Mate choice promotes inbreeding avoidance in the two-spotted spider mite

Since inbreeding in Tetranychus urticae can reduce offspring fitness, sexual
selection may favour disassortative mate choice with respect to relatedness of the mating
partners. We tested whether T. urticae shows this preference for mating with unrelated
partners. We chose an experimental set-up with high potential for female choosiness, since
females only mate once and are therefore expected to be the choosier gender. An adult
virgin female was placed together with two adult males from the same population. One male
was unrelated and the other male was related—a brother with whom she had grown up.
Significantly more copulations (64%) took place with the unrelated male. Time to mating
did not depend on the female-to-male relatedness. The remaining (non-copulating) male
tried to interfere with the ongoing mating in the majority of cases, but this interference did
not depend on the female-to-male relatedness. These results imply that T. urticae (a) can
recognize kin (via genetic and/or environmental similarity) and (b) has the potential to
avoid inbreeding through mate choice.