How can removal of an egg’s outer coat allow fertilization between different species in experiments?

Fertilization is typically a strictly species-specific process because the egg’s outer coating—such as the zona pellucida or the chorion—acts as a biochemical barrier that only recognizes and binds with sperm from the appropriate species.1

In experimental settings, removing these protective outer layers allows for heterologous fertilization (fertilization between different species) because the barrier that normally enforces species specificity is eliminated. By stripping away the egg's coat, researchers can bypass the initial binding requirements that prevent foreign sperm from interacting with the egg surface. For example, in ascidians, removing the chorion and associated test cells enables not only fertilization by different species but also allows for self-fertilization, which is otherwise prevented.1