1. Genetics of Microorganisms (P.C., R.F.M., C.R.) and Algology, Mycology and Experimental Systematics (D.B.), Institute of Plant Biology B22, University of Liege, B–4000 Liege, Belgium; and Departamento de Genética Molecular, Instituto de Fisiología Celular (D.G.-H.) and Instituto de Biología (A.R.-P.), Universidad Nacional Autónoma de Mexico, 04510 Mexico D.F., Mexico