The effector of Hippo signaling, Taz, is required for formation of the micropyle and fertilization in zebrafish

Abstract

The mechanisms that ensure fertilization of egg by a sperm are not fully understood. In all teleosts, a channel called the ‘micropyle’ is the only route of entry for sperm to enter and fertilize the egg. The micropyle forms by penetration of the vitelline envelope by a single specialized follicle cell, the micropylar cell. The mechanisms underlying micropylar cell specification and micropyle formation are poorly understood. Here, we show that an effector of the Hippo signaling pathway, the Transcriptional co-activator with a PDZ-binding domain (Taz), plays crucial roles in micropyle formation and fertilization in zebrafish ( Danio rerio). Genome editing mutants affecting taz can grow to adults. However, eggs from homozygous taz females are not fertilized even though oocytes in mutant females are histologically normal with intact animal-vegetal polarity, complete meiosis and proper ovulation. We find that taz mutant eggs have no micropyle. Taz protein is specifically enriched in mid-oogenesis in the micropylar cell located at the animal pole of wild type oocyte, where it might regulate the cytoskeleton. Taz protein and micropylar cells are not detected in taz mutant ovaries. Our work identifies a novel role for the Hippo/Taz pathway in micropylar cell specification in zebrafish, and uncovers the molecular basis of micropyle formation in teleosts.

Author summary

In many fish, sperm enters eggs through a specialized channel called the ‘micropyle’. The micropyle is formed by a special follicle cell, the ‘micropylar cell’, which sits on the top of the developing egg during oogenesis, and forms the sperm entry canal. The underlying mechanisms of this process are unknown. We find that Taz, an effector of an important signaling pathway, the Hippo pathway, is specifically enriched in micropylar cells in zebrafish, and regulates formation of the micropyle. Loss of Taz function in females results in no micropylar cells, failure to form a micropyle on eggs, which are consequently, not fertilized. Our study identifies a new role for the Hippo/Taz pathway in cell fate specification in the ovary, and reveals a potential mechanism for forming the sperm entry port. Similar mechanisms might operate in other fish as well.