Molecular cytogenetic screening of sex chromosome abnormalities in young horse populations

BACKGROUND

Chromosomal abnormalities occur in the equine population at a rate of approximately 2%. The use of molecular cytogenetic techniques allows a more accurate identification of chromosomal abnormalities, especially those with a low rate of abnormal metaphases, demonstrating that the actual incidence in equine populations is higher.

OBJECTIVES

Estimation of the number of carriers of karyotypic abnormalities in a sample from a population of young horses of various breeds, using molecular cytogenetic techniques.

STUDY DESIGN

Cross-sectional.

METHODS

Venous blood samples were collected from 500 young horses representing 5 breeds (Purebred Arabian, Hucul, Polish primitive horse [Konik], Małopolska, Coldblood, Silesian). Chromosomes and DNA were obtained from blood lymphocytes and evaluated by fluorescence in situ hybridisation (FISH) and PCR, using probes and markers for the sex chromosomes and select autosomes.

RESULTS

Nineteen horses, 18 mares and 1 stallion, were diagnosed with different chromosomal abnormalities: 17 cases of mosaic forms of sex chromosome aneuploidies with a very low incidence (0.6%-4.7%), one case of a SRY-negative 64,XY sex reversal mare, and one mare with X-autosome translocation. The percentage of sex chromosomal aberrations was established as 3.8% in the whole population, 6.08% in females and 0.49% in males.

MAIN LIMITATIONS

Limited sample size, confined to horses from Poland.

CONCLUSIONS

The rate of sex chromosomal abnormalities we identified was almost double that reported in previous population studies that used classical chromosome staining techniques. FISH allowed the detection of aneuploid cell lines which had a very low incidence. The FISH technique is a faster and more precise method for karyotype examination; however, it is usually focused on only one or two chromosomes while banding karyotyping includes the entire chromosome set.

Characterisation of eight cattle with Swyer syndrome by whole-genome sequencing

Swyer syndrome is where an individual has the karyotype of a typical male yet is phenotypically a female. The lack of a (functional) SRY gene located on the Y-chromosome is implicated in some cases of the Swyer syndrome, although many Swyer individuals with an apparently fully functional SRY gene have also been documented. The present study undertook whole genome sequence analyses of eight cattle with suspected Swyer syndrome and compared their genome to that of both a control male and female. Sequence analyses coupled with female phenotypes confirmed that all eight individuals had the 60,XY sex reversal Swyer syndrome. Seven of the eight Swyer syndrome individuals had a deletion on the Y chromosome encompassing the SRY gene (i.e., SRY-). The eighth individual had no obvious mutation in the SRY gene (SRY+) or indeed in any reported gene associated with sex reversal in mammals; a necropsy was performed on this individual. No testicles were detected during the necropsy. Histological examination of the reproductive tract revealed an immature uterine body and horns with inactive glandular tissue of normal histological appearance; both gonads were elongated, a characteristic of most reported cases of Swyer in mammals. The flanking sequence of 11 single nucleotide polymorphisms within 10 kb of the SRY gene are provided to help diagnose some cases of Swyer syndrome. These single nucleotide polymorphisms will not, however, detect all cases of Swyer syndrome since, as evidenced from the present study (and other studies), some individuals with the Swyer condition still contain the SRY gene (i.e., SRY+).

The Second Case of Non-Mosaic Trisomy of Chromosome 26 with Homologous Fusion 26q;26q in the Horse

Simple Summary

We present chromosome and DNA analysis of a normal Thoroughbred mare and her abnormal foal born with neurologic defects. We show that the foal has an abnormal karyotype with three copies of chromosome 26 (trisomy chr26), instead of the normal two. However, two of the three chr26 have fused, forming an unusual derivative chromosome. Chromosomes of the dam are normal, suggesting that the chromosome abnormality found in the foal happened during egg or sperm formation or after fertilization. Analysis of the foal and the dam with chr26 DNA markers indicates that the extra chr26 in the foal is likely of maternal origin and that the unusual derivative chromosome resulted from the fusion of two parental chr26. We demonstrate that although conventional karyotype analysis can accurately identify chromosome abnormalities, determining the mechanism and parental origin of these abnormalities requires DNA analysis. Most curiously, this is the second case of trisomy chr26 with unusual derivative chromosome in the horse, whereas all other equine trisomies have three separate copies of the chromosome involved. Because horse chr26 shares genetic similarity with human chr21, which trisomy causes Down syndrome, common features between trisomies of horse chr26 and human chr21 are discussed.

Abstract

We present cytogenetic and genotyping analysis of a Thoroughbred foal with congenital neurologic disorders and its phenotypically normal dam. We show that the foal has non-mosaic trisomy for chromosome 26 (ECA26) but normal 2n = 64 diploid number because two copies of ECA26 form a metacentric derivative chromosome der(26q;26q). The dam has normal 64,XX karyotype indicating that der(26q;26q) in the foal originates from errors in parental meiosis or post-fertilization events. Genotyping ECA26 microsatellites in the foal and its dam suggests that trisomy ECA26 is likely of maternal origin and that der(26q;26q) resulted from Robertsonian fusion. We demonstrate that conventional and molecular cytogenetic approaches can accurately identify aneuploidy with a derivative chromosome but determining the mechanism and parental origin of the rearrangement requires genotyping with chromosome-specific polymorphic markers. Most curiously, this is the second case of trisomy ECA26 with der(26q;26q) in the horse, whereas all other equine autosomal trisomies are ‘traditional’ with three separate chromosomes. We discuss possible ECA26 instability as a contributing factor for the aberration and likely ECA26-specific genetic effects on the clinical phenotype. Finally, because ECA26 shares evolutionary homology with human chromosome 21, which trisomy causes Down syndrome, cytogenetic, molecular, and phenotypic similarities between trisomies ECA26 and HSA21 are discussed.

Segmental Cervical Aplasia in a Colombian Creole Mare with Mosaic X-Chromosome Aneuploidy

A 4-year-old Colombian Creole mare was presented for diagnosis because the external orifice of her cervix was not detectable when a uterine lavage as therapy for uterine fluid accumulation was attempted. Clinical and ultrasonographic evaluation of the genital tract revealed that ovaries were of normal size and showed structures suggestive of regular ovarian activity. However, granular free-floating fluid material distending the uterus was detected by ultrasound. Upon vaginal examination, the normal external cervical morphology was not evident. The vagina ended in a blind bag with a small papilla with no evident external cervical os. Cytology of uterine fluid obtained by transvaginal aspiration showed findings compatible with mucometra. Cytogenetic analysis revealed an abnormal karyotype (63,X and 64,XX both 45% and 65,XXX 10%). A diagnosis of congenital segmental cervical aplasia was proposed possibly related to the mosaicism detected. To our knowledge, this is the first case of this reproductive pathology in a mare with regular ovarian activity and confirmed aneuploidy in mosaic form of the X sex chromosome.

Horse Clinical Cytogenetics: Recurrent Themes and Novel Findings

Clinical cytogenetic studies in horses have been ongoing for over half a century and clearly demonstrate that chromosomal disorders are among the most common non-infectious causes of decreased fertility, infertility, and congenital defects. Large-scale cytogenetic surveys show that almost 30% of horses with reproductive or developmental problems have chromosome aberrations, whereas abnormal karyotypes are found in only 2-5% of the general population. Among the many chromosome abnormalities reported in the horse, most are unique or rare. However, all surveys agree that there are two recurrent conditions: X-monosomy and SRY-negative XY male-to-female sex reversal, making up approximately 35% and 11% of all chromosome abnormalities, respectively. The two are signature conditions for the horse and rare or absent in other domestic species. The progress in equine genomics and the development of molecular tools, have qualitatively improved clinical cytogenetics today, allowing for refined characterization of aberrations and understanding the underlying molecular mechanisms. While cutting-edge genomics tools promise further improvements in chromosome analysis, they will not entirely replace traditional cytogenetics, which still is the most straightforward, cost-effective, and fastest approach for the initial evaluation of potential breeding animals and horses with reproductive or developmental disorders.

Impaired Reproductive Function in Equines: From Genetics to Genomics

Fertility is one of the key factors in the economic and productive success of the equine industry. Despite this, studies on the genetic causes affecting reproductive performance are scarce, especially in mares, where the genetic architecture of the reproductive traits is extremely complex. Today, with the increasing availability of new genomic methodologies for this species, we are presented with an interesting opportunity to understand the genetic basis of equine reproductive disorders. These include, among others, novel techniques for detecting chromosomal abnormalities, whose association with infertility in horses was established over 50 years ago; new sequencing technologies permitting an accurate detection of point mutations influencing fertility, as well as the study of inbreeding and molecular homozygosity, which has been widely suggested as one of the main causes of low reproductive performance in horses. Finally, over the last few years, reproductive performance has also been associated with copy number variants and candidate genes detected by genome-wide association studies on fertility traits. However, such studies are still scarce, probably because they depend on the existence of large and accurate phenotypic datasets of reproductive and/or fertility traits, which are still difficult to obtain in equines.

Two Novel Cases of Autosomal Translocations in the Horse: Warmblood Family Segregating t(4;30) and a Cloned Arabian with a de novo t(12;25)

We report 2 novel autosomal translocations in the horse. In Case 1, a breeding stallion with a balanced t(4p;30) had produced normal foals and those with congenital abnormalities. Of his 9 phenotypically normal offspring, 4 had normal karyotypes, 4 had balanced t(4p;30), and 1 carried an unbalanced translocation with tertiary trisomy of 4p. We argue that unbalanced forms of t(4p;30) are more tolerated and result in viable congenital abnormalities, without causing embryonic death like all other known equine autosomal translocations. In Case 2, two stallions produced by somatic cell nuclear transfer from the same donor were karyotyped because of fertility issues. A balanced translocation t(12q;25) was found in one, but not in the other clone. The findings underscore the importance of routine cytogenetic screening of breeding animals and animals produced by assisted reproductive technologies. These cases will contribute to molecular studies of translocation breakpoints and their genetic consequences in the horse.

Urogenital Hypoplasia and X Chromosome Monosomy in a Draft Horse Filly

A 5-month-old draft horse filly was presented with incontinence and severe perivulvar dermatitis, which developed during the previous 2 months. Left-sided ectopic ureter entering in the caudal vaginal lumen, signs of cervix hypoplasia, and urine accumulation in the uterus were found during initial vaginal endoscopy. Therefore, a left ureter-nephrectomy was conducted under general anesthesia. Additionally, a cytogenetic examination was performed, which showed a XO monosomy with a 63,X/64,XX mosaic. This is the first case report presenting a chromosome abnormality in a draft horse filly combined with a left-sided ectopic ureter. Cytogenetic evaluation is recommended in any female horse with developmental abnormalities of the cervix, uterus, ovaries, or with irregularities of estrus.

Novel Complex Unbalanced Dicentric X-Autosome Rearrangement in a Thoroughbred Mare with a Mild Effect on the Phenotype

Complex structural X chromosome abnormalities are rare in humans and animals, and not recurrent. Yet, each case provides a fascinating opportunity to evaluate X chromosome content and functional status in relation to the effect on the phenotype. Here, we report the first equine case of a complex unbalanced X-autosome rearrangement in a healthy but short in stature Thoroughbred mare. Studies of about 200 cells by chromosome banding and FISH revealed an abnormal 2n = 63,X,der(X;16) karyotype with a large dicentric derivative chromosome (der). The der was comprised of normal Xp material, a palindromic duplication of Xq12q21, and a translocation of chromosome 16 to the inverted Xq12q21 segment by the centromere, whereas the distal Xq22q29 was deleted from the der. Microsatellite genotyping determined a paternal origin of the der. While there was no option to experimentally investigate the status of X chromosome inactivation (XCI), the observed mild phenotype of this case suggested the following scenario to retain an almost normal genetic balance: active normal X, inactivated X-portion of the der, but without XCI spreading into the translocated chromosome 16. Cases like this present unique resources to acquire information about species-specific features of X regulation and the role of X-linked genes in development, health, and disease.

Genetics of Equine Reproductive Diseases

Reproductive disorders are genetically heterogeneous and complex; available genetic tests are limited to chromosome analysis and 1 susceptibility gene. Cytogenetic analysis should be the first test to confirm or rule out chromosomal aberrations. No causative genes/mutations are known. The only available genetic test for stallion subfertility is based on a susceptibility gene FKBP6. The ongoing progress in equine genomics will improve the status of genetic testing. However, because subfertile phenotypes do not facilitate collection of large numbers of samples or pedigrees, and clinical causes of many cases remain unknown, further progress requires constructive cross-talk between geneticists, clinicians, breeders, and owners.

X chromosome aneuploidy and micronuclei in fertile mares

Abnormalities of chromosomes are an important and well documented cause of disorders of sexual development, fertility problems and congenital anomalies in mammals. Detection of low-level 63,X/64,XX mosaicism during routine cytogenetic evaluation is a challenge because its clinical significance is not yet fully clear. This study describes the prevalence and levels of 63,X mosaicism for a cohort of fertile mares and compares the results with eight problem mares for which no clinical cause of sub-fertility was found. The study design allowed for the analysis of micronuclei which are biomarkers of genomic instability and can disturb cell divisions, drive cancer development or cause congenital diseases. Although 27% of the fertile mares were identified to be 63,X mosaics, the results showed that the rates of abnormal cells were very low (1-3%). Levels of abnormal cells in problem mares with 63,X mosaicism were similar or higher. The average rate of micronuclei in the blood of the fertile mares was ∼1%, well below the baseline (5%) which was proposed for peripheral blood of normal healthy humans. We found weak to modest, but not significant, correlations between the age of fertile mares and 63,X cells (Kendall's tau b = 0.2905; p > 0.05) as well as the rate of micronuclei (Kendall's tau b = 0.1896; p > 0.05). Likewise, the correlation between presence of a 63,X cell line and micronuclei rate was not significant (Kendall's tau b = 0.3201; p > 0.05). The presence of 63,X cells in rates greater than 3% may indeed indicate a higher risk for sub-fertility and eventually for associated health problems in such mares. Detection and elimination of mares with high level of X aneuploidies from breeding may have a positive effect on the fertility within the general horse population. This data may support the evaluation of problem mares with mosaic karyotypes involving the X chromosome.

Androgen Receptor Gene Variants in New Cases of Equine Androgen Insensitivity Syndrome

In the domestic horse; failure of normal masculinization and virilization due to deficiency of androgenic action leads to a specific disorder of sexual development known as equine androgen insensitivity syndrome (AIS). Affected individuals appear to demonstrate an incoherency between their genetic sex and sexual phenotype; i.e., XY-sex chromosome constitution and female phenotypic appearance. AIS is well documented in humans. Here we report the finding of two novel genetic variants for the AR-gene identified in a Tennessee Walking Horse and a Thoroughbred horse mare; each in individual clinical cases of horse AIS syndrome.

Genetics, Evolution, and Physiology of Donkeys and Mules

The genus Equus is made up of donkeys, horses, and zebras. Despite significant variation in chromosome number across these species, interspecies breeding results in healthy, although infertile, hybrid offspring. Most notable among these are the horse-donkey hybrids, the mule and hinny. Donkeys presently are used for everything from companion animals to beasts of burden. Although closely related from an evolutionary standpoint, differences in anatomy and physiology preclude the assumption that they can be treated identically to the domestic horse. Veterinarians should be aware of these differences and adjust their practice accordingly.

Fertility and 63,X Mosaicism in a Haflinger Sibship

Chromosomal abnormalities are notable causes of infertility in horses. Mares show various degrees of estrous behavior, and ultrasound examination often reveals an underdeveloped genital tract. This article reports investigations on fertility in a Haflinger sibship with a healthy, normally developed, fertile mare with at least three healthy offspring. Chromosomal analysis performed incidentally and blinded for this mare revealed 63,X/64,XX/65,XXX mosaicism. Two closely related mares were also mosaics (63,X/64,XX), and one of them was a carrier of a marker chromosome. Repeated examinations of the mare and seven relatives (four mares and three stallions) did not provide evidence for sub- or in-fertility. They had no developmental abnormalities or conspicuous body conditions. Peripheral blood samples were collected for analysis of the karyotype and molecular analyses. Chromosomes were Giemsa stained and 4',6-diamidino-2-phenylindole banded to identify numerical or structural aberrations of chromosomes and identification of sex chromosomes, respectively. Fluorescence in situ hybridization was performed with an equine Y-chromosome painting probe to identify and count the sex chromosomes, and polymerase chain reaction analysis was used to test for the presence of the SRY gene and investigating chimerism. The present article demonstrates the necessity of further studies analyzing chromosomal X0 mosaics to improve the predictive value of chromosomal aberrations on fertility.

An Autosomal Translocation 73,XY,t(12;20)(q11;q11) in an Infertile Male Llama (Lama glama) With Teratozoospermia

Structural chromosome abnormalities, such as translocations and inversions occasionally occur in all livestock species and are typically associated with reproductive and developmental disorders. Curiously, only a few structural chromosome aberrations have been reported in camelids, and most involved sex chromosomes. This can be attributed to a high diploid number (2n = 74) and complex chromosome morphology, which makes unambiguous identification of camelid chromosomes difficult. Additionally, molecular tools for camelid cytogenetics are sparse and have become available only recently. Here we present a case report about an infertile male llama with teratozoospermia and abnormal chromosome number 2n = 73,XY. This llama carries an autosomal translocation of chromosomes 12 and 20, which is the likely cause of defective spermatogenesis and infertility in this individual. Our analysis underlines the power of molecular cytogenetics methods over conventional banding-based chromosome analysis for explicit identification of normal and aberrant chromosomes in camelid karyotypes. This is the first case of a translocation and the first autosomal aberration reported in any camelid species. It is proof of principle that, like in other mammalian species, structural chromosome abnormalities contribute to reproductive disorders in camelids.

A new molecular screening tool for the detection of chromosomal abnormalities in donkeys

Chromosomal abnormalities are a major cause of infertility and reproductive problems in equids. Nowadays, their detection is rising due to the use of new diagnostic tools based on molecular markers instead of karyotyping. Reports of this kind of genetic aberrations in domestic donkeys (Equus asinus) are extremely scarce, despite their importance in human activities. In the present study, we analysed the implementation of a short-tandem-repeat (STR)-based molecular method initially developed for horses, as a diagnostic tool to detect chromosomal abnormalities in donkeys. The frequency of five X-linked (LEX003, LEX026, TKY38, TKY270 and UCEDQ502) and one Y-linked (ECAYM2) molecular markers and one Y-linked gene (sex-determining region Y, SRY) was characterized in 121 donkeys from two diverse breeds, the Spanish Andalusian and the African Moroccan breeds. The molecular panel showed 100% sensitivity and 99.67% specificity in detecting 10 different chromosomal abnormalities in the species. In conclusion, this methodology is a valid, rapid and low-cost tool for the detection and characterization of chromosomal abnormalities in domestic donkeys.

Chromosome Aberrations and Fertility Disorders in Domestic Animals

The association between chromosomal abnormalities and reduced fertility in domestic animals is well recorded and has been studied for decades. Chromosome aberrations directly affect meiosis, gametogenesis, and the viability of zygotes and embryos. In some instances, balanced structural rearrangements can be transmitted, causing fertility problems in subsequent generations. Here, we aim to give a comprehensive overview of the current status and future prospects of clinical cytogenetics of animal reproduction by focusing on the advances in molecular cytogenetics during the genomics era. We describe how advancing knowledge about animal genomes has improved our understanding of connections between gross structural or molecular chromosome variations and reproductive disorders. Further, we expand on a key area of reproduction genetics: cytogenetics of animal gametes and embryos. Finally, we describe how traditional cytogenetics is interfacing with advanced genomics approaches, such as array technologies and next-generation sequencing, and speculate about the future prospects.

A Non-Reciprocal Autosomal Translocation 64,XX, t(4;10)(q21;p15) in an Arabian Mare with Repeated Early Embryonic Loss

Balanced autosomal translocations are a known cause for repeated early embryonic loss (REEL) in horses. In most cases, carriers of such translocations are phenotypically normal, but the chromosomal aberration negatively affects gametogenesis giving rise to both genetically balanced and unbalanced gametes. The latter, if involved in fertilization, result in REEL, whereas gametes with the balanced form of translocation will pass the defect into next generation. Therefore, in order to reduce the incidence of REEL, identification of translocation carriers is critical. Here, we report about a phenotypically normal 3-year-old Arabian mare that had repeated resorption of conceptuses prior to day 45 of gestation and was diagnosed with REEL. Conventional and molecular cytogenetic analyses revealed that the mare had normal chromosome number 64,XX but carried a non-mosaic and non-reciprocal autosomal translocation t(4;10)(q21;p15). This is a novel translocation described in horses with REEL and the first such report in Arabians. Previous cases of REEL due to autosomal translocations have exclusively involved Thoroughbreds. The findings underscore the importance of routine cytogenetic screening of breeding animals.

Equine Embryo Sexing and Ultrasonographic Foetal Sexing - Interests and Applicability

The ability to choose the sex of the offspring is of upmost economic importance for horse breeders. Unlike other species, horses present several reproductive peculiarities that interfere with assisted reproductive technologies used in other large animals (such as bovine) and make them difficult to apply. Thus, there is a great interest to determine the sex of the offspring as soon as possible. This has led to the development of several technologies to serve this purpose, which can be classified into two categories. One is equine embryo sexing by either non-invasive biotechnological methods, such as monitoring of X-linked enzymes before X chromosome inactivation and detection of sex-specific antigen, or by invasive biotechnological methods, such as cytogenetic analysis and polymerase chain reaction (PCR). The other one is equine foetus sexing using ultrasound scanning in different stages of its development (early, mid or late), by different approaches (transrectally or transabdominally). This can be performed with classic B-mode ultrasound machines or using 3D-mode and Doppler-mode scanners. This review article offers a comprehensive overview of the current status of these procedures as well as an assessment of their interests and applicability.

Chromosomal polymorphism in mammals: an evolutionary perspective

Although chromosome rearrangements (CRs) are central to studies of genome evolution, our understanding of the evolutionary consequences of the early stages of karyotypic differentiation (i.e. polymorphism), especially the non-meiotic impacts, is surprisingly limited. We review the available data on chromosomal polymorphisms in mammals so as to identify taxa that hold promise for developing a more comprehensive understanding of chromosomal change. In doing so, we address several key questions: (i) to what extent are mammalian karyotypes polymorphic, and what types of rearrangements are principally involved? (ii) Are some mammalian lineages more prone to chromosomal polymorphism than others? More specifically, do (karyotypically) polymorphic mammalian species belong to lineages that are also characterized by past, extensive karyotype repatterning? (iii) How long can chromosomal polymorphisms persist in mammals? We discuss the evolutionary implications of these questions and propose several research avenues that may shed light on the role of chromosome change in the diversification of mammalian populations and species.

Reproductive Disorders in Horses

Reproductive disease is relatively common in the horse, resulting in a variable, yet significant, economic impact on individual horsemen as well as the entire industry. Diverse expertise from the veterinary community ensures and improves individual and population health of the horse. From a pathology and diagnostics perspective, this review provides a comprehensive overview of pathology of the male and female equine reproductive tract. Recognition by clinical and gross features is emphasized, although some essential histologic parameters are included, as appropriate. Where relevant, discussion of ancillary diagnostic tests and approaches are included for some diseases and lesions.

Male horse meiosis: metaphase I chromosome configuration and chiasmata distribution

Chromosome configurations and chiasma frequency during the metaphase I stage of spermatogenesis in the male horse are characterized in this work. The genome-wide frequency and distribution of chiasmata was detected as 49.45 ± 2.07 for 14 fertile stallions. All X and Y chromosomes shared a single chiasma at their pseudoautosomal region, while 1-4 chiasmata were observed in autosomal chromosomes. The chiasma frequency and distribution were further studied for 8 different bivalents identified by FISH in 5 fertile stallions. Genetic length was calculated from chiasmata data for the whole genome as well as for these 8 chromosomes. The findings complement the genetic linkage data and provide insight into the genetic basis of spermatogenesis in normal stallions.

Development and application of camelid molecular cytogenetic tools

Cytogenetic chromosome maps offer molecular tools for genome analysis and clinical cytogenetics and are of particular importance for species with difficult karyotypes, such as camelids (2n = 74). Building on the available human-camel zoo-fluorescence in situ hybridization (FISH) data, we developed the first cytogenetic map for the alpaca (Lama pacos, LPA) genome by isolating and identifying 151 alpaca bacterial artificial chromosome (BAC) clones corresponding to 44 specific genes. The genes were mapped by FISH to 31 alpaca autosomes and the sex chromosomes; 11 chromosomes had 2 markers, which were ordered by dual-color FISH. The STS gene mapped to Xpter/Ypter, demarcating the pseudoautosomal region, whereas no markers were assigned to chromosomes 14, 21, 22, 28, and 36. The chromosome-specific markers were applied in clinical cytogenetics to identify LPA20, the major histocompatibility complex (MHC)-carrying chromosome, as a part of an autosomal translocation in a sterile male llama (Lama glama, LGL; 2n = 73,XY). FISH with LPAX BACs and LPA36 paints, as well as comparative genomic hybridization, were also used to investigate the origin of the minute chromosome, an abnormally small LPA36 in infertile female alpacas. This collection of cytogenetically mapped markers represents a new tool for camelid clinical cytogenetics and has applications for the improvement of the alpaca genome map and sequence assembly.

Cytogenetic and molecular characterization of Y isochromosome in a 63XO/64Xi(Yq) mosaic karyotype of an intersex horse

Sex chromosome aberrations commonly lead to abnormal sexual development. Here we cytogenetically and molecularly characterized Y isochromosome in an intersex horse. Blood lymphocyte analysis showed a mosaic karyotype with 96% 63,XO and 4% 64,Xi(Y) cells. Molecular analysis of the isochromosome was carried out by fluorescence in situ hybridization and polymerase chain reaction with male-specific and pseudoautosomal markers from the horse Y chromosome. We found that the isochromosome was monocentric, composed of 2 long arms, carrying 2 sets of genes of the pseudoautosomal region (PAR) and the male-specific region of the Y (MSY), including the SRY - thus being genetically equivalent to Y disomy. Sequence analysis of a 1,955-bp region including the SRY exon, the promoter and the UTRs, revealed no mutations in the aberrant Y. The presence of an intact SRY in a small proportion of cells is the proposed cause for the intersex phenotype. Given that the i(Yq) was present in a mosaic form, both post-zygotic and meiotic mechanisms of its origin were proposed. We speculated that nonmosaic 64,Xi(Yq) karyotypes might be rare or absent because of the likely instability of the i(Yq) during cell division. Genetic and phenotypic implications of Y isochromosome formation in other mammals are discussed in the light of the diversity of Y chromosome organization between species.

The pseudoautosomal region and sex chromosome aneuploidies in domestic species

The pseudoautosomal region (PAR) is a unique and specialized segment on the mammalian sex chromosomes with known functions in male meiosis and fertility. Detailed molecular studies of the region in human and mouse show dramatic differences between the 2 PARs. Recent mapping efforts in horse, dog/cat, cattle/ruminants, pig and alpaca indicate that the PAR also varies in size and gene content between other species. Given that PAR genes escape X inactivation, these differences might critically affect the genetic consequences, such as embryonic survival and postnatal phenotypes of sex chromosome aneuploidies. The aim of this review is to combine the available information about the organization of the PAR in domestic species with the cytogenetic data on sex chromosome aneuploidies. We show that viable XO individuals are relatively frequently found in species with small PARs, such as horses, humans and mice but are rare or absent in species in which the PAR is substantially larger, like in cattle/ruminants, dogs, pigs, and alpacas. No similar correlation can be detected between the PAR size and the X chromosome trisomy in different species. Recent evidence about the likely involvement of PAR genes in placenta formation, early embryonic development and genomic imprinting are presented.

Dystocia and fetotomy associated with cerebral aplasia in a greater one-horned rhinoceros (Rhinoceros unicornis)

The captive greater one-horned rhinoceros population consists of 176 animals. Since 1971, a total of 226 calves were born into this captive population. However, 24% of the offspring born were either stillborn or did not survive the first 3 months. The causes for this high rate of stillbirth and neonate mortality have not yet been documented. Here, we report on the veterinary management of a dystocia and foetotomy resulting from a malpositioned greater one-horned rhinoceros foetus. The dead foetus presented with a forelimb flexed at the shoulder joint, with all other joints extended. The foetus was dissected into five parts and extracted during two anaesthesias on two consecutive days. The dam recovered fully and came into oestrous 31 days after surgery. Post-mortem and CT examination of the malformed foetal head revealed cranioschisis with cerebral aplasia and cerebellar hypoplasia. The cerebral aplasia presented here and in other recent cases suggests that neural tube defects and cranial malformations may be associated with more captive rhinoceros stillbirths than previously considered. Epidemiologic studies of these phenomena and possible nutritional deficiencies or hereditary defects are warranted.

Equine clinical genomics: A clinician's primer

The objective of this review is to introduce equine clinicians to the rapidly evolving field of clinical genomics with a vision of improving the health and welfare of the domestic horse. For 15 years a consortium of veterinary geneticists and clinicians has worked together under the umbrella of The Horse Genome Project. This group, encompassing 22 laboratories in 12 countries, has made rapid progress, developing several iterations of linkage, physical and comparative gene maps of the horse with increasing levels of detail. In early 2006, the research was greatly facilitated when the US National Human Genome Research Institute of the National Institutes of Health added the horse to the list of mammalian species scheduled for whole genome sequencing. The genome of the domestic horse has now been sequenced and is available to researchers worldwide in publicly accessible databases. This achievement creates the potential for transformative change within the horse industry, particularly in the fields of internal medicine, sports medicine and reproduction. The genome sequence has enabled the development of new genome-wide tools and resources for studying inherited diseases of the horse. To date, researchers have identified 11 mutations causing 10 clinical syndromes in the horse. Testing is commercially available for all but one of these diseases. Future research will probably identify the genetic bases for other equine diseases, produce new diagnostic tests and generate novel therapeutics for some of these conditions. This will enable equine clinicians to play a critical role in ensuring the thoughtful and appropriate application of this knowledge as they assist clients with breeding and clinical decision-making.

Identification of chromosome abnormalities in the horse using a panel of chromosome-specific painting probes generated by microdissection

Fluorescent in situ hybridisation (FISH) using a panel of molecular probes for all chromosome pairs obtained by chromosome microdissection of the domestic horse ( Equus caballus ) was used to diagnose karyotype abnormalities in 35 horses (32 mares, 2 stallions and 1 intersex), which were selected for the study due to infertility (23 horses), reduced fertility (10 horses) and developmental anomalies (2 horses). The use of the FISH technique with probes for each horse chromosome pair enabled the diagnosis of many different chromosome aberrations in this population. Among the horses analysed, 21 animals had normal karyotype - 64,XX (19 mares) and 64,XY (2 stallions). Fourteen animals, constituting 40% of the population studied, showed the following chromosome abnormalities: 63,X (1 mare); 63,X/64,XX (6 mares); 63,X/64,XX/65,XXX (3 mares); 63,X/65,XXX (1 mare); 64,XX/65,XX+Xp (1 mare); 63,X/64,XX/65,XX+Xq (1 mare), and 63,X/64,XX/65,XX+delY (1 intersex). When only the mares studied because of complete infertility were taken into consideration, this proportion exceeded 56%. Due to the increased frequency of the above-mentioned aberrations in the mosaic form of two or more lines, it was necessary to analyse a large number (100-300) of metaphase spreads. The use of specific molecular probes obtained by chromosome microdissection made these diagnoses much easier.