Changes in food resource allocation patterns with selection for litter size - a mouse model
Chapter 1 – Introduction
Quantitative genetics developed early in the last century with the rediscovery of Mendelian genetics (Lynch and Walsh, 1998). The major goal of applied quantitative genetics is the identification of ‘elite’ individuals which serve as parents for the next generation. Because parents pass on their genes and not their genotypes, an animal is assigned a ‘breeding value’, which refers to its genotypic value (Falconer and Mackay, 1996). The current methodologies that are used to estimate these breeding values assume additivity and linearity of relationships between traits. However, the phenotypic relationship between traits is often found to be curvilinear. Sölkner and James (1994) suggest that this may result from various physiological limitations and feedback mechanisms, such as the situation where traits are competing for a limited amount of resources.
Resources come from food intake or body stores. Weiner (1992) proposed ’the barrel model’ of an organism’s resource allocation pattern (Figure 1) [not displayed online]. Inputs constraints (food intake, digestion and absorption) are engaged in series whereas outputs (maintenance, growth, production) are parallel. If the sum of output rates does not match the input, the balance is buffered by energy storage of the body. In the long run, however, energy expenditure must balance energy intake. According to Beilharz et al. (1993), in a limited resource situation, when too many resources are allocated towards one trait, less resources will be left for other resource demanding processes to perform optimally. This must result in a negative relationship between the traits. With artificial selection, when ‘genetically forced’ to produce highly, resources may be reallocated from other processes, leaving the animal lacking in ability to respond to other demands (Dunnington, 1990).
As a consequence, genetic selection for increased levels of production is often compromised by unfavourable relationships between production and fitness traits and by limits to selection. This unfavourable relationship is aggravated when selection is on high production combined with high food efficiency, i.e., high production efficiency. An example of such an unfavourable relationship is a prolonged period of anoestrus after weaning in genetically lean lines of pigs and an increased pre-weaning mortality rate of the piglets. These problems result from excessive mobilisation of body reserves of the dam and problems of development and adaptation of their offspring (Ten Napel, 1996).
To understand why selection for production traits is compromised by undesirable side effects and selection limits, research needs to be directed to the biological aspects of selection, or in other words, to the physiological processes on which a gene and thus genetic selection acts. Without this knowledge genetic improvement through artificial selection is essentially a black box technique. Knowledge of biological backgrounds will offer the opportunity to understand, anticipate and eventually avoid negative side effects of selection.
The present study investigates changes in food resource allocation patterns with long-term selection for litter size using a mouse model. It is hypothesised that animals selected for high litter size allocate a higher amount of resources to the trait selected for, leaving less resources to respond to other demands. The mice used in this study originate from two lines of the Norwegian mouse selection experiment (Vangen, 1993): a line selected for more than 90 generations for high litter size at birth (S-line) and an unselected control line (C-line). The average number of live-born pups is about 20 in the S-line and 10 in the C-line.
The subgoals of this study are:
* To investigate changes in resource allocation patterns in non-reproductive males and females. * To investigate changes in body composition in relation to resource allocation patterns in non-reproductive males and females. * To investigate changes in behavioural strategies in relation to resource allocation patterns in non-reproductive females. * To investigate changes in resource allocation patterns in lactating females. * To investigate offspring development in relation to resource allocation patterns.
The outline of this study is as follows:
Chapter 2 presents a general overview of undesirable side effects of selection for high production efficiency in broilers, pigs and dairy cattle, with respect to metabolic, reproduction and health traits. These side effects are discussed in relation to the resource allocation theory proposed by Beilharz et al. (1993).
Chapters 3 to 6 focus on non-reproductive animals. In Chapter 3, changes in food resource allocation patterns with selection for litter size are investigated in non-reproductive male and female mice of the C- and the S-line. Growth and food intake curves are fitted to individual data. Differences in these curves between species or lines can mostly be explained by differences in mature size. Therefore, parameters are scaled by individual estimates of mature body weight. Differences that remain after scaling are a consequence of what have been called ‘specific genetic factors’ (Taylor, 1985). The presence of such specific genetic factors indicates if selection for increased litter size has disproportionally changed the resource allocation pattern.
The existence of specific genetic factors for body weight and food intake is indicated by variation in efficiency parameters such as growth efficiency and maintenance requirements. In Chapter 4, estimation of residual food intake (RFI) (Luiting and Urff, 1991) and Parks’ (1982) estimates of growth efficiency and maintenance requirements are used to quantify these factors in the same individuals as which are used in Chapter 3.
It can be suggested that the foetus acts as a parasite that would deplete the dam from her energy stores (Hammond, 1944). Usually, during the first half of pregnancy, body protein and lipid content is increased for support of fetal growth and lactation (Luz and Griggio, 1996). Since S-line females have to support a litter size that has been highly increased by artificial selection, in Chapter 5 it is investigated whether body composition at maturity has been affected as a correlated effect of selection for high litter size, to sufficiently support the fetuses during pregnancy and lactation. Furthermore, part of the observed differences between individuals in RFI may be attributable to differing proportions of body protein and lipid (Luiting, 1990). Therefore, in Chapter 5, it is investigated if the differences between the C- and S-lines in body composition of non-reproducing adult males and females are consistent with expectations from the observed differences between the lines in RFI (Chapter 4).
Several studies have indicated that a higher RFI is related to a higher level of activity. Differences in activity may suggest underlying differences in coping behaviour in response to unexpected stresses. Chapter 6 investigates whether coping strategies in non-reproductive females have been affected as a correlated effect of selection for increased litter size. For this reason, females of the C- and the S-line are subjected to three non-social tests and a social confrontation test.
Chapter 7 focuses on reproductive females. Changes in food resource allocation patterns with selection for litter size are investigated in lactating female mice of the C- and the S-line and related to offspring development. To manipulate experimentally the energy burden of lactation, in each line, half of the females support litters that are standardised at birth to eight pups per litter (when larger than eight pups), and half of the females support the natural litter size. Food resource allocation patterns are quantified by RFI. The consequences of food resource allocation patterns for offspring development was indicated by pre-weaning mortality rates and degree of maturity of the pups from birth to weaning.
Chapter 8 is a general discussion of the results of Chapters 3 to 5 and Chapter 7. The introduction to this chapter describes how selection for high production efficiency may compromise fitness. Thereafter, the main results of the four chapters are briefly summarised and the results are discussed in relation to pig production. During the course of this study, body composition was measured in, and behavioural tests were carried out on lactating females of the C- and the S-line, but the results of these trials are not included as single chapters in this thesis. However, the main results of the body composition measurements on lactating females are included in Chapter 8.
Chapter 9 concludes this study with recommendations for further research.
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