Long-lived Hosts Research Articles

The high cost of mutualism: effects of four species of East African ant symbionts on their myrmecophyte host tree

Three recent meta-analyses of protective plant-ant mutualisms report a surprisingly weak relationship between herbivore protection and measured demographic benefits to ant-plants, suggesting high tolerance for herbivory, substantial costs of ant-mediated defense, and/or benefits that are realized episodically rather than continuously. Experimental manipulations of protective ant-plant associations typically last for less than a year, yet virtually all specialized myrmecophytes are long-lived perennials for which the costs and benefits of maintaining ant symbionts could accrue at different rates over the host's lifetime. To complement long-term monitoring studies, we experimentally excluded each of four ant symbionts from their long-lived myrmecophyte host trees (Acacia drepanolobium) for 4.5 years. Ant species varied in their effectiveness against herbivores and in their effects on intermediate-term growth and reproduction, but the level of herbivore protection provided was a poor predictor of the net impact they had on host trees. Removal of the three Crematogaster species resulted in cumulative gains in host tree growth and/or reproduction over the course of the experiment, despite the fact that two of those species significantly reduce chronic herbivore damage. In contrast, although T. penzigi is a relatively poor defender, the low cost of maintaining this ant symbiont apparently eliminated negative impacts on overall tree growth and reproduction, resulting in enhanced allocation to new branch growth by the final census. Acacia drepanolobium is evidently highly tolerant of herbivory by insects and small browsers, and the costs of maintaining Crematogaster colonies exceeded the benefits received during the study. No experimental trees were killed by elephants, but elephant damage was uniquely associated with reduced tree growth, and at least one ant species (C. mimosae) strongly deterred elephant browsing. We hypothesize that rare but catastrophic damage by elephants may be more important than chronic herbivory in maintaining the costly myrmecophyte habit in this system.

Evidence for the cure of HIV infection by CCR5Δ32/Δ32 stem cell transplantation

AbstractHIV entry into CD4+ cells requires interaction with a cellular receptor, generally either CCR5 or CXCR4. We have previously reported the case of an HIV-infected patient in whom viral replication remained absent despite discontinuation of antiretroviral therapy after transplantation with CCR5Δ32/Δ32 stem cells. However, it was expected that the long-lived viral reservoir would lead to HIV rebound and disease progression during the process of immune reconstitution. In the present study, we demonstrate successful reconstitution of CD4+ T cells at the systemic level as well as in the gut mucosal immune system after CCR5Δ32/Δ32 stem cell transplantation, while the patient remains without any sign of HIV infection. This was observed although recovered CD4+ T cells contain a high proportion of activated memory CD4+ T cells, ie, the preferential targets of HIV, and are susceptible to productive infection with CXCR4-tropic HIV. Furthermore, during the process of immune reconstitution, we found evidence for the replacement of long-lived host tissue cells with donor-derived cells, indicating that the size of the viral reservoir has been reduced over time. In conclusion, our results strongly suggest that cure of HIV has been achieved in this patient.

Maintenance, spillover and spillback transmission of bovine tuberculosis in multi-host wildlife complexes: A New Zealand case study

The causative agent of bovine tuberculosis (bTB; Mycobacterium bovis) has a broad host range. The role of each animal species in spreading the disease depends on how transmission occurs, on the abundance of each host, and on the interactions between hosts. This paper explores differences in the roles individual host species can play in allowing M. bovis infection to persist and spread within a multi-species complex, using New Zealand as a case study. In New Zealand, four wild mammal species are frequently infected. Of these the brushtail possum is now regarded as the only true “maintenance” host. Red deer and ferrets can become maintenance hosts where their densities are exceptionally high, but more often they are “spillover” hosts, with most infection arising from moderately frequent inter-species transmission from possums. The latter situation is even more strongly the case for feral pigs. Spillover hosts may occasionally play a crucial epidemiological role by transmitting infection back to a potential maintenance host (spillback). Three key factors make spillback transmission far more epidemiologically important than its low frequency of occurrence might suggest—amplification of the reservoir of bTB, far greater spatial spread than by the maintenance host, and greater persistence of bTB in long-lived spillover hosts extending the risk of spillback far into the future. The risk of spillback is undoubtedly low, but it nonetheless determines the nature, scale and duration of management required. Eradication of the disease may require management of both the infection in maintenance hosts and reduction or elimination of any risk of spillback.

ECOLOGY AND DYNAMICS OF THE BLOOD PARASITE, HEPATOZOON TUATARAE (APICOMPLEXA), IN TUATARA (SPHENODON PUNCTATUS) ON STEPHENS ISLAND, NEW ZEALAND

We explored infection patterns and temporal dynamics of the protozoan blood parasite Hepatozoon tuatarae (Apicomplexa) infecting the tuatara (Sphenodon punctatus), a protected reptile living on Stephens Island, New Zealand. In March 2006, we surveyed tuatara in five study sites to examine spatial variation in infection prevalence, and four times, from May 2005 to November 2006, we recaptured marked individuals within three study sites to examine the temporal dynamics of infection. We also examined how blood-parasite infection patterns were influenced by host sex, body size, and host infestation with ticks (Amblyomma sphenodonti) and mites (Neotrombicula spp.), which are potential vectors of the blood parasite. Infection prevalence (16.9-24% infected) and intensity (<0.01-0.1% blood cells infected) were low in all samples. Infection intensity varied among the five sampled sites in March 2006, but prevalence did not. Neither infection prevalence nor intensity varied with time, and infections were detected in consecutive samples from recaptured individuals for up to 18 mo. Neither survey showed an influence of host sex on infection, but both surveys showed infection intensity declined with increasing host body size, as did infection prevalence in the spatial survey. In the temporal survey, we found a positive relationship between the tick numbers on hosts and blood-parasite infection intensity, which were stronger in two of the sampling periods and among larger hosts. These data suggest that exposure and susceptibility to infection decreases with host size and that ticks, but not mites, are probably the vectors in this ancient host-parasite association of a long-lived (>50 yr) host.

The Evolution of Host‐Parasite Range

Understanding the coevolution of hosts and parasites is one of the key challenges for evolutionary biology. In particular, it is important to understand the processes that generate and maintain variation. Here, we examine a coevolutionary model of hosts and parasites where infection does not depend on absolute rates of transmission and defense but is approximately all-or-nothing, depending on the relative levels of defense and infectivity of the host and the parasite. We show that considerable diversity can be generated and maintained because of epidemiological feedbacks, with strains differing in the range of host and parasite types they can respectively infect or resist. Parasites with broad and narrow ranges therefore coexist, as do broadly and narrowly resistant hosts, but this diversity occurs without the assumption of highly specific gene interactions. In contrast to gene-for-gene models, cycling in strain types is found only under a restrictive set of circumstances. The generation of diversity in both hosts and parasites is dependent on the shape of the trade-off relationships but is more likely in long-lived hosts and chronic disease with long infectious periods. Overall, our model shows that significant diversity in infectivity and resistance range can evolve and be maintained from initially monomorphic populations.

Local context drives infection of grasses by vector‐borne generalist viruses

Host characteristics commonly determine infection risk, but infection can also be mediated by regional- or local-scale variation in the biotic and abiotic environment. Experiments can clarify the relative importance of these factors. We quantified drivers of infection by barley and cereal yellow dwarf viruses (B/CYDV), a group of generalist, vector-borne grass pathogens, at hierarchically nested spatial scales (10(5)-1 m) by planting individuals of six common grass species into five Pacific Coast grassland sites spanning 7 degrees of latitude (> 5000 total hosts) and applying a factorial combination of nitrogen and phosphorus fertilizer. Infection differed most among experimental blocks (10(2)-10(3) m scale), suggesting that local factors control infection risk; infection increased with cover of long-lived hosts and phosphorus, but not nitrogen, fertilization. For B/CYDV, local context more strongly predicts infection risk than host species traits or regional context; such spatially nested experiments can clarify the factors underlying variation in infection risk.

Nothing in the Biology of Glycans Makes Sense, Except in the Light of Evolution

All cells in nature display cell surface glycans that are involved in host biological processes, and provide pathogen binding sites. Thus, glycans are trapped in evolutionary “Red Queen” effects, with long‐lived hosts evading their more rapidly evolving pathogens by changing glycan expression, without compromising intrinsic function. This helps explain glycan structural variations in nature, which contributes to biological diversity. Sometimes, one glycan type is eliminated in an evolutionary lineage, e.g., human‐specific loss of the sialic acid Neu5Gc, which affects pathogen recognition. Many human pathogens also disguise themselves with human‐like sialic acids (molecular mimicry) by convergent evolution, thus co‐opting host Siglecs normally dedicated to recognizing self. Such differences between humans and great apes involving sialic acids have implications for unusual features of human immunity. Also, incorporation of Neu5Gc into humans occurs from food, and a similar process contaminates biotherapeutic molecules and cells. As all humans have complement‐fixing anti‐Neu5Gc antibodies, this may contribute to some diseases, and reactions to biotherapeutic products. Finally, we hypothesize that such antibodies restrict transmission of enveloped viruses, and could change fertility, allowing sympatric speciation (collaboration with Pascal Gagneux).

The evolution of covert, silent infection as a parasite strategy

Many parasites and pathogens cause silent/covert infections in addition to the more obvious infectious disease-causing pathology. Here, we consider how assumptions concerning superinfection, protection and seasonal host birth and transmission rates affect the evolution of such covert infections as a parasite strategy. Regardless of whether there is vertical infection or effects on sterility, overt infection is always disadvantageous in relatively constant host populations unless it provides protection from superinfection. If covert infections are protective, all individuals will enter the covert stage if there is enough vertical transmission, and revert to overt infections after a ‘latent’ period (susceptible, exposed, infected epidemiology). Seasonal variation in transmission rates selects for non-protective covert infections in relatively long-lived hosts with low birth rates typical of many mammals. Variable host population density caused by seasonal birth rates may also select for covert transmission, but in this case it is most likely in short-lived fecund hosts. The covert infections of some insects may therefore be explained by their outbreak population dynamics. However, our models consistently predict proportions of covert infection, which are lower than some of those observed in nature. Higher proportions of covert infection may occur if there is a direct link between covert infection and overt transmission success, the covert infection is protective or the covert state is the result of suppression by the host. Relatively low proportions of covert transmission may, however, be explained as a parasite strategy when transmission opportunities vary.

Disease, predation and demography: assessing the impacts of bovine tuberculosis on African buffalo by monitoring at individual and population levels

Summary 1. Understanding the effects of disease is critical to determining appropriate management responses, but estimating those effects in wildlife species is challenging. We used bovine tuberculosis (BTB) in the African buffalo Syncerus caffer population of Kruger National Park, South Africa, as a case study to highlight the issues associated with estimating chronic disease effects in a long-lived host. 2. We used known and radiocollared buffalo, aerial census data, and a natural gradient in pathogen prevalence to investigate if: (i) at the individual level, BTB infection reduces reproduction; (ii) BTB infection increases vulnerability to predation; and (iii) at the population level, increased BTB prevalence causes reduced population growth. 3. There was only a marginal reduction in calving success associated with BTB infection, as indexed by the probability of sighting a known adult female with or without a calf ( P = 0·065). 4. Since 1991, BTB prevalence increased from 27 to 45% in the southern region and from 4 to 28% in the central region of Kruger National Park. The prevalence in the northern regions was only 1·5% in 1998. Buffalo population growth rates, however, were neither statistically different among regions nor declining over time. 5. Lions Panthera leo did not appear to preferentially kill test-positive buffalo. The best (Akaike’s Information Criterion corrected for small sample size) AIC c model with BTB as a covariate [exp( β ) = 0·49; 95% CI = (0·24‐1·02)] suggested that the mortality hazard for positive individuals was no greater than for test-negative individuals. 6. Synthesis and applications . Test accuracy, time-varying disease status, and movement among populations are some of the issues that make the detection of chronic disease impacts challenging. For these reasons, the demographic impacts of bovine tuberculosis in the Kruger National Park remain undetectable despite 6 years of study on known individuals and 40 years of population counts. However, the rainfall and forage conditions during this study were relatively good and the impacts of many chronic diseases may be a non-linear function of environmental conditions such that they are only detectable in stressful periods.

Virulence in clonal plants: conflicting selection pressures at work?

The evolution of pathogen virulence has received considerable attention during the last decades (Van Baalen and Sabelis 1995; Frank 1996; Day and Proulx 2004). In this note we will discuss two major theories of virulence evolution in the context of clonal plant— pathogen interactions and argue that they can lead to contradictory predictions when applied to very long-lived hosts such as clonally propagating plants. We propose that clonal plants and their pathogens may be especially suitable systems for empirically testing hypotheses of virulence evolution. A generally accepted theory of virulence evolution is based on the trade-off between pathogen transmission and within-host pathogen reproduction (Frank 1996; Lipsitch and Moxon 1997; Day 2003; Andre and Hochberg 2005), the latter being positively correlated with pathogen virulence (see Table 1 for definitions). Pathogens can benefit for a long time from hosts with low intrinsic mortality rates, provided that they exploit them prudently (low virulence), while severe host exploitation (high virulence) shortens the time during which a pathogen can benefit from the host. This effect can be seen as a mortality cost of virulence (Day and Proulx 2004). As a consequence, high host longevity should select for lower levels of virulence. Conversely, increased virulence should be favoured in shortlived hosts (Gandon et al. 2001 and references therein; Day and Proulx 2004), due to the necessity to spread quickly to new hosts and due to the lower mortality costs of virulence (Ewald 1994; Day and Proulx 2004). Depending on the specific host-pathogen interaction a whole range of evolutionary stable strategies can be expected. The optimal strategy

Correlated evolution of life history and host range in the nonphotosynthetic parasitic flowering plants Orobanche and Phelipanche (Orobanchaceae)

Most members of the nonphotosynthetic parasitic genera Orobanche and Phelipanche (Orobanchaceae) have narrow host ranges, and, as they grow on perennial hosts, are (at least potentially) perennial themselves. A few species, however, have wide host ranges and grow on annual hosts, and are thus (at least facultatively) annuals themselves. Among the latter are the weedy species, which include economically important pest taxa such as Orobanche crenata or Phelipanche aegyptiaca. Using a phylogenetically based maximum likelihood approach, which takes phylogenetic and branch length uncertainty into account, we can show that the life trait host range and life history evolve in a correlated fashion. This supports the hypothesis that parasite specialization is associated with predictable resources (i.e. long-lived hosts) and generalism with unpredictable ones (i.e. short-lived hosts), a pattern often found in animal parasites. The mechanisms and temporal sequence of the life trait changes and their interrelations remain speculative.

The Results Are In: Bacterial Parasite Strives for Balance in Host Infection

When horror-movie writers run out of ideas, they can always turn to parasites. Imagine the possibilities with flesh-eating bacteria, suicide-inducing hairworms, scalp-burrowing botflies—and castrating parasites. Such debilitating effects are an inevitable consequence of infection, but it is in the parasite's interest to avoid killing the host until it can transmit a new crop of pathogens. In the “tradeoff hypothesis” for the evolution of virulence—how quickly a parasite kills its host—host exploitation and parasite reproduction are balanced to maximize the parasite's lifetime transmission success. But lifetime transmission success, an indicator of parasite fitness, has proven difficult to measure, leaving scant direct evidence for an optimal level of virulence. In a new study, Knut Helge Jensen, Dieter Ebert, and colleagues worked with water fleas (Daphnia magna) and the castrating bacterium Pasteuria ramosa to investigate the relationship between parasite fitness and virulence. Castrating parasites divert resources from host reproduction toward their own reproductive ends. In the case of P. ramosa, that means generating transmission-stage parasites, or spores. Many castrators also boost host growth in a phenomenon called gigantism, which presumably helps support the outsized resource needs of a parasite that can account for as much as 25% of its host's body weight. In keeping with the tradeoff hypothesis, the researchers predicted that the parasite should castrate early to optimize the appropriation of host resources, and produce intermediate levels of virulence to keep the host alive long enough to maximize spore production. They used a castrating parasite that produces copious quantities of spores but doesn't release them until the host dies, so they could estimate lifetime parasite reproduction and relate it to virulence. They found significant variation in parasite virulence, and present direct evidence linking parasite reproductive success to an optimal level of virulence, with transmission success peaking at an intermediate level of virulence. To determine the relationship between virulence and lifetime production of transmission-stage parasites, the researchers exposed a Daphnia clone to bacterial spores and tracked individual host mortality. Infected Daphnia sustained far more casualties than either unexposed controls or exposed but uninfected individuals, with deaths beginning at 23 days old and ending at 74 days old. (The first control died at 96 days old.) Early host death (high virulence) was bad news for P. ramosa, since the parasite needs several weeks to produce spores. But it also didn't fare terribly well with a long-lived host (low virulence), suggesting that the bacteria in these hosts grew too slowly to reach the optimal time of killing. The highest spore production was detected in Daphnia expiring at middle age, likely reflecting the benefits of using host resources for spore production—which can be impressive. One clutch of host eggs corresponds to an estimated 4.5 million P. ramosa spores, according to a recent study. Because total spore production can be used as a proxy for parasite fitness, the researchers concluded that maximum parasite fitness derives from an intermediate level of virulence. To determine whether genetic variation among parasite lines correlated with observed differences in virulence, the researchers infected hosts with spores collected from early dying and late-dying Daphnia. They found that spores from the early killing infections did indeed produce significantly higher death rates than did the late-killing spores. These results indicate that genetic variation in the parasite can influence variation in virulence, and thus affect the evolution of virulence. The correlation between an optimal level of virulence and parasite fitness may result from the strong tradeoff between host and parasite fitness, the researchers explain, which emerges as these adversaries battle for the resources needed for reproduction. This experimental evidence for the long-held assumption that parasite fitness directly relates to virulence has important implications for a wide range of virulence-related phenomena. Estimating likely changes in parasite virulence is essential for formulating public health strategies to contain emerging parasitic diseases and for developing effective drugs and vaccines. In future studies, the researchers plan to investigate how selection pressures, such as those caused by drugs, influence parasite fitness and cause changes in virulence.

Additive effects of ectoparasites over reproductive attempts in the long‐lived alpine swift

Summary 1 Parasitism is a non-negligible cost of reproduction in wild organisms, and hosts are selected to partition resources optimally between current and future reproduction. While parents can compensate for the cost of parasitism by increasing their current reproductive investment, such change in resource allocation is expected to carry-over costs on future reproduction. 2 Life history theory predicts that because long-lived organisms have a high residual reproductive value, they should be more reluctant to increase parental effort in response to parasites. Also, when rearing successive infested broods, the cost of parasitism can cumulate over the years and hence be exacerbated by past infestations. 3 We tested these two predictions in the alpine swift Apus melba, a long-lived colonial bird that is infested intensely by the nest-based blood sucking louse-fly Crataerina melbae. For this purpose, we manipulated ectoparasite load over 3 consecutive years and measured reproductive parameters in successive breeding attempts of adults assigned randomly to ‘parasitized’ and ‘deparasitized’ treatments. 4 In current reproduction, fathers of experimentally parasitized broods produced a similar number of offspring as fathers from the deparasitized treatment, but the rearing period was prolonged by 4 days. Fathers that were assigned to the parasitized treatment in year x produced significantly fewer fledglings the following year x + 1 than those of the deparasitized treatment. The number of young produced by fathers in year x + 1 was correlated negatively with the number of days they cared for their brood in the previous year x. We also found a significant interaction between treatments performed over 2 successive years, with fathers of parasitized broods suffering a larger fitness loss if in the past they had already cared for a parasitized brood rather than for a deparasitized one. Similar effects of parasitism, although partly non-significant (0·05 0·10), were found in mothers. 5 Altogether, our results show that parasites can modify resource allocation between current and future reproduction in long-lived hosts, and that the cost of parasitism can cumulate over the years. It emphasizes the fact that effects of parasites can depend on past infestations and become apparent in future reproduction only.

Characterization of the memory/activated T cells that mediate the long-lived host response against tuberculosis after bacillus Calmette-Guérin or DNA vaccination.

The memory/activated T cells, which mediate the long-lived host response against tuberculosis, in mice immunized with either bacillus Calmette-Guérin (BCG) or mycobacterium heat-shock protein 65 (hsp 65) antigen expressed from plasmid DNA (DNA-hsp 65), were characterized. Protection against Mycobacterium tuberculosis challenge by DNA-hsp 65 vaccination was associated with the presence of lymph node T-cell populations in which CD8+/CD44hi interferon-gamma (IFN-gamma)-producing/cytotoxic cells were prominent even after 8 or 15 months of plasmid DNA-mediated immunizations, whereas after BCG vaccination the majority were CD4+/CD44lo IFN-gamma-producing T cells. When the cells were separated into CD4+CD8- and CD8+CD4- and then into CD44hi and CD44lo types, CD44lo cells were essentially unable to transfer protection in adoptive transfer experiments, the most protective CD44hi cells were CD8+CD4- and those from DNA-vaccinated mice were much more protective than those from BCG-immunized mice. The frequency of protective T cells and the level of protection were increased up to 8 months and decreased after 15 months following DNA or BCG immunizations.

Hemlock Woolly Adelgid (Homoptera: Adelgidae) Dispersion and the Failure of Binomial Sampling to Estimate Population Density

Data collected in 1994 and 1995 from eastern hemlock stands in Pennsylvania and Virginia were used to evaluate the binomial sampling method for hemlock woolly adelgid. An empirical distribution model and a model based on the negative binomial distribution were used to describe the relationship between mean hemlock woolly adelgid density ( x ) and the proportion of samples with at least T hemlock woolly adelgid ( PT ). There were significant differences in the x versus PT relationship between locations and hemlock woolly adelgid generations regardless of the value of T (1–10) or model used. The inconsistency of the relationship may be a result of the respective phenologies of hemlock woolly adelgid and eastern hemlock, and of the effect that continued hemlock woolly adelgid feeding has on the production of new foliage. Binomial sampling procedures have been successfully developed for many pests of agricultural crops. The technique may not be suitable for long-lived hosts where pest attack significantly alters the food resource.

How Does Immigration Influence Local Adaptation? A Reexamination of a Familiar Paradigm

How Does Immigration Influence Local Adaptation? A Reexamination of a Familiar Paradigm

Epiphytes on an old Huon pine tree (Lagarostrobos franklinii) in Tasmanian rainforest

Abstract The epiphytic flora of an old Huon pine (Lagarostrobos franklinii) in western Tasmania comprises 76 lichens, 55 bryophytes, and 16 vascular plant species. There is a vertical zonation of the cryptogams on the tree, with little overlap between basal and canopy floras. Bryophytes are the dominant life form at the base of the tree but there is a marked increase in the diversity of lichens, and in the ratio of lichens to bryophytes, with increasing height. The longevity of the tree is manifest in the development of a wide range of microhabitats but it seems unlikely that any of the cryptogams require such longevity per se to become established as rainforest epiphytes. However, several of the vascular species on the tree are normally terrestrial and their occurrence as epiphytes is dependent on the development of sufficient peat to support their root systems. Such a process is likely to be very slow, requiring a long-lived host species. The study illustrates the richness of lichens and bryophytes in t...

Human facial beauty

It is hypothesized that human faces judged to be attractive by people possess two features-averageness and symmetry-that promoted adaptive mate selection in human evolutionary history by way of production of offspring with parasite resistance. Facial composites made by combining individual faces are judged to be attractive, and more attractive than the majority of individual faces. The composites possess both symmetry and averageness of features. Facial averageness may reflect high individual protein heterozygosity and thus an array of proteins to which parasites must adapt. Heterozygosity may be an important defense of long-lived hosts against parasites when it occurs in portions of the genome that do not code for the essential features of complex adaptations. In this case heterozygosity can create a hostile microenvironment for parasites without disrupting adaptation. Facial bilateral symmetry is hypothesized to affect positive beauty judgments because symmetry is a certification of overall phenotypic quality and developmental health, which may be importantly influenced by parasites. Certain secondary sexual traits are influenced by testosterone, a hormone that reduces immunocompetence. Symmetry and size of the secondary sexual traits of the face (e.g., cheek bones) are expected to correlate positively and advertise immunocompetence honestly and therefore to affect positive beauty judgments. Facial attractiveness is predicted to correlate with attractive, nonfacial secondary sexual traits; other predictions from the view that parasite-driven selection led to the evolution of psychological adaptations of human beauty perception are discussed. The view that human physical attractiveness and judgments about human physical attractiveness evolved in the context of parasite-driven selection leads to the hypothesis that both adults and children have a species-typical adaptation to the problem of identifying and favoring healthy individuals and avoiding parasite-susceptible individuals. It is proposed that this adaptation guides human decisions about nepotism and reciprocity in relation to physical attractiveness.

Herbivore deme formation on individual trees: a test case.

We examined the deme-formation hypothesis, which states that sessile herbivores on long-lived hosts become locally adapted to the defensive phenotypes of individual trees. We showed a five-fold increase in resistance by individual pinyon pines (Pinus edulis) to the pinyon pine needle scale (Matsucoccus acalyptus). Although such variation could represent a significant selection pressure favoring deme formation, two lines of evidence led to rejection of the hypothesis. First, there were no significant differences in mortality among scale populations in a reciprocal transfer experiment. Second, a seven-year experiment showed that mortality of newly founded, incipient scale populations was similar to established scale populations. While our experiments fail to support the deme-formation hypothesis, they do demonstrate significant variation in the resistance traits of a natural tree population. Although we feel that demeformation is still probable in this system, it is likely to occur on a larger geographic scale than individual trees as proposed by Edmunds and Alstad.

Lecithochirium furcolabiatum (Jones, 1933), Dawes 1947: the miracidium and mother sporocyst

Experimental infections of the marine topshell Gibbula umbilicalis with Lecithochirium furcolabiatum (Digenea: Hemiuroidea) have allowed the development of a model system which will enable further studies of the molluscan host response. The long-lived intertidal prosobranch host is easily maintained in the laboratory, and experimental infection rates of 98% were consistently achieved. The miracidium and mother sporocyst have been studied at both light and ultrastructural levels, providing the first account of the morphology of these stages in Hemiuridae. The ingested egg hatches within the host intestine, treatment with L-cysteine and alkaline pH stimulating miracidial emergence in vitro. The general body surface of the miracidium is devoid of spines or cilia, the latter being restricted to four plates near the anterior extremity. The miracidium swims actively prior to penetration of the gut wall, the sporocyst being released from the miracidial epidermal coat within the haemocoel. Within 5 weeks of infection, the filamentous mother sporocyst contains 1 to 3 oval germ balls, daughter sporocysts being recorded free within the digestive gland haemocoel 7 weeks later. Twenty three weeks after ingestion of eggs, the daughter sporocyst extends into the host gill filaments, containing cystophorous cercariae ready for emergence.