Self and nonself recognition affects clonal reproduction and competition in the pea aphid

The spatial interaction of clonal organisms is an unsolved but crucial
topic in evolutionary biology. We evaluated the interactions
between aphid clones using a color mutant (yellow) and an original (green)
clone. Colonies founded by two aphids of the same clone and mixed
colonies, founded by a green aphid and a yellow
aphid, were set up to observe population growth for 15
days. We confirmed positive competition effects, with mixed
colonies increasing in size more rapidly than clonal colonies. In
mixed colonies where reproduction started
simultaneously, green aphids overwhelmed yellow aphids in number,
and yellow aphids restrained reproduction. However, when yellow
aphids started to reproduce earlier, they outnumbered
the green aphids. To test whether aphids have the ability to control
reproduction according to the densities of self and non-self clones, one
yellow aphid or one antennae-excised yellow aphid was transferred into a
highly dense green clone colony. Intact yellow aphids produced fewer
nymphs in crowded green colonies, whereas the fecundity of
antennae-excised aphids did not change. Thus, we conclude that aphid
clones can discriminate between self and non-self
clones, and can regulate their reproduction, depending on whether they are
superior or inferior in number to their competitors.