Phylogeny, reproductive isolation and kin recognition in the social amoeba Dictyostelium purpureum

Abstract

Little is known about the population structure of social microorganisms, yet such studies are particularly interesting for the ways that genetic variation impacts their social evolution. Dictyostelium, a eukaryotic microbe widely used as a developmental model, has a social fruiting stage in which some formerly independent individuals die to help others. To assess genetic variation within the social amoeba Dictyostelium purpureum, we sequenced approximately 4000 base pairs of ribosomal DNA (rDNA) from 37 isolates collected in Texas, Virginia, and Japan. Our analysis showed extensive genetic variation between populations and clear evidence of phylogenetic structure. We identified three major phylogenetic groups that were more different than other accepted species pairs. Tests using pairs of clones showed that both sexual macrocyst and asexual fruiting body formation were influenced by genetic divergence. Macrocysts were less likely to form between pairs of clones from different groups than from the same group. There was also a correlation between the genetic divergence of a pair of clones and their degree of mixing within fruiting bodies. These observations suggest that cryptic species might occur within D. purpureum and, more importantly, reveal how genetic variation impacts social interactions.

Figure 1

Bayesian tree of 21 unique haplotypes of D. purpureum. Published sequences of D. macrocephalum, D. citrinum, and D. discoideum served as outgroups. Houston clone QSpu10 is not shown, but could belong to either haplotype number 19 or 20. Nodes with posterior probabilities below 50% are not shown. Letters refer to specific phylogenetic groups (see text) and symbols refer to geographic locations of clones.

Figure 2

Relationship between genetic distance of a given pair of clones and the average relatedness in fruiting bodies arising from their pairwise mixture for 13 within phylogenetic group experiments (see Mehdiabadi et al. 2006). Genetic distance or pairwise sequence divergence represents the number of base differences per site from analysis between sequences. Methods on calculations of relatedness in fruiting bodies are reported in Mehdiabadi et al. (2006). Relatedness of 0.50 denotes complete mixing between a pair of clones, whereas a relatedness of 1.00 signifies complete sorting between a pair of clones.