Maternal behavior of a different sort
Watching National Geographic Nature programs and “Nature” on PBS has allowed us to witness numerous examples of the strenuous efforts that the mothers (and sometimes fathers as well) of different species undertake to care for and protect their young. And we’re certainly familiar with the outpouring of stinging or biting workers when intruders threaten the nests of bees and ants. However, when it comes to fruit flies, those tiny, non-social, non-biting, and rapidly reproducing, flying insects that arrive unbidden to feed and lay their eggs on our ripening fruit, one wouldn’t think of expecting much in the way of protective action by the female parents of these insects.
Some recent research, however, has informed us of the unexpected maternal behavior these insects are capable of. These little pests (which, it should be acknowledged, have played a vital role in helping us understand genetic mechanisms) are themselves plagued by parasitic wasps. When such wasps find these fruit flies they seek out their developing larvae (the second stage of embryonic development) and inject their eggs into these larvae. There the wasps’ young develop while eating the fruit fly’s larvae from the inside out.
As shown by this new study, however, if the female fruit fly sees a female parasitic wasp in the neighborhood it stops laying its eggs on newly ripening fruit and seeks out fruit in which the sugars have had a longer time to ferment. This means the alcohol content of the food their larval offspring feed upon will be high. And because alcohol is toxic to the wasp’s larvae, the fruit fly’s larvae are protected from this gruesome infection. In this way these fruit fly mothers are immunizing (or medicating) their offspring against the parasitic wasps larvae. Indeed, the larvae themselves will seek out food with more alcohol content if they get infected by the parasitic wasp, and thus are capable of self-medicating when given the opportunity.
Using various mutant fruit flies the researchers showed that it was sight and not smell which allowed the fruit fly to detect the presence of these parasitic wasps and change the location on which to lay their eggs. They also found that the female fruit fly’s choice of high-alcohol-content fruit continued for up to four days after seeing the parasitic wasps, unless they used a mutant fruit fly that lacked long-term memory. Surprisingly, male parasitic wasps were without effect on the fruit fly’s choice of food source for their offspring.
However, the fruit flies and their developing larvae are not totally immune to alcohol themselves as their success rate in raising offspring is reduced when their larvae feed on alcohol-containing fruit. In the absence of parasitic wasps, and given a free choice of food sources containing no alcohol, 3% alcohol, or 6% alcohol, the flies generally chose food with 3% alcohol. Thus the fruit fly’s behavior is equivalent to making a calculated choice in the presence of the parasitic wasp: Either risk having no living offspring by laying one’s eggs on an alcohol-free food source or accept a reduction in the number of one’s descendants knowing that those that survive on alcohol-containing fruit stand a better chance of resisting infection by the wasp.
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