The Evolution of Soldiers in Aphids

Abstract

1. Defensive individuals, termed soldiers, have recently been discovered in aphids, Soldiers are typically early instar larvae, and in many species the soldiers are reproductively sterile and morphologically and behaviourally specialized. 2. Since aphids reproduce parthenogenetically, we might expect soldier production to be more widespread in aphids than it is. We suggest that a more useful way to think about these problems is to attempt to understand how a clone (rather than an individual) should invest in defence and reproduction. 3. Known soldiers are currently restricted to two families of aphids, the Pemphigidae and Hormaphididae, although they are distributed widely among genera within these families. We discuss the use of a phylogenetic perspective to aid comparative studies of soldier production and we demonstrate this approach using current estimates of phylogenetic affinities among aphids. We show that the distribution of soldier production requires a minimum of six to nine evolutionary origins plus at least one loss. 4. At least four main types of soldiers exist and we present and discuss this diversity of soldiers. 5. Most soldier-producing species produce soldiers within plant galls and we discuss the importance of galls for the evolution of soldiers. 6. We summarize the evidence on the interactions between soldiers and predators and between soldier-producing aphids and ants. 7. We present an optimality model for soldier investment strategies to help guide investigations of the ecological factors selecting for soldiers. 8. The proximate mechanisms of soldier production are currently very poorly understood and we suggest several avenues for further research.

Multiple Origins of Eusociality Among Sponge-Dwelling Shrimps (Synalpheus)

“…Our results are consistent with a long history of research supporting competition and enemy pressure as primary selective pressures favoring the evolution of sociality and reproductive skew in social insects (Lin and Michener 1972;West-Eberhard 1975;Evans 1977;Strassmann and Queller 1989;Alexander et al 1991;Crespi 1994;Stern and Foster 1996;Brockmann 1997;Shellman-Reeve 1997;Thorne 1997;Queller and Strassmann 1998) and cooperatively breeding vertebrates (Emlen 1982(Emlen , 1991Brown 1987). In many vertebrates and some insects, cooperative breeding appears to be favored by habitat saturation, that is, the extreme scarcity of suitable, unoccupied territory.…”

“…Intense pressures from enemies, including competition for valuable nest resources and predation, long have been recognized as primary environmental factors selecting for cooperation in social insects (Lin and Michener 1972;Alexander 1974;Wilson 1975;Evans 1977), and recent findings of eusociality outside the Hymenoptera and termites tend to support this view (Alexander et al 1991;Stern and Foster 1996;Crespi and Mound 1997). Cooperative social groups often acquire and defend contested resources more effectively than individuals or less organized groups do (Lin and Michener 1972;Alexander 1974;Evans 1977;Emlen 1991).…”

“…Eusocial-like caste systems also occur in some clonal aphids (Stern and Foster 1996). The multiple origins of eusociality in the Hymenoptera, and particularly the range in social systems found within lower-level taxa such as the halictid bees, have made the Hymenoptera especially valuable for comparative approaches to understanding the evolution of social systems (e.g., Michener 1969;Wilson 1971;Carpenter 1989Carpenter , 1991Choe and Crespi 1997).…”

Facultative bacterial symbionts in aphids confer resistance to parasitic wasps

“…Further, resistance may confer indirect benefits; fewer parasitoids emerging reduces the risk of parasitism for nearby clone mates. Kin selection has been well documented in other aphids with high genetic relatedness within colonies (33).…”

Meat and bone meal still used in animal feed

“…The coated elastic honeydew droplets can be pushed, rolled and squeezed without leaking or wetting. Smith 3 noted that this behaviour was initially observed by Buckton in 1876 and may have promoted the evolution of social organization in aphids 7 . Practically, it is worthwhile examining whether aphid's wax is better at coating liquid droplets than are the spores of Lycopodium.…”