Anssi Vainikka

The role of juvenile hormone in immune function and pheromone production trade-offs: a test of the immunocompetence handicap principle

The immunocompetence handicap hypothesis postulates that secondary sexual traits are honest signals of mate quality because the hormones (e.g. testosterone) needed to develop secondary sexual traits have immunosuppressive effects. The best support for predictions arising from the immunocompetence handicap hypothesis so far comes from studies of insects, although they lack male–specific hormones such as testosterone. In our previous studies, we found that female mealworm beetles prefer pheromones of immunocompetent males. Here, we tested how juvenile hormone (JH) affects male investment in secondary sexual characteristics and immune functions in the mealworm beetle, Tenebrio molitor . We injected male mealworm beetles with JH (type III) and found that injection increased the attractiveness of male pheromones but simultaneously suppressed immune functions (phenoloxidase activity and encapsulation). Our results suggest that JH, which is involved in the control of reproduction and morphogenesis, also plays a central role in the regulation of a trade–off between the immune system and sexual advertisement in insects. Thus, the results reflect a general mechanism by which the immunocompetence handicap hypothesis may work in insects.

Do pheromones reveal male immunocompetence

Pheromones function not only as mate attractors, but they may also relay important information to prospective mates. It has been shown that vertebrates can distinguish, via olfactory mechanisms, major histocompatibility complex types in their prospective mates. However, whether pheromones can transmit information about immunocompetence is unknown. Here, we show that female mealworm beetles (Tenebrio molitor) prefer pheromones from males with better immunocompetence, indicated by a faster encapsulation rate against a novel antigen, and higher levels of phenoloxidase in haemolymph. Thus, the present study indicates that pheromones could transmit information about males parasite resistance ability and may work as a reliable sexual ornament for female choice.

Do male mealworm beetles, Tenebrio molitor, sustain the honesty of pheromone signals under immune challenge?

Immune function is potentially costly and traded-off against the expression of sexual signals, thus, making sexual advertisement a condition-dependent and honest indicator of current immunocompetence. We conducted a series of laboratory experiments using mature mealworm beetles, Tenebrio molitor, to examine whether an immunological challenge (nylon implant(s), microbial lipopolysacharides, microlatex beads) could affect the attractiveness of male pheromones or male’s survival in ad libitum and ‘water-only’ food regimes or whether pheromones signal more long-term qualities of males. We treated another set of newly-emerged males with a single nylon implant and gave first ad libitum food but later only water. We used several immune challengers and feeding regimes to examine if the expected trade-off between immune function and sexual signaling would be expressed differently depending on the resource level and the type of the immune challenge. None of the treatments affected the female preference for male pheromones or the male longevity except for the acute mortality caused by two-implant treatments. However, males lost less body mass when immune challenged and given an opportunity to feed. We conclude that females seem to prefer certain males, and the effect of immune challenge in males cannot systematically override these preferences.

In vitro embryo survival and early viability of larvae in relation to male sexual ornaments and parasite resistance in roach, Rutilus rutilus L.

According to the ‘good genes’ hypothesis, sexual ornaments provide an indication of the ‘quality’ of the bearer. In roach, Rutilus rutilus, breeding tubercles (BTs) may signal resistance against the digenean parasite, Rhipidocotyle campanula. Life history theory predicts that there should be a trade‐off between parasite resistance and other life history traits. In roach, this could imply a trade‐off between parasite resistance in mature fish and some larval feature. We studied embryo survival and the early viability of larvae of male roach in relation to expression of BTs and parasite resistance in maternal half‐sibling families. Highly ornamented males had higher resistance against R. campanula than less ornamented males, but the BTs were not related to either embryo survival or larval viability. However, sires having higher resistance to R. campanula had lower larval viability. These results suggest that BTs of male roach do not indicate ‘quality’ in terms of early survival or viability, but rather in terms of adult parasite resistance.

What do male tench, Tinca tinca, advertise with morphological ornaments?

In contrast to females, tench Tinca tinca (L.) males have large pelvic fins with a thickened and bent second fin ray. Males also produce notable ventral protuberances during breeding, but the function of these male ornaments is not known. Using wild-caught fish, we found that both the size of pelvic fins and ventral protuberance were dependent on body mass/length ratio but not necessarily on condition. Plasma testosterone concentration and relative gonad mass were positively correlated with condition factor. Plasma testosterone concentration was not related to measures of non-specific immune function, but correlated positively with the size of pelvic fins corrected for body length. However, the studied male ornaments were not related to the measures of immune defence or to the load of Diplostomum spp. We did not find evidence for male–male dominance or female preference for large male characteristics within the natural variation in these traits. Thus, our study suggests that also other mechanisms than sexual selection on good genes might contribute to the maintenance of sexual dimorphism in tench.