Plasma inhibin and FSH concentrations in young Merino rams with and without the Booroola F gene

The results of this study show that plasma FSH and inhibin concentrations in young Merino rams carrying a single copy of the Booroola F gene did not differ from Merino rams without the F gene. With respect to FSH, these results therefore support the finding of Montgomery et al. (1989) but are at variance with the conclusions of Seck et al. (1988). As the latter authors have noted, the substantial contribution to overall between-animal variation from the betweensire within-genotype source, can result in anomalous findings if the genotype comparisons are based on small numbers of sires and/or offspring. 212 Unfortunately, all studies on the effect of the F gene in males reported to date suffer this deficiency to varying degrees.
Plasma inhibin concentrations did not differ between genotypes, between litter sizes, and there was no significant between-sire effect at any of the ages. The only significant source of variation was that due to date of birth when inhibin was measured at 3 weeks of age. There was a range of 9 days in date of birth so the variation at 3 weeks was probably a simple reflection of increases in inhibin concentrations with the age of the young rams, Thus the effect of sire on FSH patterns, and the lack of an effect of the F gene, are inconsistent with the hypothesis that these effects are expressed through testicular production of inhibin. Another avenue will have to be investigated if the physiological bases of these genet ic effects are to be revealed.
The correlation coefficients between FSH and inhibin concentrations were all small and not significantly different from zero, with the exception of poor relationship at 3 weeks of age (r = 0.34). Lack of correlation between the concentrations of these two hormones has also been reported for cyclic ewe (McNeilly et al. 1989). In fact, although a correlation might be expected between two hormones that influence each others secretion, it is not evident a priori whether the correlation should be positive (FSH stimulating inhibin secretion) or negative (inhibin reducing FSH secretion). This question might be resolved by studies of the dynamic relationships between the plasma patterns of the two hormones, but even this is unlikely if, as appears to be the case for the ewe, inhibin plays only a long-term, almost passive, role in the control of FSH secretion.