Male tortoiseshell and calico (T-C) cats. Animal models of sex chromosome mosaics, aneuploids, polyploids, and chimerics

Half-chromatid mutation as a possible cause of mosaic males and females in Hymenoptera and rare fertile male tortoiseshell cats

Half-chromatid mutations occur when a single base change in a gamete is transmitted to the zygote, which, after DNA replication and cleavage, will result in a mosaic individual. These mutations will be passed on through the germ plasm and also may be expressed somatically. Half-chromatid mutation has been suggested to account for the observed lower frequency of males than expected for lethal X-linked recessive disorders in humans, such as Lesch-Nyhan syndrome, incontinentia pigmenti, and Duchene muscular dystrophy. Although attention has been paid to half-chromatid mutation in humans, it otherwise has been ignored. Here I show that half-chromatid mutation in haplodiploid organisms, such as Hymenoptera, has some interesting and important consequences: (i) since all genes follow the X-linked pattern of inheritance, half-chromatid mutations should be relatively easier to detect; (ii) recessive mutations of all viabilities may be expected; (iii) mosaics of both sexes are expected in haplodiploids with half-chromatid mutation; (iv) gynandromorphs could result from half-chromatid mutation at the sex-determination locus, in species with single-locus complementary sex-determination. Finally, half-chromatid mutation can account for the rare fertile male tortoiseshell phenotype observed in the domestic cat, Felis catus, and which still has not been fully accounted for by other mechanisms.

Disorders of sexual development in the cat: Current state of knowledge and diagnostic approach

Practical relevance:

Any congenital or developmental abnormality of any part of the male or female reproductive tract is a ‘disorder of sexual development’ (DSD). The tricolored male cat phenotype, cryptorchidism, gonadal hypoplasia and incidental abnormalities such as cystic remnants or embryonic ducts are well-known feline DSDs.

Clinical challenges:

Full characterization of DSDs requires sex chromosome determination and identification of genes related to development of the gonads, internal tubular genitalia and external genitalia. Fortunately, affected cats are seen sporadically and the clinical effects are usually minimal.

Classification:

The classification nomenclature has changed. In place of intersex, hermaphrodite, pseudohermaphrodite and sex reversal, the newer standard classification, based on sex chromosomes, designates sex chromosome DSD when there is an abnormality in the sex chromosomes, and XX (female) and XY (male) DSDs where there is not. Identification of the gonadal type (testes, ovaries, ovotestes or gonadal dysgenesis) and documentation of the internal and external genital components completes the classification.

Evidence base:

The original basis of the DSD classification was a consensus reached in humans. It was quickly accepted in veterinary pathology, courtesy of its logic and ease of application, and it has subsequently begun to appear in peer-reviewed papers and clinical reviews. This article reviewing the various disorders in cats is based on application of the classification and draws on the feline peer-reviewed literature encompassing chromosome analysis and definition of reproductive abnormalities, syndromes and diseases.

Mosaicism in Cutaneous Disorders

Genetic mosaicism arises when a zygote harbors two or more distinct genotypes, typically due to de novo, somatic mutation during embryogenesis. The clinical manifestations largely depend on the differentiation status of the mutated cell; earlier mutations target pluripotent cells and generate more widespread disease affecting multiple organ systems. If gonadal tissue is spared-as in somatic genomic mosaicism-the mutation and its effects are limited to the proband, whereas mosaicism also affecting the gametes, such as germline or gonosomal mosaicism, is transmissible. Mosaicism is easily appreciated in cutaneous disorders, as phenotypically distinct mutant cells often give rise to lesions in patterns determined by the affected cell type. Genetic investigation of cutaneous mosaic disorders has identified pathways central to disease pathogenesis, revealing novel therapeutic targets. In this review, we discuss examples of cutaneous mosaicism, approaches to gene discovery in these disorders, and insights into molecular pathobiology that have potential for clinical translation.

Experimentally produced diploid〈-〉triploid mouse chimaeras develop up to adulthood

Spontaneous diploid-triploid chimaeras occur sporadically in various mammalian species including man, but so far have never been produced experimentally. In order to get a deeper insight into the developmental consequences of this anomaly, we have developed two procedures that enabled for the first time to produce routinely diploid-triploid embryos, foetuses, and animals in the mouse. These procedures are: (1) aggregation of cleaving diploid embryos with triploid embryos produced by suppression of the second polar body in zygotes, and (2) fusion of a haploid karyoplast with one blastomere of the two-cell diploid embryos. The first procedure yielded 23 living and 6 dead postimplantation embryos and foetuses (age: 8th-19th day) out of which 22 were chimaeric. In addition, three chimaeric neonates reached adulthood. Two animals were fertile, and one--an overt chimaera--was an infertile male. The rate of postimplantation development of aggregation chimaeras was normal or only slightly retarded, and with one exception the foetuses were morphologically normal. Generally, the highest contribution of the 3n component in extra-embryonic structures was noted in the yolk sac, and usually it was higher than its contribution to the organs of the body. Chimaerism was most often noted in the liver, the heart, the intestine, and the lungs. Participation of triploid cells to all tissues studied, both in the body and in extra-embryonic structures, appeared to decrease slightly as development progressed. The second procedure yielded 10 foetuses and 6 adults. Three foetuses were chimaeric. Six fertile adults were probably non-chimaeras: the triploid component was absent in the coat and in the blood.

Cats as an aid to teaching genetics

I have used an exercise involving domestic cats in the General Genetics course at the University of Nebraska-Lincoln for the past 5 years. Using a coherent set of traits in an organism familiar to the students makes it easy to illustrate principles of transmission and population genetics. The one-semester course consists primarily of sophomores and juniors who have either taken a one-semester introductory biology course, a one-semester cell biology course, or have a strong high school biology background. The students are given a handout and asked to determine the genotype at seven unlinked loci of at least one cat. To fill out the form, the students have to grasp such concepts as dominance, incomplete dominance, temperature-sensitive mutations, epistatic interactions, sex linkage, and variable expressivity. Completing the form reinforces these concepts as they observe the cat's phenotype and fill in the genotype. I then analyze the collected data and use it in my lectures on population genetics to illustrate the Hardy-Weinberg equilibrium, calculate allele frequencies, and use statistics. This allows the students to look at population genetics in a very positive light and provides concrete examples of some often misunderstood principles.

Male tortoiseshell cats in the United Kingdom

A questionnaire concerning the coat colour and sex of cats being vaccinated or neutered was sent to 2585 veterinary practices; 393 (15.2 per cent) were returned and information was obtained about 9816 cats. Of 4598 males, 20 were recorded as tortoiseshell (0.43 per cent). The frequency of the orange gene was 19.7 per cent assuming that male tortoiseshell cats had two X chromosomes. The chromosome complement and/or gonadal histology of 14 male tortoiseshell cats is described. Cytogenetic analysis of 11 animals revealed six with a 38,XX/38,XY complement, two with 39,XXY, two with 38,XX, and one with a 38,XY complement.

Genetic mapping in mammals: chromosome map of domestic cat

A genetic map of 31 biochemical loci located on 17 feline syntenic (linkage) groups has been derived by somatic cell genetic analysis of cat-rodent hybrids. Most of these syntenic groups have been assigned to one of the 19 feline chromosomes. Comparative linkage analysis of the feline biochemical loci and homologous human loci revealed considerable conservation of linkage associations between the primates and the Felidae (order Carnivora). Many of these same linkage groups have not been conserved in the murine genome. The genetic and evolutionary implications of comparative mapping analysis among mammalian species are discussed.

Digynic triploidy in rabbit blastocysts after delayed insemination

The origin of triploid (3N) 5½-day blastocysts in rabbits is inferred from the segregation of sex chromosomes and of an autosomal M-marker whose properties are described. 39 triploids and no tetraploids were scored among 1454 chromosomally scored blastocysts. A delay of 8 h between an ovulatory injection and subsequent insemination raised the estimated normal incidence of 0·59% triploid blastocysts to 3·13%. The increase is ascribed primarily to digyny (17 blastocysts), and to diandry probably mediated by dispermy (1 blastocyst). The triploid components of the two 2N/3N mosaics and the one 3N/6N were digynic. Neither superovulation nor insemination of excessive numbers of spermatozoa could be shown to give rise to triploid embryos. The diandric triploid was X Y Y, the first of this constitution apparently reported in the rabbit. There was some evidence that X X Y triploid blastocysts up to 5½-day gestation are more viable than X X X. In the 2N/3N mosaics each component had been entered by one spermatozoon, and the dliploid component could not have been merely a contamination by dliploid maternal somatic cells. In 2N/4N, 2N/4N/8N and 3N/6N mosaics, each polyploid component showed an exact doubling of the marker chromosome constitution of a component of lower ploidy; their origin is ascribed to doubling or redoubling of chromosome number in isolated embryonic cells. With earlier data included, 49·08 (± s.e. 1·96)% of 652 diploid blastocysts were X Y. 460 non-experimental weaned rabbits were all cliploid.