Pemphigus spyrothecae

The aphid soldier exhibits its first distinguishing physical traits as a first larval instar, the phase following its complete maturation in the egg. There are two types of first instar larvae within galls: one type of larvae is thick-legged and attacks insects introduced into galls. Another type of larvae is normal-legged.[10] Monomorphic first-instar larvae of Pemphigus dorocola attack moth larvae, a predator, when it is experimentally introduced to the gall.[11] After observing a physical resemblance between these aphids and the thick-legged P. spyrothecae larvae, Aoki suggested that these insects also defended the gall. His prediction was confirmed, as he later observed a caste between normal-legged first-instar larvae as "reproductives-to-be" and the thick-legged larvae as defenders, or in his words, "soldiers".[11] This was later deemed the first discovery of aphid soldiers from a locality outside of East Asia. These soldiers have different morphological states from other members of the species.[12] First, the first-instar soldiers tend to be more aggressive. They also have thick hind legs and a stylet, which is used to attack invaders.[13][14]

Pemphigus spyrothecae

P. spyrothecae is capable of fortress defense through manipulation of the composition of aphids in galls. When introduced to a single predator, colonies with soldiers usually killed the predator, incurring the loss of a few soldiers. In colonies with only non-soldiers, predators killed the aphids and ate them.[15][16]

In a later study, Foster and Rhoden (1997) examined the effectiveness of soldiers in fortress defense.[17] They manipulated the numbers of soldiers and non-soldiers in sets of galls that remained in contact with poplar trees at the research site. When there was a combination of soldiers and non-soldiers, the gall was ten times less likely to be attacked by a predator compared to a gall with only non-soldier aphids. These results led Foster and Rhoden to conclude that soldiers effectively defend against predation under natural conditions. Earlier laboratory experiments demonstrated that soldiers could prevent specialist gall predators from attacking the gall. However, in this study, Foster and Rhoden found that such protection could also take place in the field. Five different predators attack the P. spyrothecae galls: specialist A. minki, generalists S. ribesii and A. nemoralis and two other unidentified generalist predators.[13]

P. syprothecae, like other aphid species that attack predators, maintain a singular thrusting motion as they pierce the predator with their stylets.[18] Repeated stabbing rarely occurs. This species can also utilize its legs—sometimes all six—in order to squeeze the predator. The purpose of this motion is to tear the predator's cuticle and ultimately rupture its internal structures.[19] Although detailed quantitative observations were not included in the Rhoden and Foster study, the attacking behavior was deemed costly to the soldiers. Thus, a cost-benefit analysis most likely plays a role in a soldier's decision to attack the predator.

Since P. spyrothecae clonally reproduce, genetic relatedness within colonies is rather simple: an individual aphid is either a clone of its neighbors or it is not. Since every generation within the gall displays a high degree of relatedness due to cloning, any deviation from a colony's genetic uniformity (excluding mutation) can be traced back to intergall migration.[13] The ease of intergall migration is rooted in the fact that the colonies are present in the galls on the primary host for a long period of time, only migrating once the summer has passed.[13]

The genetic relationships within aphid colonies help explain the evolution of the altruistic soldier hierarchy of P. spyrothecae.[13] More specifically, with the utilization of field trapping and microsatellites, the researchers were able to examine the degree of clonal mixing in the colony. Clonal mixing is defined as the "mixing" of genes "individual from different clones… which will dilute the benefit of cooperation by wasting it on unrelated clones" (Johnson 1525).[13] The researchers predicted that the reason that galls were a critical factor to the evolution of soldiers... Given the fact that a gall was defendable and valuable, the authors anticipated that this served as a barrier to the intrusion of other foreign clones via clonal mixing. Johnson et al. found that while soldiers typically migrate and have the capability to moult and reproduce outside of its own gall, clonal mixing was low overall. Given this insignificant level of clonal mixing, there is a low chance for barriers to the evolution and propagation of an altruistic soldier caste within the species. Ultimately, Johnson et al. (2002) became the first researchers to utilize species-specific nuclear DNA markers in order to get a clear sense of the degree of clonal diversity within the P. spyrothecae galls; with the additional usage of trapping data, their prediction of intergall migration was also confirmed.[13]

Since trapping data confirmed that all migrants are soldiers, Johnson et al. infer that these aphids can grow and produce winged emigrants in galls as an adaptive strategy of cloning instead of a chance event. This confirms the earlier belief that soldiers had evolved from specialized migrants.[20] However, further studies on colonies where the level of clonal mixing is predicted to be high would enable researchers to gain a better understanding of the relationships between the degree of clonal mixing and soldier investment.

Nest repair serves as a major example of eusociality within colonies of P. spyrothecae.[1] After prematurely creating an opening in a gall of the same size and presentation of a natural opening, the researchers observed a higher degree of mortality from predation. In this circumstance, there was no evidence of greater investment in soldiers. So, the researchers concluded that the aphids do not adapt to short-term changes in the colony via a reduction of clonal mixing (an increase in soldier reproduction). However, they did note that the holes were repaired by compensatory regrowth from nearby areas of the gall that were either unharmed and/or protected from predation. The researchers found that the soldiers were most likely responsible for the repair, highlighting the expanse of the altruism within this complex soldier caste.[1]