Ants improve the reproduction of inferior morphs to maintain a polymorphism in symbiont aphids - Scientific Reports
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Identifying stable polymorphisms is essential for understanding biodiversity. Distinctive polymorphisms are rare in nature because a superior morph should dominate a population. In addition to the three known mechanisms for polymorphism persistence, we recently reported a fourth mechanism: protection of the polymorphism by symbionts. Attending ants preferentially protect polymorphic aphid colonies consisting of green and red morphs. Here, we show that attending ants manipulate the reproductive rate of their preferred green morphs to equal that of the red morphs, leading to the persistence of the polymorphism within the colonies. We could not, however, explain how the ants maintained the polymorphism in aphid colonies regardless of inter-morph competition. Manipulation by symbionts may be important for the maintenance of polymorphisms and the resulting biodiversity in certain symbiotic systems.

From the article:

Ant-attended aphids are known to excrete high-quality honeydew when ants are present. Ant attendance has a negative effect on the growth and reproduction of the attended aphids. Therefore, trade-offs should occur between the quality of honeydew and the growth and fecundity of aphid individuals. Thus, if attending ants prefer the morph excreting a high-quality honeydew, such trade-offs and resulting competitive interactions are expected between the color morphs in M. yomogicola. The morph excreting high-quality honeydew is known to have a lower reproductive rate than the other morphs[9,10]. This fact implies that if the attending ants prefer one morph, this morph is expected to excrete high-quality honeydew. Note that any such difference between morphs leads to the exclusion of the inferior morphs. Surprisingly, nearly all colonies consist of both green and red morphs in the field.