The effect of hybrid transgression on environmental tolerance in experimental yeast crosses

Evidence is rapidly accumulating that hybridization generates adaptive variation. Transgressive segregation in hybrids could promote the colonization of new environments. Here, we use an assay to select hybrid genotypes that can proliferate in environmental conditions beyond the conditions tolerated by their parents, and we directly compete them against parental genotypes in habitats across environmental clines. We made 45 different hybrid swarms by crossing yeast strains (both Saccharomyces cerevisiae and S. paradoxus) with different genetic and phenotypic divergence. We compared the ability of hybrids and parents to colonize seven types of increasingly extreme environmental clines, representing both natural and novel challenges (mimicking pollution events). We found that a significant majority of hybrids had greater environmental ranges compared to the average of both their parents' ranges (mid-parent transgression), but only a minority of hybrids had ranges exceeding their best parent (best-parent transgression). Transgression was affected by the specific strains involved in the cross and by the test environment. Genetic and phenotypic crossing distance predicted the extent of transgression in only two of the seven environments. We isolated a set of potentially transgressive hybrids selected at the extreme ends of the clines and found that many could directly outcompete their parents across whole clines and were between 1.5- and 3-fold fitter on average. Saccharomyces yeast is a good model for quantitative and replicable experimental speciation studies, which may be useful in a world where hybridization is becoming increasingly common due to the relocation of plants and animals by humans.

Antagonistic coevolution across productivity gradients: an experimental test of the effects of dispersal

Coevolution commonly occurs in spatially heterogeneous environments, resulting in variable selection pressures acting on coevolving species. Dispersal across such environments is predicted to have a major impact on local coevolutionary dynamics. Here, we address how co-dispersal of coevolving populations of host and parasite across an environmental productivity gradient affected coevolution in experimental populations of bacteria and their parasitic viruses (phages). The rate of coevolution between bacteria and phages was greater in high-productivity environments. High-productivity immigrants ( approximately 2% of the recipient population) caused coevolutionary dynamics (rates of coevolution and degree of generalist evolution) in low-productivity environments to be largely indistinguishable from high-productivity environments, whereas immigration from low-productivity environments ( approximately 0.5% of the population) had no discernable impact. These results could not be explained by demography alone, but rather high-productivity immigrants had a selective advantage in low-productivity environments, but not vice versa. Coevolutionary interactions in high-productivity environments are therefore likely to have a disproportionate impact on coevolution across the landscape as a whole.

The impact of parasite dispersal on antagonistic host-parasite coevolution

Coevolving populations of hosts and parasites are often subdivided into a set of patches connected by dispersal. Higher relative rates of parasite compared with host dispersal are expected to lead to parasite local adaptation. However, we know of no studies that have considered the implications of higher relative rates of parasite dispersal for other aspects of the coevolutionary process, such as the rate of coevolution and extent of evolutionary escalation of resistance and infectivity traits. We investigated the effect of phage dispersal on coevolution in experimental metapopulations of the bacterium Pseudomonas fluorescens SBW25 and its viral parasite, phage SBW25Phi2. Both the rate of coevolution and the breadth of evolved infectivity and resistance ranges peaked at intermediate rates of parasite dispersal. These results suggest that parasite dispersal can enhance the evolutionary potential of parasites through provision of novel genetic variation, but that high rates of parasite dispersal can impede the evolution of parasites by homogenizing genetic variation between patches, thereby constraining coevolution.

Diversity and productivity peak at intermediate dispersal rate in evolving metacommunities

Positive relationships between species diversity and productivity have been reported for a number of ecosystems. Theoretical and experimental studies have attempted to determine the mechanisms that generate this pattern over short timescales, but little attention has been given to the problem of understanding how diversity and productivity are linked over evolutionary timescales. Here, we investigate the role of dispersal in determining both diversity and productivity over evolutionary timescales, using experimental metacommunities of the bacterium Pseudomonas fluorescens assembled by divergent natural selection. We show that both regional diversity and productivity peak at an intermediate dispersal rate. Moreover, we demonstrate that these two patterns are linked: selection at intermediate rates of dispersal leads to high niche differentiation between genotypes, allowing greater coverage of the heterogeneous environment and a higher regional productivity. We argue that processes that operate over both ecological and evolutionary timescales should be jointly considered when attempting to understand the emergence of ecosystem-level properties such as diversity-function relationships.

Kin selection and the evolution of virulence

Social interactions between conspecific parasites are partly dependent on the relatedness of interacting parasites (kin selection), which, in turn, is predicted to affect the extent of damage they cause their hosts (virulence). High relatedness is generally assumed to favour less competitive interactions, but the relationship between relatedness and virulence is crucially dependent on the social behaviour in question. Here, we discuss the rather limited body of experimental work that addresses how kin-selected social behaviours affect virulence. First, if prudent use of host resources (a form of cooperation) maximizes the transmission success of the parasite population, decreased relatedness is predicted to result in increased host exploitation and virulence. Experimental support for this well-established theoretical result is surprisingly limited. Second, if parasite within-host growth rate is a positive function of cooperation (that is, when individuals need to donate public goods, such as extracellular enzymes), virulence is predicted to increase with increasing relatedness. The limited studies testing this hypothesis are broadly consistent with this prediction. Finally, there is some empirical evidence supporting theory that suggests that spiteful behaviours are maximized at intermediate degrees of relatedness, which, in turn, leads to minimal virulence because of the reduced growth rate of the infecting population. We highlight the need for further thorough experimentation on the role of kin selection in the evolution of virulence and identify additional biological complexities to these simple frameworks.

Experimental adaptation to high and low quality environments under different scales of temporal variation

We investigated the role of the scale of temporal variation in the evolution of generalism in populations of the bacterium Pseudomonas fluorescens. Replicate populations were propagated as batch cultures for approximately 1400 generations (192 days), in either high quality media only, low quality media only, or were alternated between the two at a range of temporal scales (between 1 and 48 days). Populations evolved in alternating media showed fitness increases in both media and the rate of alternation during selection had no effect on average fitness in either media. Moreover, the fitness of these populations in high quality media was the same as for populations evolved only in high quality media and likewise for low quality media. Populations evolved only in high or low quality media did not show fitness improvements in their nonselective media. These results indicate that cost-free generalists can evolve under a wide range of temporal variation.

Spatial heterogeneity and the stability of host-parasite coexistence

Spatially heterogeneous environments can theoretically promote more stable coexistence of hosts and parasites by reducing the risk of parasite attack either through providing permanent spatial refuges or through providing ephemeral refuges by reducing dispersal. In experimental populations of Pseudomonas aeruginosa and the bacteriophage PP7, spatial heterogeneity promoted stable coexistence of host and parasite, while coexistence was significantly less stable in the homogeneous environment. Phage populations were found to be persisting on subpopulations of sensitive bacteria. Transferring populations to fresh microcosms every 24 h prevented the development of permanent spatial refuges. However, the lower dispersal rates in the heterogeneous environment were found to reduce parasite transmission thereby creating ephemeral refuges from phage attack. These results suggest that spatial heterogeneity can stabilize an otherwise unstable host-parasite interaction even in the absence of permanent spatial refuges.