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D. S. ELLIOTT, J. E. LEGATES and L. C. ULBERG

Summary.

Low fertility in mice selected for small body size (L) at 6 weeks of age was due to fewer matings, more ovulation failures in females mating, a lower ovulation rate and a lower fertilization rate than in unselected mice (C2) maintained as a control. The L males apparently had less libido than C2 males. This may have contributed to fewer matings in the L line. Failure to mate or failure to ovulate, if mating, could not be explained by differences in age or in body weight. It is suggested that a thyroid deficiency was responsible.

Females of the line selected for large body size (H) at 6 weeks of age ovulated more ova than C2 females. Essentially no foetal death occurred, after implantation, except in the H line. A lower percentage of the foetuses was living at 18 days of gestation in females of this line than in the females of the C2 line.

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PT Saunders, JM Turner, M Ruggiu, M Taggart, PS Burgoyne, D Elliott and HJ Cooke

The autosomal gene DAZL is a member of a family of genes (DAZL, DAZ, BOULE), all of which contain a consensus RNA binding domain and are expressed in germ cells. Adult male and female mice null for Dazl lack gametes. In order to define more precisely the developmental stages in germ cells that require Dazl expression, the patterns of germ cell loss in immature male and female wild-type (+/+, WT) and Dazl -/- (DazlKO) mice were analysed. In females, loss of germ cells occurred during fetal life and was coincident with progression of cells through meiotic prophase. In males, testes were recovered from WT and DazlKO males obtained before and during the first wave of spermatogenesis (days 2-19). Mitotically active germ cells were present up to and including day 19. Functional differentiation of spermatogonia associated with detection of c-kit positive cells did not depend upon expression of Dazl. RBMY-positive cells (A, intermediate, B spermatogonia, zygotene and preleptotene spermatocytes) were reduced in DazlKO compared with WT testes. Staining of cell squashes from day 19 testes with anti-gamma-H2AX and anti-SCP3 antibodies showed that germ cells from DazlKO males were unable to progress beyond the leptotene stage of meiotic prophase I. It was concluded that in the absence of Dazl, germ cells can complete mitosis, and embark on functional differentiation but that, in both sexes, progression through meiotic prophase requires this RNA binding protein.