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Fertilization
in a Chiton: Acrosome-Mediated Sperm-Egg Fusion
John Buckland-Nicks,
Ron Koss, and Fu-Shiang Chia
Gamete Research 21:
199-212 (1988)
Department of Biology, St Francis
Xavier University, PO Box 3000, Antigonish, Nova Scotia, Canada, B2G 1C0
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Contrary
to the widely accepted view that chiton sperm lack acrosomes and that fertilization
in this group accurs via a micropyle, we demonstrate here that fertilization
in Tonicella lineata occurs by acrosome-mediated sperm-egg fusion.
The acrosome is a small vesicle containing two granules located at the
tip of the sperm. The eggs have an elaborate hull (=chorion), which is
formed into cupules that remain covered by follicle cells until maturity.
When dissected ripe eggs were exposed to sperm in vitro, the sperm were
attracted only to open cupules, inside which they swam through one of seven
channels to the base where they penetrated the hull. The acrosome fired
on contact with, or in, the hull, and during passage through it the apical
granule was exhausted while the basal granule was exposed. If sperm contacted
follicle cells between the cupules, the acrosome did not react. The vitelline
layer beneath the hull contains pores arranged in a regular pattern. Embedded
in the base of each pore is an egg microvillus. Having penetrated the hull
the sperm anterior filament located a pore and fused with the tip of the
egg microvillus projecting into it. This created a membranous tube, through
which the sperm nucleus was injected into the egg. The egg membrane appeared
to be raised up into a small fertilization cone around the penetrating
sperm, the vitteline layer became slightly elevated, and some cortical
granules were released by exocytosis.
For a reprint of this article,
please e-mail jbucklan@stfx.ca
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