Facultative Endosymbiont Serratia symbiotica Inhibits the Apterization of Pea Aphid To Enhance Its Spread.

Aphids display wing polyphenism, and the mother can produce a wingless morph for reproduction and a winged morph for dispersal. It is believed that the wingless morph is an adaptive status under favorable conditions and is determined prenatally. In this study, we have found that winged nymphs of the pea aphid, Acyrthosiphon pisum, can change from winged to wingless during normal development. Our results showed that winged nymphs could become the wingless morph by apterization in response to changes from stressful to favorable conditions. The acquired wingless aphids had higher fecundity than the winged morph. However, this process of regression from winged to wingless morph was inhibited by Serratia symbiotica. The existence of the symbiont did not affect the body mass and fecundity of adult aphids, but it increased the body weight of nymphs and temporally increased the quantity of a primary symbiont, Buchnera aphidicola. Our results showed that despite temporal improvement of living conditions causing the induction of apterization of winged nymphs, the inhibition effect of S. symbiotica on this process was activated simultaneously. This finding, for the first time, reveals that the wingless morph can be changed postnatally, which explains a novel regulating mechanism of wing polyphenism driven by external abiotic stimuli and internal biotic regulation together in aphids. Wing polyphenism is an important adaptative response to environmental changes for aphids. Endosymbionts are widespread in aphids and also confer the ability to withstand unfavorable conditions. However, little is known about whether endosymbionts are involved in the wing polyphenism. In this study, we report a new finding that winged nymphs of the pea aphid could turn into adults without wings or wing-related structures through apterization when winged nymphs escaped from stressful to favorable environments. Further analysis revealed that the facultative symbiont S. symbiotica could prevent the temporal determination of the host in wing suppression by inhibiting apterization, to enhance its spread. Our findings provide a novel angle to understanding the wing polyphenism regulation of aphids.