The Questions of Developmental Biology

Sometimes steps 2 and 3 are reversed (as in mammalian fertilization) and the sperm binds to the
egg before releasing the contents of the acrosome. After these five steps are accomplished, the
haploid sperm and egg nuclei can meet, and the reactions that initiate development can begin.
In many species, the meeting of sperm and egg is not a simple matter. Many marine
organisms release their gametes into the environment. That environment may be as small as a tide
pool or as large as an ocean. Moreover, it is shared with other species that may shed their sex
cells at the same time. These organisms are faced with two problems: How can sperm and eggs
meet in such a dilute concentration, and how can sperm be prevented from trying to fertilize eggs
of another species? Two major mechanisms have evolved to solve these problems: speciesspecific
attraction of sperm and species-specific sperm activation.
Sperm attraction: Action at a distance
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
cnidarian Orthopyxis 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.