New insights into sperm rheotaxis, agglutination and bundle formation in Sharkasi chickens based on an in vitro study

Abstract

Fertility in birds is dependent on their ability to store adequate populations of viable sperm for extended durations in sperm storage tubules (SSTs). The exact mechanisms by which sperm enter, reside, and egress from the SSTs are still controversial. Sharkasi chicken sperm showed a high tendency to agglutinate, forming motile thread-like bundles comprising many cells. Since it is difficult to observe sperm motility and behavior inside the opaque oviduct, we employed a microfluidic device with a microchannel cross-section resembling close to that of sperm glands allowing for the study of sperm agglutination and motility behavior. This study discusses how sperm bundles are formed, how they move, and what role they may have in extending sperm residency inside the SSTs. We investigated sperm velocity and rheotaxis behavior when a fluid flow was generated inside a microfluidic channel by hydrostatic pressure (flow velocity = 33 µm/s). Spermatozoa tended to swim against the flow (positive rheotaxis) and sperm bundles had significantly lower velocity compared to lonesome sperm. Sperm bundles were observed to swim in a spiral-like motion and to grow in length and thickness as more lonesome sperm are recruited. Sperm bundles were observed approaching and adhering to the sidewalls of the microfluidic channels to avoid being swept with fluid flow velocity > 33 µm/s. Scanning and transmission electron microscopy revealed that sperm bundles were supported by a copious dense substance. The findings show the distinct motility of Sharkasi chicken sperm, as well as sperm's capacity to agglutinate and form motile bundles, which provides a better understanding of long-term sperm storage in the SSTs.

Sperm tendency to agglutinate in motile bundles in relation to sperm competition and fertility duration in chickens

“…The gradual egress of spermatozoa into the oviduct lumen is thought to be caused by random detachment of agglutinated sperm 4 . Therefore, we suggest that the agglutination behavior of Sharkasi sperm gives them superiority over Dandarawi sperm when present in a competitive situation to fertilize the ova because sperm are lost from the SSTs when the agglutination state is not sustained.. El-sherry et al 26 reported that sperm agglutination in motile bundles increases the number of sperm showing positive rheotaxis which is suggested to benefit sperm during the vaginal selection process (passage of sperm through the vagina) and the SSts uptake (sperm penetration into SSTs). According to this assumption, more sperm from Sharkasi will occupy vacant areas of the SSTs compared to Dandarawi sperm.…”

“…1 and 2 ). A detailed description of the sperm bundles is reported by El-sherry et al 26 . With time, the bundles grow and increase in diameter and length (Supplementary Video 1 ).…”

“…The results of the current study showed clear differences between Sharkasi and Dandarawi chickens in the degree of sperm agglutination represented by differences in the thickness and length of the sperm bundles, which were revealed by the examination under the phase-contrast microscope and confirmed by the examination using the scanning electron microscope and fluorescence microscopy. We have previously described for the first time how the formation and movement of these bundles occur in our laboratory 26 . It was also observed that, from our previous study, once the semen is released from the male genital tract, sperm agglomerate and is sequestered from the semen plasma.…”

“…Moreover, irrespective of this assumption, the higher tendency of Sharkasi sperm to agglutinate inside the SSTs keeps their residence for longer periods which in turn increases their chances to fertilize more ova than Dandarawi sperm. In other words 26 , lonesome sperm is suggested to exit from the SSTs first (followed by those detached from agglutinated sperm bundles. So, lonesome sperm (few Sharkasi and most of Dandarawi sperm) is suggested to egress the SSTs and compete to fertilize the first egg, then the sperm detached from agglutinated bundles remaining in the SSTs (few Dandarawi and most of Sharkasi sperm) compete to fertilize the subsequent eggs which gives higher opportunity to Sharkasi sperm to fertilize more ova.…”

“…Acridine orange stain 26,76 . Sperm samples (n = 6 for each stains) were collected, diluted, Semen samples from both strains were extended at dilution rates of 1:40.…”