Belg. J. Zool., 137 (1) : 41-45 January 2007 Fitness-heterozygosity associations differ between male and female winter moths Operophtera brumata L. Stefan Van Dongen 1 , Thierry Backeljau 2 , Erik Matthysen 1 & Andre A. Dhondt 3 1 Department of Biology, University of Antwerp 2 Royal Belgian Institute of Natural Sciences, Brussels, Belgium 3 Present address : Laboratory of Ornithology, Cornell University, 159 Sapsuckerwoods Road, Ithaca, NY 14850, U.S.A. Corresponding author : Stefan Van Dongen, group of Evolutionary Biology, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium. Email : stefan.vandongen@ua.ac.be. ABSTRACT. The association between heterozygosity and fitness is positive but weak on average and varies between studies. Inbreeding has been invoked as the driving force between the positive heterozygosity-fitness associations, yet in spatio-temporally stable environments a negative correlation is expected. Furthermore, different patterns can arise because of the effects of natural selection on different loci and variation can be expected among groups of individuals that experience different levels of stress. In this paper we report on fitness-heterozygosity associations in the winter moth for six allozyme loci. The relationship is estimated for males and females separately, in four areas differing in their degree of fragmentation, and variation among loci is modelled. We introduce a linear mixed model framework to achieve this analysis. This approach differs from more traditional (multiple) regres- sion analyses and allows testing specific interactions. We show that fitness, as estimated by body size, is negatively correlated with heterozygosity, but only so in females. This association does not vary significantly among loci and the four areas. We speculate that a trade-off between fitness-consequences of inbreeding and outbreeding at different stages of the winter moth life cycle could explain the observed patterns. KEY WORDS : fitness, heterozygosity, inbreeding, outbreeding, natural selection, genetic drift, winter moth, Operophtera INTRODUCTION The extent to which the environment interacts with the genotype affecting fitness can influence population dynamics and even extinction risks of small endangered populations. Yet, this issue remains poorly investigated and often ignored in ecology and evolutionary biology (COULSON et al., 1998a). One aspect of the association between genotype and fitness that has received relatively much attention in the literature are correlations with degree of heterozygosity or inbreeding. Yet, the predicted positive association between genetic diversity and fitness is far from general (BRITTEN, 1996; MITTON, 1997), and little is know about the factors that affect this association (but see e.g., PALMER, 1996; MITTON, 1997; LESBARRÉRES et al., 2005). Recently, LESBARRÉRES et al. (2005) found heterogeneity in fitness-heterozygosity associations among geographic areas and SACCHERI et al (2005) found indications for differences between the two sexes. How an association between heterozygosity and fitness can become established is a subject of many debates. Observed individual heterozygosity may reflect genome- wide heterozygosity and thus the overall levels of inbreeding and may reduce fitness (CHARLESWORTH & CHARLESWORTH, 1987). Yet, a large number of loci should be scored to obtain a reliable estimate of genome-wide heterozygosity drastically reducing power to detect a fit- ness-heterozygosity association. Failing to do so drasti- cally reduces the power to detect an association. Since many studies do find an association with few loci, rather natural selection phenomena on single loci, or effects at tightly linked areas on the chromosome, are often thought to be responsible for the observed fitness-heterozygosity association (MITTON, 1997). It is, however, unlikely that a single mechanism explains such a widespread phenome- non. Indeed, some studies using presumed selectively neutral microsatellite markers also found positive fitness- heterozygosity associations (COLTMAN et al., 1998; COULSON et al., 1998b; LESBARRÉRES et al., 2005) favour- ing the hypothesis of genome-wide effects of inbreeding over the selection theory. On the other hand, some detailed studies of the kinetics of specific metabolic enzymes have shown clear effects of selection on single locus genotypes (see MITTON, 1997 for references and details). HALDANE (1954) proposed that heterozygotes at enzyme loci would be more efficient than homozygotes at controlling flux in metabolic pathways. Under this hypothesis it is assumed that different genotypes have dif- ferent properties under different environmental conditions and that heterozygotes would be superior when condi- tions fluctuate between those favouring the different homozygotes (MITTON, 1997). When environmental dif- ferences are persistent in space, however, genetic differ- entiation (either through the effects of genetic drift or nat- ural selection) between the environments is expected to evolve, and mating between genetically distant individu- als becomes disadvantageous as it disrupts local adapta- tion (i.e. outbreeding depression, e.g. STRAUSS & KAR- BAN, 1994; COULSON et al., 1998b). Therefore, in spatio- temporally stable selective environments, a negative asso- ciation between fitness and heterozygosity is expected. In theory it should be possible to differentiate effects due to drift and selection. Genetic drift is expected to affect dif- ferent loci in a similar way whereas the effects of natural