The effects of relatedness on progeny sex ratio in spider mites

Theory predicts that optimal sex allocation in subdivided populations is dependent on the genetic relatedness among competing offspring such that when relatedness is high, progeny sex allocation should be more biased than when relatedness is low. In the laboratory we compared the progeny sex ratio of four groups of five ovipositing females of various presumed degrees of relatedness (sisters of inbred laboratory population, sisters of outbred field population, non‐sisters of the same field population and non‐sisters each from a different, geographically distant field population). We found a greater female bias in the progeny of inbred sisters than for sisters from the field population; the progeny sex ratio was also more strongly female biased for sisters than for non‐sisters from the field population. These differences in sex ratio are in line with theoretical predictions. Our results indicate that spider mites are capable of some method of discrimination between kin and non‐kin.