Abstract
ABSTRACTDuring their chemotactic swimming toward eggs, sperm cells detect their species-specific chemoattractant and sense concentration gradients by unknown mechanisms. After sensing the attractant, sperm cells commonly demonstrate a series of responses involving different swimming patterns by changing flagellar beats, gradually approaching a swimming path toward the eggs, which is the source of chemoattractants. Shiba et al. observed a rapid increase in intracellular Ca2+ concentrations in Ciona spermatozoa after sensing chemoattractants; however, the biochemical processes occurring inside the sperm cells are unclear. In the present study, we focused on the timing and sensing mechanism of chemical signal detection in Ciona. One of the most crucial problems to be solved is defining the initial epoch of chemotactic responses. We adopted a high rate of video recording (600 Hz) for detailed analysis of sperm motion and a novel method for detecting subtle signs of beat forms and moving paths of sperm heads. From these analyses, we estimated a virtual sensing point of the attractant before initiation of motility responses and found that the time delay from sensing to motility responses was almost constant. To evaluate the efficiency of this constant delay model, we performed computer simulation of chemotactic behaviors of Ciona spermatozoa.
Highlights
High efficiency in chemotactic responses and orientation toward eggs are the primary tasks of animal spermatozoa
We defined the nearest position of the swimming orbits from the source of attractant, sperm-activating and -attracting factor (SAAF); hereafter, we refer to this specific position and time as h5h0 (h050), and t5T0, respectively
Further analysis of the statistical variations in response time based on our working hypothesis, suggest that there is a mechanism to sense the change in SAAF concentration (TS) 0.18 s before time for response (TR)
Summary
High efficiency in chemotactic responses and orientation toward eggs are the primary tasks of animal spermatozoa. Because the patterns of swimming behavior during chemotaxis are similar among several animal species, it is assumed that there are a series of common mechanisms, from receiving external cue signals and transmitting intracellular signals to responding by controlling flagellar beat patterns. In invertebrate animals, spermatozoa swim in constant spiral or circular orbits without the stimuli of attractants. When they sense chemoattractants, they demonstrate a series of responses to change flagellar beating patterns and swimming paths, i.e., short turning and straight swimming, and a recovery to the original swimming patterns drawing circular orbits, which gradually leads sperms toward eggs, the source of chemoattractants
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