High-resolution mapping of sperm function defects in the t complex fourth inversion

Structural variants of the mouse Chr 17-specific t complex, known as t haplotypes, express factors that alter the ability of sperm to carry out their roles in the normal fertilization process. In previous studies of males carrying heterospecific combinations of the t complex, we discovered a unique M. spretus/t haplotype phenotype of male sterility. In additional studies with mice carrying a series of M. spretus-M. m. domesticus recombinant Chr 17 homologs and a complete t haplotype (S-+/t), we monitored physiological aspects of sperm function to map a locus (Hst6) responsible for expression of the t-specific "curlicue" sperm flagellar curvature phenotype to 1 cM within the fourth inversion of the t complex. In the present report, we quantitatively analyze the in vitro capability of sperm from mice with similar S-+/t Chr 17 genotypes to fertilize zona pellucida-free mouse eggs. The results identify a locus, Stop1, mapping distal to Pim1, with acute effects on the ability of sperm to penetrate the oolemma. The data suggest that Stop1 is a complex locus consisting of at least two genetic elements, a proximal one overlapping the Hst6 locus, and another, distal to the Hst6 locus. Further quantitative analyses of the "curlicue" phenotype produced by sperm derived from these same animals indicate that expression of this chronic flagellar curvature phenotype also derives from at least two elements, both mapping within the Hst6 locus. Thus, these studies provide higher resolution mapping of the molecular basis of t haplotype-specific sperm dysfunction emanating from In(17)4.