Question

What are the three main steps used for the recognition of egg, which contributes as a barrier to cross-species fertilization in sea urchin.

Answer 1

Sea urchins are echinoderns which live on the seabed.

The eggs released by the female urchins freely floats in the sea. This unfertilised egg meets with the free floating sperm released by the male and fuses to form the zygote.

The main steps used for recognition of the egg are:

1. Chemotaxis-Chemotraction of the sperm to the egg by certain soluble molecules secreted by the egg. These molecules are secreted only when the second meiotic division is completed in the eggs. These chemotactic factors attract only the sperms of the same species.

2. Acrosomal reaction - exocytosis of acrosomal vesicles in the sperm to release enzymes, followed by extension of the acrosomal process. This is known as acrosomal reaction. The acrosomal enzymes are specific to the species

3.​​Species specific identification

The acrosomal process of sperm contacts the surface of egg(zona pellucida or vitelline layer). The acrosomal protein bindin mediates a species specific recognition at this point.  

The sperm is then passed through the zona pellucida or the vitelline layer and fusion of egg and sperm happens.

Since all the steps are species specific, it prevents cross-species fertilisation.