DOI: 10.5061/dryad.4b8gthtjx
title: Aggregation of symbionts on hosts depends on interaction type and host traits

description: Symbionts tend to be aggregated on their hosts, such that few hosts harbor
 the majority of symbionts. This ubiquitous pattern can result from
 stochastic processes, but aggregation patterns may also depend on the type
 of host-symbiont interaction, plus traits that affect host exposure and
 susceptibility to symbionts. Untangling how aggregation patterns both
 within and among populations depend on stochastic processes, interaction
 type and host traits remains an outstanding challenge. Here, we address
 this challenge by using null models to compare aggregation patterns in a
 neutral system of Balanomorpha barnacles attached to patellid limpets and
 a host-parasite system of Trinidadian guppies (Poecilia reticulata) and
 their Gyrodactylus spp. monogeneans. We first used a model to predict
 patterns of symbiont-host aggregation due to random partitioning of
 symbionts to hosts. This null model accurately predicted the aggregation
 of barnacles on limpets, but the degree of aggregation varied across 303
 quadrats. Quadrats with larger limpets had less aggregated barnacles,
 whereas aggregation increased with variation in limpet size. Across 84
 guppy populations, Gyrodactylus spp. parasites were significantly less
 aggregated than predicted by the null model. As in the neutral
 limpet-barnacle system, aggregation decreased with mean host size.
 Parasites were also significantly less aggregated on males than females
 because male guppies tended to have higher prevalence and lower parasite
 burdens than predicted by the null model. Together, these results suggest
 stochastic processes can explain aggregation patterns in neutral but not
 parasitic systems, though in both systems host traits affect aggregation
 patterns. Because the distribution of symbionts on hosts can affect
 symbiont evolution via intraspecific interactions, and reciprocally host
 behavior and evolution via host-symbiont interactions, identifying the
 drivers of aggregation enriches our understanding of host-symbiont
 interactions.
description: # Aggregation of symbionts on hosts depends on interaction type and host
 traits
 [https://doi.org/10.5061/dryad.4b8gthtjx](https://doi.org/10.5061/dryad.4b8gthtjx) Here, we test whether the processes that drive aggregation differ between a neutralistic and parasitic host-symbiont system. We further test how the tolerance-linked sex and exposure-linked size differences of guppy *Poecilia reticulata* hosts can drive differences in the aggregation of *Gyrodactylus* spp. parasites among their hosts. For the guppy-Gyrodactlid portion of the manuscript we are using the guppygyro.csv dataset. This data is a combination of population level data from Stephenson, J. F., Oosterhout, C. van, Mohammed, R. S. & Cable, J. Parasites of Trinidadian guppies: evidence for sex- and age-specific trait-mediated indirect effects of predators. Ecology 96, 489–498 (2015) and an additional, unpublished data set. For the guppy-Gyrodactlid portion of the manuscript we are using the guppygyro.csv dataset guppygyro.csv. This data is a combination of population level data from Stephenson, J. F., Oosterhout, C. van, Mohammed, R. S. & Cable, J. Parasites of Trinidadian guppies: evidence for sex- and age-specific trait-mediated indirect effects of predators. Ecology 96, 489–498 (2015) and an additional, unpublished data set limpetbarnacle.csv. We additionally have a separate CSV file named "guppysexescorr.csv." This dataset is just a subset of the larger guppy dataset to test for correlations between the aggregation of parasites between males and female guppies from the same site. ## Description of the data and file structure **guppygyro.csv**: contains all data pertaining to the Trinidadian guppy and gyrodactylus parasite data. Included variables: **Site**: A factor that identifies the drainage, river, course, site, and year the population data was taken. It has a level per population. **drainage**: The drainage where the population was collected from. Six levels **river**: The river where the population was collected from. 13 levels. **year**: The year the population was collected in. 5 levels. **season**: The season the population was collected. 2 levels. **Course**: The watercourse the population was collected in. 2 levels. Upper indicates low predation populations while lower indicates high predation populations in line with previous publications from the Trinidadian guppy system **H**: The total number of hosts in the population **P**: The total number of parasites in the population **MeanP**: The mean number of parasites infecting hosts in the population **VarP**: The mean number of parasites infecting hosts in the population **MeanLen**: The mean length of all individuals in the population (millimeters) **VarLen**: The variance in length of the population (millimeters<sup>2</sup>) **MeanW**: The mean weight of all individuals in the population (grams) **VarW**: The variance in weight of the population. (grams<sup>2</sup>) fs var log med: The log median variance predicted by the feasible set. \\newline sex: The sex of individuals in the population. Populations were separated into males and females to conduct each analysis and calculate body condition. This allowed us to better understand aggregation differences between sexes. 2 levels. **jonvar**: The variation in condition of individuals in the population **ResBC**: The body condition given the residual mass index. **jonres**: The residual of a regression between the body condition given by index from Johnson et al. 2011 and the mean length of the population. We did this to control for any body condition bias we may see between populations just due to size differences. **diffvar**: The difference in variances between the observed and expected distribution (the feasible set) of parasites in the population. Positive values indicates the population has a higher variance (More aggregation) than expected by the feasible set, negative values means the feasible set predicted more variance than is present in the observed data **logMean**: The log10 mean of parasites in the population. **jonresRS**: The scaled jonres variable from above. **MeanLenRS**: The scaled mean length variable from above. **ExpPrev**: The expected prevalence of the population given by the feasible set \\newline **ObsPrev**: The observed prevalence of the population **MaxF**: The expected number of parasites on the individual with the highest parasite burden in the population given by the feasible set **MaxE**: The observed number of parasites on the individual with the highest parasite burden in the population We also include an additional guppysexcorr.csv file for an analysis examining the correlation between aggregation metrics between males and females from the same site. A list of the data in this file is included below: **Site**: A factor that identifies the drainage, river, course, site, and year the population data was taken. It has a level per population. **FemaleDiff**: The difference in variances between the observed and expected distribution (the feasible set) of parasites in the population of females at a given site. **MaleDiff**: The difference in variances between the observed and expected distribution (the feasible set) of parasites in the population of males at a given site. Lastly, we have a file titled limpetbarnacle.csv with all data pertaining to the limpet-barnacle host-symbiont analysis part of the manuscript. A list of the data in this file is included below: **Site**: A factor that identifies the site where the community was collected. **QUADRAT**: A factor that identifies the quadrat site location of each community of limpets and barnacles. It has a level per community. **H**: The total number of limpet hosts in each community. **P**: The total number of barnacle symbionts in each community. **MeanP**: The mean abundance of barnacles living on the limpets for each community. **varP**: The variance in abundance of barnacles living on the limpets for each population. **MeanW**: The mean width measurement for the limpets in each community. Measured in (millimeters) **VarW**: The variance in width measurement for the limpets in each community. Measured in (millimeters<sup>2</sup>) **MeanL**: The mean length measurement for the limpets in each community. Measured in (millimeters) **VarL**: The variance in length measurement for the limpets in each community. Measured in (millimeters<sup>2</sup>) **fs var log med**: The log median variance predicted by the feasible set for each limpet-barnacle community. <br> <br> ##