Identification of Proteins in the Accessory Sex Gland Fluid Associated With Fertility Indexes of Dairy Bulls: A Proteomic Approach

We evaluated the expression of proteins in the accessory sex gland fluid (AGF) and their relationships with fertility indexes of dairy bulls. Fertility was normalized as the percentage point deviation of their nonreturn rates (PD) from the average fertility of all bulls from a given artificial insemination center. Services associated with each sire ranged from 269 to 77 321 and PD values from +7.7% to −18.1%. AGF, from 37 bulls, was obtained with an artificial vagina after cannulation of the vasa deferentia. Proteins from AGF were separated by 2‐dimensional SDS‐PAGE followed by staining with Coomassie blue and analysis of polypeptide maps using PDQuest software. Bulls were divided in groups based on PD values and the optical density of spots in the AGF gels used as independent variables to predict bull fertility. Proteins were identified by capillary liquid chromatography nanoelectrospray ionization tandem mass spectrometry (CapLC‐MS/MS). An average of 52 ± 5 spots was detected in the AGF gels, but there were no spots unique to groups of either high‐ (PD ≥ 0) or low‐ (PD < 0) fertility sires. The former were neither less nor more homogeneous than the latter based on correlations of all matched spots between pairs of AGF maps. However, high fertility of dairy bulls was significantly associated with lower expression of 14‐kDa spermadhesin Z13 isoforms and higher amounts of 55‐kDa osteopontin and 58‐kDa phospholipase A2 (PLA2) isoforms. The average intensity of 5 spots identified as BSP 30 kDa in the AGF gels had a quadratic association with fertility indexes (R2 = .18; P = .03). PD values of bulls were related (R2 = .56) to the quantity of spermadhesin, osteopontin, and BSP 30 kDa in the AGF polypeptide maps. Bull fertility was also determined by another equation (R2 = .53) with spermadhesin, BSP 30 kDa, and PLA2 as independent variables. We conclude that interactions among several proteins in accessory sex gland fluid explain a significant proportion of the variation in fertility scores of mature dairy sires.