A fragment of the mitochondrial cytochrome b gene of avian malaria (genera Haemoproteus and Plasmodium) was ampli¢ed from blood samples of 12 species of passerine birds from the genera Acrocephalus, Phylloscopus and Parus. By sequencing 478 nucleotides of the obtained fragments, we found 17 di¡erent mitochondrial haplotypes of Haemoproteus or Plasmodium among the 12 bird species investigated. Only one out of the 17 haplotypes was found in more than one host species, this exception being a haplotype detected in both blue tits (Parus caeruleus) and great tits (Parus major). The phylogenetic tree which was constructed grouped the sequences into two clades, most probably representing Haemoproteus and Plasmodium, respectively. We found two to four di¡erent parasite mitochondrial DNA (mtDNA) haplotypes in four bird species. The phylogenetic tree obtained from the mtDNA of the parasites matched the phylogenetic tree of the bird hosts poorly. For example, the two tit species and the willow warbler (Phylloscopus trochilus) carried parasites di¡ering by only 0.6% sequence divergence, suggesting that Haemoproteus shift both between species within the same genus and also between species in di¡erent families. Hence, host shifts seem to have occurred repeatedly in this parasite^host system. We discuss this in terms of the possible evolutionary consequences for these bird species.

Recovery from infection is not always complete, and mild chronic infection may persist. Although the direct costs of such infections are apparently small, the potential for any long-term effects on Darwinian fitness is poorly understood. In a wild population of great reed warblers, we found that low-level chronic malaria infection reduced life span as well as the lifetime number and quality of offspring. These delayed fitness effects of malaria appear to be mediated by telomere degradation, a result supported by controlled infection experiments on birds in captivity. The results of this study imply that chronic infection may be causing a series of small adverse effects that accumulate and eventually impair phenotypic quality and Darwinian fitness.

Summary 1.We have used molecular methods to unravel a remarkable diversity of parasite lineages in a long-term population study of great reed warblers Acrocephalus arundinaceus that was not foreseen from traditional microscopic examination of blood smears . This diversity includes eight Haemoproteus and 10 Plasmodium lineages of which most probably represent good biological species. 2. Contrary to expectation, the relative frequency of parasite lineages seemed not to change over the 17-year study period and we found no effects of the parasites on a male secondary sexual ornament (song repertoire size) and two measures of fitness (adult survival and production of recruited offspring). 3. We discuss whether the absence of fitness consequences of the parasites might relate to the fact that we have studied the host at the breeding sites in Europe, whereas the transmission seems to take place at the wintering sites in Africa, where the naïve birds encounter the parasites for the first time and the resulting primary infections likely make them sicker than during the chronic phase of the infection. 4. The prevalence of the three most common lineages appeared to fluctuate in parallel with a periodicity of approximately 3-4 years. Theoretical models based on intrinsic interactions between parasite antigen and host immune genes cannot explain such dynamics, suggesting that knowledge of extrinsic parameters such as vector distribution and alternative hosts are required to understand these patterns.

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Malaria parasites are a widespread and species-rich group infecting many wild populations of mammals, birds and reptiles. Studies on humans have demonstrated that genetic factors play a key role in the susceptibility and outcome of malaria infections. Until the present study, it has not been examined whether genetic variation in hosts is important for the outcome of malaria infections in natural avian populations. We investigated associations between major histocompatibility complex (MHC) genes and prevalence of three different avian malaria parasites (Haemoproteus payevskyi (GRW1), Plasmodium sp. (GRW2) and Plasmodium sp. (GRW4)) in a long-term study of great reed warblers Acrocephalus arundinaceus. We hypothesized that the MHC genes could either give full protection against a malaria infection, or confer protection against lethal malaria and direct the infection towards being milder. We found a positive association between numbers of MHC class I alleles (a measure of level of heterozygosity) and prevalence of the GRW2 parasite, suggesting the latter scenario. There was also a positive association between a specific MHC allele (B4b), previously shown to be under frequency-dependent selection in the study population, and prevalence of GRW2. These associations suggest that individuals carrying either a large number of MHC alleles or a specific MHC allele are protected against lethal malaria infections.

Sexual selection hypotheses stipulate that the major histocompatibility complex genes (MHC) constitute a key molecular underpinning for mate choice in vertebrates. The last four decades saw growing empirical literature on the role of MHC diversity and dissimilarity in mate choice for a wide range of vertebrate animals, but with mixed support for its significance in natural populations. Using formal phylogenetic metaanalysis and meta-regression techniques, we quantitatively review the existing literature on MHC-dependent mating preferences in nonhuman vertebrates with a focus on the role of MHC diversity and dissimilarity. Overall, we found small, statistically nonsignificant, average effect sizes for both diversity-and dissimilarity-based mate choice (r = 0.113 and 0.064, respectively). Importantly, however, meta-regression models revealed statistically significant support regarding female choice for diversity, and choice for dissimilarity (regardless of choosy sex) only when dissimilarity is characterized across multiple loci. Little difference was found among vertebrate taxa; however, the lack of statistical power meant statistically significant effects were limited to some taxa. We found little sign of publication bias; thus, our results are likely to be robust. In light of our quantitative assessment, methodological improvements and fruitful future avenues of research are highlighted.

Abstract. Heterozygosity-fitness correlations (HFCs) at noncoding genetic markers are commonly assumed to reflect fitness effects of heterozygosity at genomewide distributed genes in partially inbred populations. However, in populations with much linkage disequilibrium (LD), HFCs may arise also as a consequence of selection on fitness loci in the local chromosomal vicinity of the markers. Recent data suggest that relatively high levels of LD may prevail in many ecological situations. Consequently, LD may be an important factor, together with partial inbreeding, in causing HFCs in natural populations. In the present study, we evaluate whether LD can generate HFCs in a small and newly founded population of great reed warblers (Acrocephalus arundinaceus). For this purpose dyads of full siblings of which only one individual survived to adult age (i.e., returned to breed at the study area) were scored at 19 microsatellite loci, and at a gene region of hypothesized importance for survival, the major histocompatibility complex (MHC). By examining siblings, we controlled for variation in the inbreeding coefficient and thus excluded genome-wide fitness effects in our analyses. We found that recruited individuals had significantly higher multilocus heterozygosity (MLH), and mean d 2 (a microsatellite-specific variable), than their nonrecruited siblings. There was a tendency for the survivors to have a more diverse MHC than the nonsurvivors. Single-locus analyses showed that the strength of the genotype-survival association was especially pronounced at four microsatellite loci. By using genotype data from the entire breeding population, we detected significant LD between five of 162 pairs of microsatellite loci after accounting for multiple tests. Our present finding of a significant within-family multilocus heterozygosity-survival association in a nonequilibrium population supports the view that LD generates HFCs in natural populations.Key words. Heterozygosity, inbreeding depression, linkage disequilibrium, major histocompatibility complex, microsatellite.Received June 19, 2003. Accepted December 11, 2003 Correlations between heterozygosity at a set of molecular markers and fitness-associated traits have been detected in several natural and experimental populations over the last decades (Allendorf and Leary

Invasive species can displace natives, and thus identifying the traits that make aliens successful is crucial for predicting and preventing biodiversity loss. Pathogens may play an important role in the invasive process, facilitating colonization of their hosts in new continents and islands. According to the Novel Weapon Hypothesis, colonizers may out-compete local native species by bringing with them novel pathogens to which native species are not adapted. In contrast, the Enemy Release Hypothesis suggests that flourishing colonizers are successful because they have left their pathogens behind. To assess the role of avian malaria and related haemosporidian parasites in the global spread of a common invasive bird, we examined the prevalence and genetic diversity of haemosporidian parasites (order Haemosporida, genera Plasmodium and Haemoproteus) infecting house sparrows (Passer domesticus). We sampled house sparrows (N = 1820) from 58 locations on 6 continents. All the samples were tested using PCR-based methods; blood films from the PCR-positive birds were examined microscopically to identify parasite species. The results show that haemosporidian parasites in the house sparrows' native range are replaced by species from local host-generalist parasite fauna in the alien environments of North and South America. Furthermore, sparrows in colonized regions displayed a lower diversity and prevalence of parasite infections. Because the house sparrow lost its native parasites when colonizing the American continents, the release from these natural enemies may have facilitated its invasion in the last two centuries. Our findings therefore reject the Novel Weapon Hypothesis and are concordant with the Enemy Release Hypothesis.