Zinc protection of fertilized eggs is an ancient feature of sexual reproduction in animals

Abstract

One of the earliest and most prevalent barriers to successful reproduction is polyspermy, or fertilization of an egg by multiple sperm. To prevent these supernumerary fertilizations, eggs have evolved multiple mechanisms. It has recently been proposed that zinc released by mammalian eggs at fertilization may block additional sperm from entering. Here, we demonstrate that eggs from amphibia and teleost fish also release zinc. Using Xenopus laevis as a model, we document that zinc reversibly blocks fertilization. Finally, we demonstrate that extracellular zinc similarly disrupts early embryonic development in eggs from diverse phyla, including: Cnidaria, Echinodermata, and Chordata. Our study reveals that a fundamental strategy protecting human eggs from fertilization by multiple sperm may have evolved more than 650 million years ago.

Sperm MMP‐2 is indispensable for fast electrical block to polyspermy at fertilization in Xenopus tropicalis

“…Third, a slow block to polyspermy by the formation of a fertilization envelope may be delayed or insufficient during fertilization by ΔHPX (−/−) sperm. Although we have not yet determined the details of exocytosis of cortical granules, which is necessary for the formation of the fertilization envelope and S‐layer (Grey et al, 1976; Quill & Hedrick, 1996; Stewart‐Savage & Grey, 1987), in addition to zinc ion release (Wozniak et al, 2020), it is unlikely that the slow block was delayed, because both cortical contractions induced by the increase in [Ca 2+ ] i and egg rotation in the perivitelline space surrounded by the fertilization envelope occurred without delay in comparison with fertilization by WT sperm. Taken together, it is most probable that polyspermy induced by ΔHPX (−/−) sperm reduces voltage sensitivity against the positive fertilization potential.…”

“…Sperm reaches the egg surface at intervals of 11–40 s, while a Ca 2+ wave reaches over the whole egg surface within 5 min after fertilization to induce exocytosis of cortical granules (Watabe et al, 2019). Since the formation of the fertilization envelope (Grey et al, 1976; Wyrick et al, 1974), the S‐layer (Stewart‐Savage & Grey, 1987), and zinc ion release (Wozniak et al, 2020) are induced by exocytosis of cortical granules, only a few sperm seem to contact the egg surface before completion of the slow block caused by the Ca 2+ wave induced by the first fertilizing sperm.…”

“…Since fertilization is prevented when the egg membrane potential is clamped positively, known as voltage‐dependent fertilization, the entry of extra sperm is blocked on the egg plasma membrane, known as the voltage‐dependent polyspermy block (Charbonneau et al, 1983; Cross & Elinson, 1980; Glahn & Nuccitelli, 2003; Iwao, 1989; Watabe et al, 2019). After fast, electrical blocking on the egg membrane, a slow, but permanent block, mediated by the formation of the S‐layer (Stewart‐Savage & Grey, 1987) and fertilization envelope (Grey et al, 1976; Wyrick et al, 1974) over the egg plasma membrane in addition to zinc ion release (Wozniak et al, 2020) are caused by the exocytosis of cortical granules.…”

TMEM16A activation for the fast block to polyspermy in the African clawed frog does not require conventional activation of egg PLCs

“…For procedures requiring the removal of jelly, eggs were incubated at room temperature for 5 minutes before insemination with sperm suspension prepared as described previously (Wozniak et al, 2020). Activated eggs, identified by their ability to roll so the animal pole faced up and displayed a contracted animal pole, were used for western blot preparations.…”