Mapping Sperm Mobility QTLs: The Search For The Holy Grail(s)

Sperm mobility is a quantitative trait in poultry.  Sperm mobility is defined as the movement of a sperm cell population against resistance at body temperature.  Phenotype is measured with the sperm mobility assay.  A sperm suspension is overlaid upon pre-warmed 6% (w/v) Accudenz in a cuvette.  The cuvette is incubated for 5 min at 41 C.  Next, the absorbance of the Accudenz layer is measured.  The number of mobile sperm within the overlaid sperm suspension is proportional to absorbance.  Proportion of mobile sperm can be estimated as follows.  Once absorbance is measured, cuvette contents are mixed and absorbance is read a second time.  The proportion of mobile sperm within the semen sample is the quotient of the first and second absorbance measurements.  The proportion of mobile sperm within chicken semen ranges from zero to 80%.  This range has been explained in previous work in terms of the interaction between sperm cell glycolytic ability and duration of sperm passage through the reproductive tract's deferent duct.  It appears that such passage puts sperm cells at risk of premature mitochondrial failure.  Such failure renders affected sperm immobile (straight line velocity < 30 micrometers per second) without significant effect upon sperm viability.  Thus, some sperm cells fail in all males prior to ejaculation and do so in a stochastic manner.  What differs among males within a population is the extent to which this happens.  This extent is subject to genetic selection, and low line males have a deferent duct transit that is approximately twice that of high line males, i.e. 2 versus 1 d.  Likewise, as evidenced by proteome analysis, the glycolytic ability of sperm from low line males is compromised.  The goal of on-going research is to associate SNPloytpe with phenotype by conducting a multi-generational breeding experiment.  F1 roosters will be produced by a reciprocal line cross.  F2 male progeny will be produced by randomly mating F1 sires with F1 dams.  F2 male progeny will be phenotyped with the sperm mobility assay.  Data will be used to construct a phenotypic distribution.  A normal distribution is anticipated.  Thus, males within the lower and upper tails, i.e. those whose phenotype is less than or greater than a standard deviation from the population mean, will have reproductive tract throughput measured and SNPlotype determined.  It is anticipated that a SNPloytpe will be discovered that predicts the low sperm mobility phenotype.