Polyspermy, the fertilization of an egg by more than one sperm, is a common problem faced by the eggs of all sexually reproducing species and causes severe chromosomal defects and embryonic mortality. The effects of polyspermy are evident in the development of cleavage furrows shortly after fertilization.
In normal fertilization, the embryonic cleavage furrows develop symmetrically and transverse the entire embryo (center photo at 8 cell stage). By contrast, the cleavage furrows develop irregularly following fertilization by more than one sperm (right photo) (Wozniak, Mayfield, & Carlson, unpublished).
Our lab strives to uncover the crucial signaling events ensuring that only one sperm enters the egg and thereby allowing for normal embryonic development. Eggs can possess two mechanisms to prevent more than one sperm from entering at fertilization. One such mechanism, called the fast block to polyspermy, exists in many species in the form of a prolonged membrane depolarization of the egg.
Fertilization-evoked depolarization in X. laevis eggs (recording by Wozniak, unpublished). Line denotes 0 mV.
Here’s an example of a polyspermic fertilization. Notice that during the first depolarization, a second sperm enters to create a step in the recording. This embryo developed asymmetric cleavage furrows as expected with polyspermy.
A recording made from an egg fertilized by two sperm (recording by Mayfield, unpublished). Line denotes 0 mV.
The channels that mediate this depolarization and the signaling pathways that regulate it are unknown. To uncover the earliest signaling events evoked by fertilization, we use the African clawed frog Xenopus laevis as the model system. Frogs naturally fertilize externally, facilitating the study of their fertilization in the lab. Furthermore, X. laevis are particularly well-suited for the experiments proposed here due to the large size of their eggs (~1.2 mm diameter) and ease of exogenous protein expression.
Our lab is interested in elucidating the signaling events that occur within the first minutes of sperm entry into the egg. We use various techniques including electrophysiology and fluorescence imaging to establish the timeline of events.