Australia: The Land Where Time Began

A biography of the Australian continent 

Slime Moulds – Co-existence of Diverse Genotypes in Cellular Slime Moulds Can be Explained by Tradeoffs of Fitness Between Spores and Nonaggregating Cells

Cellular clime moulds, such as Dictyostelium discoideum, are amoebae that include single-cellular and multicellular stages in their life cycle, the multicellular stage being achieved when they are starving, at which time the individual amoebae aggregate. In the aggregate they are not necessarily clonal; there is strong selection of the individuals to be represented in the reproductive spores. A reduction in overall genotypic diversity would result from this process, which is not consistent with the wide range of diversity that is observed in nature. Tarnita et al. suggest that an additional component of fitness is provided by the cells that fail to aggregate, in which case this can solve the inconsistency: Strong selection for aggregation only occurs in environments where food is replenished slowly. Otherwise unicellularity is strongly selected.

Individual amoebae aggregate upon starvation forming a fruiting body comprised of dead stalk cells and reproductive spores. This process has been described in terms of altruism. Chimeric fruiting bodies result from nonkin not being perfectly discriminated against. Complex interactions among genotypes within chimeras have been documented, which should theoretically reduce genetic diversity. This is, however, not consistent with the wide diversity of genotypes observed in nature. Recent work has shown that the loner cells that do not participate in aggregation, which is a component of Dictyostelium discoideum fitness that has not been well studied, can be selected for, depending on environmental conditions. Tarnita et al. suggest that the existence of loners in all cellular slime moulds could resolve the apparent diversity paradox in 2 ways. The apparent genotypic skew in the spores of chimeras could simply be the result of different investments into spores verses loners. Alternatively, coexistence of multiple genotypes can occur in ecosystems in which multiple local environments differ in the characteristics of food recovery and are connected globally by weak-to-moderate dispersal. Tarnita et al. argue that the defining of altruistic behaviour, winners and losers, is made impossible by the presence of loners.

Sources & Further reading

Tarnita, C. E., A. Washburne, R. Martinez-Garcia, A. E. Sgro and S. A. Levin (2015). "Fitness tradeoffs between spores and nonaggregating cells can explain the coexistence of diverse genotypes in cellular slime molds (slime mould) " Proceedings of the National Academy of Sciences 112(9): 2776-2781.