A genetic linkage map of the ovine X chromosome containing type I and type II markers has been constructed. The map contains 7 known gene markers and 14 microsatellite markers with a recombination length of 141.9 cM. Segregation of polymorphic markers was observed in a three-generation pedigree containing 480 animals. The maximum number of informative meioses was 912. Additional information was obtained for some markers by following segregation in the AgResearch International Mapping Flock, consisting of nine three-generation full-sib pedigrees. A pseudoautosomal region containing two markers has been identified at one end of the linkage map. Comparisons with mouse and human X chromosomes confirms the observation of Ohno (1973} that the gene content of the mammalian X chromosome is retained. In particular, the conserved grouping of the genes PHKA1, ATP7A, and XIST observed in both the human and the mouse X chromosome appears to be conserved in the sheep X chromosome, and XIST has been mapped to near the center of the chromosome. This study provides the first reported genetic linkage map combining both type I and type 11 markers for any ruminant X chromosome.Until very recently, the X chromosomes of placental mammals were considered to be constrained by Ohno's rule (Ohno 1973), which postulated that genes on the X chromosome of one mammal would be on the X chromosomes of all other mammals. This rule was refined with the discovery that several genes on the short arm of the human X chromosome (Xp) are autosomal in marsupials and monotremes (McKay et al. 1992;Fitzgerald et al. 1993;Graves and Foster 1994). The apparent conservation of gene content of the X chromosomes was thought to be related to the requirement of X-inactivation in females for dosage compensation. The only known exception to this conservation in placental mammals has been a chloride channel gene (CLCN4), which has been shown recently to map to the X chromosome in humans and Mus spretus mice, but to chromosome 7 in C57BL/6 mice (Palmer et al. 1995;Rugarli et al. 1995). The X chromosomes of humans and mice are the most well-characterized and many known X-linked genes and other unique DNA sequences have been mapped in these species (Herman et al. 1994;Willard et al. 1994). Based on mapped gene orders, evolutionary relationships between mice and human X 3Corresponding author. E-MAIL galloways@agresearch.cri.nz; FAX 64-3-477-5413. chromosomes have been explained by five breakpoints and rearrangement of the resulting six blocks of genes (Amar et al. 1988). This concept has been refined recently to accommodate eight conserved segments with seven breakpoints (Blair et al. 1994(Blair et al. , 1995. It is of interest to determine whether the conserved blocks of genes identified in humans and mice remain conserved in other mammalian species, and whether the conservation of these groups has been maintained throughout evolution.The X chromosomes of mammalian species other than human and mouse are less wellcharacterized. In cattle, the genes DMD (Duchenne muscul...
A multiply auxotrophic strain, hOG45, was derived from Candida albicans ATCC 10261. Prototrophic revertants of this multiple auxotroph were selected after mutagenesis. These prototrophic revertants were distinguishable from the original prototroph, ATCC 10261, because of their mitotic instability. They gave rise to auxotrophic derivatives which displayed one or more of the auxotrophic requirements characteristic of hOG45. Two of the auxotrophic requirements, those for adenine and methionine, frequently reappeared together in the auxotrophic derivatives of the prototrophic revertants. This apparent linkage of ade and met was confirmed by protoplast fusion analysis of the original auxotroph. These data indicate that C. albicans ATCC 10261 is diploid, the multiple auxotroph h0G45 is homozygous for recessive auxotrophic alleles, the prototrophic revertants are multiple heterozygotes, the auxotrophic derivatives are homozygotes produced by mitotic crossing-over, and the association between the ade and met alleles is due to linkage.
A number of laboratories are now engaged in the genetic analysis of Candida albicans. This diploid yeast, the major fungal pathogen of humans, is imperfect. Parasexual techniques have been devised for complementation and recombination analysis in this organism. This paper attempts to address the question of the extent to which nonisogenic strains of C. albicans have conserved a common genetic map. This analysis is a prerequisite for the integration of work done in different laboratories and may also provide useful information on the taxonomy of the genus Candida. The paper also reports the analysis of an interspecific hybrid between C. albicans and Candida stellatoidea. The method employed in these studies was the analysis of the mitotic recombination relationships of a group of linked genes and their centromere. Strains carrying linked auxotrophic mutations were fused with isogenic and nonisogenic complementary strains to form tetraploids. The mitotic recombination analyses of these tetraploids suggest that in the isolates studied the genetic map is conserved. A comparison of tetraploid and diploid mitotic recombination analyses is also presented. Candida albicans, a common causative agent of mycotic infection, is generally considered to exist naturally only in the asexual state (2). Recently, parasexual protocols have been reported for this organism (5, 7, 8, 10). The protocols for parasexual analysis all employ enzymic digestion to produce protoplasts, followed by the induction of protoplast fusion by polyethylene glycol. Fusion of complementing auxotrophs generates prototrophic fusion products which can be selected on appropriate media. Such fusion products are either multinucleate heterokaryons or, more rarely, uninucleate cells which have undergone karyogamy. Uninucleate prototrophic fusion products are spontaneously unstable, giving rise with a low frequency to auxotrophic derivatives (8). The mitotic instability offusion products can be increased by exposure to UV irradiation (8). It has been suggested that the predominant form of mitotic instability is mitotic crossing-over (7, 10). The ploidy of C. albicans remains a matter of debate, some reports supporting haploidy (10) and others diploidy (1, 4, 7, 9, 11, 12). It has been suggested that protoplast fusion of naturally diploid cells gives rise to euploid tetraploids (8), but it may also give rise to aneuploid products (10). The first report of genetic linkage in C. albicans was provided by Sarachek et al. (10) with derivatives of C. albicans WC. The report indicated that the arg allele carried by WC-5-4 and
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