Sperm Mobility: Phenotype in Roosters (Gallus domesticus) Determinedby Concentration of Motile Sperm and Straight Line Velocity1

Previous research demonstrated that sperm mobility, i.e., the net movement of a sperm population, is a quantitative trait of the domestic fowl. However, the cellular basis for this trait was unknown. In the present work, individual motile sperm were evaluated with a Hobson SpermTracker in order to identify one or more properties of motile sperm that could account for variation in sperm mobility observed among males. A method was validated for assessing sperm motion over an erythrocyte monolayer at body temperature. A small-scale experiment with roosters from the tails and center of a normal distribution of sperm mobility phenotypes (n = 33 roosters) demonstrated that straight line velocity (VSL) and motile concentration were critical to expression of phenotype. The importance of these variables was confirmed with a large-scale experiment using a representative subpopulation (n = 100 roosters). VSL of individual sperm at 41°C ranged between 5 and 100 μm/sec. VSL averaged 32, 39, and 40 μm/sec for low, average, and high sperm mobility phenotypes. Sperm were diluted to 1.2 × 106/ml for motion analysis. Mean motile concentrations were 0.52, 0.84, and 0.95 × 106/ml for low, average, and high sperm mobility phenotypes. Motile concentration was correlated with sperm mobility (r = 0.71). VSL appeared to have an additive effect as it was correlated with straightness of sperm cell trajectory (r = 0.79).