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Efficient Heritable Gene Expression Readily Evolves in RNA Pools.

ABSTRACT: Heritable gene expression arises readily in a simple non-genetic system employing known small-RNA biochemistry. Pooled cross-templating ribonucleotides show varied chemical competence on which selection acts, even calculating only minimal effects. Evolution can be quick-computed progress toward encoded gene expression can require only days or weeks for two millimolar, partly activated complementary 5' ribonucleotides. After only one product selection cycle, early templating can become prevailing pool behavior. Subsequently, a selected templated product is efficiently amplified as a pool ages, frequently accumulated in the same order of concentration as incoming nucleotides. Pools spontaneously favor templating because sporadic nucleotide accumulations increase it-and selection increases templating in pools of all ages. Nonetheless, templated chemical competence appears most easily in young pools. Pool history is critical-pools can perish from periodic hazards (like tides), or alternatively, from hazards roughly constant in time (like rainfall). Selection is greatly enhanced in constant hazard pools-more effective if pools have varied ages. Stronger selection is disproportionately more effective. Selected evolutionary change has an uncomplicated molecular basis-progress from chemical product synthesis to templated, proto-genetic inheritance exploits identity between templating and entropic catalysis. Though discovered by computation, selection of an elevated product of template catalysis is plausible, independent of any chemical or mathematical assumption. Selected chemical variation before genetics (chance utility) therefore inaugurates inheritance, even when hindered by unstable, dilute nucleotides, erratically supplied in undependable quantities. Remarkably, such uncontrolled conditions are not necessarily hostile, but can instead accelerate appearance of primordial gene-like behavior.

SUBMITTER: Yarus M

PROVIDER: S-EPMC5501911 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Host genetic variation plays an important role in the structure and function of heritable microbial communities. Recent studies have shown that insects use immune mechanisms to regulate heritable symbionts. Here we test the hypothesis that variation in symbiont density among hosts is linked to intraspecific differences in the immune response to harboring symbionts. We show that pea aphids (Acyrthosiphon pisum) harboring the bacterial endosymbiont Regiella insecticola (but not all other species of symbionts) downregulate expression of key immune genes. We then functionally link immune expression with symbiont density using RNAi. The pea aphid species complex is comprised of multiple reproductively-isolated host plant-adapted populations. These 'biotypes' have distinct patterns of symbiont infections: for example, aphids from the Trifolium biotype are strongly associated with Regiella. Using RNAseq, we compare patterns of gene expression in response to Regiella in aphid genotypes from multiple biotypes, and we show that Trifolium aphids experience no downregulation of immune gene expression while hosting Regiella and harbor symbionts at lower densities. Using F1 hybrids between two biotypes, we find that symbiont density and immune gene expression are both intermediate in hybrids. We propose that in this system, Regiella symbionts are suppressing aphid immune mechanisms to increase their density, but that some hosts have adapted to prevent immune suppression in order to control symbiont numbers. This work therefore suggests that antagonistic coevolution can play a role in host-microbe interactions even when symbionts are transmitted vertically and provide a clear benefit to their hosts. The specific immune mechanisms that we find are downregulated in the presence of Regiella have been previously shown to combat pathogens in aphids, and thus this work also highlights the immune system's complex dual role in interacting with both beneficial and harmful microbes.

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Efficient Heritable Gene Expression Readily Evolves in RNA Pools.

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