Phenotypic Plasticity: Mixing Darwinian and Lamarckian Adaptation in our bioRxiv Weekend Preprint Review

In a paper that is bound to raise the expectations of trolls in the debate on the role of adaptation in natural selection, the paper below analyzes genome changes in a microbial population exposed to the stress of a toxic environment. Darwinian selection leads to shifts in the average genome of the population towards those that can better induce the enzymes needed to detoxify the stressed cells.

How is this Lamarckian? Actually, it isn't. Lamarck's typical example would be one in which later generations of giraffes had longer necks as a shift in the population toward a better reach to the greenery of the tops of leafy trees induced by the animals straining upward. Even when shifted to a plains environment without trees, the offspring of giraffes that stretched their necks would keep their long necks, in Lamarck's theory. But the paper below's version of Lamarckian adaptation (not really Lamarck's) would rather be one in which the giraffes could grow longer necks if they were in an environment where stretching upwards was needed, but in an environment where the giraffe had no need to stretch upwards, the offspring of long-necked giraffes would only have short necks, and the long necks would never be seen. In biology, this is termed phenotypic plasticity. I would reframe the paper's reference to Lamark, calling the makeup of the host organism's bacterial flora a part of the host's phenotype. In humans, where we surely consider our commensal bacteria part of phenotype (or even just part of our environment) and not genotype, another common, minor example of phenotypic plasticity is the ability to tan the skin under sunlight exposure.

Is phenotypic plasticity where Lamarck was right? I think not. That honor belongs to epigenetics.

One interesting feature of the paper is that its "Holobiont" organism is partially composed of microbial elements all of which could be germ plasm for another holobiont, unlike mammals which have separate populations of haploid cells. This emphasizes a key difference in natural selection of animals versus some plants and most bacteria.



Darwinian Selection Induces Lamarckian Adaptation in a Holobiont Model

Dino Osmanovic, David A Kessler, Yitzhak Rabin, Yoav Soen



Current models of animal evolution focus on selection of individuals, ignoring the much faster selection of symbiotic bacteria. Here we take host-symbiont interactions into account by introducing a Population Genetics-like model of holobionts exposed to toxic stress. The stress can be alleviated by selection of resistant individuals (host and bacteria) and by secretion of a detoxification agent (“detox”). By defining a new measure, termed the ‘Lamarckian’, we show that selection of resistant bacteria over one generation of hosts leads to stress-dependent increase in the tolerance of the hosts' offspring. This benefit is mediated by co-alleviation of toxic and physiologic stress. Prolonged exposure leads to further adaptation by ‘group selection’ of bacterial communities with higher detox per bacterium. These findings show that Lamarckian adaptation can arise via interactions between two levels of Darwinian selection within a holobiont system. The conclusions and modelling framework are applicable to diverse types of holobiont systems.

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