The ecology, role of the killer trait, and interactions of five species of the Paramecium aurelia complex inhabiting the littoral zone

Publisher Website
Google Scholar
Abstract
The field work was carried out over a 3-year period. Extensive sampling of two ponds in the Bloomington, Indiana, area was conducted using a 0.5-m2 grid to ensure the precise determination of the location, patchiness, and size of the populations of the five species of the Paramecium aurelia complex found in central Indiana. The number of killer paramecia in each collection was also determined. Competition experiments were performed with killers and sensitives at densities observed in nature.It was discovered that density-independent factors regulated the population size of paramecia in nature. Paramecia generally exist in small, clumped, and temporary resource patches. Several species often inhabit the same patch yet statistical analysis does not demonstrate that competitive exclusion occurs. The spatial and temporal heterogeneity of the environment is the likely factor allowing the very similar species to co-exist. The exception may be the species of P. tetraurelia, which exhibits the killer trait. Paramecium tetraurelia may successfully exploit specific resource patches that enable the species to reach particularly high densities compared with those in which other species of paramecia are found. Paramecium tetraurelia killers, as shown by the laboratory experiments, can outcompete sensitives. They are also rarely found with sensitive paramecia. Field collections also indicate that killers rarely have an opportunity to mate. An analysis of the genetics of the killer trait demonstrates that only infection due to cytoplasmic exchange or natural selection can maintain the killer trait in a population. Therefore, only natural selection could be responsible for the continued existence of killer paramecia in nature.