Experimental ecology and evolution of microbial diversity : the role of spatial structure

M.G.J.L. Habets

Research output: Thesisinternal PhD, WU

Abstract

In the light of the competitive exclusion principle, which states that complete competitors cannot coexist, many explanations have been sought to explain the high diversity found in nature. The most common explanation is the niche differentiation hypothesis: coexistence is obtained through differentiation of species in ecological niches. Spatial structure is thought to be a factor capable of providing opportunities for niche differentiation. We have focused on four aspects of spatial structure enabling genetic diversity to emerge and /or to be maintained.
First of all, population fragmentation, resulting from growth in spatially structured habitats, can increase diversity, because the resulting smaller subpopulations, due to their smaller population size, are more likely to adaptively diverge. By allowing small and large populations of E. coli to evolve for 500 generations in two different nutrient environments, we test this hypothesis. The results demonstrate higher variance in fitness among small populations, and consequently more heterogeneous adaptive trajectories for small populations, some of which surprisingly lead to higher fitness peaks than reached by even the best adapted large population.
In a short-term invasion experiment between a superior E. coli competitor and its inferior ancestor, we demonstrate that populations residing in structured environments experience slower invasion dynamics of beneficial mutations than well-mixed populations due to limited dispersal, and therefore local competition. Moreover, our results demonstrate a deceleration of invasion with increasing size of the invading subpopulation. This is caused by a decrease of inter specific competition relative to intra specific competition. Since inferior competitors are present in the community for a longer period of time, they can recombine with other persisting lineages or obtain new mutations, some of which might be beneficial. It is therefore possible that polymorphisms arise which would not have had the opportunity to emerge in a well-mixed environment. Even though both population fragmentation and slower competitive dynamics can increase the emergence of diversity, they do not provide a means for their maintenance.
Environmental heterogeneity on the other hand can cause maintenance of diversity. Environmental heterogeneity can be introduced by spatial structure, e.g. by providing gradients in biotic and abiotic factors, thereby increasing the number of niches. By allowing E. coli populations to evolve for 900 generations in either a well-mixed environment or two structured environments (with or without dispersal), we demonstrate stable coexistence of diversity in structured populations without dispersal. This can be attributed to negative frequency-dependent fitness interactions among niche specialists that either inhabit existing niches provided by the heterogeneous environment or new niches constructed by organisms inhabiting the environment.
In addition to examining aspects of spatial structure that provide means for populations to diversify, we examine a specific consequence of slower dynamics and environmental heterogeneity: the probability of mutators to hitchhike to fixation. Understanding the emergence of mutators is not only scientifically important, but also relevant for human health, since high frequencies of mutators have been found in bacterial populations and drug resistant mutants arise more often in mutator populations. E. coli mutator populations were introduced at different starting frequencies in a well-mixed environment and two structured environments differing in their dispersal rate. Contrary to expectations, we find an advantage in the rate of invasion for mutators in well-mixed environments. Faster competitive dynamics may allow a rapid increase of population size and hence a greater supply of mutations for subsequent adaptation. Due to a delay in mutator extinction in structured environments at low frequencies, mutators may gain from fluctuating conditions.






Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Hoekstra, Rolf, Promotor
  • de Visser, Arjan, Co-promotor
Award date1 Feb 2008
Place of PublicationS.l.
Print ISBNs9789085048619
Publication statusPublished - 2008

Keywords

  • microorganisms
  • diversity
  • biodiversity
  • evolution
  • ecology
  • adaptation
  • heterogeneity
  • mutants
  • microbial diversity

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