What factors mediate sperm pairing in monodelphis domestica?
New World marsupials are unusual in that their sperm join and form pairs following spermiogenesis. Paired sperm are precisely and firmly joined together at their heads, their tails free to beat more vigorously together than singly. We investigated the effect on sperm pairing of environmental factors (temperature, pH, Ca++). Sperm pairing was unaffected by ambient temperatures slightly above or slightly below (35 and 31°C, respectively) body temperature (33°C), Ca++ in culture medium (DMEM with 0, 100 and 200 μM Ca++), and pH (DMEM at pH 6.7, 7.1, 7.4 or 8.1). We did, however, determine that a pH gradient exists in the epididymis in which sperm pairing occurs stepwise by region (unpaired in the caput, pairing in the corpus, and paired in the cauda). We also examined the role in sperm pairing of a cell-surface protein (SED1, or secreted protein containing EGF repeats and Discoidin/F5/8 complement domains) known to mediate sperm-zona adhesion. Using confocal analysis of immunohistochemically treated sperm, we detected SED1 on the sperm surface. Preliminary results indicated that the SED1 signal is stronger at the interface between paired sperm, which coincides with the cell membrane closest to the acrosome. We are currently determining whether or not application of the same polyclonal anti-SED1 antibody will cause paired sperm to unpair in vitro.
Cruz, Y.P., M.M. Sasaki, A. Chock, et al. 2006. "What factors mediate sperm pairing in monodelphis domestica?" Developmental Biology 295(1): 465.