1. Leibniz Institute for Farm Animal Biology (R.I.), 18196 Dummerstorf, Germany;
2. School of Molecular and Biomedical Science (R.I.), University of Adelaide, SA5005, Australia;
3. Department of Animal Sciences (K.T.), Wageningen University, 6709 WD Wageningen, The Netherlands;
4. Department of Biology (G.E.H.), Morgan State University, Baltimore, Maryland 21251