Abstract
Most swarmers (swimming cells) of the stramenopile group, ranging from unicellular protist to giant kelps (brown algae), have two heterogeneous flagella: a long anterior flagellum (AF) and a relatively shorter posterior flagellum (PF). These flagellated cells often exhibit phototaxis upon light stimulation, although the mechanism by which how the phototactic response is regulated remains largely unknown. A flavoprotein concentrating at the paraflagellar body (PFB) on the basal part of the PF, which can emit green autofluorescence under blue light irradiance, has been proposed as a possible blue light photoreceptor for brown algal phototaxis although the nature of the flavoprotein still remains elusive. Recently, we identified helmchrome as a PF-specific flavoprotein protein in a LC-MS/MS-based proteomics study of brown algal flagella (Fu et al. 2014). To verify the conservation of helmchrome, in the present study, the absence or presence and the localization of helmchrome in swarmers of various algal species were investigated. The results showed that helmchrome was only detected in phototactic swarmers but not the non-phototactic ones of the stramenopile group. Electron microscopy further revealed that the helmchrome detectable swarmers bear a conserved PFB-eyespot complex, which may serve as structural basis for light sensing. It is speculated that all three conserved properties: helmchrome, the PFB structure, and the eyespot apparatus, will be essential parts for phototaxis of stramenopile swarmers.




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Abbreviations
- AUREO:
-
Aureochrome
- bZIP:
-
Basic leucine zipper
- ChR:
-
Channelrhodopsin
- DAPI:
-
4′,6-Diamidino-2-phenylindole
- HELMC:
-
Helmchrome
- LOV:
-
Light-oxygen-voltage sensing
- PAC:
-
Photoactivated adenylyl cyclase
- PAS:
-
Per-ARNT-Sim
- PFB:
-
Paraflagellar body
- PHOT:
-
Phototropin
- PHY:
-
Phytochrome
- RGS:
-
Regulator of G protein signaling
- VVD:
-
Vivid
- WC:
-
White collar
- ZnF:
-
Zinc finger
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Acknowledgments
We would like to thank Dr. Hidetoshi Okuyama, Faculty of Environmental Earth Science, Hokkaido University, for kindly providing the Thraustochytrid-like 12B strain. We also thank Dr. J. Mark Cock, University Pierre et Marie Curie and Centre National de la Recherche Scientifique, France, who kindly gave us many valuable comments and corrected our English.
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Supplementary Fig. S1
Immunofluorescence microscopy images of 25 swarmers investigated. Images of α-tubulin (red), helmchrome (green), and merged with DAPI (blue) are shown for each sample. The cells are orientated with the AF extending upwards and the PF extending opposite. a Female gamete of Scytosiphon lomentaria. b Female gamete of Culteria cylindrica. c Zoospores of Desmarestia ligulata. d Sperm of Silvetia babingtonii. e Zoospore of Chorda filum. f Zoospore of Saccharina angustata. g Zoospore of Undaria pinnatifida. h Zoospore of Alaria crassifolia. i Zoospore of Agarum clathratum. j Dinobryon sertularia. k Dinobryon sociale. l Zoospore of Thraustochytrid-like 12B. m Zoospore of Sicyoidochytrium sp. n Zoospore of Ulkenia amoeboidea. o Zoospore of Parietichytrium sp. p HG193 Symbiodinium sp. q HG243 Symbiodinium sp. r HG241 Bysmatrum sp. s TM106 Gymnodinium sp. t Male gamete of Ulva pertusa. u Female gamete of Ulva pertusa. v Female gamete of Bryopsis plumosa. w Chrysochromulina hirta. Scale bars, 5 μm. (GIF 266 kb).
Supplementary Fig. S2
Electron microscopy image of a Ulva pertusa gamete. Note the association of eyespot and flagella is different from that of phototactic brown algal swarmers. Ch, chloroplast; Es, eyespot; F, flagellum; M, mitochondria. Scale bar, 500 nm. (GIF 95 kb).
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Fu, G., Nagasato, C., Yamagishi, T. et al. Ubiquitous distribution of helmchrome in phototactic swarmers of the stramenopiles. Protoplasma 253, 929–941 (2016). https://doi.org/10.1007/s00709-015-0857-7
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DOI: https://doi.org/10.1007/s00709-015-0857-7