Biology subjects

Nelson, P. G.

Publications and source records attributed to Nelson, P. G..

2 recordsLinked to original sources

bioRxiv2018

Defensive symbiosis and the evolution of virulence

Although environments rife with enemies should cause selection for defensive traits, such enemy-rich environments should also select for greater virulence in co-occurring symbionts, yet many defensive symbionts cause little to no damage while protecting their hosts from enemies. Thus, co-infection of a defensive symbiont and a parasite is predicted to select both for increased virulence in co-infecting symbionts and for increased defense in the protective symbiont. Why then do we observe defensive mutualists that protect hosts while causing little damage? To address this question, we build a symbiont-centered model that incorporates the evolution of two independent traits: defense and virulence. Virulence is modeled as a continuous trait spanning mutualism (negative virulence) and parasitism (virulence) and thus accounts for the entire range of direct effects that symbionts have on host mortality. Defense is modeled as a continuous trait that ameliorates the costs to the host associated with co-infection with a deleterious parasite. We obtain the counterintuitive result that the evolution of increased defense in one symbiont largely leads to the evolution of lower virulence in both symbionts and may even facilitate pathogens evolving to mutualism. However, results are context-dependent and when defensive traits are costly, the evolution of greater defense may also lead to higher virulence.

evolutionary biology↗

bioRxiv2017

Evolutionary capacitance emerges spontaneously during adaptation to environmental changes

All biological populations are to a greater or lesser degree evolvable, but the forces that shape evolvability, especially the evolution of evolvability as an adaptive response to a changing environment, have been a source of controversy. One source of enhanced evolvability is the benign status of \"cryptic sequences\" typically expressed at low levels due to molecular errors, but with the potential to be expressed more fully following mutational co-option. A genome enriched for benign cryptic sequences has a more benign mutational neighborhood, via the possibility of co-option, and thus enhanced evolvability. Whether selection for evolvability itself can be the cause of a more benign mutational neighborhood remains an open question. Here, we show that environmental change can cause the evolution of increased evolvability, despite our use of a strong-selection weak mutation regime that precludes, by design, the adaptive evolution of evolvability. Instead, enhanced evolvability arises as a byproduct of environmental change via a novel mechanism that we call \"emergent evolutionary capacitance\". When the environment changes, increased molecular error rates evolve as a strategy to rapidly change phenotypes, with the side effect of purging deleterious cryptic sequences and enhancing the mutational neighborhood for future adaptation. The behavior is strikingly similar to that seen in a model system for capacitance, the yeast prion [PSI+].

evolutionary biology↗