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J. L. Spearow and G. E. Bradford

Summary. Selection for litter size (Line SI) and for post-weaning body weight gain (Line G) increased spontaneous ovulation rate in mature females by 69 and 73%, respectively, over that of randomly bred control mice (Line C). Inbreeding from S1 mice with selection for litter size produced highly inbred lines with elevated ovulation rates. Inbreeding from Line C mice produced a 21% divergence among lines, but did not depress the mean ovulation rate. Crosses of these lines revealed little heterosis in ovulation rate. LH receptors were induced by treating females from 22 days of age with diethylstilboestrol for 4 days and FSH for 2 days. The in-vitro binding of 1 25I-labelled hCG per μg DNA decreased 56% in response to selection for litter size and increased 57% in response to selection for body weight gain, indicating high susceptibility of this trait to genetic change. Inbreeding from Line C mice produced a 135% divergence amongst lines, but did not depress the mean LH receptor induction. Body weight had significant effects on ovulation rate and LH receptor induction.

These results show that selection for litter size and for rapid post-weaning body weight gain increases ovulation rate, but we suggest that different mechanisms are involved in these responses.

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Trish Berger, C. C. Calvert and G. E. Bradford

Summary. Male reproductive capacity was examined in 5 lines of mice which differed markedly in mature body weight (30–73 g) due to selection for growth and/or substitution of the high growth gene (hg). Testes weight ranged from 0·75–0·77% of body weight in control lines to 0·33% in the largest mice which had the hg gene in a growth selected background. Both selection for growth and the hg gene in a control background reduced absolute testes size (0·200 and 0·207 g vs 0·236 and 0·228 g in control lines) as well as relative testes size although body weight was increased by at least 50%. Although the combination of growth background and hg gene reduced sperm production per g testis compared to the outbred control, the primary cause of reduced sperm production per mouse in lines containing either the growth background or the hg gene alone was reduced absolute testes size. At 24, but not at 11, weeks of age, the hg gene reduced sperm motility. In these lines, high genetic potential for post-weaning growth was associated with decreased male reproductive capacity.

Keywords: testis size; spermatogenesis; body weight; mice

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SANDRA M. MURR, I. I. GESCHWIND and G. E. BRADFORD

Summary.

Concentrations of plasma LH and FSH were measured by radioimmunoassay in four strains of mice maintained in controlled lighting. Gonadotrophin levels in an unselected control line (Line C) were measured under the following conditions: induced cycles; spontaneous cycles; cycles in absence of males and post-partum cycles. In mice with induced cycles, LH reached a mean of about 40 ng/ml between 16.00 and 17.00 hours of pro-oestrus. Levels of FSH reached a peak of around 2800 ng/ml about 2 hr later, between 19.00 and 20.00 hours, suggesting that the function of FSH is to stimulate growth of the crop of follicles which will ovulate during the succeeding oestrous cycle. Spontaneously cycling mice of Line C also had a mean LH concentration of about 40 ng/ml, but this peak began 1 hr later at 17.00 hours of pro-oestrus and persisted for about 4 hr. No well-defined FSH peak was found. Only two of seventy individually caged females killed during pro-oestrus had LH or FSH levels greater than the mean di-oestrous levels in induced and spontaneously cycling animals. Within 24 hr of parturition, there was no one time when the majority of mice showed elevated levels of LH or FSH. The timing and magnitude of gonadotrophin release during pro-oestrus of induced cycles in lines successfully selected for small litter size, high embryo survival, and high ovulation rate were the same as for Line C, suggesting that the principal effect of selection was probably to alter the sensitivity of the target organs.

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L. Zarco, G. H. Stabenfeldt, S. Basu, G. E. Bradford and H. Kindahl

Summary. Pregnant (N = 10) and non-pregnant (N = 10) ewes were bled every 2 h from Days 12 to 17 after oestrus (oestrus = Day 0). Plasma concentrations of progesterone, 15-keto-13,14-dihydro-PGF-2α and 11-ketotetranor-PGF metabolites were determined in all samples. The number of PGF-2α pulses in non-pregnant ewes was 8·2 ± 0·4 (mean ± s.e.m.) with an interpulse interval of 10·7 ± 0·7 h. Two or 3 pulses of low frequency (interpulse interval = 13·4 ± 1·6 h) occurred in most non-pregnant ewes before the onset of luteolysis; the interpulse interval then decreased to 7·9 ± 0·4 h for the 6·0 ± 0·3 pulses temporally associated with luteolysis. In contrast, the number of PGF-2α pulses in pregnant ewes was lower (2·5 ± 0·7, 0–8) and the interpulse intervals longer (18·9 ± 6·1 h). Most pulses occurred on Days 14 and 15 in the pregnant and non-pregnant ewes.

The mean concentrations of both PGF-2α metabolites in non-pregnant ewes were highest on Day 15 while basal levels of both metabolites remained constant at all times. In pregnant ewes, the mean concentrations of both metabolites were highest on Day 14; basal concentrations of both metabolites were also highest on Day 14. The mean concentrations of 15-keto-13,14-dihydro-PGF-2α were higher in pregnant than in non-pregnant ewes on Days 13 and 14 (P < 0·05) and higher in non-pregnant than pregnant ewes on Day 15 (P < 0·05). The basal concentrations of the 15-keto metabolite were higher in pregnant than non-pregnant ewes at Days 13, 14, 15, 16 and 17 (P < 0·05). Both the mean and the basal concentrations of 11-ketotetranor-PGF metabolites were higher in pregnant than in non-pregnant ewes on Day 14 (P < 0·05).

It is concluded that uterine production of PGF-2α peaks at Days 14–15 after oestrus in pregnant and non-pregnant ewes. Patterns of release differ, however, in that non-pregnant ewes have a pulsatile PGF-2α pattern superimposed on a constant baseline, while pregnant ewes have an increasing basal secretory pattern which is more nearly continuous, i.e. not pulsatile in form. Modification of pulsatile PGF-2α synthesis and release is therefore a key aspect of prolongation of luteal function at the beginning of pregnancy in the ewe.

Keywords: PGF-2α; sheep; pregnancy

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L. Zarco, G. H. Stabenfeldt, J. F. Quirke, H. Kindahl and G. E. Bradford

Summary. Ewes (N = 32) were bled every 2 h from 5 days before expected oestrus until the end of oestrus. Plasma concentrations were determined for progesterone to monitor luteal activity and for the prostaglandin F-2α (PGF-2α) metabolites, 15-keto-13,14-dihydro-PGF-2α and 11-ketotetranor-PGF to determine uterine synthesis and release of PGF-2α. Most of the variation in cycle length was associated with the time of onset of luteolysis, the timing of events after luteolysis being constant and not related to cycle length. The time of occurrence of the first PGF-2α pulse and the interval between this pulse and the start of luteolysis were the two main determinants responsible for oestrous cycle length. Several PGF-2α pulses with interpulse intervals of 15·9 h occurred before the onset of functional luteolysis compared with 7·7 h for pulses associated with luteolysis. The numbers of PGF-2α pulses and interpulse intervals were similar for oestrous cycles of different lengths. While a gradual decline in progesterone concentrations was observed before functional luteolysis in the ewes with longer cycles, this did not appear to be an integral part of the stimulus which initiates the pulse frequency of PGF-2α required for luteolysis. We therefore suggest that differences in oestrous cycle length in the ewe are determined by the time of the onset of PGF-2α pulsatile release, and especially by the time of increased pulse frequency.

Keywords: PGF-2α; luteolysis; sheep; oestrous cycle

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G. E. BRADFORD, R. HART, J. F. QUIRKE and R. B. LAND

Summary.

Results of egg transfer and of reciprocal crossing among breeds of sheep with a range of 1 week in mean gestation period indicate that the genotype of the fetus is the major determinant of the duration of gestation, over a range of litter sizes from 1 to 5 and a wide range of combinations of size of donor and recipient breeds. It is concluded that the fetus accounts for at least two-thirds of the genetic variation in gestation period in this species.

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J. F. Quirke, H. H. Meyer, A. Lahlou-Kassi, J. P. Hanrahan, G. E. Bradfords and G. H. Stabenfeldt

Summary. Ovulation rate, in mixed-age groups of prolific and non-prolific ewe breed types, after administration of a range of doses of PMSG (0, 375, 750 and 1500 i.u.) during the follicular phase of the oestrous cycle, were compared in Ireland, Morocco and New Zealand. The ewes in Ireland and Morocco were from the Finnish Landrace and Galway, and D'Man and Timhadite breeds, respectively. In New Zealand Booroola Merino × Romney ewes which had been previously identified as heterozygous carriers (F +) of the Booroola high fecundity gene and purebred Romneys were used to represent the prolific and non-prolific genotypes respectively; in addition a group of Booroola Merino × Romney non-carriers (++) of the major gene were also included for comparison. Ovulation rate at the oestrus which preceded stimulation with PMSG was also measured in all animals.

In all 3 locations the ewes of the prolific genotype had a greater ovulation rate after PMSG stimulation than did the non-prolific controls. However, this association between prolificacy and response to PMSG was removed when ovulation rate after PMSG was transformed by dividing by the ovulation rate observed before PMSG administration. Despite the differences in the genetic basis of their high prolificacy the pattern of response to PMSG over the range of dosages used was similar in Finnish Landrace, D'Man and Booroola Merino × Romney (F +) ewes and all breeds had means of about 10 ovulations in response to 1500 i.u. PMSG. Amongst the non-prolific breeds, the Timhadite was the most responsive to PMSG although it had the lowest natural ovulation rate.

Information on fertility and litter size was available for all breeds in Ireland and New Zealand. In all breeds fertility was depressed at the highest dose of PMSG. Litter size at birth increased in all non-prolific breeds as the dose of PMSG was raised. For the prolific breeds, increasing the dose of PMSG from 750 to 1500 i.u. resulted in a substantial reduction in the number of lambs born.