Abstract
Phototaxis in the broadest sense means positive or negative displacement along a light gradient or vector. Prokaryotes most often use a biased random walk strategy, employing type I sensory rhodopsin photoreceptors and two-component signalling to regulate flagellar reversal. This strategy only allows phototaxis along steep light gradients, as found in microbial mats or sediments. Some filamentous cyanobacteria evolved the ability to steer towards a light vector. Even these cyanobacteria, however, can only navigate in two dimensions, gliding on a surface. In contrast, eukaryotes evolved the capacity to follow a light vector in three dimensions in open water. This strategy requires a polarized organism with a stable form, helical swimming with cilia and a shading or focusing body adjacent to a light sensor to allow for discrimination of light direction. Such arrangement and the ability of three-dimensional phototactic navigation evolved at least eight times independently in eukaryotes. The origin of three-dimensional phototaxis often followed a transition from a benthic to a pelagic lifestyle and the acquisition of chloroplasts either via primary or secondary endosymbiosis. Based on our understanding of the mechanism of phototaxis in single-celled eukaryotes and animal larvae, it is possible to define a series of elementary evolutionary steps, each of potential selective advantage, which can lead to pelagic phototactic navigation. We can conclude that it is relatively easy to evolve phototaxis once cell polarity, ciliary swimming and a stable cell shape are present.
Highlights
Phototaxis in the broadest sense means positive or negative displacement along a light gradient or vector
Eukaryotes evolved the capacity to follow a light vector in three dimensions in open water
This strategy requires a polarized organism with a stable form, helical swimming with cilia and a shading or focusing body adjacent to a light sensor to allow for discrimination of light direction
Summary
Most prokaryotes are unable to sense the direction of light, because at a small scale it is very difficult to make a detector that can distinguish a single light direction. Some gliding filamentous prokaryotes can even sense light direction and make directed turns, but their phototactic movement is very slow. Some species among both eubacteria and archaebacteria (archaea) are phototactic (Scharf & Wolff 1994; Armitage & Hellingwerf 2003). In most cases the mechanism of phototaxis is a biased random walk, analogous to bacterial chemotaxis Halophilic archaebacteria, such as Halobacterium salinarum, use sensory rhodopsins (SRs) for phototaxis (Luecke et al 2001; Spudich 2006). The positive response is probably mediated by a bacteriophytochrome photoreceptor, TaxD1 This protein has two chromophore-binding GAF domains, which bind. The slow steering of these cyanobacterial filaments is the only light-direction sensing behaviour prokaryotes could evolve owing to the difficulty in detecting light direction at this small scale
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More From: Philosophical Transactions of the Royal Society B: Biological Sciences
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