Breeding and Genetics Symposium: a systems biology definition for chicken semen quality

Rooster semen is an effluent from paired reproductive tracts. Each tract includes a testis, epididymis, and deferent duct. Upon ejaculation, efficacy of sperm propulsion varies among roosters. This phenotype is sperm mobility, that is, the movement of sperm against resistance at body temperature. The present work 1) compares reproductive tract throughput between lines of chickens selected for low and high sperm mobility, 2) demonstrates how semen quality can be defined in terms of an interaction between reproductive tract throughput and the proportion of mobile sperm ejaculated, 3) confirms that phenotype can be linked to genomewide differences in SNPlotype, and 4) shows how breeding can affect semen quality. Sperm mobility phenotype distributions were based on the average of duplicate observations per male (n = 241 and 262 roosters for low and high lines, respectively). Distributions were skewed and normal for low and high lines, respectively. Subsequent analyses used these base populations as sources for test subjects. In the first analysis, 10 males were selected from the mode of each distribution, and sperm mobility data were evaluated by nested ANOVA. Variation was observed between lines (P < 0.0001) but not among males within lines (P = 0.980). Sperm mobility data along with data from paired reproductive tracts were used to estimate combined reproductive tract throughput. Whereas testicular output was 1.2-fold greater in the low line (P = 0.037), the output of mobile sperm per day was 10.5-fold greater in the high line (P < 0.0001). Deferent duct transit differed between tails of the low line (P < 0.0001) but not between the tails of the high line (P = 0.514). Males from the mode and upper tail of the low line were SNPlotyped using a 60k chip by DNA Landmarks. These test subjects were used to associate phenotype with SNPlotype because founder effects and genetic drift could be discounted. Loci of interest were found on multiple chromosomes. Loci on chromosome Z were of particular interest because roosters are homozygous for this sex chromosome and a pronounced maternal effect was observed in a prior heritability study. Midrange phenotypes were produced by crossing low and high sperm mobility lines. Our experimental outcomes demonstrate that genes affect reproductive tract function as well as sperm cell attributes and thereby make semen quality subject to genetic selection.