Abalone sperm

More recently, an abalone sperm attractant has been identified. Sperm of the red abalone Haliotis rufescens responds to soluble factors released into the seawater by conspecific eggs. Bioassay-guided purification revealed... 

The steps in abalone sperm-egg interaction were deduced from light and electron microscopic studies (Figure 4). First, the sperm plasma membrane covering the tip... 

The abalone sperm AR can be artificially induced by raising the calcium ion concentration of seawater from the normal 10 mM to 50 mM in seawater buffered with 10 mM Tris at pH 8.2. Unlike other species used for fertilization studies, the abalone AR does not lead to the rapid death of the sperm. In abalone sperm, the acrosomal compartment is sealed off from the respiratory compartment acrosome-reacted, sperm will continue to swim for days if stored in the cold room at 4°C. The acrosomal exudate of these sperm is composed predominantly of soluble ly sin and 18K protein. Reducing and denaturing polyacrylamide gel electrophoresis of whole sperm, AV exudate, and purified lysin shows that abalone spermatocytes make a substantial investment in the synthesis of these two acrosomal proteins (Figure 6). 

Figure 6. Polyacrylamide gel electrophoresis of abalone sperm and AV contents. Lanes A and E are standard proteins of known molecular mass. Lane B, whole sperm dissolved in SDS. Lane C, the acrosome vesicle content released to seawater when exocytosis of the sperm is induced by high calcium ion concentration. Lane D, purified 16-kDa lysin. (from Lewis et al., 1982).

Lee, Y.-H. and Vacquier, V.D. (1992). The divergence of species-specific abalone sperm lysins is promoted by positive Darwinian selection. Biol. Bull. 7S2 97-105. 

Y.-H. and Vacquier, V.D. (1995). Evolution and systematics in Haliotidae (Mollusca, Gastropoda) Inferences from DNA sequences of sperm lysin. Marine Biology 724 267-279. Y.-H., Ota,T., and Vacquier, V.D. (1995). Positive selection is a general phenomenon in the evolution of abalone sperm lysin. Molecular Biology and Evolution 72 231-239. 

Lewis, C.A., Talbot, C. F., and Vacquier, V.D. (1982). A protein from abalone sperm dissolves the egg vitelline layer by a nonenzymatic mechanism. Dev. Biol. 92 227-240. 

Shaw, A., Fortes, P.G.A., Stout, C.D., and Vacquier, V.D. (1995). Crystal structure and subunit dynamics of the abalone sperm lysin dimer Egg envelopes dissociate dimers. J. Cell Biol. 750 1117-1126. 

Shiroya, Y. and Sakai, Y.T. (1993). Organization of actin filaments in the axial rod of abalone sperm revealed by quick freeze technique. Dev. Growth and Differ. 55 323-329. 

Vacquier, V.D. and Lee, Y.-H. (1993). Abalone sperm lysin Unusual mode of evolution of a gamete recognition protein. Zygote 7 181-196. 

In Chapter 2, Vacquier and coauthors present a detailed account of their research on two acrosomal proteins of abalone sperm. This research has significantly advanced our understanding of how species-specificity evolves. Knowledge of the behavior and structure of these sperm proteins has suggested possible mechanisms of evolution of species-specific gamete interactions and has demonstrated that these proteins may play essential roles in establishing reproductive isolation between species. 

Figure 4. Scheme showing sperm-egg interaction in the abalone. 1. The sperm binds to the egg VE by the plasma membrane at the tip of the AV (AG), (F, flagellum M, mitochondrion N, nucleus). 2. The sperm acrosome reacts releasing lysin and the 18K protein from its anterior tip. 3. Lysin disrupts the fibers of the VE and the 18K coats the extending acrosome process as it extends. 4. The sperm passes through the hole in the VE and the membrane covering the tip of the acrosomal process fuses with the egg (from Vacquier and Lee, 1993). 

Figure 8. The fusion of artificial phospholipid vesicles induced by 18K protein (a) and lysin (b) at the three concentrations indicated above (c, buffer alone). The upper panels are with sperm proteins from the red abalone (Hr H. refescens) the lower panels are with sperm proteins from the green abalone (Hf H. fulgens). The 18K proteins are more potent fusagens than lysin. Although the two 18K proteins are only 33.8% identical in primary structure, their five predicted amphipathic helices have similar hydrophobic moments (from Swanson and Vacquier, 1995a).

To summarize, variable structural features of lysins suggest ways to attack the elucidation of the molecular mechanism of species-specific sperm-egg recognition in abalones. The invariant structural features of lysins suggest ways to explore the molecular mechanism lysin uses to destroy nonenzymatically the integrity of the VE to allow the sperm to pass through this protective envelope and contact the egg cell membrane. 

Swanson, W.J. and Vacquier, V.D. (1997). The abalone egg vitelline envelope receptor for sperm lysin is a giant, multivalent molecule. Proc. Natl. Acad. Sci. USA. 94 6724-6730. 

Vacquier, V. D., Gamer, K.R., and Stout, C D. (1990). Species-specific sequences of abalone lysin, the sperm protein that creates a hole in the egg envelope. Proc. Natl. Acad. Sci. USA 87 5792-5796. 

Sperm penetrate the zona pellucida only after completion of the acrosome reaction. A similar process occurs in nonmammalian species, where sperm must penetrate the vitelline coat. In abalone this is accomplished by release of lysin, an acrosomal protein that disperses the vitelline coat by a noncatalytic mechanism (Lewis et al., 1982 Shaw et al., 1993). In contrast, the generally accepted model for mammalian sperm penetration of the zona pellucida is the acrosin hypothesis in which proteolysis of zona pellucida matrix glycoproteins by acrosin, the acrosomal serine esterase, plays a trailblazing role in the sperm penetration process (Yanag-... 

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