Have genetic trade-offs in host use been overlooked in arthropods?

A popular hypothesis to explain the high degree of host specialisation observed among mites and insects is the existence of host-associated fitness trade-offs. According to this theory, adaptation to a host results in a relatively poorer performance on alternative hosts due to the antagonistic pleiotropic action of one or more genes. Evidence in favour of the genetic trade-off hypothesis is however scarce. Recent ecological work has shown that the optimisation of adult performance drives the evolution of host choice in at least some phytophagous insects. Yet, most ecological and evolutionary studies on host choice assume that females maximise their fitness by optimising offspring performance. In this paper, we investigate whether a general lack of attention for the role of adult performance in host choice may have diminished the chance of detecting genetic trade-offs. We reviewed the literature on genetic trade-offs and showed that most studies neglected host specific variation in adult performance. Moreover, studies that considered both adult and offspring performance had a higher chance of detecting genetic fitness trade-offs. Our results also suggested that studies on asexual reproducing species tend to detect trade-offs more often than studies on obligate sexual reproducing species. We argue that future studies on genetic trade-offs should take all fitness parameters into account in order to be conclusive. This approach may reveal (i) that genetic trade-offs are more common than hitherto reported and/or (ii) that genetic trade-offs are more common, or more easily detected among asexual reproducing species like mites and aphids.