Slime Moulds – Farming Bacteria

Australia: The Land Where Time Began

Slime Moulds – Farming Bacteria

It has been shown that the slime mould Dictyostelium farm bacteria, which according to Boomsma sheds light on the trade-offs governing incipient domestication.

Dictyostelium is a unicellular amoeba that lives in the soil which reproduces sexually (Bloomfield et al., 2010), though best known for social reproduction which involves the aggregation of cells to form a motile multicellular slug which subsequently produces a fruiting body that contains asexual spores (Kessin, 2001). They do this when their local bacterial prey which they feed upon in a “quite catholic” manner (Brock et al., 2011), has been exhausted. The species Dictyostelium discoideum is a particularly important model system, for biologists who are studying the origin of multicellularity (Kessin, 2001) and for evolutionary biologists in the study of conflicts and cooperation occurring under the conditions of kin selection (Gilbert et al., 2007). It is demonstrated (Brock et al., 2011) that a significant fraction of D. discoideum spores carry bacteria with which they inoculate new habitat with food, though this husbandry has remained clone-specific, which is practiced by about ¹/₃ of the strains that have been examined.

It is indicated by molecular and experimental evidence that different strains are either farmers or non-farmers and that substantial fitness benefits are conferred on Dictyostelium by spore-borne bacteria when the spores land of patches where there is a scarcity of suitable food. As patches cannot be exhaustively grazed before the cells aggregate in the process of reproduction this form of farming involves significant costs for the farming strains of Dictyostelium. Shorter distances are also covered by slugs that are loaded with bacteria, and when spores germinate in places where there are already food bacteria the investment in co-transmission is wasted. According to Boomsma¹ Dictyostelium farming polymorphism is an example of a mixed evolutionarily stable strategy (Maynard Smith, 1979). In spite of the obvious risks of bacterial exploitation of the dispersal opportunities that are provided by the host, evidently the Dictyostelium-bacterial symbiosis is driven by mutualistic advantages.

Dictyostelium slime moulds are an ancient sister group to fungi and animals combined. Therefore the question arises why the benefits of farming is such a mixed bag. According to Boomsma what has stalled developments in the direction of farming that is unambiguously profitable is that slime moulds didn’t specialise on a single bacterial species, and if they had specialised the bacterial species involved in the symbiosis could have co-speciated with its host to the point where it lost its free-living stages. Slime moulds have not adapted to farming, though their bacterial ‘crops’ have continued to be subjected to selection for independent growth. There is a lack of substrate facilitation, reinforcement of growth, or removal of competitors, as occurs in the classic fungus-farming symbioses (Mueller et al., 2005) and in the more recently discovered incipient practices of fungus-farming by snails, Littoraria, and red alga farming by damselfish, Stegastes (Hata, Watanabe & Kato, 2010).

The mutualism that is observed in damselfish has resulted in at least 1 case of monoculture in which the crops have no free-living relatives. According to Boomsma¹ underlying the point that transmission from parent to offspring is not sufficient to install absolute co-dependency in mutualism, is that husbandry by slime mould has not achieved this status. It appears that some form of monoculture farming is essential before symbionts give up their ability of living free, as being eaten is only profitable if it benefits clone mates that are nursed and dispersed. These kin-selected benefits are made consistent by monocultural commitment (Aanen et al., 2009). Therefore in slime moulds the limitations of bacterial husbandry (Brock et al., 2011) clarify a major cornerstone of the understanding of mutualistic interactions. Further study is needed to unravel the molecular mechanisms that underlie or prevent bacterial transmission, and the elucidation of the dynamics of food transmission in slugs that are genetic mixtures of several strains (Gilbert et al., 2007).

In slime moulds and humans, the farmers did not become isolated reproductively from the non-farmers, and their crops and livestock did not lose the ability to hybridise with wild members of the same species, as has occurred in the symbioses of insect fungus-farming (Mueller et al., 2005; Aanen et al., 2009). Boomsma¹ suggests slime moulds may lack the necessary degree of multicellular complexity to evolve specialisations for nursing traits for particular crops, while humans lacked evolutionary time and consistent selection for extreme specialisation in the crops. The farming by the social ants and termites were subject to neither of these constraints.

Dictyostelium may possess as yet unknown adaptations, the discovery of which would illuminate fundamental questions of conflict and cooperation across the boundaries of species, though Dictyostelium do not actively rear their crops. Ancestral slime moulds were among the earliest organisms to colonise terrestrial habitats, therefore the history of this bacterial husbandry symbiosis could possibly extend back further than other system of farming.

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