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ANIMAL GENETICS |
Departamento de Ciencia Animal, Universidad Politécnica de Valencia, 46071, Valencia Spain
Abstract
The aim of this work was to determine whether prenatal survival depends on the genotype of the mother or of the embryo and to identify the critical periods for prenatal mortality in two lines of rabbits divergently selected by high (H) and low (L) uterine capacity. Does from H (n = 124) and L (n = 115) lines were slaughtered at 72 h of gestation. Embryos recovered at 72 h of gestation were transferred to the oviducts of recipient does from the H (n = 23) and L (n = 19) lines. Each recipient does received eight embryos from the H line into one oviduct and eight embryos from the L line into the other. Recipient does were slaughtered on d 28 of gestation. No differences were found between lines in the embryo recovery either in ovulation rate (OR) or in fertilization rate of ova recovered. Recovery rate was higher for the H line (0.80 vs. 0.72, P < 0.01). The number of embryos recovered, fitting ovulation rate as a covariate, was also higher for the H line (9.74 vs. 8.78, P < 0.05).
The H line showed a more advanced embryonic stage of development, having a higher percentage of blastocysts (PB) and a lower percentage of compact morulae (PCM) (38% vs. 20%, P < 0.001 for PB, and 51% vs. 64%, P < 0.01 for PCM). The percentage of early morulae was low and similar in both lines. Neither donor nor recipient lines affected embryonic survival from 72 h to 7 d of gestation. Fetal survival was affected by the recipient line (P < 0.05). An interaction between donor and recipient was found. Embryos from the H donor line had a better fetal survival rate than embryos from the L donor line (P < 0.05) in H recipient females. Within L recipient females, embryos from H and L donor lines showed similar fetal survival. Fetal survival was divided into early (from d 7 to 17 of gestation) and late (from d 17 to 28 of gestation). The high recipient line showed a higher early fetal survival than the L recipient line (P < 0.05). The same effect was observed for late fetal survival, but the difference between H and L recipient lines was lower (P < 0.10). Thus, fetal survival depends mainly on the maternal genotype, and the embryo genotype only affects fetal survival when embryo transfer is performed to a favorable maternal environment. Selection for uterine capacity in rabbits leads to modification of early embryonic survival and of early and late fetal survival, but differences are higher for early than for late fetal survival.
Key Words: Embryonic Effects • Maternal Effects • Prenatal Survival • Rabbit • Selection • Uterine Capacity
Introduction
Several studies have been carried out to better understand genetic factors affecting prenatal survival in rabbits (Torres et al., 1987
), pigs (Wilson et al., 1998), and mice (Ernst et al., 2000), although interactions that are established between mother and fetus make it difficult to study. It seems that the maternal genotype plays the most important role, whereas the embryonic genotype has a minor effect (reviewed by Bradford, 1979; Blasco et al., 1993). Nevertheless, some authors have recently shown that embryos have an active role during gestation. For example, in rabbits, Hoffman et al. (1998) found that the presence of the blastocyst in implantation sites led to a reduced expression of a mucin that has antiadhesive properties, and in pigs, Wilson and Ford (1997) observed that the amount of estradiol produced by the embryo was positively correlated with the concentration of IGF-I in the uterine flushings, which has been reported as affecting early embryonic development (Lewis et al., 1992). Thus, the relative importance of maternal and embryonic genotype on prenatal survival remains unclear.
Embryo transfer is a useful tool to study genetic factors affecting prenatal survival. In rabbits, ovulation is induced by coitus, and interhorn migration does not occur, so this species is particularly suitable for the study of genetic factors affecting prenatal survival. The aim of this work was to determine whether prenatal survival depends on the genotype of the mother or on the genotype of the embryo and to identify the critical periods for prenatal mortality by using two lines of rabbits divergently selected for high (H) and low (L) uterine capacity (UC).
Materials and Methods
Animals
Animals used as donors and recipients came from an experiment of divergent selection on UC. Uterine capacity has been defined as the maximal number of fetuses that the dam is able to support at birth when ovulation rate is not a limiting factor (Christenson et al., 1987). In rabbits, Blasco et al. (1994) proposed litter size in unilaterally ovariectomized females as an estimator of UC. As transuterine migration is not found in rabbits, unilateral ovariectomy and consequent ovarian hypertrophy results in overcrowding of the uterine horn corresponding to the functional ovary. The two lines were generated from a synthetic population bred at the experimental farm of the Universidad Politécnica de Valencia. Both were divergently selected for 10 generations, and selection was relaxed from the 11th through the 15th generations. A detailed description of the selection procedure was reported by Argente et al. (1997). Animals were housed at the experimental farm of the Universidad Politécnica de Valencia in individual metal cages. Animals were kept under controlled 16-h light:8-h dark photoperiods and fed a commercial diet.
Embryo Recovery.
Contemporary does from the lines selected for high and low UC were used. Does came from the 13th, 14th, and 15th generations: 124 does from the H line and 115 does from the L line. Unilateral ovariectomies, as described by Blasco et al. (1994), were performed on females from the 13th generation at 14 to 16 wk of age (ULO). Females from the 14th and 15th generations were not unilaterally ovariectomized (INTACT). All data were recorded from first to fifth gestation. Natural matings were carried out with males from the same line as the donor females. Does were slaughtered by stunning and exsanguination at 72 to 75 h postcoitum. The entire reproductive tract was removed after slaughter and the number of corpora lutea was recorded. The oviducts and the first one-third of the uterine horns were excised and flushed once with 5 mL of Dulbecco’s PBS (Sigma, Alcobendas, Madrid, Spain) supplemented with CaCl2 (0.132 g/L), 0.2% of BSA (Sigma, Alcobendas, Madrid, Spain), and antibiotics (300,000 IU of penicillin G sodium, 700,000 IU of penicillin G procaine, and 1,250 mg of dihydrostreptomycin sulfate; Penivet 1, Divasa Farmavic, Barcelona, Spain) at room temperature. Embryos were counted and classified as normal or abnormal according to morphological criteria; early morulae, compact morulae, and blastocysts were classified as normal when they presented homogenous cellular masses and intact zona pellucida (Hafez, 1993). Embryo classification was always carried out by the same operator.
Embryo Transfers.
Recipient females came from the 15th generation: 23 does from the H line and 19 does from the L line were used. Nulliparous females 19 to 20 wk of age were used as recipients. Twenty-one days before the transfer, recipients were synchronized by i.m. administration of 1 µg of busereline acetate (Hoechst, Marion Roussel, Madrid, Spain). In rabbits, vulva color allows determination of whether a female is receptive (Moody and McNitt, 1988); so, females that presented vulva color associated with receptive status were induced to ovulate with a second injection of busereline acetate 72 h before transfer. To perform the transfers, rabbits were anaesthetized with an i.m. injection of Xylazine (Rompun 2%, Bayer AG, Leverkusen, Germany) at a rate of 0.2 mL/kg of BW; 5 min later, this injection was followed by an i.v. dose of 2 to 3 mL of Ketamine HCL and clorbutol (Imalgène 500, Merial-Lyon, France) in the marginal ear vein. Embryo transfers were performed using the laparoscopic technique described by Besenfelder and Brem (1993). This technique allows transferring embryos with minor abdominal surgery. Only embryos from INTACT females that were classified as normal were transferred. The number of embryos transferred per Fallopian tube was standardized to eight, so each recipient received 16 embryos (eight embryos from the H line into one oviduct and eight embryos from the L line into the other). Clutter et al. (1990) reported differences between the right and the left side of the reproductive tract in mice. To avoid these possible differences, transfers to the right or left uterine horn were randomized. Embryo transfers were performed using a mixture of different embryonic stages (early morulae, compact morulae, and blastocysts) to avoid a possible effect of embryo type.
Traits
Traits Recorded at Embryo Recovery.
Ovulation rate (OR) was estimated as the number of corpora lutea. The total number of embryos recovered (TE = normal embryos + abnormal embryos) was recorded. Traits calculated were: fertilization rate (FR = TE/oocytes + normal embryos + abnormal embryos) and recovery rate (RR = oocytes + normal embryos + abnormal embryos/OR). For each INTACT female, percentages of early morulae (PEM = number of early morulae/TE), compact morulae (PCM = number of compact morulae/TE), and blastocysts (PB = number of blastocysts/TE) were calculated.
Traits Recorded at Embryo Transfer.
For each uterine horn, the number of live fetuses (LF1) was recorded around 14 d of gestation, as observed by laparoscopy. The number of implanted embryos (IE) was estimated as the total number of embryos in the uterus (LF1 + dead embryos). Recipient females were slaughtered by stunning and exsanguination at d 28 of gestation, and for each uterine horn, the number of live fetuses (LF3) and dead fetuses were recorded. The number of live fetuses at d 17 of gestation (LF2) was estimated as LF3 + the number of dead fetuses at d 28 of gestation, since the presence of dead fetuses at d 28 of gestation represents mortality occurring after d 17 postcoitum (Adams, 1960a,b). Pregnancy rate was defined as the number of uterine horn of all does in the group having at least one live fetus at d 28 of gestation divided by the total number of uterine horns in the group.
Statistical Analysis
Traits Recorded at Embryo Recovery.
Those females that did not ovulate (two does from the H line and three from the L line) or did not have embryos (three females from the H line and two from the L line) were excluded from statistical analyses. For significance testing, FR, RR, PEM, PCM, and PB were transformed following Freeman and Tukey (1950) transformation, but results are reported in the original scale. Least squares analyses were performed on the variables OR, FR, RR, and TE using a model with fixed effects of line (H and L), treatment (ULO and INTACT), lactation-parity status at slaughtering (nulliparous, nonlactating after first parity, nonlactating with more than one parity, lactating with more than one parity), and year-season nested within treatment (ULO summer of 2000, ULO winter of 2000/2001, INTACT winter of 2000/2001, INTACT spring of 2001, and INTACT summer of 2001). Additional analyses of ET included OR as a covariate. For the traits PEM, PCM, and PB, the model was the same without treatment effect. The GLM procedure of SAS (SAS Inst., Inc., Cary, NC) was used.
Traits Recorded at Embryo Transfer.
A chi-squared test was performed to determine the effect of the recipient line (high recipient, low recipient) and the donor line (high donor, low donor) on pregnancy rate. Only pregnant uterine horns that had at least one fetus alive at 28 d of gestation were included in the analyses of the other traits. Least squares analyses were performed on the variables IE, LF1, LF2, and LF3 with the fixed effects of recipient line (high recipient, low recipient), donor line (high donor, low donor), their interaction, and the random effect of female. In order to study fetal survival, additional analyses of LF2 and LF3 included IE as a covariate. The number of live fetuses at d 28 of gestation was also analyzed, with LF2 as a covariate. The MIXED procedure of SAS was used for these analyses.
Results and Discussion
Embryo Recovery
Ovulation rate, FR, and RR were higher for INTACT than for ULO females (13.04 vs. 12.08, P < 0.01; 98% vs. 91%, P < 0.001; 80% vs. 72%, P < 0.05, respectively). Santacreu et al. (1997) reported that unilateral ovariectomy led to a slightly lower ovulation rate in rabbits, but did not affect either fertilization rate or embryo recovery. The lower FR and RR found in ULO females brought about a lower total number of embryos recovered per doe for ULO vs. INTACT females (8.28 vs. 10.24, P < 0.001). It may be, as suggested by Lamberson et al. (1989) in mice, that unilateral ovariectomy increased the number of immature ova.
Both lines (H and L) presented OR (Table 1) similar to those earlier reported by Santacreu et al. (2000) in intact females. The RR was high and similar to that obtained by other authors in maternal rabbit lines (Mocé et al., 2002). The H line presented a higher recovery rate than the L line (Table 1). Although the lines had a similar OR and FR, the number of total embryos recovered at 72 h of gestation was higher for the H line (Table 1). These differences for the total number of embryos recovered were maintained when ovulation rate was fitted as a covariate (Table 1). A preliminary study done by Santacreu et al. (1997) using unilaterally ovariectomized females from the fourth to seventh generations of this selection experiment showed that at 30 h of gestation, the number of embryos recovered was similar for both lines. Thus, it seems that selection for UC affects early embryo survival (30 to 72 h of gestation). The transition from maternal to zygotic control of development (reviewed by Kanka, 2003) is a critical step for the embryonic development success that takes place during this time. In rabbits, this transition has been described at the 8- to 16-cell stage (Manes, 1973; Brunet-Simon et al., 2001) at approximately 48 h postcoitus.
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). The percentage of early morulae was low and similar in both lines (11% vs. 16% for H and L lines, respectively; Figure 1). These results corroborate those obtained in mice by several authors. For example, Al-Shorepy et al. (1992) observed that lines selected for litter size or for its components showed a more advanced stage of development at d 3.5 of gestation than an unselected control line. Also in mice, Durrant et al. (1980) observed that the percentage of embryos developed beyond the eight-cell stage at d 2 of gestation was greater in a line selected for large litter size than in an unselected control line. Moler et al. (1981) reported that the percentage of blastocysts was higher in a mice strain selected for high embryo survival than in a line selected for small litter size. Several causes, depending on the mother and/or on the embryo, may account for the more advanced developmental stage reported for the H line. Protein patterns of uterine and oviductal secretion change throughout gestation; the role of most of the proteins secreted remains unclear, but some of them affect early embryonic development. For example, IGF-I has been reported as affecting early embryonic development (Lewis et al., 1992, in pigs; Herrler et al., 1998, in rabbits). Furthermore, in mouse embryos, Goldbard and Warner (1982) identified the preimplantation embryo development gene, which affected the cleavage divisions. Therefore, results obtained in the present experiment point out that selection for UC leads to modification of early embryonic survival and of the developmental stage, but it remains to be determined whether these differences are due to mother or embryo.
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). Santacreu et al. (2000) working with animals from 11th generation of the same lines (H and L) reported a prenatal survival at birth of 68% for the H line and 52% for the L line, which are similar to those observed in the present work. Embryo transfer in mice is less successful (Moler et al., 1981; Barkley and Fitzgerald, 1990; Ernst et al., 2000). In rabbits, ovulation is induced by coitus and it is also possible to know whether a doe is receptive by checking vulva color (Moody and McNitt, 1988). These two characteristics make it easier to synchronize recipient and donor females in rabbits than in mice, and thus to study factors affecting prenatal survival. It was observed that neither the recipient line nor the donor line affected pregnancy rate. Both recipient lines had similar pregnancy rates (82% for H line and 88% for L line), and neither depended on the donor line (both H and L donors had an 85% pregnancy rate). This result does not agree with Moler et al. (1981), who, working with two lines of mice differing in prenatal survival and derived from the same base population, found a significant effect of recipient females on pregnancy rate (44 and 78%). However, Moler et al. (1981) indicated that those differences could be due to an asynchrony between development of the embryo and recipient uterus. The previously mentioned characteristics of rabbits prevent these asynchronies.
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and 3). Because the uterine horns received an equal number of embryos and implantation took place at d 7 of gestation in rabbits, this result indicates that embryo survival from 72 h to d 7 of gestation was the same for the two lines. Results obtained in the embryo recoveries and in the embryo transfers point out that selection for UC would affect early embryo survival but not late embryo survival.
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and 3) are similar to those obtained for IE. This agrees with Barkley et al. (1990), who, working with mouse strains with different prenatal survival rates derived from the same base population, did not find any effect of maternal or embryonic genotypes on number of fetuses at d 10 of gestation. We did not find any significant effect of the recipient line on number of live fetuses at d 17 or 28 (LF2 and LF3, respectively); nevertheless, the observed differences were relevant (Figure 2), so larger samples are needed to assess this effect. Donor line and interaction between donor and recipient did not have any effect on LF2 and LF3 (Figure 3). The number of live fetuses at d 28 of gestation was studied including IE as a covariate in order to study fetal survival. Fetal survival was significantly affected by the recipient line (Table 2). Santacreu et al. (2000), working with animals from the 11th generation of the same lines, found significant differences between lines in prenatal survival (0.68 vs. 0.52) due to a significantly higher embryo survival (0.89 vs. 0.77) and perhaps to a higher fetal survival rate (0.75 vs. 0.65, differences not significant). An interaction between donor and recipient line was observed. Within H recipient females, embryos from the H donor line showed a greater fetal survival rate than did embryos from the L donor line (5.34 vs. 4.7 fetuses for H and L donor lines, respectively; P < 0.05). Within L recipient females, embryos from H and L donor lines showed similar fetal survival rates (4.06 vs. 4.46 fetuses when transferring embryos from H and L donor lines, respectively, P = 0.22). Thus, it seemed that the genotype of the embryo had an effect on fetal survival only in a favorable maternal environment. These results are in agreement with those reported by Moler et al. (1981), who detected a recipient and recipient x donor interaction effect on survival in mice to term.
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). The H recipient line had a higher early fetal survival than the L recipient line (Table 2). The same effect was detected for late fetal survival, but the difference between the H and L recipient lines was lower. It appears that selection for UC leads to differences in fetal survival and that this difference is mainly produced by modification of early fetal survival. In summary, it was observed that at 72 h of gestation, there were differences between both lines for the number of embryos recovered, and no differences were found from 72 h of gestation to 7 d of gestation. Thus, it seems that differences in embryo survival appear before 72 h of gestation. Moreover, it was observed that divergent selection for UC leads to differences in the early embryonic stage of development. The line selected for H uterine capacity showed a more advanced embryonic stage of development than did the L line. Selection for UC also led to differences in fetal survival rate, but differences between both recipient lines were higher for early than for late fetal survival. Furthermore, from this experiment, it was concluded that fetal survival depends mainly on the genotype of the mother. The genotype of the embryo only affects fetal survival when embryos are transferred to a favorable maternal environment.
Implications
Results obtained in the present experiment point out that selection for uterine capacity in rabbits leads to modification of early embryonic survival and of the developmental stage, but it remains to be determined whether these differences are due to mother or embryo. Selection for uterine capacity also leads to modification of fetal survival, but mainly of early fetal survival. Differences in fetal survival were due to the genotype of the mother, but it will be necessary to study, which are the maternal factors that determine fetal mortality.
Footnotes
1 This study was supported by CICYT (AGF98-0382-C02-01). The authors wish to thank COST Action 848 for providing the financial support to learn laparoscopic embryo transfers. We would also like to thank U. Besenfelder, J. S. Vicente, E. Mocé, and R. Lavara for their technical support. 
3 Present address: Departamento de Produccíon Animal y Ciencia y Tecnología de los Alimentos. Universidad Cardenal-Herrera-CEU, Edificio Seminario, 46113 Moncada, Valencia, Spain.
2 Correspondence—phone: +34-96-136-9000; fax: +34-96-139-5272; e-mail: mmoce{at}uch.ceu.es.
Received for publication March 26, 2003. Accepted for publication September 3, 2003.
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