Chromosomal anomalies
Keywords: aneuploidy, chromosome, intersex, equineThe intersex condition with abnormal genitalia is not rare in horses. In such cases, the external genitalia are sometimes ambigious (as seen here) but in others, the tract may appear to be almost normal.
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Animals with derangement of chromosomal makeup may have a wide range of karyotypes based on the absence, presence or duplication of X and Y chromosomes and sometimes autosomes as well. These anomalies may bare little expected association with the gonadal sex of the animal. For example, intersex animals with XX karyotypes may have testicles because of the presence of the SR-y gene. In other cases, the SR-y gene may be absent in an XX individual yet the gonadal sex may be male.
MANY genetic factors are involved in gender determination. For example, the following statement was copied and pasted from one of many articles on sexual differentiation:
| The essential genes affecting this process are as follows: WT1, SF1,CBX2, SOX9, fibroblast growth factor 9 (FGF9), prostaglandin D2 (PGD2), DAX1, WNT4, forkhead family transcription factor 2 (FOXL2), R-Spondin 1 (RSPO1) and b-catenin. While SOX9, FGF9 and PGD2 have more testis-promoting activity, DAX1, WNT4, FOXL2, RSPO1 and b-catenin are predominantly ovary-promoting genes. SOX9/FGF9 and WNT4/RSPO1 act as antagonistic signals in early gonadal differentiation.
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The animals in these images were all male pseudohermaphrodites i.e. the most common form of intersex where testicles are present but the external genitalia are ambiguous. None of these animals had a vagina. All showed predominantly male behavior and they had enlarged clitorises, resembling small glans penises. This resemblance is not surprising because the clitoris is the homolog of the glans penis in females. Essentially, a male pseudohermaphrodite is a manifestation of failed conversion of the default, female phenotype to the male phenotype. This is often because of abnormal Sertoli cell function and anti mullerian hormone (AMH) production.
Occasionally intersex animals are used as teasers because male pseudohermaphrodites have testicles and can have good libido.
Turners-like syndrome (XO) is not a case of intersex although but those mares have aneuploidy. The author has no images of the external genitalia of these mares simply because the external genitalia are normal in appearance. Although smaller than usual, their uteruses and ovaries are also normal in appearance. The primary source of pathology in XO mares lies with ovarian function and absence of folliculogenesis. Turners-like syndrome should be excluded when mares are not cycling during summertime.
| In most cases, owners will elect not to pursue either gonadectomy or karyotyping. Therefore many cases of chromosomal abnormalities remains incompletely diagnosed. Also, one must appreciate the fact that basic karyotyping may reveal aneuploidy and gross chromosomal anomalies but genetic anomalies will go undetected. PCR, "FISH" and other techniques are require for the accurate diagnosis of these conditions. |
