Sperm Movement, Egg Fertilization, and Oviposition
Introduction | Male Anatomy |
Female Anatomy | Fertilization |
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Mating monarchs can remain together for 16 hours or longer, and it is only at the
very end of this period that sperm are transferred. We have studied sperm movement
in female monarchs by dissecting females at different time intervals following the
end of mating. Immediately after the pair separates, sperm are in the end of the
spermatophore. They move out of the
collum and into the sperm duct about 10 minutes later. The bundles of eupyrene sperm break apart after the sperm leave the
spermatophore, an amazing process that can be observed under a microscope.
Sperm are probably propelled through the sperm duct and into the spermatheca by
a combination of their own movement and muscular contractions by the female. They
are stored in the spermatheca until they are released to fertilize eggs, which may
occur weeks or even months after mating. Both apyrene
and eupyrene sperm move into the spermatheca, even though only the eupyrene sperm
can fertilize eggs.
Fertilization occurs just before an egg is about to be laid. As the egg passes down
the common oviduct, a few sperm are released from the spermatheca. It’s not clear
exactly how this happens, but it is likely that the spermatheca has muscles that
contract and force out the sperm. The egg is oriented so that the micropyle is opposite
the opening of the spermatheca. When the sperm reach the egg, they move through
the micropyle and fertilization occurs. We don’t know if more than a single sperm
actually go into the egg, but even if they do, only one would fertilize it and the
others would break down.
Once the egg is fertilized, the new monarch starts to form and the female deposits
the egg (oviposits) on an appropriate hostplant. Recent work by Haribal and Renwick
(1995) at Cornell University has shown that flavonol glycosides in the milkweed
stimulate oviposition by female monarchs. They probably detect these chemicals with
taste receptors on their feet and possibly the ovipositor at the end of their abdomen.
Females are probably quite choosy about characteristics of the plants on which they
lay eggs, since hostplant quality can have a big effect on larval development and
survival.
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