ARS | Publication request: Genetic variation in testis size and testicular development

Submitted to: Meeting Abstract
Publication Type: Abstract
Publication Acceptance Date: August 1, 2008
Publication Date: August 7, 2008
Citation: Ford, J.J. 2008. Genetic variation in testis size and testicular development. In: Proceedings of Midwest Boar Stud Managers Conference, August 7-8, 2008, St. Louis, Missouri. CDROM.

Technical Abstract: Breed differences in sperm production have been described in a number of studies with these differences reflecting variation in testicular size (Ford et al., 2006; Smital, 2008). Within a given breed or genetic line of boars, sperm production increases as testicular size increases. Furthermore, the number of Sertoli cells within the testes at puberty establishes adult testicular size (Lunstra et al., 2003). Sertoli cells provide physical and nutritional support for developing germ cells in their early development toward sperm cells. Sertoli cells first become apparent in the testes on day 26 of gestation. Thereafter, these cells continue to increase in number until boars reach puberty after which they undergo final maturation. This maturation process is associated with accumulation of fluids within the tubules, enlargement of tubule diameter and formation of a lumen that subsequently allows transport of sperm through the tubules followed by their eventual exit from the testes into the epididymis. The potential for sperm production and testicular size is established at puberty when Sertoli cells cease to proliferate. Patterns of testicular growth diverge greatly in breeds or crossbred lines of boars that vary in mature testicular size. Meishan and Piau are breeds with somewhat smaller mature body weight and distinctly smaller testicular weight than today¿s crossbred boars; however, these two breeds differ vastly in their age at puberty. Meishan boars develop expanding seminiferous tubules (> 100 µm in diameter) by 50 days of age while Piau do not achieve this degree of testicular development until 140 days of age. Furthermore, testes of adult Piau boars contain a much greater proportion of seminiferous tubules than Meishan boars. For the three crossbred lines that have been characterized, diameter of seminiferous tubules advanced to 100 µm by 90 days of age in boars of a 4-breed composite, by 105 days in Landrace x Duroc boars and by 120 days in Hampshire x Duroc x Yorkshire boars. These different studies were conducted under well-managed conditions, but management may have had subtle effects on age for the initiation of pubertal development. Also, the number of boars in each age group is small in most of these studies. Nonetheless, these data emphasize that age of puberty and genetic potential for testicular size are not correlated traits. Heritability for testicular size was employed in a study conducted at the University of Nebraska to improved sperm production significantly by 18% in association with a 24% increase in testicular size (Huang and Johnson, 1996). These boars were selected for 11 generations for increased testicular size at 150 days of age. A correlated response was earlier age of puberty in boars of the selected line that was substantiated by earlier enlargement of seminiferous tubules and earlier onset of sperm production (Harder et al., 1995; Rathje et al., 1995). In contrast with this, Meishan crossbred boars that reach puberty early have smaller testes and lower sperm production as adults than boars with later pubertal development (Lunstra et al., 2003). Collectively, these studies emphasize that the relationship of age at puberty and mature testicular size varies with the genotype of the boars that are under investigation. Informative genetic markers for testicular size would provide a means to select boars early in life with the best potential for greater sperm production as adults. A stimulus for this area of research was the observation that plasma concentrations of follicle-stimulating hormone (FSH) were negatively correlated with testicular size (Ford et al., 1997; Ford et al., 2001). Initial efforts to identify genetic markers utilized Meishan crossbred boars (Rohrer et al., 2001; Sato et al., 2003). Evidence to date indicates the presence of quantitative trait loci (QTL) on swine chromosomes X, 3, and 8 that harbor genes for testicular weight and/or plasma FSH concentrations. Additional information relating to chromosomes 3 and 8 has not advanced since these initial reports. On chromosome X, the thyroxine-binding globulin (TBG) gene resides in the QTL for plasma FSH, and Nonneman et al. (2005) determined that alleles for TBG predicted testicular size. Boars with the TBG allele inherited from the Meishan breed had smaller testes than boars with the TBG allele from a white composite line. However, to date, the Meishan allele for TBG has not been found in breeds of pigs used for commercial swine production. Studies are in progress to identify QTL for testicular size and sperm production in crossbred boars that do not contain the Meishan breed. Worth mentioning is that the Meishan TBG allele is also predictive of greater backfat thickness (Nonneman et al., 2005; Ponsuksili et al., 2005). Comparison of testicular size in Meishan and Piau boars with current day crossbreds draws attention to how easily testicular weight increased as pigs were selected for faster growth rate and leaner carcasses. This supports the observation by Harris (1883) that testicular size changed rapidly when boars were selected for greater body mass and greater fat content. Although data are unavailable, association of fatness with small testicular size is supported by photographs of older breeds of pigs that have not been selected intensively for leanness such as the Iberian and Mangalitza breeds. This ability to modify testicular size rapidly as preference for specific carcass characteristics underwent change substantiates a moderate to high heritability for testicular size. The BX line of pigs at the U.S. Meat Animal Research Center is a composite of one-quarter Duroc, Yorkshire, maternal Landrace and terminal Landrace. Considerable variation exists for testicular size of post-pubertal boars in this line creating a resource to investigate non-invasive procedures to use early in life as a means to predict post-pubertal testicular size and sperm production. However, estimation in pubertal boars of testicular size by ultrasound has proved less informative than desired thereby leaving identification of informative QTL as the most promising selection tool to develop for improving sperm production of boars used in commercial swine production.