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J A Graves

Publications and source records attributed to J A Graves.

At least 19 recordsLinked to original sources
PubMed2000

Regulation of the yeast INO1 gene. The products of the INO2, INO4 and OPI1 regulatory genes are not required for repression in response to inositol.

The ino2Delta, ino4Delta, opi1Delta, and sin3Delta mutations all affect expression of INO1, a structural gene for inositol-1-phosphate synthase. These same mutations affect other genes of phospholipid biosynthesis that, like INO1, contain the repeated element UAS(INO) (consensus 5' CATGTGAAAT 3'). In this study, we evaluated the effects of these four mutations, singly and in all possible combinations, on growth and expression of INO1. All strains carrying an ino2Delta or ino4Delta mutation, or both, failed to grow in medium lacking inositol. However, when grown in liquid culture in medium containing limiting amounts of inositol, the opi1Delta ino4Delta strain exhibited a level of INO1 expression comparable to, or higher than, the wild-type strain growing under the same conditions. Furthermore, INO1 expression in the opi1Delta ino4Delta strain was repressed in cells grown in medium fully supplemented with both inositol and choline. Similar results were obtained using the opi1Delta ino2Delta ino4Delta strain. Regulation of INO1 was also observed in the absence of the SIN3 gene product. Therefore, while Opi1p, Sin3p, and the Ino2p/Ino4p complex all affect the overall level of INO1 expression in an antagonistic manner, they do not appear to be responsible for transmitting the signal that leads to repression of INO1 in response to inositol. Various models for Opi1p function were tested and no evidence for binding of Opi1p to UAS(INO), or to Ino2p or Ino4p, was obtained.

Fungal Proteins↗
PubMed1999

Concerted regulation and molecular evolution of the duplicated SNRPB'/B and SNRPN loci.

The human small nuclear ribonucleoprotein SNRPB ' /B gene is alternatively spliced to produce the SmB or SmB' spliceosomal core proteins. An ancestral duplication gave rise to the closely related SNRPN paralog whose protein product, SmN, replaces SmB'/B in brain. However, the precise evolutionary and functional relationship between these loci has not been clear. Genomic, cDNA and protein analyses presented here in chicken, two marsupials (South American opossum and tammar wallaby), and hedgehog, suggest that the vertebrate ancestral locus produced the SmB' isoform. Interestingly, three eutherians exhibit radically distinct splice choice expression profiles, producing either exclusively SmB in mouse, both SmB and SmB' in human, or exclusively SmB' in hedgehog. The human SNRPB ' /B locus is biallelically unmethylated, unlike the imprinted SNRPN locus which is unmethyl-ated only on the expressed paternal allele. Western analysis demonstrates that a compensatory feedback loop dramatically upregulates SmB'/B levels in response to the loss of SmN in Prader-Willi syndrome brain tissue, potentially reducing the phenotypic severity of this syndrome. These findings imply that these two genes encoding small nuclear ribonucleoprotein components are subject to dosage compensation. Therefore, a more global regulatory network may govern the maintenance of stoichiometric levels of spliceosomal components and may constrain their evolution.

Alternative Splicing↗
PubMed1999

Temperature-dependent sex determination: upregulation of SOX9 expression after commitment to male development.

In mammals, birds and reptiles the morphological development of the gonads appear to be conserved. This conservation is evident despite the different sex determining switches employed by these vertebrate groups. Mammals exhibit chromosomal sex determination (CSD) where the key sex determining switch is the Y-linked gene, SRY. Although SRY is the trigger for testis determination in mammals, it is not conserved in other vertebrate groups. However, a gene closely related to SRY, the highly conserved transcription factor, SOX9, plays an important role in the testis pathway of mammals and birds. In contrast to the CSD mechanism evident in mammals and birds, many reptiles exhibit temperature dependent sex determination (TSD) where the egg incubation temperature triggers sex determination. Here we examine the expression of SOX9 during gonadogenesis in the American alligator, (Alligator mississippiensis), a reptile that exhibits TSD. Alligator SOX9 is expressed in the embryonic testis but not in the ovary. However, the timing of SOX9 upregulation in the developing testis is not consistent with a role for this gene in the early stages of alligator sex determination. Since SOX9 upregulation in male embryos coincides with the structural organisation of the testis, SOX9 may operate farther downstream in the vertebrate sex differentiation pathway than previously postulated.

Alligators and Crocodiles↗
PubMed1999

Temperature-dependent sex determination in the American alligator: AMH precedes SOX9 expression.

Gonadal morphogenesis is very similar among mammals, birds, and reptiles. Despite this similarity, each group utilises quite different genetic triggers for sex determination. In mammals, testis development is initiated by action of the Y-chromosome gene SRY. Current evidence suggests that SRY may act together with a related gene, SOX9, to activate another gene(s) in the pathway of testicular differentiation. A downstream candidate for regulation by SRY and SOX9 is AMH. In mouse, Sox9 is expressed in the Sertoli cells of the embryonic testis and it precedes the onset of Amh expression. During mouse gonadogenesis, Amh is confined to the embryonic testis, although it later shows postnatal expression in the ovary. Reptiles such as the American alligator, which exhibit temperature-dependent sex determination (TSD) do not have dimorphic sex chromosomes and apparently no SRY orthologue. SOX9 is expressed during testis differentiation in the alligator; however, it appears to be expressed too late to cause testis determination. Here we describe the cloning and expression of the alligator AMH gene and show that AMH expression precedes SOX9 expression during testis differentiation. This is the opposite to that observed in the mouse where SOX9 precedes AMH expression. The data presented here, as well as findings from recent expression studies in the chick, suggest that AMH expression is not regulated by SOX9 in the non-mammalian vertebrates.

Alligators and Crocodiles↗
PubMed1999

Sex chromosomes and sex-determining genes: insights from marsupials and monotremes.

Comparative studies of the genes involved in sex determination in the three extant classes of mammals, and other vertebrates, has allowed us to identify genes that are highly conserved in vertebrate sex determination and those that have recently evolved roles in one lineage. Analysis of the conservation and function of candidate genes in different vertebrate groups has been crucial to our understanding of their function and positioning in a conserved vertebrate sex-determining pathway. Here we review comparisons between genes in the sex-determining pathway in different vertebrates, and ask how these comparisons affect our views on the role of each gene in vertebrate sex determination.

Animals↗
PubMed1999

Comparative chromosome painting between marsupial orders: relationships with a 2n = 14 ancestral marsupial karyotype.

A 2n = 14 karyotype is shared by some species in each of the marsupial orders in Australian and American superfamilies, suggesting that the ancestral marsupial chromosome complement was 2n = 14. We have used chromosome painting between distantly related marsupial species to discover whether genome arrangements in 2n = 14 species in two Australian orders support this hypothesis. Cross-species chromosome painting was used to investigate chromosome rearrangements between a macropodid species Macropus eugenii (2n = 16) and a wombat species in a different suborder (Lasiorhinus latifrons, 2n = 14), and a dasyurid species in a different order (Sminthopsis macroura, 2n = 14). We demonstrate that many chromosome regions are conserved between all three species, and deduce how the similar 2n = 14 karyotypes of species in the two orders are related to a common ancestral 2n = 14 karyotype.

Animals↗
PubMed1999

Karyotype relationships between four distantly related marsupials revealed by reciprocal chromosome painting.

Marsupial karyotypes have shown extensive conservation even between distantly related groups with a high diversity of life forms and reproductive biology. Banding analysis has been the main test for assessing their homologies and chromosome rearrangements. More recently, cross-species reciprocal chromosome painting has been developed and applied to several mammalian species and has shown homologies and rearrangements not revealed by banding analysis. Karyotype relationships between four marsupial species, Sminthopsis crassicaudata, Potorous tridactylus, Trichosurus vulpecula and Macropus eugenii, which are from different families in two orders, were investigated and presented in the form of comparative chromosome maps. These show that only a limited number of chromosomal rearrangements have occurred during their evolution. A karyotype phylogeny of the four marsupials was derived from these maps. A comparison between published gene location and the comparative chromosome maps for these species is presented and inconsistencies with previous gene mapping data indicated.

Animals↗
PubMed1999

Comparative painting reveals strong chromosome homology over 80 million years of bird evolution.

Chickens and the great flightless emu belong to two distantly related orders of birds in the carinate and ratite subclasses that diverged at least 80 million years ago. In the first ZOO-FISH study between bird species, we hybridized single chromosome paints from the chicken (Gallus domesticus) onto the emu chromosomes. We found that the nine macrochromosomes show remarkable homology between the two species, indicating strong conservation of karyotype through evolution. One chicken macrochromosome (4) was represented by a macro- and a microchromosome in the emu, suggesting that microchromosomes and macrochromosomes are interconvertible. The chicken Z chromosome paint hybridized to the emu Z and most of the W, confirming that ratite sex chromosomes are largely homologous; the centromeric region of the W which hybridized weakly may represent the location of the sex determining gene(s).

Animals↗
PubMed1999

Chromosome painting in marsupials: genome conservation in the kangaroo family.

In order to deduce the ancestral genome arrangement in the karyotypically diverse marsupial family Macropodidae, and to assess chromosome change in this family, chromosome-specific paints from the tammar wallaby (2n = 16) were hybridized to metaphase spreads from the two species proposed to represent the 2n = 22 ancestral karyotype, as well as species with derived 2n = 20 and 2n = 14 karyotypes. Identical patterns were observed in the two 2n = 22 species, from which the rearrangements to form the three derived karyotypes may be easily deduced to be 1, 3 and 4 different fusions, respectively. The identical Thylogale and Dorcopsis genomes may both be used to represent the pleisiomorphic macropodid chromosome complement. Variation in the X chromosome was also investigated by hybridizing an X-Y shared tammar wallaby 12-kb repeat element to chromosomes from the other four macropodid species, finding that it hybridized only to the most closely related species, and therefore is of recent origin.

Animals↗
PubMed1999

Chromosome evolution in kangaroos (Marsupialia: Macropodidae): cross species chromosome painting between the tammar wallaby and rock wallaby spp. with the 2n = 22 ancestral macropodid karyotype.

Marsupial mammals show extraordinary karyotype stability, with 2n = 14 considered ancestral. However, macropodid marsupials (kangaroos and wallabies) exhibit a considerable variety of karyotypes, with a hypothesised ancestral karyotype of 2n = 22. Speciation and karyotypic diversity in rock wallabies (Petrogale) is exceptional. We used cross species chromosome painting to examine the chromosome evolution between the tammar wallaby (2n = 16) and three 2n = 22 rock wallaby species groups with the putative ancestral karyotype. Hybridization of chromosome paints prepared from flow sorted chromosomes of the tammar wallaby to Petrogale spp., showed that this ancestral karyotype is largely conserved among 2n = 22 rock wallaby species, and confirmed the identity of ancestral chromosomes which fused to produce the bi-armed chromosomes of the 2n = 16 tammar wallaby. These results illustrate the fission-fusion process of karyotype evolution characteristic of the kangaroo group.

Animals↗
PubMed1999

Mammalian Y chromosome evolution and the male-specific functions of Y chromosome-borne genes.

All mammals have an XY chromosomal sex determining system, in which a small Y chromosome triggers male development, and contains genes required for spermatogenesis. The X and Y chromosomes were originally homologous, but diverged during evolution as the Y chromosome was degraded progressively. Comparisons among the sex chromosomes of different mammal groups indicate that the X and Y chromosomes received additions of material from other chromosomes. Genes on the Y chromosome originated from the ancient X-Y pair, or from these additions, or were copies of genes on one of the autosomes. Only genes with important male-specific functions, such as sex determination and spermatogenesis, are selected for and retained on the differential region of the Y chromosome. The mammalian sex determining gene, SRY, controls the testis determination pathway, which includes at least one related gene. Several candidate spermatogenesis genes have been identified, but so far the only one that is conserved on the Y chromosome of all therian mammals is RBM (RNA-binding motif gene, Y chromosome).

Animals↗
PubMed1998

Evolution of the mammalian Y chromosome and sex-determining genes.

In mammals, male sex determination, as well as spermatogenesis, is controlled by genes on the Y chromosome. Evolutionary comparisons may be used to detect and test candidate genes for these functions, under the hypothesis that the rapid evolution of the mammalian Y chromosome causes it to contain few genes other than those with a critical function in male reproduction. Comparisons of the gene content of sex chromosomes from the three major groups of extant mammals (placentals, marsupials, and monotremes) show that part of the X chromosome, and a corresponding region of the Y, is shared by all mammals and must be very ancient, but part was added relatively recently. Evolution of the mammalian Y took place in several cycles of addition and attrition, as autosomal regions were added to the pseudoautosomal region of one sex chromosome, recombined onto the other, and degraded on the Y. This explains why most genes and pseudogenes on the Y chromosome have relatives on the X. The gene SRY itself is apparently no exception, being closely related to the highly conserved X-linked gene SOX3. Comparisons of SRY/SOX base sequence and gene location in the three groups of mammals suggest that SRY evolved from SOX3 relatively recently by mutation and loss of all sequences outside the HMG box. It is suggested here that, rather than acting as a transcriptional activator, the SRY gene acts to inhibit its paralogue SOX3, which in turn inhibits an ancient autosomal sex-determining gene SOX9.

Animals↗
PubMed1998

Undermethylation associated with retroelement activation and chromosome remodelling in an interspecific mammalian hybrid.

Genetic models predict that genomic rearrangement in hybrids can facilitate reproductive isolation and the formation of new species by preventing gene flow between the parent species and hybrid (sunflowers are an example). The mechanism underlying hybridization-induced chromosome remodelling is as yet unknown, although mobile element activity has been shown to be involved in DNA rearrangement in some dysgenic Drosophila hybrids. It has been proposed that DNA methylation evolved as a means of repressing the movement of mobile elements (the host defence model). If such a protective mechanism were to fail, mobile elements could be activated, and could cause major and rapid genome alterations. Here we demonstrate the occurrence of genome-wide undermethylation, retroviral element amplification and chromosome remodelling in an interspecific mammalian hybrid (Macropus eugenii x Wallabia bicolor). Atypically extended centromeres of Macropus eugenii derived autosomes in the hybrid were composed primarily of an unmethylated, amplified retroviral element not detectable in either parent species. These results, taken with the observation of deficient methylation and de novo chromosome change in other mammalian hybrids, indicate that the failure of DNA methylation and subsequent mobile-element activity in hybrids could facilitate rapid karyotypic evolution.

Animals↗
PubMed1998

De novo insertion of an intron into the mammalian sex determining gene, SRY.

Two theories have been proposed to explain the evolution of introns within eukaryotic genes. The introns early theory, or "exon theory of genes," proposes that introns are ancient and that recombination within introns provided new exon structure, and thus new genes. The introns late theory, or "insertional theory of introns," proposes that ancient genes existed as uninterrupted exons and that introns have been introduced during the course of evolution. There is still controversy as to how intron-exon structure evolved and whether the majority of introns are ancient or novel. Although there is extensive evidence in support of the introns early theory, phylogenetic comparisons of several genes indicate recent gain and loss of introns within these genes. However, no example has been shown of a protein coding gene, intronless in its ancestral form, which has acquired an intron in a derived form. The mammalian sex determining gene, SRY, is intronless in all mammals studied to date, as is the gene from which it recently evolved. However, we report here comparisons of genomic and cDNA sequences that now provide evidence of a de novo insertion of an intron into the SRY gene of dasyurid marsupials. This recently (approximately 45 million years ago) inserted sequence is not homologous with known transposable elements. Our data demonstrate that introns may be inserted as spliced units within a developmentally crucial gene without disrupting its function.

Amino Acid Sequence↗
PubMed1998

Interactions between SRY and SOX genes in mammalian sex determination.

The SRY gene on the mammalian Y chromosome undoubtedly acts to determine testis, but it is still quite unclear how. It was originally supposed that SRY acts directly to activate other genes in the testis-determining pathway. This paper presents an alternative hypothesis that SRY functions indirectly, by interacting with related genes SOX3 (from which SRY evolved) and SOX9 (which appears to be intimately involved in vertebrate gonad differentiation). Specifically, I propose that in females SOX3 inhibits SOX9 function, but in males, SRY inhibits SOX3 and permits SOX9 to enact its testis-determining role. This hypothesis makes testable predictions of the phenotypes of XX and XY individuals with deficiencies or overproduction of any of the three genes, and is able to account for the difficult cases of XX(SRY-) males and transdifferentiation in the absence of SRY. The hypothesis also suggests a way that the dominant SRY sex-determining system of present-day mammals may have evolved from an ancient system relying on SOX3 dosage.

Animals↗