Sex ratio and morphological polymorphism in an isolated, endemic Teladorsagia circumcincta population.

Teladorsagia circumcincta is a polygamous nematode that exhibits morphological polymorphism. Sex ratio is typically female biased and the male nematodes occur in association with the genetically similar, minor morphotypes Teladorsagia davtiani and Teladorsagia trifurcata. In experimental infections, sex ratio (proportion male) and the proportion of minor male morphs observed have been shown to be influenced by both host and nematode-related factors. As similar investigations from natural systems are rare, this study examined whether sex ratio and minor male morph frequency were associated with host age and sex and nematode infra-population size in the isolated Soay sheep population on St Kilda. Count data for Teladorsagia were analysed for sheep of all age classes and both sexes from the winters of three consecutive population crashes (1999, 2002 and 2005). Generally, the intensity of Teladorsagia nematodes increased with host age until the age of 2 years before decreasing. In 2005, abundance of nematodes was generally higher than in the previous crashes, nematode sex ratio was negatively associated with host age and tended to be negatively associated with nematode intensity. Within the male nematode subpopulation, T. circumcincta always predominated, followed by T. davtiani and then T. trifurcata, with little variation in the relative proportions between hosts. The presence of each minor morph was primarily associated with the intensity of male T. circumcincta and, in those hosts where all three male morphs were detected, intensity of each minor morph was most associated with intensity of Teladorsagia females. Therefore, in a year when the nematode was generally more abundant, sex ratio appeared to be influenced by both host and nematode-related factors, whereas in all years examined, the frequency of morphological polymorphism was primarily density dependent.