Flagellar motility during sperm chemotaxis and phototaxis in fucalean algae

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, <i>Hormosira banksii</i>, 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 <i>H. banksii</i>. 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.