation. This information should contribute extensively toward a general understanding of the genetic control of development in amniotes.

Animals↗