Search

A 2-yr experiment was conducted to determine whether isolation of ewes from rams is necessary to achieve a high response to the ram effect and whether ewes respond as well in May as in June. The experiment was conducted at two locations, with the same four treatments at each location. The four treatments differed with respect to ewe proximity to rams before mating (isolated vs adjacent) and date of joining with novel breeding rams (May 15 vs June 15). The proximity treatment at one location was changed in the 2nd yr; teaser rams were joined with the ewes instead of being adjacent to them. Overall, 86% of the eligible ewes were judged to have responded to the ram effect. A period of isolation before mating did not increase response compared with ewes that remained adjacent to, or in contact with, rams (86 vs 85%). Response was greater (P less than .05) in June and in the 2nd yr (P = .05). A physiological response, different from that generally described, was identified. Ewes ovulated approximately 8 d (8.0 +/- .19 d) after joining with breeding rams. The subsequent ovulation, accompanied by estrus, occurred approximately 15 d later (15.3 +/- .29 d). Eighty-five percent (87/102) of the ewes sampled responded in this manner. However, 82% (31/38) of a sample of these ewes had at least one morphologically normal corpus luteum when examined by laparoscopy 4 d after joining. It seems that these corpora lutea were not completely functional with respect to progesterone production. The ram effect can be achieved without prior isolation of ewes from rams.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in ovulation rate, embryo survival, litter size, and fertility are presented for four lines of mice that have been selected for growth or are homozygous for a recessive gene (hg) imparting rapid postweaning gain. Two of the lines were hg/hg, one in a growth-neutral and one in a growth-selected background. The remaining two were Hg/Hg (the corresponding normal, dominant allele with no effect on postweaning gain) in the same two backgrounds. Average ovulation rates ranged from 10.9 to 17.1 eggs shed, and litter sizes ranged from 8.6 to 14.0. In the growth-selected background, the hg allele reduced the number of ovulations, implantation, and litter size by nearly three compared with the Hg/Hg controls. The impact of the hg allele in the growth-neutral background was not significant. When males of a different line selected for high litter size were mated to females of the four stocks, more than two additional eggs, implants, and pups were recorded, compared with results of mating to males of the same line as the female.

A study was undertaken to determine the effect of selection for high weaning weight on concentrations of plasma insulin-like growth factor I (IGF-I) in sheep and to evaluate the usefulness of measuring IGF-I as an aid in identification of genotypes with a higher growth potential. Lambs from two lines selected for high 120-d weight (HW and DH) and an unselected control (C) were weighed and blood samples collected monthly from birth to weaning (4 mo. of age). A clear differentiation in size occurred after 1 mo of age between lines, between sexes, and between singles and twins. At weaning, selected lines were 3.8 and 5.0 kg heavier than controls. Plasma IGF-I concentrations were 1.5 to 2 times higher (P less than .001) in males than in females after 1 mo of age. There were no significant differences in IGF-I concentration between lines or types of birth. However, line DH and single lambs on average had higher concentrations of IGF-I. Within sex and type of birth correlations between IGF-I concentrations at 0, 1, 2, 3, and 4 mo and 4-mo BW ranged from -.16 to .49 in the three lines, and most were not significant. Coefficients of variation for IGF-I concentrations (36 to 50%) were two to three times higher than those for BW (11 to 15%). Due to the high variability of IGF-I measurements, the low correlations between IGF-I concentration and BW, and the small differences in IGF-I between control and selected lines, measurement of plasma IGF-I is unlikely to be an effective aid to selection for growth rate in sheep.

Ovulation rate was measured in Barbados Blackbelly (United States strain; B), Targhee (T), Barbados X Targhee (B X T) and Barbados X Dorset-Targhee (B X DT) ewes at first and second estrus following synchronization of cycles early in the breeding season. Body weight at sponge removal differed (P less than .001) between B (30.9 kg) and T ewes (54.9 kg) and was intermediate for B X T (44.9 kg) and B X DT (43.5 kg) ewes. Ovulation rate was higher (P less than .01) at first and second estrus for B (1.86, 2.04), B X T (1.93, 2.04) and B X DT (1.72, 1.80) than for T (1.29, 1.40) ewes. Regressions of ovulation rate on body weight within the breed groups did not differ significantly from each other and the average was significant (b = .049 +/- .014 CL/kg at first estrus and b = .046 +/- .011 CL/kg at second estrus, where CL = number of corpora lutea), but differences between the groups in body weight did not explain the differences in ovulation rate. Litter size for B, B X T, B X DT and T groups was 1.71, 1.84, 1.84 and 1.28, respectively. The B X T ewes were superior to the average of the B and T ewes for ovulation rate (P less than .05) and litter size (P less than .01); there was no direct estimate of embryo survival, but the results indirectly indicate superiority of the crossbreds for this component also.(ABSTRACT TRUNCATED AT 250 WORDS)

The nature of genetic variation in litter size and its components was examined using results of long term selection and of crossing selected lines of mice. Selection for small and for large litters produced strains with mean litter size of approximately 6 and 15, respectively. The large litter size line showed a period of renewed response after 45 generations of selection, following a period of 15 generations with no response. The relative contribution of genotype of the dam and of the embryo to prenatal survival was examined using results of selection, crossing and embryo transfer. Genotype of female carrying the litter was responsible for most of the observed differences. Genes for poor survival expressed by the embryo were present in some strains, but accounted for less variation than did maternal genotype. Information from the selected lines and from crosses among them provided evidence for dominance of genes affecting fertility and prenatal survival and additivity of genes affecting ovulation rate and body weight. The information was used to suggest a model for more effective exploitation of heterosis using breeds or strains of animals of known performance for different components. The extent of genetic variation for prenatal survival suggests that natural selection in this species may not always be for maximum survival. It is suggested that the ability to reduce litter size after conception, in response to nutritional or other stress, may at times be of selective advantage, contributing to maintenance of genetic variation in survival rate.

Mating initiates a pattern of prolactin secretion composed of daily surges which appear with semicircadian rhymicity until Day 8 (Day 0 = Day of plug detection) of mouse gestation. On Day 8, the diurnal (1500-2100 h) prolactin increase diminishes in magnitude in a line selected for high embryo survival (E). By Day 9, the diurnal surge has been terminated. The nocturnal prolactin surge (2400-0900 h) is present in the line E through Day 9 of pregnancy, but on Day 10 it disappears. The temporal pattern of prolactin secretion in a randomly bred control line (C) is similar on Days 2 and 4 of pregnancy to that observed in line E. Collection of head blood and/or trunk blood had no effect on plasma prolactin concentration in either line C or E if no more than 30 seconds elapsed between cage handling and sample collection. In contrast, prolactin levels were dramatically elevated in the blood of animals placed in the same room in which serial exsanguination was performed.

Conception rate, prenatal survival and litter size were recorded for 444 ewes of two age groups from five lines of grade Targhee sheep: two unselected control lines, HC1 and DC(C); two lines selected for 20 yr for increased 120-d weight, HW and DH(W); and a line selected for 18 yr for increased multiple births, T. Line T was equal or superior to the control lines in conception rate, prenatal survival and litter size in both age groups, although most of the differences were not significant. The W selected lines were inferior to the C and T lines in fertility and tended to be lower in prenatal survival, among mature ewes, resulting in a significantly lower number of lambs born per corpus luteum in the W lines than in the other two groups. Among yearlings, C ewes were non-significantly lower in fertility than T and W ewes, while W ewes were significantly lower than C and T ewes in prenatal survival. The T line ewes had higher overall reproductive performance than either of the other two groups. Ewes with two ovulations had a significantly higher conception rate than ewes with single ovulations. Gestation period was exceptionally uniform with a coefficient of variation of 1.3% and little difference due either to line or litter size. It was concluded that selection for multiple births improved overall reproductive performance, whereas selection for increased growth rate had an adverse effect on several components of reproduction, leading to a net decline in fitness.

Three breeds of Javanese sheep are described briefly and data suggesting the segregation of a gene with large effect on ovulation rate and litter size are presented. The three breeds are Javanese Thin Tail (JTT), Javanese Fat Tail (JFT) and Semarang (SEM), the last possibly a substrain of JTT. All three breeds have mean mature ewe weights under 30 kg. Ovulation rate and litter size did not differ significantly among the three; all had litter sizes of up to 4 or 5 with a mean for mature ewes of approximately 2. Ovulation rate ranged from 1 to 5 and had an average within-breed repeatability of .8 within season and .65 between seasons. Within-breed repeatability of litter size was .35 +/- .06. Prenatal survival in pregnant ewes with two, three and four or more ovulations averaged 93, 88 and 86% over two seasons. Dams that had at least one ovulation rate or litter size record greater than or equal to 3 produced two groups of daughters in approximately equal numbers: one group with many records greater than or equal to 3 and mean ovulation rate and litter size of 2.73 and 2.31, respectively, and one group with ovulation rates and litter sizes of 1 or 2 and corresponding means of 1.39 and 1.38. Dams with ovulation rate or litter size records of only 1 or 2 produced daughters in which over 90% had records of only 1 or 2. Estimated heritabilities for the mean of approximately three ovulation rate or litter size records from these daughter-dam comparisons exceeded .7. These results suggest segregation of a Booroola-type gene, one copy of which increases ovulation rate by about 1.3 and litter size by .9 to 1.0. Relationships between duration of estrus and ovulation rate, and between timing of release of luteinizing hormone and number of eggs shed, resemble the pattern in Booroola Merino more closely than that in Finnish Landrace or Romanov, supporting the hypothesis of a major gene.

The reproductive performance of 255 Rambouillet (R), Dorset (D), Finnsheep (F) and F1 ewes born in 1978-1979 (group I) and 1979-1980 (group II) and managed in a semiconfinement fall/winter lambing system was evaluated through 4 yr of age of all ewes and through 5 yr for a portion of group I ewes. Ewes were with rams from approximately May 1 to late September each year, with a 2-wk break late in July/early August. Traits considered were fertility (ewes lambed/ewes exposed), lambing date, litter size, lamb survivial and 70-d lamb weights. Breeds and crossbred groups differed significantly in lambing date, with DR crossbred ewes earliest and F ewes latest. Repeatabilities for groups I and II were .31 and .22, .24 and .24 and .11 and .07 for lambing date, fertility and litter size, respectively. There was no significant heterosis in lambing date, although DR ewes in both groups I and II were superior to (D + R)/2, by about 1 wk on average. There was significant positive heterosis for fertility and traits of which fertility is a component in FR ewes in group I, but none in group II. The FD ewes showed negative heterosis for litter size, -.23 (P less than .05) for group I and -.09 for group II. The results indicate: F and FD ewes are not well adapted to the Mediterranean climate where this experiment was conducted; there is little, if any, useful heterosis in crosses among these three breeds for lambing date or other reproduction traits and RD and R ewes are most suitable of the groups tested, while late onset of the breeding season limits the usefulness of even 50% Finnsheep ewes for an autumn lambing system in this environment.

The effect of fetal and maternal genotype on embryo survival was studied in two lines of mice from the same base population: line E, selected for high embryo survival, and line CN-, selected for small litter size. Embryos were collected at 3.5 days postcoitus from normally mated donors of both lines and were transferred to the uteri of 2.5-day pseudopregnant recipients from each line. The number of embryos recovered and the proportion of those recovered that had been fertilized were greater in line E donors than in line CN- donors (P < .05). Development into blastocysts was slower for CN- embryos. The proportion of recipient females that were pregnant was higher for line E than for line CN- after transfer of line E embryos (P < .005). The same trend was observed following transfer of line CN- embryos (P < .10). Embryos of both lines showed greater survival when transferred to line E recipients than when transferred to line CN- recipients (P < .005), and, within line E recipients, E embryos tended to survive better than did CN- embryos (P < .10). The results suggest that although the genotype of the embryo may play a role, the maternal genotype has greater influence on embryo survival in the two lines of mice.

Ovulation rate was measured by laparoscopy at two consecutive cycles on 366 ewes 2 yr old and over and 85 yearling ewes of five lines of Targhees from the base base population; 53 yearling linecross ewes were also included. The lines were two unselected controls (HCl and DC), two selected for 21 yr for increased 120-d weight (HW and DH) and one selected for 19 yr for multiple births (T). Ewes were synchronized in late July or early August at the start of the normal breeding season with intravaginal pessaries impregnated with 60 mg methylacetoxyprogesterone and examined at first and second estrus. Ovulation had occurred in both cycles in 327 (89%) and 177 (85%) of the mature and yearling ewes, respectively. Overall mean numbers of corpora lutea at first and second estrus were 1.42 and 1.63, respectively for ewes 2 yr and over and 1.20 and 1.44 for yearlings, indicating an effect of synchronizing treatment, season, flushing, or a combination of these. Among mature ewes, ovulation rate was higher (P less than .05) in DH (+.20), HW (+.19) and T (+.16) than in controls at first estrus, and in HW (+.29) and T (+.21) but not DH (-.04) at second estrus. Among yearlings, differences were significant only at second estrus (HW, +.40; T, +.35) and again not for DH (+.08). The failure of line DH to increase in ovulation rate from first to second estrus as did other lines was transmitted to linecross progeny. Body weight within line affected ovulation rate significantly, with a greater effect at second estrus, in both age groups. Adjustment for body weight removed the difference between HW and controls but not between T and controls. Repeatability of corpora lutea count was .27 and .25 for mature and yearling ewes, respectively.

Sixty ewe lambs, 12 each of Suffolk, Dorset, Finnish Landrace, Finn-Dorset and Rambouillet born in January and February were used in a study of puberty and duration of the first breeding season. Ovulation was monitored via progesterone assay of weekly blood samples beginning in July, and estrus via daily checking with vasectomized rams. All 60 ewes ovulated but three, one Dorset and two Rambouillet failed to show estrus although each had five to seven ovulatory cycles. Several ewes showed small progesterone rises, 300 to 500 pg/ml, before first ovulation and six (10%) failed to initiate regular cyclicity following the first ovulation as defined by progesterone greater than 500 pg/ml. Mean number of ovulatory events before the first behavioral estrus ranged from 1.25 for Finnish Landrace to 2.20 for Rambouillet ewe lambs (P less than .1). Rambouillets were older (277 d; P less than .05) at first estrus than Finns (258 d) and Finn-Dorsets (260 d), but differences in age at first ovulation were not significant: Finn-Dorset, 233 d; Dorset, 234 d; Finn, 235 d; Rambouillet, 240 d; Suffolk, 245 d. Most cycles were of normal length, with 91.4% between 14 and 20 d. Repeatability of cycle length within breed group was .38. Suffolks had significantly shorter cycles (16.1 d) than the other four groups (17.0 to 17.4 d). Finnish Landrace and Finn-Dorset ewes continued to cycle much longer than the other groups.(ABSTRACT TRUNCATED AT 250 WORDS)

EVERAL investigators have reported the results of selecting for measures of growth rate or body size in mice (GOODALE 1938; MACARTHUR 1944, 1949; FALCONER 1953; RAHNEFELD et al. 1963; ROBERTS 1967). Direct response to selection has been substantial in each of these cases. Litter size has been shown to increase in several of the lines, the increase in the experiment reported by RAHNEFELD et al. (1966) being nearly three mice per litter. This suggested the possibility that litter size might be increased more rapidly by indirect selection, for body weight gain, than by direct selection, particularly since results of selection for litter size have indicated quite low heritability for this trait (FALCONER 1960; DALTON and BYWATER 1963). A line selected for rapid post-weaning gain was therefore included in an experiment in which the effects of selecting for litter size and its components were being studied. This permitted a direct comparison of the effects of these two kinds of selection, using the same base population and selecting contemporarily within the same laboratory. Results from this project of selecting for litter size and for its components have been reported (BRADFORD 1968, 1969). The present paper reports the direct and correlated responses to selection in the rapid gain line and lines derived from it. The results suggest a complex relationship between body size and reproduction in the mouse, and provide additional evidence on the nature of the genetic control of litter size. MATERIALS AND METHODS The base population was a cross of four inbred lines (C57BL/6JJ, AKR/J, C3H/J, DBA/2J). Details of the formation of the stock and of the management of the colony are given in an earlier paper (BRADFORD 1968). The diet used was commercially prepared “White Diet” with specified minimum protein and fat contents of 24% and 7% respectively. The strain selected for rapid post-weaning gain was designated line G. Eighteen males were each mated to two females each generation. Litters were counted the day of birth, and sexed at two days of age when litters of more than 10 were reduced to 10; there was no augmentation of litters with fewer than 10. Young were weaned, weighed, and ear notched at 21 days and caged, by sex, 4 to 7 per cage to 42 days of age when they were reweighed. Those individuals with the greatest weight increase between the two weighings were selected, regardless of litter, litter size or coat color genotype. Normally more than the necessary 18 males and 36 females were kept at six weeks, but only those numbers were mated except in the latest generations when numbers were increased slightly. Matings were made at approximately nine weeks of age, thus producing four generations per year. Males remained with the females for 7-10 days in the early part of