M. A. Santacreu

The genetics of prenatal survival of pigs and rabbits: a review

Abstract Current knowledge on the genetic variability of prenatal survival (PS) in pigs and rabbits is reviewed. There is a large amount of variation between lines or breeds and these differences are not always negatively correlated with ovulation rate (OR); a line with a high OR can also have a high level of PS (e.g. the Meishan pig). Crossbreeding studies show that the maternal genotype is much more important in the control of line differences in PS than the the embryo/fetus, particularly the former, enhances PS, demonstrating the importance of non-additive genetic variation in the control of this trait. Only few estimates of the within breed genetic parameters of PS are available in the literature. Heritability seems to be low, with estimates ranging from 0 to 0.23. PS is negatively correlated with OR and positively correlated with number of embryos/fetuses (NE) or litter size (LS), but estimates of genetic correlations differ widely between studies. Selection for OR generally leads to an increase of NE at mid-gestation, but not at birth. Selection on a linear index combining OR and PS has not proved to be more efficient than selection on LS. New methods, such as unilateral hysterectomy/ovariectomy, which increases the emphasis on fetal survival and aims to measure uterine capacity, are currently under study and seem to be promising.

In vivo development of vitrified rabbit embryos: Effects on prenatal survival and placental development

The aim of this work is to study the effect of the vitrification procedure on prenatal survival and on placental development at the end of gestation in rabbits (Oryctolagus cuniculus). One hundred eighty-one females were slaughtered at 72 h of gestation. Morphologically normal embryos recovered at 72 h of gestation were kept at room temperature until transfer or vitrification. Vitrified embryos (320 embryos) were transferred into a total of 24 does and fresh embryos (712 embryos) were transferred into a total of 43 does. Females were induced to ovulate 72 h before transfer when fresh embryos were transferred and 60 to 63 h before transfer when vitrified embryos were transferred. Each recipient doe received eight embryos into the left oviduct and eight embryos into the right oviduct. The number of implanted embryos was estimated by laparoscopy as number of implantation sites at Day 14 of gestation. Recipient females were slaughtered by stunning and exsanguination 25 d after the transfer, and fetuses were classified according to their status. Live fetuses and fetal and maternal placenta were weighed Pregnancy rate was defined as the total number of females having at least one live fetus at Day 28 of gestation divided by the total number of females. Prenatal survival was estimated as live fetuses at Day 28 of gestation divided by the number of transferred embryos. The pregnancy rate after transfer of vitrified embryos (92%) was similar to that achieved with fresh embryos (86%), but prenatal survival was lower for vitrified than for fresh embryos (53% vs. 34%). We did not find differences in embryo survival from 72 h to implantation. Transfer of vitrified embryos reduced fetal survival from implantation to Day 28 (57% vs. 82%). Differences in the number of live fetuses at Day 28 of gestation were mainly due to the higher fetal mortality observed soon after implantation in pregnancies resulting from the transfer of vitrified embryos. A higher percentage of decidual reactions and atrophic maternal placentas (27.5% vs. 8.3%) and also of atrophic fetal and maternal placentas (12.1% vs. 5.3%) were observed after transfer of vitrified embryos. Both treatments showed similar percentage of dead fetuses (3.3% vs. 4%). Maternal placenta of the fetuses from fresh embryos was 15% heavier than maternal placenta of fetuses from vitrified embryos.

Estimates of genetic parameters for ovulation rate, prenatal survival and litter size in rabbits from an elliptical selection experiment

Abstract Data from an elliptical selection experiment were used to estimate genetic parameters for components of second parity litter size (LS) in rabbits. The ovulation rate (TOR), the number of implanted embryos and the number of live embryos (TLE) were recorded using laparoscopy at day 12 of gestation and LS was recorded at birth. Prenatal survival (PS) was calculated as LS/TOR. Data from 119 founder does, 120 daughters of 44 does selected on a quadratic index of ovulation rate and prenatal survival and 54 contemporary unselected does were analysed by residual maximum likelihood (REML) in an animal model. Estimates of the proportion of the variance due to the common family environment ( c 2 ) were not significantly greater than zero for any trait. Heritabilities of TOR, TLE, LS and PS were estimated to be 0.21±0.11, 0.26±0.10, 0.29±0.12 and 0.23±0.10, respectively. Genetic correlations were estimated to be −0.14±0.35 between TOR and PS, 0.36±0.31 between TOR and LS and 0.87±0.08 between PS and LS. Genetic variation in prenatal survival appears to be the major cause of genetic variation in litter size in the rabbit.

Identification of Single-Nucleotide Polymorphism in the Progesterone Receptor Gene and Its Association With Reproductive Traits in Rabbits

A total of 598 F2 does from a cross between the high and low lines selected divergently for uterine capacity during 10 generations were used in a candidate gene analysis. The presence of major genes affecting the number of implanted embryos and uterine capacity has been suggested in lines divergently selected for uterine capacity. Uterine capacity is a main component of litter size. The progesterone receptor gene was tested as a candidate gene to determine whether polymorphisms explain differences in litter size and its components. Fragments of the promoter region and exons 1–8 were amplified and sequenced. One SNP was found in the promoter region, 2464G>A, three SNPs in the 5′-UTR exon 1, and a silence SNP in exon 7. The first four SNPs were segregated in two haplotypes. The allele G found in the promoter region was found in 75% of the high-line parental animals and in 29% of the low-line parental animals. The GG genotype had 0.5 kits and 0.5 implanted embryos more than the AA genotype. At 48 hr of gestation, the difference in early embryo survival and embryonic stage of development was small. However, at 72 hr of gestation, the GG genotype had 0.36 embryos more than the AA genotype and also had a more advanced embryonic stage of development, showing a lower percentage of compacted morulae and a higher percentage of blastocysts. The difference in litter size between the GG and GA genotypes was similar to the difference found between homozygote genotypes; however, differences in implanted embryos, early embryo survival, and embryo development were not detected between the GG and GA genotypes.

Selection for ovulation rate in rabbits: Genetic parameters, direct response, and correlated response on litter size

The aim of this work was to evaluate the response to 10 generations of selection for ovulation rate. Selection was based on the phenotypic value of ovulation rate, estimated at d 12 of the second gestation by laparoscopy. Selection pressure was approximately 30%. Line size was approximately 20 males and 80 females per generation. Traits recorded were ovulation rate at the second gestation, estimated by laparoscopy as the number of corpora lutea in both ovaries; ovulation rate at the last gestation, estimated postmortem; ovulation rate, analyzed as a single trait including ovulation rate at the second gestation and ovulation rate at the last gestation; right and left ovulation rates; ovulatory difference, estimated as the difference between the right and left ovulation rates; litter size, estimated as the total number of kits born and the number of kits born alive, both recorded at each parity. Totals of 1,477 and 3,031 records from 900 females were used to analyze ovulation rate and litter size, respectively, whereas 1,471 records were used to analyze ovulatory difference, right ovulation rate, and left ovulation rate. Data were analyzed using Bayesian methodology. Heritabilities of ovulation rate, litter size, number of kits born alive, right ovulation rate, left ovulation rate, and ovulatory difference were 0.16, 0.09, 0.08, 0.09, 0.04 and 0.03, respectively. Phenotypic correlations of ovulation rate with litter size, number of kits born alive, and ovulatory difference were 0.09, 0.01, and 0.14, respectively. Genetic correlations of ovulation rate with litter size and with number of kits born alive were estimated with low accuracy, and there was not much evidence for the sign of the correlation. The genetic correlation between ovulation rate and ovulatory difference was positive (P = 0.91). In 10 generations of selection, ovulation rate increased in 1.32 oocytes, with most of the response taking place in the right ovary (1.06 oocytes), but there was no correlated response on litter size (-0.15 kits). In summary, the direct response to selection for ovulation rate was relevant, but it did not modify litter size because of an increase in prenatal mortality.

Candidate gene analysis for reproductive traits in two lines of rabbits divergently selected for uterine capacity.

The objective of this work was to analyze 3 functional candidate genes for reproduction in 2 lines of rabbits divergently selected by uterine capacity. Both lines were selected for 10 generations. The selection was then relaxed until the 17th generation, when it was compounded by 61 and 63 does of the High and Low lines, respectively. We sequenced the SCGB1A1 gene, which encodes the main protein secreted by the rabbit in the uterus and seems to play an important role in implantation. We found 6 SNP in the promoter region cosegregating in 2 haplotypes in both lines with similar frequency. We also analyzed IGF1 mRNA because of its effects on embryo development, but we did not find any polymorphism between individuals of the 2 lines. The third gene analyzed was the TIMP1, which encodes a protein involved in many biological processes related to reproduction. We determined the sequence of its promoter region and found 1 SNP (g.1423A>G) segregating with different frequencies in both lines (0.60 for allele A in the High line and 0.82 for allele G in the Low line). The association study performed in an F(2) population (n = 598) generated by the cross of the 2 lines of rabbits revealed that the AA genotype had 0.88 embryos more than the GG genotype at 72 h of gestation. The difference increased to 2.23 embryos at implantation, but no difference was found between genotypes at birth. These results suggest that TIMP1 could be a candidate gene for embryo implantation and embryo survival.

Expression of progesterone receptor related to the polymorphism in the PGR gene in the rabbit reproductive tract

The association of the 2464G > A SNP found in the promoter region of the rabbit progesterone receptor gene with progesterone receptor (PR) expression was evaluated by Western blot analysis. This SNP was associated with 2 lines divergently selected for uterine capacity, the high line selected to increase uterine capacity and the low line selected to decrease uterine capacity. Two progesterone isoforms were obtained using a commercial monoclonal antibody: the PR-B isoform described previously in rabbits, and the PR-A isoform, not described previously in rabbits. The GG genotype, the genotype more frequent in the high line, showed less PR-B and PR-A expression than the AA genotype in the oviduct (GG/AA(PR-B) = 0.81 and GG/AA(PR-A) = 0.73) and uterus (around 0.70 in both isoforms). The GA genotype showed similar PR-A expression in both tissues and also similar PR-B expression in the oviduct to the GG genotype. Conversely, the GG genotype showed less PR-B expression than the GA genotype in the uterus (GG/GA(PR-B) = 0.79). Similar expression of both PR isoforms was found in the uterus at d 2 and 3 of gestation; meanwhile, an increase of both isoforms was observed in the oviduct. Similar PR-A expression was observed in the ampulla and isthmus; meanwhile, the PR-B expression in the isthmus was double that in the ampulla.

Genetic selection for ovulation rate and litter size in rabbits: Estimation of genetic parameters and direct and correlated responses1

The aim of this work was to estimate direct and correlated responses in survival rates in an experiment of selection for ovulation rate (OR) and litter size (LS) in a line of rabbits (OR_LS). From generation 0 to 6 (first selection period), females were selected only for second gestation OR estimated by laparoscopy. From generation 7 to 13 (second selection period), a 2-stage selection for OR and LS was performed. In stage 1, females having the greatest OR at second gestation were selected. In stage 2, selection was for the greatest average LS of the first 2 parities of the females selected in stage 1. Total selection pressure in females was about 30%. The line had approximately 17 males and 75 females per generation. Traits recorded were OR estimated as the number of corpora lutea in both ovaries, number of implanted embryos (IE) estimated as the number of implantation sites, LS estimated as total number of rabbits born recorded at each parity, embryo survival (ES) estimated as IE/OR, fetal survival (FS) estimated as LS/IE, and prenatal survival (PS) estimated as LS/OR. Data were analyzed using Bayesian methodology. The estimated heritabilities of LS, OR, IE, ES, FS, and PS were 0.07, 0.21, 0.10, 0.07, 0.12, and 0.16, respectively. Direct and correlated responses from this study were estimated in each period of selection as the difference between the average genetic values of last and first generation. In the first selection period, OR increased 1.36 ova, but no correlated response was observed in LS due to a decrease on FS. Correlated responses for IE, ES, FS, and PS in the first selection period were 1.11, 0.00, -0.04, and -0.01, respectively. After 7 generations of 2-stage selection for OR and LS, OR increased 1.0 ova and response in LS was 0.9 kits. Correlated responses for IE, ES, FS, and PS in the second selection period were 1.14, 0.02, 0.02, and 0.07, respectively. Two-stage selection for OR and LS can be a promising procedure to improve LS in rabbits.

Selection for ovulation rate in rabbits: Direct and correlated responses estimated with a cryopreserved control population1

The aim of this work was to evaluate the response in 10 generations of selection for ovulation rate in rabbits using a cryopreserved control population. Selection was based on the phenotypic value of ovulation rate estimated at d 12 of second gestation by laparoscopy. To produce the control population, embryos from 50 donor females and 18 males, belonging to the base generation of the line selected for ovulation rate, were recovered. A total of 467 embryos (72-h embryos) were vitrified and stored in liquid N(2) for 10 generations. The size of both populations was approximately 10 males and 50 females. The number of records used to analyze the different traits ranged from 99 to 340. Data were analyzed using Bayesian methodology. A difference between the selected and the control populations of 2.1 ova (highest posterior density interval (HPD(95%))[1.3, 2.9]) was observed in ovulation rate (OR), but it was not accompanied by a correlated response in litter size (LS; -0.3; HPD(95%) [-1.1, 0.5]). The number of implanted embryos (IE) increased with selection in 1.0 embryo (HPD(95%) [-0.6, 2.0]), but this increase was not relevant. Prenatal survival, embryonic survival, and fetal survival (FS) were calculated as LS/OR, IE/OR, and LS/IE, respectively. Prenatal survival was reduced with selection (-0.12; HPD(95%) [-0.20, -0.04]), basically because of a decrease in FS (-0.12; HPD(95%) [-0.19, -0.06]). Embryonic survival could have slightly decreased (-0.05; HPD(95%) [-0.12, 0.02]). In summary, comparison with a control population showed that ovulation rate in rabbits increased with selection without any correlated response in litter size, basically because of a decrease in fetal survival.

Selection for ovulation rate in rabbits: Genetic parameters and correlated responses on survival rates1

The aim of this work was to evaluate the correlated responses on survival rates after 10 generations of selection for ovulation rate (OR). Selection was based on the phenotypic value of ovulation rate estimated at d 12 of second gestation by laparoscopy. Traits recorded were litter size (LS), estimated as total number of rabbits born per litter in up to 5 parities; OR, estimated as the number of corpora lutea in both ovaries; the number of implanted embryos (IE), estimated as the number of implantation sites; the number of right and left IE (RIE and LIE); ovulatory difference (OD), defined as the difference between the right and the left OR, expressed as an absolute value; implantatory difference (ID), defined as the difference between RIE and LIE, expressed as an absolute value; embryonic survival (ES), calculated as IE/OR; fetal survival (FS), calculated as LS/IE; prenatal survival (PS), calculated as LS/OR. A total of 1,081 records were used to analyze ES, and 770 were used to analyze FS and PS. The number of records used to analyze the other traits ranged from 1,079 for ID to 3,031 for LS. Data were analyzed using Bayesian methodology. Genetic parameters of OR, OD, and LS were estimated in a previous paper. Estimated heritabilities of IE, ID, ES, FS, and PS were 0.11, 0.03, 0.09, 0.24, and 0.14, respectively. Estimated repeatabilities of IE, ID, and ES were 0.22, 0.12, and 0.20. Estimated phenotypic correlations of OR with ES, FS, and PS were -0.07, -0.26, and -0.28, respectively. Their estimated genetic correlations with FS and PS were negative (probability of being negative 1.00 and 0.98, respectively). Nothing can be said about the sign of the genetic correlation between OR and ES. Ovulation rate was phenotypically uncorrelated with ID. Their estimated genetic correlation was positive (probability of being positive 0.91). The genetic correlation of ID with PS and LS was not accurately estimated. Phenotypic and genetic correlations between LS and survival rates were positive (probability of being positive 1.00). In 10 generations of selection, FS decreased around 1% per generation. No correlated response in ES was observed. In summary, the decrease in FS in rabbits selected for OR seemed to be responsible for the lack of correlated response observed in LS.