The overall objective of this research was to define the cellular basis underlying heritable subfertility in roosters homozygous for the rose comb allele (R/R). Fertilization in the hen is preceded by the ascension of motile sperm through the vagina and sperm sequestration within sperm storage tubules (SST). The objective of the first set of experiments was to determine if reduced sperm sequestration could account for subfertility. Sperm sequestration differed between genotypes following intravaginal insemination (p<0.0001). However, sperm sequestration did not differ between genotypes when sperm were incubated with SST in vitro (p>0.05). Therefore, subfertility was attributed to reduced sperm transport within the vagina. To test this hypothesis, an assay was developed to evaluate fowl sperm motility in vitro. Based upon this assay, ejaculates from subfertile males contained smaller subpopulations of highly motile sperm than the ejaculates from controls (p<0.001). The objective of the next set of experiments was to characterize the motility of individual sperm and to identify a mechanism that could account for the genotypic difference in sperm cell motility. Computer-assisted sperm motion analysis evaluation revealed that ejaculates from controls contained 91% motile sperm whereas ejaculates from subfertile males contained 62% motile sperm (p<0.001). The ATP concentration in sperm from subfertile males was 63% less than that of sperm from controls (p<0.001). A link between sperm ATP concentration and immotility was investigated. First, sperm metabolism was evaluated using motility as an endpoint. The genotypic difference in sperm motility persisted when ATP synthesis was limited to glycolysis (p<0.001). Consequently, mitochondrial respiration could not account for the genotypic difference in sperm motility. In contrast, sperm uptake of [1,2-��H] 2-deoxy-D-glucose did differ between genotypes (p<0.001). The activity of key glycolytic enzymes, creatine kinase, and dynein ATPase did not differ between genotypes (p>0.05). Therefore reduced sperm motility did not appear to be due to ATP synthesis, allocation of high energy phosphate bonds along the axoneme, or ATP consumption (p>0.05). In conclusion, subfertility of roosters homozygous for the rose comb allele was attributed to decreased spermatozoal glucose transport. / Graduation date: 1997