Does selection for production traits affect the ability to cope with pathogens?
Coltherd, Jennifer Carolyn
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Phenotypic selection for production traits causes changes in the underlying genetics of the animal. As such, intensive selection on one trait may have consequences on other traits. Indeed alterations to traits seemingly unrelated to the desirable trait under selection have been documented, although the strength and direction have been inconsistent in livestock species. This leads to the question of how selection for growth may alter the ability to cope with pathogens, and whether because of its associated increase in nutrient requirements, improved nutrition could ameliorate any loss of resilience or resistance arising from this selection (Chapter One). This thesis uses a unique mouse line, divergently selected for low (Roslin low: ROL) and high (Roslin high: ROH) body weight and a chronic gastrointestinal nematode infection, Heligmosomoides bakeri, to address this question. Chapter 2 investigates the effects of dietary crude protein contents ranging from scarce to more than adequate on resilience and resistance traits of uninfected and primary infected ROH and ROL mice, using a fixed level of 250L3 as infection pressure. The data suggest that ROH mice had a greater penalty of infection on resilience, which was overcome by increased protein nutrition, and showed higher worm burdens and egg counts. Chapter 3 goes on to investigate the existence of a minimum parasite dose for the observation of loss of resilience and resistance during infection. Over a range of primary infection pressure from 0 to 250L3, it was found that an incoming parasite dose of 150L3 and over was required to reduce weight gain in ROH mice fed a low protein diet and that this loss in weight gain was ameliorated by increased protein nutrition. Resilience of ROL mice was not affected. It was also observed that worm burdens and egg counts of all mice reached a plateau at 150L3. Samples were taken for cytokine and chemokine analysis (Chapter 4) and data showed that the parasite infection did not polarise a Th2 type immune response as expected, whilst infection in ROL mice and ROH mice on low protein diets resulted in inflammatory immune response. Chapter 5 compares primary and secondary infection in ROH and ROL mice, finding that not only do ROH reduce weight gain in response to a primary and secondary infection during protein scarcity, they also show the greatest reduction in worm burdens and egg counts due to previous exposure. The data from this thesis, discussed in Chapter 6, suggests that intensive selection for high body weight can cause a loss of resilience that is sensitive to protein nutrition but that this may be due to a prioritisation of immunity over growth. Intensive selection for low body weight can cause a greater degree of resilience but cause a reduction in resistance to a pathogen challenge. Whilst this thesis therefore provides evidence that intensive selection, in either direction, can alter an animal‟s ability to cope with a pathogen challenge, future work using a non-selected control line is required to advance this hypothesis.