DOI: 10.5061/dryad.kkwh70s97
title: Experimental evolution of Dictyostelium discoideum cheating under relaxed selection

description: Many microbes interact with one another, but the difficulty of directly
 observing these interactions in nature makes interpreting their adaptive
 value complicated.  The social amoeba Dictyostelium discoideum
 forms aggregates wherein some cells are sacrificed for the benefit of
 others. Within chimeric aggregates containing multiple unrelated lineages,
 cheaters can gain an advantage by undercontributing, but the extent to
 which wild D. discoideum has adapted to cheat is not fully clear. In this
 study, we experimentally evolved D. discoideum in an environment where
 there were no selective pressures to cheat or resist cheating in chimeras.
 D. discoideum lines grown in this environment evolved reduced
 competitiveness within chimeric aggregates and reduced ability to migrate
 during the slug stage. By contrast, we did not observe a reduction in cell
 number, a trait for which selection was not relaxed.  The
 observed loss of traits that our laboratory conditions had made irrelevant
 suggests that these traits were adaptations driven and maintained by
 selective pressures D. discoideum faces in its natural environment. Our
 results suggest that D. discoideum faces social conflict in
 nature, and illustrate a general approach that could be applied to
 searching for social or non-social adaptations in other microbes.
description: Data represent the results of assays measuring various phenotypes
 of <em>D. discoideum </em>strains before and after
 experimental evolution under relaxed selection. See
 manuscript for full methods, but in short: Cheating
 assays - combine strains of interest with labelled control strain at 50:50
 ratio. Plate atop filter paper on non-nutrient plates, allow to form
 chimeric fruiting bodies.  Harvest fruiting bodies, suspend in buffer, and
 photograph using bright field and fluorescent microscopy. Calculate
 fraction of fluorescently labelled spores. Slug
 migration assays - inoculate strains of interest along a secant line on
 one end of a non-nutrient plate. Cover in opaque foil except for a small
 hole that allows light from the opposite end of the plate. Wait 8 days,
 then photograph plates. Measure distance of each fruiting body from the
 starting line. Cell count/spore count/sporulation
 efficiency assays - inoculate strains of interest on plates, wait for
 growth/development, and then quantify cells using a
 hemacytometer.
description: # Assays of phenotypes of D. discoideum experimentally evolved under
 relaxed selection --- Dataset represents five different assays performed
 on experimentally evolved D. discoideum lines. 10 wild D. discoideum
 strains were evolved (as three replicate lines each) under conditions in
 which the social cycle never occurred. After experimental evolution, the
 researchers measured various traits of the experimentally evolved lines
 and compared them to their ancestors. The chief prediction was that traits
 for which selection had been relaxed (like slug migration and cheating)
 should tend to decay, while traits that continued to be under selection in
 the lab (like cell number) should not. ## Description of the Data and file
 structure The data are in four spreadsheets corresponding to five
 different assays. --- ### data\_cheatingassay: These data represent the
 number of fluorescently labelled and total spores within chimeric fruiting
 bodies inoculated from a 50:50 mixture of a D. discoideum strain of
 interest (ancestral or experimentally evolved) and a fluorescently labeled
 control strain RFP-NC28.1. If no cheating was occurring, 50% of the spores
 should be fluorescently labelled. If more or fewer are, it indicates one
 of the strains is undercontributing. These data correspond to figure 2 in
 the manuscript. Column names: * Date - the date of the experiment,
 naturally (MM/DD/YY) * Strain - D. discoideum strain name, either NC28 or
 one of 10 wild strains used in the experimental evolution experiment. *
 Line - For experimental strains, indicates whether the data points are for
 the ancestor (Anc) or one of three replicate experimentally evolved lines
 (E1, E2, or E3) * Rep - For separating technical replicates * Filter -
 Fruiting bodies were grown on small pieces of filter paper, two of which
 were assayed for each experiment. This column distinguishes those (mostly
 to help hunt down specific photos if necessary) * Image - Five images were
 taken for each sample. This column distinguishes those (mostly to help
 hunt down specific photos if necessary) * Brightfield - A count of the
 total number of spores, as counted via a brightfield micrograph image.
 This represents both labeled and unlabeled spores. * Red - A count of the
 total number of fluorescently labeled spores, as counted via a
 fluorescence micrograph image. --- ### data\_slugmigrationassay: These
 data represent the distance travelled by D. discoideum slugs. Cells were
 inoculated along one end of a nutrient-free plate and given 8 days to move
 towards a pinhole light source on the opposite end. These data correspond
 to figure 3 in the manuscript. Column names: * Date, Strain, Line - see
 above * Plate - image names for each photographed plate (to help hunt down
 specific photos if necessary) * Migration distance - Migration distance
 (in cm) from starting line, as measured from the photograph. --- ###
 data\_cellcountassay: These data represent the total number of cells
 produced after inoculation of spores of ancestral or evolved D. discoideum
 lines on SM/5 plates. Data were collected in two separate assays with
 slightly different purposes and methodology: For cell count assays, cells
 were counted prior to aggregation so to eliminate the effect of any
 differences in sporulation. These data correspond to figure 4 in the
 manuscript. For spore count assays, cells were counted after sporulation,
 and so their number should be affected by any differences that exist
 between strains in both vegetative cell growth rate and in sporulation.
 These data correspond to supplementary figure 1 in the manuscript. Column
 names - * Strain, Line - see above * Assay - either 'Cell' or
 'Spore', to distinguish the two assays described above. *
 Totalcells - A count of the total cells/spores produced. --- ###
 data\_sporulationefficiencyassay: These data represent the total number of
 spores produced after inoculation of vegetative cells of ancestral or
 evolved D. discoideum lines on non nutrient plates. These data correspond
 to supplemental figure 2 in the manuscript. Column names - * Strain, Line
 - see above * Totalspores - A count of the total spores produced ##
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