Life-history consequences of bidirectional selection for male morph in a male-dimorpic bulb mite

Intralocus sexual conflict (IASC) arises when males and females have different trait optima. Some males pursue different alternative reproductive tactics (ARTs) with different trait optima, resulting in different strengths of IASC. Consequently, for instance daughter fitness is differentially affected by her sire’s morph. We tested if—and which—other life-history traits correlatively change in bidirectional, artificial selection experiments for ARTs. We used the male-dimorphic bulb mite Rhizoglyphus robini, the males of which are high-fitness ‘fighters’ or low-fitness ‘scramblers’. Twice in each of the five generations of selection, we assessed clutch composition (number of mites of the various life stages present) and size (total number of offspring). Furthermore, we tracked offspring from egg to adulthood in the first and final generation to detect differences between selection lines in the size and duration of stages, and in maturation time. We found that selection for male morph increased the frequency of that morph. Furthermore, compared to fighter lines, scrambler lines produced more females, which laid larger eggs (in the final generations), and maintained a higher egg-laying rate for longer. Otherwise, our results showed no consistent differences between the selection lines in clutch size and composition, life stage size or duration, or maturation time. Though we found few correlated life-history trait changes in response to selection on male morph, the differences in egg laying rate and egg size suggest that IASC between fighters is costlier to females than IASC with scramblers. We hypothesize that these differences in reproductive traits allow fighter-offspring to perform better in small, declining populations but scrambler-offspring to perform better in large, growing populations

Male nutritional history affects female fecundity in a male-dimorphic mite: Evidence for a nuptial gift?

In male-dimorphic species, males are often either armoured ‘majors’ that can monopolise access to females, or unarmoured and defenceless ‘minors’ that cannot. However, majors, unlike minors, have to spend energy to maintain their weaponry. Like-for-like, this could mean that minors have relatively more energy available to increase their reproductive output through e.g. sperm competition, or the transference of nutrients by means of a nuptial gift. Such a fitness advantage to minors could therefore contribute to explaining the coexistence of both morphs in single populations. We tested if food-deprived females of the male-dimorphic bulb mite Rhizoglyphus robini produced more eggs when mated to a minor or to a major male, and whether egg production depended on whether their mates were starved or fed prior to mating. We found no effect of male morph on female fecundity, but females did produce more eggs when mated to previously fed males. We also found that females increased in mass, but males decreased in mass over the course of the experiment. From these observations we infer that fed males are able to transfer nutrients, a nuptial gift, to their mate. This is the first observation to suggest nuptial gift transfer in mites. Though males of both morphs appeared able to produce nuptial gifts; other factors, like habitat complexity, should be considered to identify fitness benefits of minor over major males to understand why the two morphs coexist

Costs of weaponry: unarmed males sire more offspring than armed males in a male-dimorphic mite

Morphological structures used as weapons in male–male competition are not only costly to develop but are also probably costly to maintain during adulthood. Therefore, having weapons could reduce the energy available for other fitness‐enhancing actions, such as post‐copulatory investment. We tested the hypothesis that armed males make lower post‐copulatory investments than unarmed males, and that this difference will be most pronounced under food‐limited conditions. We performed two experiments using the male‐dimorphic bulb mite Rhizoglyphus robini, in which males are either armed “fighters” or unarmed “scramblers.” Firstly, we tested whether fighters and scramblers differed in their reproductive output after being starved or fed for 1 or 2 weeks. Secondly, we measured the reproductive output of scramblers and fighters (starved or fed) after one, two or three consecutive matings. Scramblers sired more offspring than fighters after 1 week, but scramblers and fighters only sired a few offspring after 2 weeks. Scramblers also sired more offspring than fighters at the first mating, and males rarely sired offspring after consecutive matings. Contrary to our hypothesis, the fecundity of starved and fed males did not differ. The higher reproductive output of scramblers suggests that, regardless of nutritional state, scramblers make larger post‐copulatory investments than fighters. Alternatively, (cryptic) female choice generally favours scramblers. Why the morphs differed in their reproductive output is unclear. Neither morph performed well relatively late in life or after multiple matings. It remains to be investigated to what extent the apparent scrambler advantage contributes to the maintenance and evolution of male morph expression