Environmental sex determination and density-dependent population regulation in the entomogenous nematodeRomanomermis culicivorax

Abstract

SUMMARYEnvironmental sex determination in the mermithid nematodeRomanomermisculicivorax is examined in the context of parasite reproductive success and population regulation. Experimental results show that the sex ratio of the nematode within its mosquito host (Culex quinquefasciatus) is dependent on parasite density. Sex ratios are biased to females at low parasite burdens and to males at high parasite burdens. Low temperature further enhances female-biased ratios. The net effect of density-dependent sex determination on parasite and host population growth is shown to be critically dependent on the frequency distribution of parasite numbers/host. Mermithid parasite distributions within natural host populations show low degrees of aggregation relative to other helminth species. The population regulation of the parasite is examined with respect to environmental sex determination and parasite-induced host mortalities by means of simple mathematical models of the dynamics of parasite transmission via its life-cycle. The significance of regulatory constraints on population growth are discussed in relation to the use of mermithids as biological control agents of insect pests or disease vectors.

Two cues for sex determination in Gammarus duebeni : Adaptive variation in environmental sex determination?

“…Morph development in the parasitic nematode Strongyloides ratti is controlled by two developmental switches: host immune status and environmental temperature (Harvey et al 2000). In Romanomermis culicivorax, sex is cued by temperature and parasite density within the host (Tingley and Anderson 1986). To our knowledge, the only other record of a dual cue for ESD comes from the fish Poecilia sphenops, in which sex determination is cued by the interaction between temperature and salinity (Baron et al 2002).…”

Superparasitism and Population Regulation of the Mosquito-Parasitic Mermithid Nematodes Romanomermis iyengari and Strelkovimermis spiculatus

“…This type of scramble competition in insect parasitoids was regarded by Taylor (1988) as having an unstable trajectory due to excessive competition. Tingley and Anderson (1986), however, suggest that mermithid nematodes may not closely follow this pattern because of their longer life cycle relative to the host. In a 3-yr field study, Micieli et al (2012) observed that the frequency over time and the level of infection by S. spiculatus were key parameters in regulating populations of Ochlerotatus albifasciatus.…”

“…When mosquito population density is low, the outcome may be more frequent superparasitism and a skewed mermithid sex ratio favoring males. Superparasitism provides mermithid population reduction, regulation, and stability by relaxing pressure on the host population (Tingley and Anderson, 1986;Sanad et al, 2013). Conversely, when mosquitoes are abundant, the already reduced mermithid population encounters less competition, less superparasitism, more nutrients and consequently produce more females to boost their population to track an expanding host population.…”

“…Manipulation of this equilibrium, therefore, is central to population regulation. Suboptimal host-parasite densities, however, can lead to male-dominant populations in some species, as is most evident for mosquito-parasitic mermithid nematodes (Petersen, 1980;Tingley and Anderson, 1986;Sanad et al, 2013), which serves to dampen parasite populations and reestablish host-parasite equilibrium.…”

“…Sex determination in mosquito mermithids occurs postinfection within the host and is dependent on nematode density (Tingley and Anderson, 1986;Sanad et al, 2013). Sex ratios are female biased at low parasite loads and male biased at high parasite loads.…”