Abstracts (first author)


Male-male aggression peaks at intermediate relatedness in a social spider mite


Author(s): Egas M, Sato Y, Sabelis MW, Mochizuki A


Inclusive fitness theory predicts that when individuals live in groups or colonies, male-male aggression should peak at intermediate levels of average relatedness in the colony. Assuming that aggression is costly and directed towards nonrelatives and that competition for reproduction acts within the colony, the benefits of aggressive behaviour are maximized in colonies with a mix of related and unrelated competitors because aggression hurts unrelated individuals often, thereby favouring reproduction of related individuals. This prediction has been tested with specific bacterial strains in laboratory settings, but not with organisms in the field. Here, we study how male-male aggression varies with average relatedness in naturally occurring colonies of the social spider mite Stigmaeopsis miscanthi. This mite lives on Chinese silver grass, constructs colonies as woven nests on the undersurface of grass leaves, and lives within these nests in kin groups. This mite species shows parental care: males defend the offspring they fathered by counterattacking the offspring from predatory mites that intruded the nest. However, males also show aggression towards conspecific males. They kill rival males inside nests, and establish their own harem. Male-male aggression (quantified as the probability of lethal combat) is highly variable among populations, and it is negatively correlated with a proxy for average genetic relatedness in the colony - winter harshness. We sampled 25 populations across a wide geographic range between Taiwan and Japan, representing a gradient of high to low within-colony relatedness. For each population the weaponry of males was measured as the length of the first pair of legs, and male-male aggression was tested by placing pairs of non-sibling males together and scoring the frequency of male death over a given period. In support of theory, male-male aggression and weapon size strongly peak at intermediate average relatedness.

Abstracts (coauthor)


Sneaking behaviour as a strategy to compete for females has been observed in many taxa from insects to mammals. Often, the less competitive males who would lose contests over females against stronger males, display sneaking behavior in that they deceive their rivals by pretending to be female. To understand the evolution of observed sneaking behaviour, it is important to elucidate the mechanisms maintaining alternative mating strategies. Recently, we found three male mating phenotypes in a population of the two-spotted spider mite: territorial, sneaking and opportunistic. Opportunistic males wander around in search of females that are about to moult into the adult phase (teleiochrysalis). Territorial males spend much time in a mounted position on the dorsum of the teleiochrysalis females, and guard them by fighting intruding rival males. Sneaker males also spend much time in a mounted position, but they never show aggressive behaviour against intruders. Intruding males easily find and attack territorial males but fail to notice or even ignore sneaker males. Here, we focus on territorial and sneaker males in the two-spotted spider mite, and investigate which males display sneaking behaviour and how effective the sneaking behaviour is against territorial males. We show that young males display sneaking behaviour more often, and that young territorial males are three times more likely to lose their mounted position to old territorial males than young sneaker males. Old males are always territorial and rarely lose their position on the female dorsum to young males. Finally, young territorial males are observed to change their mating strategy to that of sneaker in the presence of old males. Our results suggest that old males are superior to young males in contests over females, and that in response to old males young males can phenotypically switch to sneaker behavior, thereby possibly improving their mating success.


The herbivorous spider mite Tetranychus evansi is an invasive species specialized to feed on Solanaceous plants, including tomato. In its native range (South America) it is not considered as a pest, but in Africa and Southern Europe major crop losses have been recorded due to T. evansi outbreaks. Apart from predator release, two important factors appear to facilitate the spread of this species: (1) its ability to suppress tomato plant defense to below control levels, and (2) its very high population growth rate. Related species, such as the generalist two-spotted spider mite T. urticae, also have a high population growth rate, but cannot suppress tomato defence to the same extent as T. evansi.

Plants protect themselves in various ways against herbivores by producing toxins or attracting natural enemies. Therefore, suppressing these defences can benefit herbivores, and it has been shown that some herbivores indeed do so. However, T. evansi downregulates the defence of its host plant to levels that fall even below house-keeping levels of healthy unattacked plants. Downregulating host plant defence to below house-keeping levels is beneficial for herbivores, but – perhaps surprisingly - not observed before in nature. Given that herbivores do suppress plant defence but only to a limited extent, we hypothesize that suppression of host plant defense trades off with life history traits such as oviposition rate or making dense web to defend a suppressed-defence part of the host plant against competitors.

We are currently establishing T. evansi lines that differ in their ability to suppress tomato defense through crossing mites from different geographical locations and subsequent inbreeding, and through experimental evolution. Measurements of life-history characteristics can provide insight into genetic trade-offs for tomato defense suppression. In addition, competition experiments allow us to assess the relative fitness of different T. evansi strategies.


Chairman: Octávio S. Paulo
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XIV Congress of the European Society for Evolutionary Biology

Organization Team
Department of Animal Biology (DBA)
Faculty of Sciences of the University of Lisbon
P-1749-016 Lisbon


Computational Biology & Population Genomics Group