Ploidy-level variation is common and can drastically affect organismal fitness. We focus on the potential consequences of this variation for parasite resistance. First, we elucidate connections between ploidy variation and key factors determining resistance, including allelic diversity, gene expression and physiological condition. We then argue that systems featuring both natural and artificially manipulated ploidy variation should be used to evaluate whether ploidy level influences host -parasite interactions.Keywords: polyploidy; host-parasite interactions; allelic diversity; gene expression; host condition 1. INTRODUCTION Polyploidization 1 has generated variation in ploidy level within and across species [1,2], and new examples are continually being discovered, particularly in animals [3]. Intraspecific ploidy-level variation is associated with a suite of connections between polyploidy and biological phenomena [4,5], and has the potential to influence fitness-related traits [6,7]. We discuss whether ploidy variation could mediate resistance to a ubiquitous enemy-parasites (defined here as an organism that harms its host). Although current theory suggests ploidy level can profoundly influence infection dynamics and host-parasite evolution [8,9], data are scarce [10,11].
PLOIDY AND IMMUNE FUNCTIONPolyploidy (i.e. autopolyploidy) could directly influence immune response to a parasite attack in at least two ways. Firstly, the addition of a new genome may increase allelic diversity. Higher allelic diversity at immune genes could help hosts recognize a greater diversity of parasites [12]. Secondly, if the additional genome copies are expressed, then polyploids may generate higher amounts of gene products related to immune function.(a) Allelic diversity and immune function The high allelic diversity of immune genes is partly a consequence of parasite-mediated selection for rare genotypes and/or novel immune functions [13]. Parasitemediated selection may generate such diversity through mechanisms such as heterozygote advantage [14] and negative frequency-dependence [13]. Heterozygote advantage could be influenced by ploidy level because the extra alleles present in polyploids may increase the probability of heterozygosity for an individual at a given locus. The presence of an extra genome could also increase the likelihood that an individual possesses a rare genotype at resistance loci; this would be advantageous if parasite-mediated negative frequency-dependent selection favours rare genotypes.Studies from natural populations have documented connections between parasite-mediated selection and the maintenance of allelic variation at genes associated with immune function [12,15]. Genetic polymorphism can play an important role in generating variation in recognition molecules [16], and diversity is important for disease resistance in host individuals and populations [17,18]. Furthermore, measures of genome-wide genetic variation (e.g. microsatellite heterozygosity [19,20]) are often correlated with individual immune ...