Genet. Sel. Evol. 40 (2008) 241-264
DOI: 10.1051/gse:2008001

Exploring the assumptions underlying genetic variation in host nematode resistance (Open Access publication)

Andrea Beate Doeschl-Wilson¹, Dimitrios Vagenas², Ilias Kyriazakis^{2, 3} and Stephen Christopher Bishop<sup>4

1</sup>  Sustainable Livestock Systems, Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG, UK
²  Animal Health and Nutrition Department, Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG, UK
³  Faculty of Veterinary Medicine, University of Thessaly, Trikalon 224, 43100, Karditsa, Greece
⁴  Roslin Institute and Royal (Dick) School of Veterinary Studies Roslin BioCentre, Midlothian EH25 9PS, UK

(Received 18 July 2007; accepted 21 December 2007; published online 10 April 2008)

Abstract - The wide range of genetic parameter estimates for production traits and nematode resistance in sheep obtained from field studies gives rise to much speculation. Using a mathematical model describing host - parasite interactions in a genetically heterogeneous lamb population, we investigated the consequence of: (i) genetic relationships between underlying growth and immunological traits on estimated genetic parameters for performance and nematode resistance, and (ii) alterations in resource allocation on these parameter estimates. Altering genetic correlations between underlying growth and immunological traits had large impacts on estimated genetic parameters for production and resistance traits. Extreme parameter values observed from field studies could only be reproduced by assuming genetic relationships between the underlying input traits. Altering preferences in the resource allocation had less pronounced effects on the genetic parameters for the same traits. Effects were stronger when allocation shifted towards growth, in which case worm burden and faecal egg counts increased and genetic correlations between these resistance traits and body weight became stronger. Our study has implications for the biological interpretation of field data, and for the prediction of selection response from breeding for nematode resistance. It demonstrates the profound impact that moderate levels of pleiotropy and linkage may have on observed genetic parameters, and hence on outcomes of selection for nematode resistance.

Key words: gastro-intestinal parasites / genetic parameters / modelling / disease resistance / sheep

Correspondence and reprints: Andrea.Wilson@sac.ac.uk

© INRA, EDP Sciences 2008