I have an important question ...

In Insects, the molecular basis of sperm - egg interactions are still unknown,
mainly because the well - established sperm - egg binding assays with isolated
cells used in vertebrates and in a variety of invertebrates cannot be applied.
However, we use Drosophila melanogaster as a model system because of
the wealth of information available on the physiology of reproduction, on
gametogenesis and its genetic control.

So, I ask you: why we can't apply, in Drosophila, sperm - egg binding assays used in vertebrates? Which are the differences? Why we have to count on the availability of mutations that disrupt sperm - egg interactions to identify functional important molecules that would otherwise escape identification?

I hope that you undestand me...I want to put in practice (?) my scientific english and I need an answer to my question for my graduation thesis!!! Thanks!