Parasite distribution ► Population – a group of individuals of a given species at a given time and in a given area ► Population characteristics: ► natality ► mortality ► age structure ► reproductive capacity ► dispersion ► growth ► density Parasite Populations Infrapopulation of parasites ► a set of all individuals of one species of parasite on/in one host individual ► Short life span – limited by life span of the host individual ► Individual host - one or more infrapopulations of different parasitic species Parasite 1 Parasite 2 Parasite 3 Host Distribution of parasites Distribution of parasites in the host population - frequency distribution Variance < mean Variance > mean Positive binomial function Negative binomial function Variance = mean Poisson distribution random aggregatedequal Number of specimens in sample Numberofsamples (hosts) Aggregation of parasites ► Hosts = components of a suitable habitat in an inhospitable environment Some components contain more parasites than average, others less so ► Tendency to aggregate: main characteristics of parasitic infection ► The distribution of parasites over time is not static ► It cannot be expressed by a single measurement ► At multiple levels: at the level of individuals (within the population), at the level of microhabitats (within the host) ► Most hosts have a low frequency of parasites or are not parasitized at all, few hosts have a high number of parasites Aggregation of parasites Distribution of the number of nematodes (Aspiculuris tetraptera and Syphacia obvelata) in the parental mice (Mus musculus and Mus domesticus) and their hybrids (Moulia et al. 1991). Causes of aggregation ► Temporal and spatial heterogeneity of host exposure ► Intraspecific variability of the host - immune response, size, age, sex, physiology or behavior of the host, genetically fixed differences in susceptibility to parasites ► The result of a combination of several factors ► Variability in different host-parasite systems → the degree of aggregation depends on the properties of the parasites and hosts Causes of aggregation: heterogeneity in host exposure to parasites ► Ex. Karvonen et al. (2004) - heterogeneity in exposure of hosts to metacercariae Diplostomum spathaceum (Trematoda) in rainbow trout Oncorhynchus mykiss leads to parasite aggregation Causes of aggregation: parasite size Ex. Poulin & Morand (2000) 59 species of gastrointestinal nematodes in mammals Causes of aggregation: exposure time ► Ex. Experimental infection with ectoparasite (Argulus foliaceus) in Oncorhynchus mykiss - same infectious dose, different exposure time - differences in host exposure may generate an aggregated distribution Causes of aggregation: diverse distribution of the parasite in time and space ► ex. Keymer & Anderson (1979) experimental study – aggregated distribution of infectious stages in space leads to aggregated distribution of parasites in the host population Consequences of aggregation ► Advantages of aggregation for parasites ► Encounters for reproduction, the "mating rendez-vous" hypothesis - in the case of low population densities (Monogenea) ► Factor facilitating species coexistence (high aggregation at intra-species level than in inter-species level) Consequences of aggregation ► „crowding effect“ (Read, 1951) - in large infrapopulations - strong aggregation could reduce the fitness of parasites - high numbers of parasites on/in host specimen = smaller average size of parasites (e.g. Cestoda) - growth and fecundity are density dependent (for many parasites) ► Population of adult parasites in the definitive host - more small individuals with low fecundity, several large individuals with high fecundity (superior parasite genotypes?) Consequences of aggregation ► Impact on genetic variability of parasitic populations – reduction of genetic variability ► Ex. The genetic variability of parasitic helminths is lower than in free-living invertebrates ► Different heterozygosity in parasitic and free-living organims Consequences of aggregation: genetic diversity in the parasite populations Consequences of aggregation: genetic diversity of parasites Distribution of rare alleles - eggs produced from the same adult are genetically more similar than eggs of different adults Cornel et al. (2003) model of the spread of rare, recessive but beneficial, alleles of trichonstrogylid nematodes parasitizing ruminants (i.e. aggregation is associated with a certain degree of inbreeding) ► Effect on sex ratio ► Ex. in polygamous parasites Nematoda and Acanthocephala in favour of females - the result of differential mortality (females live longer) Consequences of aggregation: shift in sex ratio ► Apicomplexa - sexual stages = gametocytes - the benefit of females - for some representatives of Plasmodium, pronounced in Toxoplasma - inbreeding - selection for low production of male gametocytes - Apicomplexa in birds - not always a shift in the sex ratio Consequences of aggregation: shift in sex ratio Quantification of aggregation ► Variance to mean ratio ► Parameter of negative binomial distribution (k) ► Discrepancy index (D) ► Taylor's rule Variance to mean ratio ► The simplest and commonly used ► Ratio of variance to mean number of parasites per host ► Variance - a measure of mathematical variability within a population ► Var (M) / M = 1 random distribution ► Var (M) / M <1 uniform "under-dispersed" ► Var (M) / M> 1 aggregated "over-dispersed" ► Increasing value of the coefficient = increasing the rate of aggregation ► It quantifies variability in the intensity of infection between hosts or variability in the size of parasite infrapopulations Aggregation of metazoan parasites Mean number of parasites per host (in log) Varianceinparasitenumbers(inlog) ► Unsuitable if analyzing frequency distribution of parasites between infrapopulations of different sizes Variance to mean ratio as an indicator of aggregation Parameter of negative binomial distribution ► Defined by the average abundance (M) and the dispersion parameter k ► Negative binomial distribution s2 = M+M2/k ► With negative binomial distribution k = M2/(s2-M) ► if k → 0 aggregation increases, if k is high (> 20) random distribution (Poisson function) Comparison of frequency distribution of parasites expressed using different aggregation indices ► (1) based on parameter k ► (2) the ratio of variance to diameter Ex. 269 metazoan populations parasites in vertebrates Discrepancy index ► Poulin (1993) ► It quantifies aggregation as the deviation between the observed distribution and the hypothetical distribution of parasites, where all hosts are used equally and all parasites occur in infrapopulations of similar size x is the number of parasites on host j (hosts are sorted in order from the least infected, i.e. j = 1, to the most infected) and n is the number of hosts in the sample Discrepancy index ► Cumulative number of parasite individuals vs. cumulative number of host individuals, hosts ranked from the least to the most infected ► Relative discrepancy A/(A + B) ► Minimum index value = 0 - no aggregation ► Maximum value = 1 - aggregation is the highest Taylor’s rule ► Taylor (1961) ► Taylor’s rule - relation between abundance (M) and variance Var (M) a: intercept, b: slope of the line, exponent of the rule b represents the aggregation index There is a relationship between the parameter k and the parameters a and b of Taylor's rule Taylor's rule: parameter b ► describes how species use the environment, i.e. parasites use the host ► variable between species 1