Relations between ovarian & embryonic traits in pigs : effects of genetic selection for litter traits at birth
This thesis focuses on how the number (ovulation rate) and the size of corpora lutea (CL) interact with embryonic survival and development at 35 days of pregnancy. The aim is to unravel the underlying mechanisms that lead to lower average birth weight and lower birth weight uniformity of piglets born in large litters. Lower piglet birth weight and lower within litter piglet birth weight uniformity in large litters increases piglet mortality during lactation, a major economic and welfare concern for modern pig production. The main hypothesis of this thesis is that an increase in ovulation rate (OR) compromises not only embryonic survival but also embryonic and placental development and thereby litter characteristics at birth.First, a literature review was done on how OR might influence follicular and subsequent embryonic survival and development [Chapter 2]. Ovulation rate (OR) it is the result of factors influencing the growth and development of follicles during early antral follicle development. The pattern of growth of the ovulatory follicles seems different between animals with high and low ovulation rate, as the recruitment of ovulatory follicles is extended in sows with higher OR. This longer follicular phase may be used to recruit additional follicles for ovulation. Moreover, there appears to be considerable variation within and between animals in the size of the selected pre-ovulatory large follicles (7-11 mm), but also in morphology and steroidogenic activity. This may indicate that not all follicles are at the same stage of development at the time of selection; a variation that can be higher in animals with a higher OR. Thus, animals with a higher OR might have a higher variation in stage of follicular development at ovulation, which might indicate a higher variation in follicular quality. A higher variation in follicular quality may have also consequences for oocyte quality, but also on the corpora lutea development after ovulation and on embryonic development.The physiological consequences of the increase in ovulation rate (OR) on embryonic survival and development in multiparous sows and in gilts at 35 days of pregnancy were investigated [Chapters 3 and 4, respectively].An increase in OR resulted in a curvilinear increase in number of vital embryos at 35 days of pregnancy. This means that at a high OR (≥ 22 for sows and ≥ 26 for gilts), more ovulations are needed to achieve one more vital embryo at 35 days of pregnancy (ideally each ovulation would be represented by one vital embryo in the uterus, e.g. 20 corpora lutea equals 20 embryos). This limited increase in number of vital embryos with the increase in OR was related with the increase in early (before uterine implantation at ~ 13 days of pregnancy) and in late embryonic mortality (from 13 up to 35 days of pregnancy) with the increase in OR. In sows, each extra ovulation from 17 up to 38 led to an increase of 0.49 in early and of 0.24 in late embryonic mortality. In gilts, a minimum incidence of 3.3 in early and of 1.2 in late embryonic mortality was observed at 15 ovulations and 18 ovulations, respectively, increasing thereafter with the increase in OR (up to 34). Thus, the number of ovulations that are represented by a vital embryo at 35 days of pregnancy was smaller in sows and gilts with high OR, because of higher embryonic mortality.More importantly, also the development of the vital embryos is compromised with the increase in OR. In sows with high number of ovulations, vital embryos occupied a smaller area in the uterus (implantation site)[-0.35 cm/ovulation in implantation length of the vital embryos] and had a smaller placenta at 35 days of pregnancy [41 cm for OR ≥ 29 vs 46 cm for OR 17 to 21]. In gilts with a high number of ovulations there was more variation in the weight of vital embryos at 35 days of pregnancy [~ 0.44 g with OR ≥ 23 vs 0.35 g with OR 8 to 18]. Also, the average weight of the CL at 35 days of pregnancy was lower in gilts with higher OR [minimum of 0.42 g at28 ovulations]. Moreover, gilts with lower average CL weight had vital embryos with lower weight at 35 days of pregnancy. Thus, an increase in OR compromised the development of the corpora lutea (CL) and also the survival and development of the vital embryos at 35 days of pregnancy.The hypothesis behind these results is that in sows and gilts an increase in OR is associated with changes in follicular development, which causes ovulation to occur when part of the follicles are smaller and not completely developed and mature. These smaller follicles will liberate oocytes (eggs) of lower quality and will develop into smaller corpora lutea during pregnancy. The oocytes of lower quality will develop into embryos of lower quality. These lower quality embryos might have one of two fates: first, it is possible that the embryos die around the time of elongation (~ d12) of their more advanced littermates, which change the uterine environment making it hostile for the less developed (lower quality) embryo (early mortality); second, it is also possible that the less developed embryos reach elongation in time, but elongate less, therefore acquiring a smaller uterine implantation site and developing a smaller placenta. These embryos thus might, due to insufficient nutrient uptake, die after implantation (late embryo mortality and foetal mortality) or be born with lower birth weight.As the aim of this thesis was to unravel underlying mechanisms leading to litter characteristics at birth, the relationships between OR, corpora lutea (CL) size in pregnant sows and their litter characteristics at birth were investigated [Chapter 5].First, the possibility to assess OR and average corpora lutea (CL) diameter using transrectal ultrasonography (TUS) in multiparous sows (n = 45) in early pregnancy (23.4 days) was investigated. Results showed that it is not possible to accurately count the number of CL with transrectal ultrasonography to assess OR in pregnant sows, as the relationship between OR assessed with TUS and after slaughter and dissection of the ovaries was of only 0.24 CL per CL. However, assessment of average CL diameter is reliable, as the relationship between the diameter measured with TUS and after slaughter and dissection of the ovaries was of 1.0 mm TUS / mm at dissection.Consequently, the relationship between average CL diameter and litter characteristics at birth was investigated. Results showed that sows with a lower average CL diameter at ~24 days of pregnancy had lower average piglet birth weight (BW) and a lower within litter piglet birth weight standard deviation (BWSD), independent of the litter size they were born in. Thus, transrectal ultrasonography cannot be used to assess OR, but it can be used to assess average CL diameter in pregnant sows, which was related with piglet average piglet birth weight and with the within litter piglet birth weight standard deviation.The consequences of genetic selection for total number of piglets born (TNB), average piglet birth weight (BW), and within litter piglet birth weight standard deviation (BWSD) were investigated for ovarian (OR and CL weight), uterine (length, empty uterine space around the vital embryos) and embryonic survival (number of vital embryos, early and late embryonic mortality) and development (implantation length and area in the uterus, average and standard deviation in weight) traits in gilts at 35 days of pregnancy [Chapter 6].The results revealed the importance of OR and average CL weight for litter characteristics at birth. Genetic selection for higher TNB increased OR and the number of vital embryos at 35 days of pregnancy, but did not influenced early and late embryonic mortality. Further analyses showed that in gilts with a higher genetic merit for TNB an increase in OR resulted in a linear increase in the number of vital embryos. This means that the number of ovulations needed to increase the number of vital embryos was the same in animals with low or high OR. Unfortunately, the increase in OR was related with vital embryos acquiring a smaller uterine implantation site. Thus, the increase in OR in gilts with higher genetic merit for TNB compromised vital embryonic development at 35 days of pregnancy.Moreover, genetic selection for TNB decreased average CL weight at 35 days of pregnancy, which might be a consequence of higher OR. Phenotypically, gilts with lower CL weight had vital embryos with lower weight at35 days of pregnancy and sows with lower average CL diameter farrowed piglets with lower birth weight. Thus, it seems that the lower average piglet BW in gilts with high genetic merit for TNB might be related with lower average CL weight at 35 days of pregnancy.Genetic selection for higher average piglet BW did not influence vital embryonic development traits or uterine traits that can influence embryonic development (uterine length, empty uterine space around the vital embryos, and uterine length available per embryo) at 35 days of pregnancy. However, genetic selection for higher average piglet BW increased the average CL weight. Thus, at 35 days of pregnancy, average CL weight was the only trait influenced by genetic selection for higher BW. Interestingly, gilts with higher genetic merit for BWSD (i.e. lower genetic merit for within litter piglet birth weight uniformity) also had a higher average CL weight at 35 days of pregnancy. This might be related with the fact that these two traits are genetically correlated and might share gene(s) that are involved in CL weight.The genetic variation of OR, average CL weight, and embryonic survival and development traits in gilts at 35 days of pregnancy was also investigated [Chapter 6]. Results showed that OR, average CL weight, embryonic and uterine traits at 35 days of pregnancy were all highly heritable and can be improved by genetic selection. However, most of these traits were also genetically correlated. This means that the phenotypic relationships between OR, average CL weight and embryonic development traits described previously are partly genetic.Ovulation rate and average CL weight are important traits for embryonic development and piglet birth weight. Moreover, these traits can be measured in animals in a more practical approach with the use of transrectal ultrasonography. So, transrectal ultrasonography can be used as an breeding tool to phenotype animals for OR and average CL weight and use this information in breeding programs. For example, if the breeding goal is to improve piglet survival after birth by increasing the average piglet BW, average CL weight could be also included in the genetic selection index (together with average piglet birth weight), but also OR to prevent losses in TNB. Thus, the inclusion of average CL weight and OR in the selection index, which includes also TNB, BW and BWSD, will likely balance selection for litter traits probably improving piglet birth weight without decreasing the total number of piglets born.Thus, this thesis shows that, phenotypically, an increase in OR increases the number of embryos at 35 days of pregnancy, but up to a certain limit. Females with high OR have compromised placental and embryonic development at 35 days of pregnancy, which is likely to increase foetal mortality and to decrease piglet birth weight. Lower piglet birth weight in females with high OR might also be related with the decrease in average CL size at 35 days of pregnancy, and it might be a result from lower follicle development at ovulation. The fact that genetic improvement of piglet birth weight is associated with an increase in CL weight at 35 days of pregnancy might indicate the importance of follicular/oocyte/embryonic quality for piglet BW. Therefore, future genetic selection programs should take into consideration that genetic improvement in piglet birth weight and in within litter birth weight standard deviation (increased uniformity) in large litters is dependent not only of improvements in uterine capacity, but also in follicular quality at ovulation and corpora lutea size during pregnancy.
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| Format: | Doctoral thesis biblioteca |
| Language: | English |
| Published: |
Wageningen University
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| Subjects: | Life Science, |
| Online Access: | https://research.wur.nl/en/publications/relations-between-ovarian-amp-embryonic-traits-in-pigs-effects-of |
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