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

Since inbreeding in Tetranychus urticae can reduce offspring fitness, sexualselection may favour disassortative mate choice with respect to relatedness of the matingpartners. We tested whether T. urticae shows this preference for mating with unrelatedpartners. We chose an experimental set-up with high potential for female choosiness, sincefemales only mate once and are therefore expected to be the choosier gender. An adultvirgin female was placed together with two adult males from the same population. One malewas 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 matingdid not depend on the female-to-male relatedness. The remaining (non-copulating) maletried to interfere with the ongoing mating in the majority of cases, but this interference didnot depend on the female-to-male relatedness. These results imply that T. urticae (a) canrecognize kin (via genetic and/or environmental similarity) and (b) has the potential toavoid inbreeding through mate choice.