Restricting the gene pool

Nature has evolved clever ways to prevent animals from different species from successfully reproducing. As published in the upcoming issue of Genes & Development, molecular biologists at UC Irvine are gaining a better understanding as to how.

In the October 15th issue, Drs. Noriko Kamei and Charles Glabe report on the identification of a receptor on the surface of sea urchin eggs that regulates the species-specific adhesion of sperm.

External fertilization can be risky business, especially for marine animals whose sperm is released into an aqueous environment. Thus, there are several barriers to prevent cross-fertilization between different species of sea urchins, or even, say, sea horse sperm from fertilizing sea urchin eggs. The recognition of egg and sperm involves a number of steps (sperm attraction, activation, and adhesion to the egg surface), each of which serves as a checkpoint to restrict the gene pool to individuals of the same species.

Over 25 years ago, scientists discovered that at the tip of sea urchin sperm exists a protein that mediates binding of the sperm to the egg – a protein they named “bindin.” Each species of sea urchin has its own unique version of bindin, and scientists hypothesized that each species’ egg must, likewise, have a similarly unique bindin receptor on its surface. Since then, species-specific sperm adhesive proteins and egg receptors have been identified in other animals, including mammals, but the identity of the sea urchin egg bindin receptor has remained elusive. Until now.

Drs. Kamei and Glabe have identified the species-specific sea urchin egg bindin receptor, which they call EBR1 (egg bindin receptor 1). Drs. Kamei and Glabe searched through complementary DNA sequences from ovaries of two different species of sea urchins (S. franciscanus and S. purpuratus, or Sf and Sp, respectively). The scientists were looking for sequences that were present in one species but not the other, and therefore might encode a species-specific protein. Of the four Sf-specific DNA sequences they found, only one was sufficiently large to encode the expected size of the bindin receptor.

Drs. Kamei and Glabe undertook several different experimental approaches to test the validity of this putative bindin receptor, and all roads led to the same conclusion: EBR1 is the long sought after egg bindin receptor. Structural analysis revealed that while Sf-EBR1 and Sp-EBR1 proteins share a conserved core domain, they also have a unique region that accounts for the proteins’ species-specific activity.

“Having the sperm and egg molecules (bindin/EBR1) in hand will allow study at a molecular level of how sperm and egg interact to make fertile offspring. Such insights could provide an understanding of common structural features and functional principals of molecules involved in mammalian gamete interactions. It also may she light on how new species evolved,” explains Dr. Kamei.