Behavior out of control: Experimental evolution of resistance to host manipulation

Intraspecific variation for host immune activation by the spider mite Tetranychus evansi

Many parasites can interfere with their host's defences to maximize their fitness. Here, we investigated if there is heritable variation in the spider mite Tetranychus evansi for traits associated with how they interact with their host plant. We also determined if this variation correlates with mite fecundity. Tetranychus evansi can interfere with jasmonate (JA) defences which are the main determinant of anti-herbivore immunity in plants. We investigated (i) variation in fecundity in the presence and absence of JA defences, making use of a wild-type tomato cultivar and a JA-deficient mutant (defenseless-1), and (ii) variation in the induction of JA defences, in four T. evansi field populations and 59 inbred lines created from an outbred population originating from controlled crosses of the four field populations. We observed a strong positive genetic correlation between fecundity in the presence (on wild-type) and the absence of JA defences (on defenseless-1). However, fecundity did not correlate with the magnitude of induced JA defences in wild-type plants. Our results suggest that the performance of the specialist T. evansi is not related to their ability to manipulate plant defences, either because all lines can adequately reduce levels of defences, or because they are resistant to them.

A symbiotic aphid selfishly manipulates attending ants via dopamine in honeydew

Symbiotic relationships are widespread in nature, but the mechanisms maintaining these relationships remain to be elucidated because symbiosis incurs a maintenance cost to each participant, which lowers its reproductive rate. In host-parasite relationships, parasites are known to manipulate the host's behavior selfishly, and there is an arms race between them. Selfish manipulations also occur in symbiosis, but the effects of selfish manipulations on symbiosis are not fully understood. Here, we show that an ant-associated aphid manipulates attending ants to receive stronger protection. Aphid honeydew regurgitated by ants contains dopamine (DA). The ants showed low aggressiveness before contact with the aphids, but it rose after contact. Administration of DA to the ants increased ant aggressiveness as the concentration increased, while an antagonist of DA inhibited this effect. The other 3 amines showed no effect on aggressiveness. A previous study showed that attending ants selfishly manipulate aphids by increasing the reproductive rate of green morph to obtain high-quality honeydew. These results suggest that mutual selfish manipulation benefits both participants and is likely to strengthen symbiosis. The selfishness of each participant may contribute to sustaining this symbiosis because their selfishness increases their long-term fitness.

Parasites, pathogens, and other symbionts of copepods

There is a large diversity of eukaryotic symbionts of copepods, dominated by epizootic protists such as ciliates, and metazoan parasites. Eukaryotic endoparasites, copepod-associated bacteria, and viruses are less well known, partly due to technical limitations. However, new molecular techniques, combined with a range of other approaches, provide a complementary toolkit for understanding the complete symbiome of copepods and how the symbiome relates to their ecological roles, relationships with other biota, and responses to environmental change. In this review we provide the most complete overview of the copepod symbiome to date, including microeukaryotes, metazoan parasites, bacteria, and viruses, and provide extensive literature databases to inform future studies.

Tapeworm manipulation of copepod behaviour: parasite genotype has a larger effect than host genotype

Compared with uninfected individuals, infected animals can exhibit altered phenotypes. The changes often appear beneficial to parasites, leading to the notion that modified host phenotypes are extended parasite phenotypes, shaped by parasite genes. However, the phenotype of a parasitized individual may reflect parasitic manipulation, host responses to infection or both, and disentangling the contribution of parasite genes versus host genes to these altered phenotypes is challenging. Using a tapeworm (Schistocephalus solidus) infecting its copepod first intermediate host, I performed a full-factorial, cross-infection experiment with five host and five parasite genotypes. I found that a behavioural trait modified by infection, copepod activity, was affected by both host and parasite genotype. There was no clear evidence for host genotype by parasite genotype interactions. Several observations indicated that host behaviour was chiefly determined by parasite genes: (i) all infected copepods, regardless of host or parasite genotype, exhibited behavioural changes, (ii) parasitism reduced the differences among copepod genotypes, and (iii) within infected copepods, parasite genotype had twice as large an effect on behaviour as host genotype. I conclude that the altered behaviour of infected copepods primarily represents an extended parasite phenotype, and I discuss how genetic variation in parasitic host manipulation could be maintained.