DOI: 10.5061/dryad.j3tx95xcv
title: Intraspecific variation in symbiont density in an insect-microbe symbiosis

description: Many insects host vertically-transmitted microbes, which can confer
 benefits to their hosts but are costly to maintain and regulate. A key
 feature of these symbioses is variation: for example, symbiont density can
 vary among host and symbiont genotypes. However, the evolutionary forces
 maintaining this variation remain unclear. We studied variation in
 symbiont density using the pea aphid (Acyrthosiphon pisum) and the
 bacterium Regiella insecticola, a symbiont that can protect its
 host against fungal pathogens. We found that relative symbiont
 density varies both between two Regiella phylogenetic
 clades and among aphid ‘biotypes’. Higher-density symbiont infections are
 correlated with stronger survival costs, but variation in density has
 little effect on the protection Regiella provides
 against fungi. Instead, we found that in some aphid genotypes, a dramatic
 decline in symbiont density precedes the loss of a symbiont infection.
 Together, our data suggest that the optimal density of a symbiont
 infection is likely different from the perspective of aphid and microbial
 fitness. Regiella might prevent loss by maintaining high
 within-host densities, but hosts do not appear to benefit from higher
 symbiont numbers and may be advantaged by losing costly symbionts in
 certain environments. The standing variation in symbiont density
 observed in natural populations could therefore be maintained by
 antagonistic coevolutionary interactions between hosts and their symbiotic
 microbes. 
description: <strong>Regiella strain densities</strong> - Density
 data for the <em>Regiella</em> strain variation experiment
 (Figure 1A). Column A (Sample)
 gives <em>Regiella</em> genotype names. Each genotype's
 phylogenetic clade (either 1 or 2) is shown in Column B (Clade). Column C
 (neg_delta_Ct) gives <em>Regiella</em> densitiy as qPCR
 output, which can be interpreted on a log2 scale. The neg_delta_Ct value
 is equal to 1 - {(Ct value for ef1a [aphid]) - (Ct value for hrpA
 [Regiella])}. 
 <strong>Density_survival_correlation </strong>- Shows
 the values used to calculate the correlation
 betwen <em>Regiella </em>density and aphid survival (Figure
 1B). Symbiont Sample and Clade are given in Columns A and B, respectively.
 Density is given as the average negative delta Ct value as above.
 Survival_Cost data are taken from Parker et al. (2017)
 Evolution.  <strong>G_host x G_symbiont
 densities</strong> - Density data for the G_host by G_symbiont
 experiment (Figure 1C). Sample names are given in Column A. Host genotype
 and Symbiont genotype associated with each sample are given in Columns B
 and C, respectively, along with Symbiont clade in Column D. Symbiont
 density is reported as neg_delta_Ct as above (Column E). 
 <strong>Host genotype densities </strong> - Density
 data for the host biotype experiment (Figure 2A). Aphid genotype and the
 biotype from which each aphid line comes from are given in Columns A and
 B, respectively. Density data is shown in column C as negative delta Ct
 values, as above.  <strong>Sporulation &
 Survival</strong> - Gives raw data for the fungal infection data
 (Figures 2B and 2C). Aphid line names are given in Column A, and the aphid
 genotype, biotype of each aphid line, and symbiont status (control or
 with <em>Regiella</em>) are shown in columns B, C, and D,
 respectively. Treatment is given in column E (Control or Fungus). The
 random number assigned to each Petri dish is given in Column F. The Number
 of aphids placed in each dish at the start of the experiment, and the
 number of aphids that survived the humidity treatment by Day 2 of the
 infection, are given in Columns G and H. The Number of aphids from each
 dish that produced a sporulating cadaver by the end of the experiment is
 given in Column I. 
 <strong>correlations</strong> - Values used to
 calculate correlations between symbiont density, survival, and fungal
 resistance (Figures 2D and 2E). Aphid genotype and biotype are given in
 Columns A and B. Density is as above, in Column C. Columns D and E give
 the survival of each genotype without and
 with <em>Regiella</em>, respectively, and the difference
 between these values in Column F (survival cost). Columns G and H give the
 percent sporulated aphids of each genotype without and
 with <em>Regiella</em>, respectively, with the difference
 between these values (protection) given in Column I. 
 <strong>Regiella time-course densities </strong>-
 Density data in the symbiont loss experiment (Figure 3). Experimental
 Block is given in Column A, with aphid genotype in Column B (either 317 or
 663). <em>Regiella </em>density (negative delta Ct values as
 above) are shown in Columsn C and D at generations 2 and 4, respectively.
 Whether a line ultimately lost or retained symbionts by the end of the
 experiment (Generation 5) is shown in Column E.