Experimental demonstration that pre‐ and post‐conceptional mechanisms influence sex ratio in mouse embryos

Abstract

Previously we have demonstrated in two monotocous species (bovine and sheep), a relationship between time of insemination, moment of ovulation, and embryo sex ratio. Here, we have analyzed in a polytocous specie (mice) if in addition to pre-conceptional mechanisms, also post-conceptional ones affect the offspring sex ratio. To verify this hypothesis we carried out two experiments. In the first experiment, we analyzed the effect of mating dynamics on the sex ratio of mice with synchronic male and female embryo development. Females were mated before and after ovulation and sacrificed 13 days later for sex determination of embryos and reabsorptions. A decreased litter size, and an increased offspring sex ratio in matings occurring later in oestrus, supported the view that a biased sex ratio may occur as the result of behavioral differences between the populations of X- or Y-bearing spermatozoa. In the second experiment, embryos developmentally synchronic and asynchronic with the recipient female endometrium were transferred, and again, 13 days later, females were sacrificed for sex determination of embryos and reabsorptions. The male proportion per litter found, indicated that our developmentally asynchronic transfers favored a sex ratio disbalance at birth. When combined, these results become the first experimental evidence supporting the view that both pre- and post-conceptional mechanisms of sex ratio distortion in polytocous species are not mutually exclusive and both may explain, under different conditions, sex ratio deviations at birth.

Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels around the time of conception

“…005 level (James 2000a). Similar phenomena have been reported in other species viz white-tailed deer (Verme & Ozoga 1981), Barbary macaque (Paul & Kuester 1987), golden hamster (Pratt et al 1987), Norway rat (Hedricks & McClintock 1990), mouse (Jimenez et al 2003) and pig (Brooks et al 1991). It is suggested that hormonal changes across the cycle are responsible for the observed variation of offspring sex ratio.…”

Temperature-related birth sex ratio bias in historical Sami: warm years bring more sons

“…The phenomenon may, however, turn out to be rather complex, as we found contrasting effects of concurrent and previous year's temperature on birth sex ratio. This might be associated with the action of both pre-and postconception mechanisms of sex ratio variation (Jimenéz et al 2003). Temperature may influence primary sex determination by the variable fertilization success of X-and Y-bearing sperm (McLachlan & Storey 2003).…”

The variances of the distributions of the combinations of the sexes within mammalian litters: Notes to mark the centenary of the problem

“…Norway rat (Hedricks & McClintock 1990); and mouse (Jimenez et al 2003). If this proposition were also correct in women (and the direct evidence for it is strong (James 2000)), then, as a consequence, we should expect three further propositions to be true viz a.…”

“…However, it is possible to arrange experimentally that there is only a short time window for fertilization and that therefore no dam had the opportunity for resorption and re-insemination. From data of this sort, several workers have (apparently inadvertently and independently) made the highly important discovery that sex ratio variation exists at the time of formation of the zygote in the golden hamster (Pratt et al 1987;Huck et al 1990); the sheep (Gutierrez-Adan et al 1999); the mouse (Tarin et al 1999;Jimenez et al 2003); the white-tailed deer (Verme & Ozoga 1981;De Young et al 2004); the American bison (Rutberg 1986); the springbok (Kruger et al 2005); domestic cattle (Gutierrez-Adan et al 1999); the Barbary macaque (Paul & Kuester 1987) and, with less certainty, the human being (See James 2006 b for references).…”