Sperm motility in birds: insights from fowl sperm.

This review outlines how computer-assisted sperm motion analysis helped explain variation in fertility among roosters, investigate sperm cell energetics, and predict sperm allocation within the oviduct. The context for these experiments was the analysis of sperm mobility, a quantitative trait discovered in the mid-1990s. Sperm mobility denotes the movement of a sperm cell population against resistance at body temperature. Whereas mobile sperm must be motile, not all motile sperm are mobile. For example, straight-line velocity (VSL) must be > 30 microm/s for sperm to penetrate an Accudenz solution within a cuvette from an overlaid sperm suspension. Absorbance of the Accudenz solution is proportional to the number of mobile sperm within the semen sample. Phenotypic variation among roosters was explicable in terms of VSL distributions, which in turn reflected varying proportions of sperm containing dysfunctional mitochondria. In the case of mobile sperm, motility was enabled by calcium cycling, which drives phospholipase A2 activity and thereby controls the release of endogenous fatty acids for beta-oxidation within mitochondria. In view of sperm storage tubule ultrastructure and sperm rheotaxis, experimental outcomes led to the realization that sperm residence within the hen's sperm storage tubules was most readily explained in terms of progressive motility and that decreased VSL may account for sperm egress. It is noteworthy that male fertility can be altered by genetic selection using in vitro sperm mobility as the selection criterion. Sperm immobility may arise from formation of the mitochondrial permeability transition pore. Ongoing work addresses underlying mechanisms and related genes.