Parasitism, reproduction and sexual selection of roach, Rutilus rutilus L

Abstract

Raine Kortetin väitöstutkimus lisää tietämystä kalan kutukyhmyistä, lisääntymisbiologiasta, seksuaalivalinnasta ja immuunipuolustuksesta. Tutkimuksessa on ensimmäistä kertaa osoitettu, että sekä loisvälitteinen että ns. immunokompetenssihaittateoria voivat olla toimivia saman systeemin seksuaalivalinnassa. Tutkimustyön tuloksia voidaan mahdollisesti soveltaa esimerkiksi kalanviljelyssä ja -jalostuksessa sekä uhanalaisten kalalajien suojelussa.

One of the main issues in immunological ecology is the role of immune defence in the evolution of life-history traits, i.e. the investment in reproduction at the cost of immune function. Moreover, theories of parasite-mediated sexual selection suggest that secondary sexual characters are connected to parasitism. For this thesis, I studied aspects of fish reproduction and sexual selection with respect to defence against parasites and pathogens. As a model species I used roach, Rutilus rutilus. In the seasonal cycle of fish immune defence, the relative size of the spleen and the chemotactic migration activity of head kidney granulocytes decreased immediately before and after spawning, respectively. However, breeding-related changes in other measurements (phagocytosis activity of head kidney granulocytes, blood IgM concentration, white cell count and red cell count) were absent or inconsistent. This suggests that the different parts of the immune system may have different temporal patterns. Moreover, epidermal papilloma disease outbreak peaked around the spawning period in both sexes. Males and large fish had a higher prevalence of papilloma tumours. The gender dependence in the disease prevalence found in this study may indicate differences in stress and immunocompetence between male and female roach. Sexual ornamentation of male roach, the breeding tubercles, signalled a long-term resistance of males against the locally most abundant parasite in the study, Rhipidocotyle campanula. Moreover, breeding tubercle ornamentation was associated with dominance status in intrasexual prespawning interactions. In addition, raised circulating androgen levels were related to high breeding tubercle ornamentation, but interestingly, also to increased prevalence of papilloma disease. Thus, I propose that, in the present system, the character that a male roach signals by his sexual ornamentation may be resistance against parasites (R. campanula), and that papillomatosis occurring during spawning is an element which contributes to the honesty of the signal. The results in the thesis support the hypothesis of a trade-off between reproduction and immune defence and the hypothesis of parasite-mediated sexual selection. Thus, parasites and pathogens may have an appreciable impact on the evolution of life histories and sexual selection of fish.