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J. M. Bedford
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R. T. F. Bernard
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R. M. Baxter
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The Soricidae are generally considered to comprise two subfamilies – Crocidurinae and Soricinae – each of which has distinctive reproductive characteristics. Although Myosorex varius is classified as a crocidurine, the features of its reproductive system call that classification into question. Compared with three other shrew genera, Myosorex exhibited a number of specific features including a relatively prolonged time (about 22 h) for ovulation to be induced by hCG injection and the smallest diameter (75 μm) recorded for any mammal egg. Moreover, the relative testis mass and the number of epididymal spermatozoa were somewhat greater than in some other shrews studied recently. However, many reproductive features in Myosorex have a 'hybrid' character. The glans penis has spines similar to those evident in crocidurines but absent in soricines, yet the accessory sperm storage site, midway along the vas deferens, is similar to that in soricine shrews. The ultrastructure of Myosorex spermatozoa was primarily soricine, despite an unduly large acrosome, which reaches its apogee in the Crocidurinae. Whereas the Fallopian tube displays a crocidurine-type isthmus characterized by deep crypts throughout, the ampulla is richly endowed with ciliated crypts, which in soricines contain spermatozoa. The first polar body persists in the Myosorex ovum, as it does in the soricines Cryptotis and Blarina, but not in the crocidurine Suncus and Crocidura. However, the cumulus oophorus of Myosorex appears largely crocidurine by virtue of its persistent intercellular junctions, long term stability, and the absence of a matrix, lacking only the unique perizonal space that finally characterizes the cumulus of the crocidurines, Suncus and Crocidura. The 'hybrid' character of the reproductive system of Myosorex is more consistent with the proposal that the genus is a survivor of a primitive subfamily – the Crocidosoricinae – from which present day Soricinae and Crocidurinae have arisen.

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R. Webb
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G. Baxter
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D. McBride
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M. Ritchie
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A. J. Springbett
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Summary. Three experiments were carried out during seasonal anoestrus in Finnish Landrace and Scottish Blackface ewes, to establish whether the differences between the breeds in ovulation rate are functional during the non-breeding season and are therefore independent of the mechanism controlling ovulation.

In Expt 1, follicles ⩾2 mm in diameter were dissected from the ovaries of both breeds and incubated individually for 2 h to assess their ability to secrete oestradiol and testosterone. In both breeds, follicles producing ⩾500 pg oestrogen/ml/h (oestrogen-active) were readily identifiable from a population producing less (oestrogen-inactive). The number of oestrogen-active follicles in each breed was similar to the number of ovulations near the end of the breeding season. Oestrogen-active follicles also had more luteinizing hormone (LH) receptors and larger diameters than oestrogen-inactive follicles. There were, however, no significant differences between the two follicle types in follicular fluid or in-vitro testosterone concentrations.

In Expt 2, seasonally anoestrous Scottish Blackface ewes were unilaterally ovariectomized; the second ovary was removed 7 days later. Follicles from both ovaries were processed as described for Expt 1; oestrogen-active follicles were categorized according to their ability to produce >500 pg/ml/h. There were twice as many oestrogen-active follicles in the second ovary as in the first ovary; the number of oestrogen-active follicles in the second ovary was also similar to the total number of oestrogen-active follicles in both ovaries of the Scottish Blackface ewes in Expt 1. There were no significant differences between the first and second ovaries for any of the other parameters measured in oestrogen-active follicles. There were no significant changes in peripheral gonadotrophin concentrations measured 24 h after removal of the first ovary.

In Expt 3, seasonally anoestrous ewes of both breeds were challenged with an ovulatory dose of human chorionic gonadotrophin (hCG) (750 iu). There was a significant difference in the mean number of ovulations between the breeds and it was representative for the breed (Finnish Landrace 2·6 ± 0·2; Scottish Blackface 1·6 ± 0·2 mean ovulations per ewe). None of the saline-treated controls ovulated.

The results demonstrated that the mechanism controlling the number of mature, oestrogen-active follicles, and hence ovulation rate, is functional during seasonal anoestrus. This conclusion was confirmed by the observation that compensatory ovarian hypertrophy also occurs during seasonal anoestrus.

Keywords: sheep; follicle; ovulation rate; steroidogenesis; seasonal anoestrus

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C. S. Haley
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G. J. Lee
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M. Fordyce
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G. Baxter
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R. B. Land
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R. Webb
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Summary. A high and a low response line in sheep were selected on the basis of the mean concentration of LH in 10-week-old Finn–Dorset ram lambs after an i.v. injection of 5 μg GnRH. After 8 male generations the mean LH response of the high line was more than 5-fold that of the low line and the heritability of the selected trait was estimated at 0·44 ± 0·015. Highly significant line differences in mean LH response to GnRH were also found in males at 20 weeks of age and females at 10 and 20 weeks of age and the genetic correlations between the four LH response traits appear to be close to unity. Large line differences in the mean FSH response to GnRH were also found in both males and females at 10 and 20 weeks of age. Selection had little effect on the physical characteristics of lambs. High-response line ewes entering their first breeding season at about 7 months of age showed oestrus earlier in the season and had higher ovulation rates and numbers of lambs born per ewe lambing than did low-response line ewes. In the second breeding season, at about 19 months of age, the only line difference was a higher ovulation rate early in the breeding season in high-line ewes. It is suggested that these changes may be mediated by a more rapid response in high-line ewes to increased GnRH stimulation at puberty or at the beginning of the breeding season.

Keywords: genetic selection; LH release; GnRH; sheep

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