All Clone-mates are not Created Equal: Fitness Discounting Theory Predicts Pea Aphid Colony Structure

As many animals form aggregations, group-living is believed to be adaptive. It is not clear, though, if clonal aggregations should have spatial structure, as protecting clone-mates is the genetic equivalent of protecting self. 'Fitness discounting' theory states that immediate reproductive opportunities are of greater value than are delayed opportunities. Thus, we hypothesized that spatial structure should exist in colonies of unequal-aged, clonal organisms like aphids. We predicted that, compared to reproductive (5 ^th instar) individuals, young (2 ^nd and 3 ^rd instar) juveniles (i. e., the youngest instars capable of emitting an alarm signal) should occupy the most dangerous feeding positions. As individuals approach reproductive maturity and alarm signals decline (4 ^th instar), they should occupy increasingly safer feeding positions. We tested these predictions by documenting the spatial distribution of two (green and pink) pea aphid, Acyrthosiphon pisum, asexual lineages ("clones") at 1, 3, 6, 24, 48, 72, 96, and 120 h after host plant colonization. Confirming our hypothesis, we found that early (2 ^nd and 3 ^rd) instar aphids occupied feeding positions with the highest predation risk. Upon reaching the penultimate (4 ^th) instar, individuals dispersed from the colony to colonize other leaves. Thus, pea aphid colonies are not random aggregations; aphid colony structure can be explained by fitness discounting theory.