How do gametes from the same species recognize each other?

Answer 1

Species-specific sperm attraction has been documented in numerous species, including cnidarians, molluscs, echinoderms, and urochordates (Miller 1985; Yoshida et al. 1993). In many species, sperm are attracted toward eggs of their species by chemotaxis, that is, by following a gradient of a chemical secreted by the egg. In 1978, Miller demonstrated that the eggs of the cnidarianOrthopyxis caliculata not only secrete a chemotactic factor but also regulate the timing of its release. Developing oocytes at various stages in their maturation were fixed on microscope slides, and sperm were released at a certain distance from the eggs. Miller found that when sperm were added to oocytes that had not yet completed their second meiotic division, there was no attraction of sperm to eggs. However, after the second meiotic division was finished and the eggs were ready to be fertilized, the sperm migrated toward them. Thus, these oocytes control not only the type of sperm they attract, but also the time at which they attract them.

The mechanisms of chemotaxis differamong species (see Metz 1978; Ward and Kopf 1993). One chemotactic molecule, a 14-amino acid peptide called resact, has been isolated from the egg jelly of the sea urchin Arbacia punctulata (Ward et al. 1985). Resact diffuses readily in seawater and has a profound effect at very low concentrations when added to a suspension of Arbacia sperm (Figure 7.9). When a drop of seawater containing Arbacia sperm is placed on a microscope slide, the sperm generally swim in circles about 50 μm in diameter. Within seconds after a minute amount of resact is injected into the drop, sperm migrate into the region of the injection and congregate there. As resact continues to diffuse from the area of injection, more sperm are recruited into the growing cluster. Resact is specific for A. punctulata and does not attract sperm of other species. A. punctulata sperm have receptors in their plasma membranes that bind resact (Ramarao and Garbers 1985; Bentley et al. 1986) and can swim up a concentration gradient of this compound until they reach the egg.

Answer 2

Gametes from the same species recognize each other through specific proteins on their surfaces that act as molecular signals. These proteins enable the gametes to bind to each other and initiate the process of fertilization, ensuring that mating and reproduction occur successfully within the same species.