Disorders of Sex Development (DSD)

The disorders of sex development (DSD) and impaired fertil­ity (IF) are serious concerns in animal breeding, particularly where artificial insemination is generally performed. There are three main categories of DSDs (originally adapted from humans) occurring in domestic and companion animals.

Fig. 18.2 Development of male and female reproductive organs under the influence of various genetic, endocrine and growth factors. [The bold lines showed the male gonad and duct development and dotted lines showed for females. Major genes involved in male reproductive organs development are Sry, Sox9, SF1, and the development of female repro­ductive organs is brought about by Wnt4, FoxL2, Dax1, RSPO1,

BMP4. DHH (Desert hedgehog) gene, FGF9 (Fibroblast Growth Fac­tor), AMH (Anti-Mullerian hormone) and MIS (Mullerian Inhibiting Substance) promote the regression of the Mullerian duct. Testosterone released from the Leydig cell further potentiates to develop the male reproductive system. INSL3 (insulin-like peptide 3) is a relaxin peptide family member that helps descend the testis]

and lymphocyte chimerism (XX/XY). The XX DSDs are analogous to Turner’s syndrome in humans which are observed in goats, pigs, horses and dogs. The XX DSDs are characterized by the presence of both male (testicles or ovotestes) and female (uterus, oviducts and ductus deferens) reproductive organs. The XY DSDs (analogous to Klinefelter’s syndrome) include androgen insensitivity syn­drome (AIS) and persistent Mullerian duct syndrome (PMDS) and are characterized by cryptorchidism and hypo­spadias. Under certain circumstances, animals with XX kar­yotype may express H-Y antigen and result in hermaphroditic females.

18.3.1 Freemartins

One of the common examples of sex chromosome DSD is freemartinism. The freemartin is a sterile female calf result due to fusion of the chorioallantoic portions of the placenta occurring around days 28 and 30 of gestation in heterosexual twin births. The fusion of the placenta facilitates the common blood supply between twin foetuses. The male determining factors (testosterone, H-Y antigen, MIT) of the male foetus suppress the development of genitalia in the female foetus. The sterile female calf may have urogenital tract, vagina and external genitalia. The freemartins are common in cattle, goats and camelids.

18.3.2 Mammalian Hybrids

Interspecies mammalian hybrids are developed for various purposes. Some of them are sterile, and some are fertile depending on the chromosome numbers of breeding parents (Table 18.2). Some examples of such hybrids are mule, hinny, tigon and liger. The mule (male donkey X female horse) and hinny (male horse X female donkey) have 63 chromosomes. They are sterile due to chromosomal incompatibility despite their fertile parents (horse and donkey have 64 and 62 chromosomes, respectively). Mules can exhibit oestrus symptoms and can maintain horse embryos. Tigon is obtained from the breeding of a male tiger and a female lion. Liger is the result of breeding a male lion to a female tiger. Both lion and tiger have 38 chromosomes; hence, liger and tigon also have the same number of chromosomes and are fertile.

Table 18.2 Chromosome number (2n) in different domestic and wild animals and birds

18.4