1. Department of Zoology, University of Geneva, 30 quai Ernest Ansermet, CH-1211 Geneva, Switzerland
2. Present address: LBMC-ENS Lyon, 46, allée d’ Italie, F-69364 Lyon Cedex 07, France
3. Present address: Swiss Institute of Bioinformatics (SIB-ISB), 1, rue Michel Servet, CH-1211 Geneva, Switzerland