Population abundance and sex ratio in dioecious helminth parasites

Abstract

Parasite populations are highly fragmented in space and time, and consist of aggregates of genetically similar individuals sharing the same host. To avoid inbreeding, theory predicts that female-biased sex ratios should be strongly favoured when either or both prevalence and intensity of infection are low. Other models indicate that if sex ratios are selected to increase the probability of mating, they should be less biased at a high intensity of infection in polygamous parasites, since at high intensities all females are mated. To test these predictions, the relationship between sex ratio and both the prevalence and intensity of infection was examined in comparative studies across 193 populations of nematode and acanthocephalan parasites. Sex ratios in these two dioecious, polygamous taxa are usually female biased. Among natural populations, no significant relationship was observed once the confounding effects of phylogeny had been removed. However, among experimental populations of nematodes, a negative relationship was found between intensity of infection and sex ratio, even after controlling for phylogeny. In other words, at high intensities, populations of nematodes are less female biased. This result must be treated with caution because of the unusually high numbers of worms per host in experimental infections. Nevertheless, combined with information on the proximate mechanisms regulating sex ratios in these parasites, it suggests a link between the characteristics of parasite populations and their sex ratio.

Stochastic and spatial dynamics of nematode parasites in farmed ruminants

“…We have discussed elsewhere the implications for the spread of drug resistance (Cornell et al 2000(Cornell et al , 2003. One might also expect this to lead to a skew in the parasite sex ratio (Poulin 1997), which has not been observed in farmed conditions. To model the observed levels of parasite aggregation, it is necessary to introduce host heterogeneity ( Our model is a stochastic spatial extension of a standard deterministic model in which immune response is a simple function of age (Smith 1988).…”

Sex ratios and sex-biased infection behaviour in the entomopathogenic nematode genus Steinernema

“…The male proportion never reached 50% (except with S. glaseri for early emergers on filter paper where it was 50.2%), and in most cases there was a significant deviation from equality of the sexes. The sex ratios of nematode parasites tend to be female-biased (Poulin, 1997). With the notable exception of Steinernema hermaphroditum where males comprise less than 6% of the population (Griffin et al, 2001), a similarly slight female bias (typically 60% female) has frequently been recorded in experimental infections of Steinernema spp.…”

“…The sex ratio of acanthocephalan and nematode parasites is frequently biased towards females in both natural and experimental infections (Poulin, 1997), while that of schistosomes is typically male-biased (Mone and Boissier, 2004). The primary sex ratio of animals is expected to be balanced (Fisher, 1930) but later events such as differential survival and infection rates may result in biased sex ratios of parasites within hosts.…”

Age Distribution and Seasonal Dynamics of Abomasal Helminths in Wild Red Deer from Central Spain

“…quadrispiculata. A decrease in ratios for male and female nematodes with the age of the host has been considered an indicator of senescence within a nematode population (Halvorsen and Bye, 1999) given the longer life expectancy of female worms (Poulin et al, 1997). The suggestion seems reasonable, because fawns showed an increased ratio from the time they acquired the first infections (fall) extending into spring, when relative abundance is comparable to that found in adult hosts.…”