Social aphids

 

Overview

Aphids are among the few insect groups outside of the Hymenoptera that express advanced sociality. Sociality was first described in aphids in the mid-1970’s (Aoki 1977). Almost all social species are found in dense clonal groups within galls on their host plants, and within these, specialized nymphal soldiers are produced that defend the clone from enemies. Their discovery attested to both the central importance of kin selection in social evolution as well as the decisive roles played by ecological factors, such as predation and cooperative “fortress defense” (Queller and Strassmann 1998; Kutsusake et al. 2004).  Recent work in my lab has suggested an important role for intraspecific competition and conflict in aphid sociality. Current and future work builds on these discoveries.

Do social aphids experience a tragedy of the commons? That a substantial fraction of aphids within Pemphigus groups are unrelated to natal altruists seems paradoxical. We quantified the costs of cheating in social aphids. Research found that costs can be severe – both the fecundity of reproductive aphids and overall group size are diminished by invading cheaters. This pattern is similar to that of a “tragedy of the commons” in which publically-available resources are exploited by the selfish, competitive actions of group members.

Is aphid sociality a case of “fortified imperialism”? Because aphids are clonal, it is easy to imagine that the origins of sociality in aphids were fostered by the uniquely high relatedness of uniclonal groups. However, we found that, remarkably, the tendency to disperse between clones evolved prior to the emergence of soldiers in the North American genus Pemphigus. Perhaps the evolution of soldiers in aphids has been facilitated by behavioral or morphological traits that evolved under the context of dispersal and intraspecific competition (Thorne et al. 2003).

How do social aphids deter attack from much larger predators? What role do plant secondary compounds play in aphid defense? How do herbivorous insects like soldier aphids kill much larger arthropods? Our preliminary research, described above, suggests a fascinating clue: soldiers readily kill Drosophila larvae (which act as surrogates in our experiments for the dipteran larvae that are natural enemies of aphids), and these larvae exhibit symptoms of immunological toxicity (e.g, elevated phenoloxidase relative to controls; phenoloxidase is the rate-limiting enzyme in the melanization cascade).  Further, we recently found that the function of the plant-derived plant secondary metabolites discovered in soldiers by co-PI Tooker may be to act as inducers of the melanization response (Lawson et al., in preparation). In other words, social aphids may be co-opting the chemical defenses of their host plants for altruistic defense, and these defenses may be targeted at vulnerabilities in insect immunity. Aphids may be the social analogs to monarch butterflies, in that a history of antagonistic chemical co-evolution between herbivore and plant has facilitated the entry into new ecological niches.

Selected publications


  1. 1.Grogan K, Chhatre V, & Abbot P. 2010. The costs and consequences of Dispersal and cheating in aphid Societies.  J. Evol biol. 23: 185-193

  2. 2.Abbot P. 2009. On dispersal and altruism in social aphids. Evolution, 63: 2687-2696.

  3. 3.Abbot P & Chhatre V. 2007. Kin structure provides no explanation for intruders in social aphids. Molecular Ecology, 16: 3659-3670

  4. 4.Abbot P & Withgott JH. 2004. Phylogenetic and molecular evidence for allochronic speciation in gall-forming aphids (Pemphigus). Evolution 58:  539-553.

  5. 5.Abbot P, Withgott JH, & Moran NA. 2001. Genetic conflict and conditional altruism in social aphid colonies. Proceedings of the National Academy of Sciences USA 98: 12068-12071.