Altered testicular cell type composition in males of two outbred mouse lines selected for high fertility.

Recently we described two outbred mouse lines which have been selected for high fertility. These mouse models doubled the number of offspring per litter. Although selected for a primarily female-trait of high fertility (increased litter size), we were interested whether also males of the fertility lines show differences within their reproductive organs. We investigated males from two outbred mouse lines which have been selected for the phenotype "high fertility" for more than 170 generations. In the present study, we analysed the testicular cell type composition by flow cytometry. We further investigated the weights of reproductive organs, histomorphometry of testis as well as studied sperm motility parameters using a thermal stress assay as well as a sperm hyperactivation assay. Here, we describe that males of the fertility line (FL) 1 show an increased percentage of diploid cells within the testis. Flow cytometric analysis identified this enlarged cell population as Leydig cells. Testis weights were unaffected whereas the weights of seminal vesicles of FL1 and FL2 were increased compared to Ctrl bucks. FL2 males show decreased diameter of tubulus seminiferi and an enhanced spermatid/Sertoli cell index. Sperm motility parameters of FL1 and Ctrl males are initially indistinguishable but FL1 spermatozoa show a better performance in a thermal stress experiment over a 5 hours observation period. These data indicate that although selected for a primarily female-trait of high fertility also males from the fertility lines are effected by defined alterations in their reproductive organs. Some of these alterations are FL1-specific others are FL2-associated, indicating that different molecular strategies warrant the high-fertility phenotype on the female as well as on the male side.