Egg activation in physiological polyspermy

Abstract

Fertilization is indispensable not only for restoring diploid genomes but also for the initiation of early embryonic cell cycles in sexual reproduction. While most animals exhibit monospermy, which is ensured by polyspermy blocks to prevent the entry of extra sperm into the egg at fertilization, several animals exhibit physiological polyspermy, in which the entry of several sperm is permitted but only one sperm nucleus participates in the formation of a zygote nucleus. Polyspermy requires that the sperm transmit the egg activation signal more slowly, thus allowing the egg to accept several sperm. An increase in intracellular Ca 2C concentration induced by the fertilizing sperm is both necessary and sufficient for egg activation in polyspermy. Multiple small Ca 2C waves induced by several fertilizing sperm result in a long-lasting Ca 2C rise, which is a characteristic of polyspermic amphibian eggs. We introduced a novel soluble sperm factor for egg activation, sperm-specific citrate synthase, into polyspermic newt eggs to cause Ca 2C waves. Citrate synthase may perform dual functions: as an enzyme in mitochondria and as a Ca 2C-inducing factor in egg cytoplasm. We also discuss the close relationship between the mode of fertilization and the Ca 2C rise at egg activation and consider changes in this process through evolution in vertebrates.

Egg activation in physiologically polyspermic newt eggs: involvement of IP3 receptor, PLCγ, and microtubules in calcium wave induction

“…The injection of acetyl CoA might induce a Ca 2+ increase through sensitization of the IP 3 receptors on the ER to induce Ca 2+ releases (Missiaen et al, 1997). The local increase of Ca 2+ probably causes a successive Ca 2+ release from the neighboring ER by directly stimulating the IP 3 receptors or through activation of PLC (Iwao, 2012). In this connection, b-tubulin has been shown to bind Src homology 2 (SH2) domains (Itoh et al, 1996) and pleckstrin homology (PH) domains of PLCg1 to promote its activity (Chang et al, 2005).…”

“…They complete meiosis with degradation of cyclin B and c-Mos when the eggs are activated by an increase in intracellular the Ca 2+ concentration ([Ca 2+ ] i ) at fertilization. The Ca 2+ increase is the most important and primary trigger for egg activation in both monospermic and physiologically polyspermic eggs (Iwao, 2000a(Iwao, , 2012Tarin, 2000). Most animals exhibit monospermy in which several blocks to polyspermy prevent extra sperm from entering the egg before sperm-egg fusion.…”

“…In such cases, the egg must detect the arrival and entry of the first sperm, and then undergo activation to rapidly initiate blocking mechanisms against polyspermy. The fertilizing sperm provides an activation signal on the egg membrane, which rapidly propagates through the entire egg (Iwao, 2012). In contrast, physiologically polyspermic eggs have no block to polyspermy before the fusion with sperm (Iwao, 2000a, b;Wong and Wessel, 2006).…”

Formation of triploid plants via possible polyspermy

“…44 In addition to fast and slow polyspermy blocks, which operate at the plasmogamy step, the polyspermy barrier at the karyogamy step has been demonstrated in polyspermic zygotes of some animal taxa, in which only one sperm nucleus fuses with the egg nucleus, and the remaining extra sperm nuclei degenerate or disappear. 22,45,46 Interestingly, in some gymnosperms such as Pinus nigra and Picea glauca, two sperms enter the egg, but only one sperm nucleus migrates toward the egg nucleus and fuses with it. These indicate that selective karyogamy producing a diploid zygote operates as a polyspermy barrier in some animals and plants.…”

Focused Proteomics on Egg Membrane Microdomains to Elucidate the Cellular and Molecular Mechanisms of Fertilization in the African Clawed Frog Xenopus laevis

“…Therefore, it has been thought that gamete interaction at fertilization may act as a ligand-like signal to stimulate an egg-surface receptor so that the intracellular PTK signaling is triggered. On the other hand, eggs of mammalian species (e.g., mouse) and some other nonmammalian species (e.g., bird, newt) seem to employ sperm-derived factor(s), which would be incorporated into the egg cytoplasm, for the sperm-induced calcium responses: molecules identifi ed so far include phospholipase Cζ ( Swann and Lai 2013 ) and citrate synthase (Iwao 2012 ). This observation may refl ect that gamete membrane interaction-mediated and gamete membrane fusion-mediated egg activation systems employ their specifi c molecular machinery and, perhaps more importantly, some other differences of fundamental importance that are found between these species: aquatic or terrestrial life, and external or internal fertilization.…”