Cytogenetic Evaluation

CHAPTER 10Cytogenetic Evaluation



TERI L. LEAR


Many factors can lead to reproductive failure in horses; however, one factor commonly overlooked is a chromosome or karyotypic abnormality. Normal horses have 64 chromosomes including two sex chromosomes, XX in females and XY in males. When horse chromosomes are analyzed for abnormalities, the chromosomes are paired according to their size, morphology, and banding patterns into a standardized karyotype (ISCN 1990). The horse karyotype is composed of 13 pairs of metacentric or submetacentric and 18 pairs of acrocentric autosomes, plus a submetacentric X and an acrocentric Y chromosome.


Mares that are infertile or exhibit reduced fertility may have an abnormal chromosome complement, particularly with regard to the sex chromosomes (Table 10-1). The most common abnormalities affecting mares are X monosomy, often referred to as XO syndrome or equine Turner’s syndrome; sex chromosome mosaicism; and sex reversal syndrome. Sex chromosome abnormalities sometimes cause the failure of reproductive organs to function normally, or they can result in unviable gametes lacking a normal sex chromosome complement. Other chromosome abnormalities affecting mare fertility, such as partial X chromosome deletions and autosomal duplications and translocations, are rare but have been identified. These abnormalities result in failure to produce gametes with complete sets of chromosomes, or they produce developmental abnormalities in the embryo.


Table 10-1 Common Chromosome Abnormalities in Mares



















Chromosome Number Common Name Phenotype
63,X XO syndrome Normal external genitalia; irregular or absent estrous cycles; gonadal dysgenesis; no follicular development; poor conformation; small stature
63,X/64,XX XO mosaicism Normal appearance but with gonadal dysgenesis; rare follicular activity/estrous cycles
64,XY Sex reversal syndrome Normal external genitalia; no estrous cycles; gonadal dysgenesis; intraabdominal testes/ovotestes; may exhibit stallion-like behavior and secondary sex characteristics

Most samples submitted for karyotyping are from horses with poor reproductive performance. The incidence of chromosome abnormalities in the general horse population is unknown, since no systematic survey has been done. However, Power (1990) summarized the number of chromosome abnormalities observed in samples submitted for clinical analyses from 47 publications, and in 29% of the cases a chromosome abnormality was observed. Among the chromosome abnormalities observed, 36% were X monosomy, 28% were sex reversal syndrome, 5% were autosomal abnormalities, and the remaining 31% were mosaic for sex chromosomes. Equine heterosexual twins with XX/XY blood chimerism have been observed occasionally; however, this chimerism does not appear to affect sexual development or fertility in the female co-twin such as that seen in the freemartin condition described in cattle and sheep (Power, 1990).



CLINICAL SIGNS



Sex Chromosome Abnormalities


Mares exhibiting chronic primary infertility, failure to cycle regularly or at all, small ovaries, or lack of follicular activity, may have a numerical chromosome abnormality known as XO monosomy. These individuals have a karyotype of 63,XO instead of the normal 64,XX. Some XO mares may also exhibit small body size, angular limb deformities, and poor conformation. This is the most common chromosome abnormality described in horses and is thought to be the equine equivalent of human Turner’s syndrome.


A few cases of X chromosome duplication, or trisomy, have been described in the horse. These mares have a karyotype of 65,XXX and appear phenotypically normal but are infertile.


Another variation of numerical sex chromosome abnormalities is X chromosome mosaicism, which may be the third most common sex chromosome abnormality described in the horse. Mares with mosaicism may exhibit one or more of the symptoms described above and have a variety of cells, each with different numbers of sex chromosomes, such as 63,X;64,XX or 63,X;64,XX;65,XXX.


Some infertile mares may have a more common but complicated abnormality called XY sex-reversal syndrome. XY sex-reversed individuals have a female phenotype and a male karyotype of 64,XY and can exhibit a wide range of phenotypes. Some mares may be quite feminine with normal external genitalia, underdeveloped uterus, and slightly infantile ovaries with follicles present, whereas others exhibit classic gonadal dysgenesis, ovotestes, testicular feminization, and sometimes, masculine behavior. This condition has been attributed to a deletion of the sex-determining region (SRY gene) on the Y chromosome. Although a Y chromosome is present, SRY deletion causes these genotypic males to develop into phenotypic females with gonadal dysgenesis (Pailhoux et al, 1995).



Autosomal Chromosome Abnormalities (Non–Sex Chromosomes)

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  4. Low-Dose Insemination Techniques

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Jun 4, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Cytogenetic Evaluation

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