Sperm Movement, Egg Fertilization and Oviposition

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 chemical 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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