The Path to Fertilization: SPAM 1 and Sperm Pairing in \u3cem\u3eMonodelphis domestica\u3c/em\u3e

New World marsupials such as the lab opossum, Monodelphis domestica, have a remarkable characteristic unknown in other vertebrate animals: sperm pairing. After maturation in the male reproductive tract, sperm undergo a series of maneuvers, resulting in the precise alignment and adhesion of two adjacent sperm heads. Work in our lab suggests that such maneuvers are facilitated by temporary but simultaneous sperm adhesion to a planar surface, ensuring that sperm-sperm adhesion occurs only at the acrosomal surface of the sperm head. Work by others indicates that paired sperm swim faster and more efficiently than unpaired sperm. Paired sperm unpair just before encountering an egg during fertilization. Thus, the sperm pairing adaptation ensures successful reproduction. The mechanics of sperm pairing remain unknown. A candidate adhesive agent is Sperm Adhesion Molecule 1 (SPAM1), a glycoprotein that has been localized in the male and female reproductive tract, and on the sperm heads of many mammals. This study hypothesized that if SPAM1 plays a role in pairing, it should be present in the lining of the tubes that sperm travel in both the male and female reproductive tracts. SPAM 1 should also be detectable on the acrosomal surface. Results support this hypothesis, reinforcing the notion that SPAM1 has an adhesive role in sperm pairing. Moreover, in vitro experiments designed to disable SPAM1 function via exposure to anti-SPAM1 antibody demonstrated a time-dependent relationship between exposure and sperm unpairing. Taken together, these results point to a role for SPAM1 in sperm pairing in the lab opossum