Flagellar motility during sperm chemotaxis and phototaxis in fucalean algae

ABSTRACT

The swimming of male gametes of brown algae is controlled by two heterokont flagella, regardless of reproductive system, but the patterns of flagellar movement can vary. We investigated sperm chemotaxis and phototaxis in four oogamous Australasian fucalean species. Sperm morphology was similar to that of male gametes of isogamous and anisogamous species but their chemotactic behaviours were much closer to those of other oogamous species than those of isogamous and anisogamous species. Moreover, unlike anisogamous species, sperm chemotaxis of the oogamous species, Hormosira banksii, was not inhibited by a phosphodiesterase inhibitor, theophylline, suggesting variability in regulatory mechanisms of sperm chemotaxis amongst brown algae. In our previous work, lower extracellular Ca²⁺, supernatant of female gametes and theophylline induced the reversal of phototactic signs in anisogamous male gametes. In contrast, lower extracellular Ca²⁺ and theophylline did not significantly affect phototaxis in sperm of the oogamous H. banksii. Moreover, high extracellular Ca²⁺ and supernatant of eggs disrupted the negative phototaxis but did not reverse their phototactic sign. These results suggest that the regulation of phototaxis also varies amongst brown algal gametes. Further investigation of brown algal species is warranted to assess the generality of these patterns of chemotaxis and phototaxis among anisogamous and oogamous gametes.

KEYWORDS:

• Brown algae
• fucoid
• heterokont
• macroalgae
• oogamy
• phototaxis
• sex pheromone
• sexual reproduction

Acknowledgements

We thank Jasmine Bursic and Paul Tinkler for assistance with collections.

Disclosure statement

No potential conflict of interest was reported by the authors.

supplementary-material

The following supplementary material is accessible via the Supplementary Content tab on the article’s online page at: https://doi.org/10.1080/09670262.2020.1767307

Supplementary table 1. Diversity of cell size and flagellar length in four species of fucalean algae (mean ± SD, n = 7).

Supplemental figure 1. Diversity of sperm structures. Arrowheads indicate the position of eyespot. Scale bars: 10 µm.

Supplemental figure 2. Chemotactic responses of sperm in Hormosira banksii under addition of 5 mM theophylline. The two seconds trajectories of sperm are shown. * indicates the position of an egg. Scale bar: 200 µm.

Supplemental figure 3–4. Phototaxis of sperm in Hormosira banksii for different times after spawning. Fig. 3. 0.5 s swimming trajectories of sperm without and with blue light (blue arrows). Red and blue dots indicate start and end point of their swimming, respectively. Fig. 4. Mean (± SD) swimming velocities of sperm in the dark and under blue light (n = 28). SME (Dark vs. Blue light), **p<0.01, ***p<0.001. Scale bars: 200 µm.

Supplemental Figs 5–6. Phototaxis of sperm in Hormosira banksii under several conditions of observation medium. Fig. 5. 0.5 s swimming trajectories of sperm without and with blue light (blue arrows). Red and blue dots indicate start and end point of their swimming, respectively. Fig. 6. Mean (± SD) swimming velocities of sperm in the dark and under blue light (n = 28). SME (Dark vs. Blue light), *p<0.05, ***p<0.001. Scale bars: 200 µm.

Author contributions

N. Kinoshita-Terauchi: conceptualization, investigation and formal analysis and drafting and editing manuscript; A. Bellgrove: brown algal sampling, investigation, statistical analysis drafting and editing manuscript; K. Shiba: drafting and editing manuscript; K. Inaba: conceptualization, drafting and editing manuscript.

Supplemental Material

The following supplementary material is accessible via the Supplementary Content tab on the article’s online page at https://doi.org/10.1080/09670262.2020.1767307.

Additional information

Funding

This work was partially supported by the Japan Society for the Promotion of Science (JSPS) Fellowship to N.K-T., JSPS Grant-in-Aid for Scientific Research (A) and for Innovative Areas to K.I. and funding from Deakin University and the Centre for Integrative Ecology to A.B.