Rapid evolution of tolerance to toxic Microcystis in two cladoceran grazers.

Xiaodong Jiang,Han Gao,Lihua Zhang,Huishuang Liang,Xiao Zhu

doi:10.1038/srep25319

Abstract

Evolutionary adaptation could assist organisms to cope with environmental changes, yet few experimental systems allow us to directly track evolutionary trajectory. Using experimental evolution, evolutionary tolerance to Microcystis aeruginosa was investigated in two cladocerans (Daphnia pulex and Simocephalus vetulus) to test the hypothesis that cladoceran grazers rapidly adapt to toxic cyanobacteria. After exposure for either three or six months, both grazers evolved a higher tolerance. The intrinsic rate of population increases in S. vetulus feeding on cyanobacteria was negatively correlated with that on green algae, which suggests that evolutionary adaptation in tolerance would carry a cost in the absence of cyanobacteria. However, the cyanobacterial selection resulted in a general increase in D. pulex when fed both cyanobacteria and green algae. Following a three-month relaxation of selection, S. vetulus in the selection line exhibited reverse evolution back to their original state when their diets were switched back to pure green algae. The present experimental evolution, both forwards and reverse, not only demonstrates the evolutionary responses of cladoceran grazers to toxic cyanobacterial cells in the laboratory, but also indicates that the grazer-cyanobacteria interaction would be an effective system to empirically study rapid evolution to environmental changes.

Highlights

Environmental changes have shifted the growth/reproduction times of organisms, altered the geographic distributions of populations, changed the composition of communities, and shaped the nature of species interactions[1,2]
The present results of experimental evolution are consistent with previous reports on the adaptive evolution of tolerance to toxic cyanobacteria from the viewpoints of local adaptation[14,15] and paleolimnological records[16]
Our results clearly demonstrated that this evolutionary process occurred within a short time span may be a general response to toxic cyanobacteria for cladoceran grazers via extending test animal from Daphnia[18] to other species

Summary

Introduction

Environmental changes have shifted the growth/reproduction times of organisms, altered the geographic distributions of populations, changed the composition of communities, and shaped the nature of species interactions[1,2]. A wave of recent studies have highlighted that adaptive phenotypic evolution can be rapid across a wide range of taxa[7]. These exciting results imply that adaptive evolution could assist organisms to cope with environmental changes to a greater extent than previously thought[8]. CyanoHABs produce and release a wide variety of toxins or secondary compounds[12], which can have harmful effects on aquatic grazers, such as impaired feeding, physiological dysfunction, depressed reproduction, and increased mortality[13]. The direction of selection may not be very clear in such reconstructed populations because grazer performance integrates the adaptive evolution to toxic cyanobacteria during blooms, the reversal during non-blooms, and other selection forces, such as predation, competition and so on. We seek to extend this observation by asking the following questions: (1) Can cladoceran grazers rapidly adapt to toxic Microcystis within the time span of a typical bloom? (2) If so, is there a cost or trade-off for evolved tolerance? (3) Do they return to ancestral status during a non-bloom period?

Methods

Results

Conclusion