Three-dimensional motion of avian spermatozoa

Observations have been made on spermatozoa from the domestic fowl, quail an d pigeon (non-passerine birds) and also from the starling and zebra finch ( passerine birds). In free motion, all these spermatozoa roll (spin) continu ously about the progression axis, whether or not they are close to a plane surface. Furthermore, the direction of roll is consistently clockwise (as s een from ahead). The flagellar wave has been shown to be helical and dextra l (as predicted) for domestic fowl sperm when they swim rapidly in low visc osity salines. Calculations have shown that their forward velocity is consi stent with their induced angular velocity but that the size of the sperm he ad is suboptimal for progression speed under these conditions. Dextrally he lical waves also occur on the distal flagellum of fowl, quail and pigeon sp erm in high viscosity solutions. But in other cases, the mechanism of torqu e-generation is more problematical. The problem is most profound for passer ine sperm, in that typically these cells spin rapidly while seeming to rema in virtually straight. Because there is no evidence for a helical wave on t hese flagella, we have considered other possible means whereby rotation abo ut the local flagellar axis (self-spin) might be achieved. Sometimes, passe rine sperm, while maintaining their spinning motion, adopt a fixed curvatur e; this must be an instance of bend-transfer circumferentially around the a xonemal cylinder-though the mechanism is obscure. It is suggested that the self-spin phenomenon may be occurring in non-passerine sperm that in some c ircumstances spin persistently, yet without expressing regular helical wave s. More complex waves are apparent in non-passerine sperm swimming in high viscosity solutions: added to the small scale bends is a large scale, sinis trally helical curvature of the flagellum. It is argued that the flagellum follows this sinistrally helical path (i.e. "screws" though the fluid) beca use of the shape of the sperm head and the angle at which the flagellum is inserted into it. These conclusions concerning avian sperm motility are tho ught to have relevance to other animal groups. Also reported are relevant a spects of flagellar ultrastructure for pigeon and starling sperm. (C) 1999 Wiley-Liss, Inc.