PARASITISM AND HETEROZYGOSITY INFLUENCE THE SECONDARY SEXUAL CHARACTERS OF THE EUROPEAN MINNOW, PHOXINUS-PHOXINUS (L) (CYPRINIDAE)

The Hamilton & Zuk hypothesis (HAMILTON & ZUK 1982) was tested in the European minnow, Phoxinus phoxinus (L.), where the male breeding chara cters include tubercles, which are horny growths on the skin of the he ad, and a bright red abdomen. Two trematode parasite species, neither directly transmitted, were present: Diplostomum phoxini in the brains of 100% of the fish and Macrolecithus papilliger in the guts of 44%. T he heterozygosity of the fish at 11 loci was determined. The number of M. papilliger was negatively related to redness and positively relate d to the size of tubercles. The number of D. phoxini was nor related t o redness but was also positively related to the size of tubercles. He terozygosity was positively related to redness and negatively related to the size of tubercles. This is the first report of such inverse rel ationships with heterozygosity. The number of tubercles was positively related to the size and condition of a fish. We suggest that males un able to develop exaggerated characters to display to females, e.g. a b right red abdomen, have the alternative strategy of emphasizing weapon s for use in male-male competition. This effect could explain other re ported positive correlations between parasite burden and sexual displa y.