-I 425 Epidemiology and genetic variability of two species of nematodes (Heligmosomoides polygyrus and Syphacia stroma) of Apodemus spp. C. D. M. MULLER-GRAF 1 ,2*, P. DURAND 2 ,3, c. FELIU\ J.-P. HUGOT 5 , C. J. O'CALLAGHANl, F. RENAUD 3 , F. SANTALLA 4 and S. MORAND 2 1 Department of Biological Sciences, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK 2 Centre de Biologie et d'Ecologie Tropicale et Mediterraneenne, Laboratoire de Biologie Animale, CNRS UMR 5555, Universite de Perpignan, Avenue de Villeneuve, 66860 Perpignan Cedex, France 3 Laboratoire de Parasitologie Comparee, CNRS UMR 555 Universite Montpellier 2, Place E. Bataillon, 34095 Montpellier Cedex OS, France 4 Laboratori de Parasitologia, Facultat de Farmacia, Universitat de Barcelona, Av. Diagonal, sin. 08028 Barcelona, Spain 5 Museum National d'Histoire Naturelle, Mammiferes et Oiseaux, (FR CNRS 1541) 55, rue Buffon, 75231 Paris Cedex OS, France SUMMARY The epidemiology and genetic variability of 2 parasitic nematodes Heligmosomoides polygyrus and Syphacia stroma of Apodemus spp. were investigated. Both are parasites of the same host, exhibit a direct life-cycle and are dioecious. However, H. polygyrus has a diploid and S. stroma a haplodiploid mode of reproduction. Haplodiploidy may lead to a more female biased sex ratio and reduced genetic variability. Levels of genetic diversity were analysed using both isoenzyme electrophoresis and RAPDs (random amplified polymorphic DNA). Both parasites showed a female biased sex ratio with a stronger bias for the haplodiploid parasite. Results showed significantly fewer genetic polymorphisms as measured by RAPDs for the haplodiploid parasite S. stroma in comparison with H. polygyrus. Despite the observed female biased sex ratio this could not be explained by a significant amount of inbreeding. Heterozygote deficiency for individual allozyme loci - which could indicate inbreeding - was not found in either parasite species. Other features of the particular life- history of these species are likely to have an impact on the sex ratio and genetic variability too. Key words: trichostrongyloids, oxyuroid, haplodiploidy, genetic diversity, sex ratio. INTRODUCTION Over recent years studies have begun to investigate parasite life-histories in a comparative way Skorping, Read & Keymer, 1991 ; Poulin, 1995; Morand, 1996; Morand & Sorci, 1998). Parasite life-histories are likely to be reflected in the epidemiological and genetic structure of parasite species. However, the picture is not clear and empirical research is needed to study the interactions. Factors like demography, life-cycles, mechanisms of dispersal, host specificity, mating patterns and effective population size may have an impact (Nadler, 1995). We were interested to study whether a life-history trait, such as the mode of reproduction, influences the epidemiology, for instance, the sex ratio of the parasite, and levels of genetic diversity. It is difficult and challenging to assess the impact of life-history patterns. In order to control for the confounding effect caused by host parameters, it seems appropriate to use a com- parative approach in which 2 different parasite * Corresponding author: Laboratoire d'Ecologie, CNR5- UMR 7625, Pierre et Marie Curie Universite, Case 237,7 quai 5t Bernard, 75252 Paris Cedex OS, France. Tel: 0033 (0) 144 273 809. Fax: 0033 (0) 144 273 516. E-mail: cmuller@snv.jussieu.fr species of the same host are investigated simul- taneously. We have compared 2 nematodes of the same host with a similar life-cycle, but with different modes of reproduction. We studied Heligmosomoides polygyrus (Dujardin, 1845) (== Nematospiroides dubius) , a trichostrongyloid, a nematode with a diploid mode of reproduction, and Syphacia stroma (von Linstow, 1884), an oxyuroid parasite with a haplodiploid mode of reproduction. Both parasites exhibit a direct life- cycle, are dioecious and parasitize the same host (Apodemus spp.). The characteristics of a haplo- diploid mode of reproduction (males are haploid, from unfertilized eggs; females are diploid, from fertilized eggs) may have an impact on the epi- demiology (i.e. sex ratio) and the levels of genetic diversity of this parasite species as compared to a diploid species. A persistent component of haplo- diploidy is considered to be an elevated level of inbreeding (Adamson, 1989). Haplodiploids are often colonizing species with small semi-isolated subpopulations. New habitats are colonized by females which can then produce males partheno- genetically. Since not many males are necessary and local mate competition may lead to unnecessary cost, only a small number of males is needed (Hamilton, Parasitology (1999), 118, 425-432. Printed in the United Kingdom © 1999 Cambridge University Press