Population dynamics of a thelastomatid nematode of cockroaches

Complex and Diverse Drivers of Parasite Loads in a Cosmopolitan Insect

The goal of parasite epidemiologists is to understand the factors that determine host infection levels. Potential infection determinants exist at many scales, including spatial and temporal environmental variation, among-host differences, and interactions between symbionts infecting the same host. All of these factors can impact levels of parasitism, but frequently only a subset is considered in any host-parasite system. We examined several potential determinants of pinworm infection in wild Australian cockroaches (Periplaneta australasiae) from multiple biological scales: (1) habitat; (2) season; (3) cockroach body size, developmental stage, and sex; and (4) interactions between 2 pinworm species (Leidynema appendiculata and Thelastoma sp.). Over 1 yr, we collected 239 cockroaches from 2 separate rooms in an Illinois greenhouse. We used generalized linear mixed-effects models (GLMMs) to evaluate simultaneously the influence of these factors on pinworm abundance, and nearly all had significant effects. Overall, the abundance of L. appendiculata was greater than Thelastoma sp., but the relative abundance of the 2 species was reversed in each room (i.e., a taxon × habitat effect). Abundance varied over 4 trapping seasons and increased with cockroach size. Adult cockroaches had more pinworms than nymphs, and there was also a significant taxon × stage effect: adult cockroaches had fewer pinworms than expected for their larger size, and this reduction was greater in Thelastoma sp. than in L. appendiculata. Cockroach sex had no effect on infection. Although females had more worms than males, this difference could be explained by the larger size of females. Finally, after controlling for all other potential determinants of infection, we found a strong negative association between Thelastoma sp. and L. appendiculata; cockroaches tended to be infected with either 1 pinworm species or the other. Our work underscores the importance of measuring potential determinants of infection from as many scales as possible. Such approaches are necessary to unravel the complexities of host-parasite interactions.

Lethal fighting in nematodes is dependent on developmental pathway: male-male fighting in the entomopathogenic nematode Steinernema longicaudum

Aggressive encounters occur between competitors (particularly males) throughout the animal kingdom, and in some species can result in severe injury and death. Here we describe for the first time lethal interactions between male nematodes and provide evidence that the expression of this behaviour is developmentally controlled. Males of the entomopathogenic nematode Steinernema longicaudum coil around each other, resulting in injuries, paralysis and frequently death. The probability of death occurring between pairs of males was affected by the developmental pathway followed, being much greater among males that had passed through the infective juvenile (IJ, or dauer) stage than among males that had not. Post-IJ males are found only in newly colonised hosts, typically with few competing males present. Killing those few competitors may secure valuable resources (both females and a host cadaver for nourishment of offspring). Non-IJ males develop in subsequent generations within a host cadaver, where the presence of many closely related male competitors increases the risk:benefit ratio of fighting. Thus, passage through the IJ stage primes males for enhanced aggression in circumstances where this is more likely to result in increased reproductive success. Fighting occurred between males developing in mixed-sex social groups, indicating that it is an evolved trait and not an abnormal response to absence of females. This is supported by finding high mortality of males, but not of females, across a range of population densities in insect cadavers. We propose that these nematodes, with their relatively simple organization, may be a useful model for studies of aggression.

Density-dependent sex ratio in Echinomermella matsi (Nematoda), a parasite of the sea urchin Strongylocentrotus droebachiensis

We investigated the adult sex ratio in 70 infrapopulations of the nematode Echinomermella matsi, a parasite of the green sea urchin Strongylocentrotus droebachiensis. The adult sex ratio was skewed towards female dominance at low adult intensity and towards male dominance at high adult intensity. We hypothesize that this is due to differences between the sexes in development and mortality rates, or that female recruitment is density dependent. A model with differences between the sexes in developmental and mortality rates may develop the observed sex ratios if the female developmental and mortality rates are several times that of the males. A large difference in developmental rates between the sexes appears unreasonable because the developmental rate for both sexes is low, and the predicted low female life-expectancy is unlikely because the males appear to accumulate in infrapopulations as the females age. Density dependence of female numbers is, however, supported by a significantly lower female recruitment in infrapopulations with old females. We also find that the mean male length is negatively related to measures of crowding, thereby supporting the hypothesis that competition is of importance in E. matsi infrapopulations. A female bias at low intensities of infection, a density dependence in female recruitment and the taxonomic position of E. matsi indicate that sex may be environmentally determined in this nematode.

Evidence for population self-regulation, reproductive competition and arrhenotoky in a thelastomatid nematode of cockroaches

Experimental infection of adultDrymaplaneta variegataShelford, 1909 (Blattodea) with known numbers of eggs ofProtrellus dixoniZervos, 1987 (Nematoda: Thelastomatidae) showed that each infrapopulation was regulated by a density-dependent and sex-dependent reduction in infection intensity with infrapopulation age. This reduction was not equal in initial speed or intensity between the sexes (reduction in number of males was faster) and led to infrapopulations with never more than a single adult male, 1–8 (usually 1–3) adult females and 0–28 juvenile females. In structure, these laboratory-produced infrapopulations resembled those in field-collected hosts. Parasite-mediated chemical interference competition is suggested as the cause of infrapopulation regulation.Per capitaegg production was greater in uncrowded worms which suggests that self-regulation benefits those nematodes that survive competition. Unmated females produced male offspring only (probably by arrhenotokous parthenogenesis). Only females that occurred with a male produced females although some also produced males; such females may avoid insemination or prevent fertilization of some or all eggs. Female offspring probably result from amphimixis.

PROPERTIES OF CROWDING INDICES AND STATISTICAL TOOLS TO ANALYZE PARASITE CROWDING DATA

Crowding, i.e., the size of the infrapopulation inhabiting an individual host, is a major component of parasites' environment, which often influences both morphological and life-history characters (the so-called density-dependent characters) in different parasite taxa. Although crowding equals intensity in case of a single parasite individual, mean intensity of the host population does not define mean crowding of the parasite population. Crowding indices are notoriously hard to handle statistically because of the inherently large number of nonindependent values in data. In this study, we aim to investigate the apparently paradox features of crowding indices and to make some proposals and also to introduce statistical methods to calculate confidence intervals and 1-sample and 2-sample tests for mean crowding. All methods described in this study are supported by the freely distributed statistical software Quantitative Parasitology.

Evolutionary patterns in life histories of Oxyurida

The Oxyurida comprises some 850 known species that occur in the intestine of arthropods and vertebrates (one species in annelids). Important arthropod hosts include Diplopoda, Blattodea, Gryllotalpoidea, Passalidae, Scarabaeida and Hydrophilidae. The major vertebrate hosts are lizards, tortoises, primates, rodents and lagomorphs. An underlying characteristic of the group is haplodiploid reproduction and like many haplodiploid groups, pinworms tend to have life histories that involve high levels of inbreeding. Unlike Strongylida, Ascaridida and Spirurida, which have diversified in tissue site and life cycle as well as hosts, pinworms show little variation in these features and have radiated only across host groups. Two explanations are advanced for this. Haplodiploidy and its concomitant inbreeding may act to canalise evolutionary change, although diverse groups such as the Hymenoptera belie this. Alternatively, Strongylida, Ascaridida and Spirurida are presumed to have arisen from skin-penetrating ancestors that were forced to undergo a tissue migration before reaching their primitive tissue site, the gut. This migration demanded they adapt to a variety of tissue sites and thus acted as a preadaptation to further diversification. The Oxyurida, in contrast, probably arose using oral contaminative transmission. The lack of exposure to other tissue sites may therefore have relegated pinworms to their position in the posterior gut.

Structure of the pinworm (Oxyurida: Nematoda) guild in the hindgut of the American cockroach, Periplaneta americana

The pinworm (Oxyurida: Nematoda) guild in a laboratory colony of Periplaneta americana was investigated to answer the following questions. (1) Is guild structure stable with time? (2) Is there evidence of interaction among the species? (3) Is there evidence for niche diversification with respect to spatial distribution or developmental stage of host? Four species were found: Thelastoma periplaneticola, Thelastoma bulhoesi, Hammerschmidtiella diesingi and Leidynema appendiculatum; females of the first two could not be distinguished and they were lumped and referred to as Thelastoma sp.; 328 mid-instar hosts were dissected between September 1987 and February 1990, and the number of adult females of each pinworm species was recorded to investigate changes in guild structure through time. Seventy-five percent of hosts had Thelastoma sp., 62% had H. diesingi and 40% had L. appendiculatum; mean intensities were 6.5 (Thelastoma), 2.9 (H. diesingi) and 1.9 (L. appendiculatum). Relative numbers were stable with time and intensities of the species were similar whether they occurred alone in the host or in conjunction with other species. Thelastoma sp. and L. appendiculatum occurred together less often than expected and their numbers in individual hosts were negatively correlated. Both species occurred alone more often than expected. Surprisingly, H. diesingi and L. appendiculatum occurred together more often than expected. Intestines of 44 roaches were frozen in liquid nitrogen and divided into 5 equal sections to study longitudinal distributions of the species in the gut. Most L. appendiculatum and H. diesingi were found in the first, whereas most Thelastoma sp. occurred in the second segment. However, Thelastoma sp. dominated all segments of the gut. Worms were counted in 192 hosts belonging to 4 categories (early instar, late instar, adult male and adult female). Single species infections were more common in early instar hosts but there was no evidence that different species preferred different host stages. Modal distributions in male L. appendiculatum and T. bulhoesi suggest that these species may exhibit intraspecific interference competition similar to that observed in other pinworms. Such effects may be more important than interspecific effects in maintaining guild stability.