Mixed life-history strategies in a local population of the ectoparasitic fly Carnus hemapterus

A major issue for the proper understanding of the evolution of life-cycle histories is the regulation of voltinism and its variation. Diapause characteristics are known to regulate voltinism, but the underlying mechanisms are poorly understood. This paper studies diapause duration and voltinism variation in a haematophagous diptera parasitizing 2 sympatric hosts with very different breeding phenologies. We hypothesize that bivoltinism will be more frequent in carnid flies parasitizing an early breeding, multi-brooded species than in flies parasitizing a late breeder, single-brooded species. We obtained evidence of the co-occurrence of uni- and bivoltinism in both clutches of the multi-brooded Spotless starling (Sturnus unicolor) as well as in clutches of the single-brooded European roller (Coracias garrulus). Unexpectedly, the proportion of bivoltine flies was similar in both host species. A remarkable degree of host-parasite synchronization at the population level was found for bivoltine flies. Our findings reveal the facultative nature of diapause in Carnus. We discuss the influence of abiotic conditions and host availability on polymorphism in life-history cycles and the consequences both for the parasite and the host.     

Geographical variation in dormancy in a copepod: evidence from population crosses

Populations of the freshwater copepod Mesocyclops edax inhabiting Michigan lakes are dormant during winter, whereas populations inhabiting Florida lakes develop and reproduce continuously throughout the year. A Michigan and a Florida population were exposed to dormancy inducing conditions (low temperature and short photoperiod) in the laboratory and observed for indications of dormancy. All Michigan individuals and a small percentage of the Florida individuals entered dormancy as indicated by prolonged duration of the fourth copepodid instar and cessation of feeding. I suggest that in these population these observations represent diapause, rather than quiescence. The two populations were crossbred to examine the nature of inheritance of dormancy. The F₁ hybrids exhibited an incidence of diapause approximately intermediate between the Florida and Michigan parental stocks. The backcrosses of F₁ individuals to the Michigan and Florida stocks, respectively, exhibited a high and an intermediate incidence of diapause. Survival of the F₂ crosses was very low. The present study presents evidence of genetic differentiation between the Michigan and Florida populations of M. edax with respect to ability to diapause.     

Evidence for host variation in parasite tolerance in a wild fish population

Hosts can protect themselves against parasites by actively reducing parasites burden (i.e. resistance) or by limiting the damages caused by parasites (i.e. tolerance). Disentangling between tolerance and resistance is important for predicting the evolutionary outcomes of host-parasite interaction. Dace (Leuciscus leuciscus) are often parasitized by the ectoparasite Tracheliastes polycolpus which feeds on (and destroys) fins, reducing thus the host’s condition. We tested the hypothesis that genetically-based variation in ectoparasite tolerance exists in a wild dace population. We found that moderately heterozygous dace, which are less resistant than highly heterozygous or homozygous dace, tolerated better the effect imposed by T. polycolpus for a given parasite burden. However, tolerance also varied upon environmental conditions, suggesting that genetic and environmentally-based variation exists for both resistance and tolerance in this natural host-parasite system. Moreover, a negative genetic correlation may exist between tolerance and resistance, and hence several evolutionary outcomes are possible in this interacting system.     

Geographic variation in oviposition preference for male and female host plants in a geometrid moth: implications for evolution of host choice

Several dioecious plant species exhibit sexual dimorphisms in defensive traits. However, the effects of sexual dimorphism on defense against herbivores remain poorly understood. Eurya japonica (Thunb.) (Theaceae) is a dioecious shrub that shows sexual dimorphism in the chemical defense of flower buds. Female calyces contain higher concentrations of total phenolics and condensed tannins than do male calyces. Male flower buds are edible for a florivore moth, Chloroclystis excisa (Butler) (Lepidoptera: Geometridae), whereas the female flower buds are lethal to the moth larvae. The moths prefer to oviposit on male over female E. japonica flower buds. As the moths also occur in areas lacking E. japonica, we tested whether the oviposition preference for E. japonica flower sex differed between moths sympatric and allopatric with E. japonica. The moths sympatric with E. japonica showed a stronger preference for male E. japonica than the moths allopatric with E. japonica. Our phylogeographic study using mitochondrial cytochrome oxidase subunit I gene sequences revealed little genetic differentiation between moth populations sympatric and allopatric with E. japonica. These results suggest that the adaptive oviposition preference for flower sex of E. japonica has evolved rapidly in C. excisa.     

Geographical variation in Ascaris lumbricoides fecundity and its implications for helminth control

The observation by microscopy of nematode eggs in human faeces is used to diagnose a helminthic infection, while the concentration of those eggs is used to estimate the number of worms in the host. Within a community, the prevalence of infection and the mean egg count provide useful information about the extent of a public health problem, and are being used to guide the growing efforts to control disease caused by helminths. Here, Andrew Hall and Celia Holland examine data on the relationship between the worm burdens of Ascaris lumbricoides and the concentration of eggs in faeces, and discuss the implications of the variation found for using such data to plan helminth control programmes.     

Comparative demography of a specialist and generalist fruit fly: Implications for host use and pest management

Host plants used by phytophagous insects can have significant consequences on demography parameters, overall lifetime fitness and their subsequent population dynamics. Here, we conduct a comparative demographic study between the specialist Zeugodacus cucumis (French) and generalist Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) to determine whether the host plants used by these fly species play any role in their overall lifetime fitness and explains current host use patterns. These two fly species are pests within the north-eastern region of Australia and we further aimed to use complete life-history data to determine the population parameters and models that would help identify the sensitive life-history stage that could be targeted for effective field management. Eggs collected from laboratory-reared flies were inoculated into organically grown fruits of both the primary and alternate host plant cultivars of both fly species. The proportion surviving each life stage from egg through to adult and fecundity were monitored for all cohorts from the different plant cultivars. Complete stage-base life-tables for cohorts of each fly species developing from each fruit cultivar were constructed, and the key demographic parameters and population models were analysed using PopTools matrix model programme. Our results showed that the host used by each fly species had significant consequences on fly demographic parameters and hence their overall lifetime fitness. The generalist B. tryoni was able to compensate for the fitness loss experienced at the pre-adult stage by having adults with higher fecundity, but this was not the case for the specialist Z. cucumis. Stage-base population models revealed that the population growth rate of both species was highly sensitive at the adult reproductive stage, indicating that manipulating probability of survival at this life stage would effectively manage populations of these pest species. This study provides the empirical evidence of undertaking complete life history demography studies of phytophagous insects to accurately understand their lifetime fitness consequences of using a certain host, their observed host use patterns, and overall population dynamics. We suggest that any efforts to manage dacine fruit fly pest population should consider life-history consequences of host use.     

Intra‐specific variability in life‐cycle synchronization of an ectoparasitic fly to its avian host

The role of environmental and host‐associated factors in synchronization of host–parasite life‐cycles is an important question of evolutionary ecology. Yet, only handsome of studies examined this question at the intraspecific level. Here we explore how host‐associated traits, such as breeding phenology and host breeding habitat, can influence parasite phenology and co‐occurrence at different spatial scales. We studied the system comprised of a generalist ectoparasitic fly Carnus hemapterus and one of its avian hosts, the European roller Coracias garrulus. Inter‐annual variation in phenology was larger for parasites than hosts. Host predictability in terms of occurrence and phenological regularity was moderate, suggesting that this resource can be difficult to be tracked by the parasite. A large proportion of flies consistently emerged before the appearance of suitable host resources at both the nest and population level. Consequently, we revealed low and highly variable inter‐annual host–parasite synchronization rates. Nevertheless, we found that parasites from nests of early and progressively earlier breeding European rollers were more synchronized with their hosts than parasites from nests of late and progressively later breeding hosts, respectively. Temporal trends in host suitability and parasite emergence at the population scale suggest that other mechanisms, such as dispersal or exploitation of other host species, ensure parasites access to resources and counteract asynchrony with the host at the nest scale.     

Female-biased density-dependent dispersal of a tephritid fly in a fragmented habitat and its implications for population regulation

The aim of this study was to quantify the rate of dispersal as a response to density in the specialist tephritid fly Paroxyna plantaginis (the main seed predator on its patchily distributed host plant, Tripolium vulgare, Asteraceae). Marked flies were released at three different fly densities in artificial host patches. The individual histories of recaptures were recorded as well as migration between patches and invasion by unmarked flies. The loss of marked flies relative to initial density was analysed using maximum likelihood estimation. Females generally had the highest loss rate. When comparing a density-independent model with a density-dependent model of the loss rate, the density-dependent model won four times out of six for the females but not a single time for the males. A stronger immigration rate of females relative to males supported the suggested female-biased dispersal. This indicates a sit-and-wait strategy for the territorial males and a pre-emptive competition strategy for egg-laying substrates for the females. These results may be of general importance for non-frugivorous tephritid systems with unpredictable and almost ephemeral accessibility to host plants and with a dynamics characterised by a high turnover rate and high attack levels. The study presents a method for measuring the propensity of individuals to leave an area as a response to local density. It is further an example of the consequences individual behavioural responses may have on the population dynamics of a patchy population.     

Genotypic variation in propensity for host alternation within a population of Pemphigus betae (Homoptera: Aphididae)

In a Utah canyon, the aphid Pemphigus betae shows both a complex life cycle, with alternation between a gall-forming phase on cottonwood leaves and a root-feeding phase on herbs, and a simple life cycle, with year-round residence on roots. In order to determine the extent of clonal variation in life cycle, experiments using multiple sublines of individual clones were carried out in the laboratory and in the field. Previous studies suggested that both genetic and environmental factors underlie life cycle differences among subpopulations of aphids from different sites and different life cycle phases. The current study is the first assessment of clonal variation in propensity for host alternation within a natural population of aphids. In the laboratory experiment, clones showed highly significant differences in reproductive rates and in production of the host-alternating migrants. In agreement with previous findings, clones originating from lineages that had alternated to cotton-wood hosts in the previous year had lower average density and produced more migrants than clones originating from lineages that had remained on roots during the previous year. In order to ascertain how clonal variation and site-specific environmental factors affect life cycle variation under natural conditions, clones from laboratory cultures were used to establish experimental colonies at two elevational sites within the canyon. Production of the host-alternating migrants was affected strongly by clone-x-site interaction and was affected slightly by site. Results from both experiments indicate that loss of host alternation in P. betae could be effected through genetic change, environmental change, or both.     

Genetic variation and population structure in desert bighorn sheep: implications for conservation

Bighorn sheep populations experienced a drastic reduction in both distribution and abundance until the advent of modern wildlife management, where improving viability of extant populations and translocating animals into historical habitat range have been the most important management policies. The fact that subspecies relationships among bighorn are ambiguous,together with the importance of selecting appropriate source stock and the expense of translocation projects, makes an understanding of subspecies relationships and genetic variation, within and between populations, important for the management and conservation of this species. In this study, genetic variation in 279 bighorn sheep from 13 study sites in Arizona, California, New Mexico and Alberta, Canada were examined by analyzing ten microsatellite loci to determine interpopulation differentiation and relationships between closely related taxa. All populations contained a substantial amount of genetic variation. Genetic differences between populations were large and roughly proportional to geographic distance. The significance of this to desert subspecies relationships and management is discussed.     

Hybrid origin of a cichlid population in Lake Malawi: implications for genetic variation and species diversity

The importance of species recognition to taxonomic diversity among Lake Malawi cichlids has been frequently discussed. Hybridization - the apparent breakdown of species recognition - has been observed sporadically among cichlids and has been viewed as both a constructive and a destructive force with respect to species diversity. Here we provide genetic evidence of a natural hybrid cichlid population with a unique colour phenotype and elevated levels of genetic variation. We discuss the potential evolutionary consequences of interspecific hybridization in Lake Malawi cichlids and propose that the role of hybridization in generating both genetic variability and species diversity of Lake Malawi cichlids warrants further consideration.     

Understanding long-term fruit fly (Diptera: Tephritidae) population dynamics: implications for areawide management

Fruit flies (Diptera: Tephritidae) are devastating agricultural pests worldwide but studies on their long-term population dynamics are sparse. Our aim was to determine the mechanisms driving long-term population dynamics as a prerequisite for ecologically based areawide pest management. The population density of three pestiferous Anastrepha species [Anastrepha ludens (Loew), Anastrepha obliqua (Macquart), and Anastrepha serpentina (Wiedemann)] was determined in grapefruit (Citrus x paradisi Macfad.), mango (Mangifera indica L.), and sapodilla [Manilkara zapota (L.) P. Royen] orchards in central Veracruz, México, on a weekly basis over an 11-yr period. Fly populations exhibited relatively stable dynamics over time. Population dynamics were mainly driven by a direct density-dependent effect and a seasonal feedback process. We discovered direct and delayed influences that were correlated with both local (rainfall and air temperature) and global climatic variation (El Ni?o Southern Oscillation [ENSO] and North Atlantic Oscillation [NAO]), and detected differences among species and location of orchards with respect to the magnitude and nature (linear or nonlinear) of the observed effects, suggesting that highly mobile pest outbreaks become uncertain in response to significant climatic events at both global and local levels. That both NAO and ENSO affected Anastrepha population dynamics, coupled with the high mobility of Anastrepha adults and the discovery that when measured as rate of population change, local population fluctuations exhibited stable dynamics over time, suggests potential management scenarios for the species studied lie beyond the local scale and should be approached from an areawide perspective. Localized efforts, from individual growers will probably prove ineffective, and nonsustainable.     

Geographical variation in species' population responses to changes in temperature and precipitation

Despite increasing concerns about the vulnerability of species'' populations to climate change, there has been little overall synthesis of how individual population responses to variation in climate differ between taxa, with trophic level or geographically. To address this, we extracted data from 132 long-term (greater than or equal to 20 years) studies of population responses to temperature and precipitation covering 236 animal and plant species across terrestrial and freshwater habitats. Our results identify likely geographical differences in the effects of climate change on populations and communities in line with macroecological theory. Temperature tended to have a greater overall impact on populations than precipitation, although the effects of increased precipitation varied strongly with latitude, being most positive at low latitudes. Population responses to increased temperature were generally positive, but did not vary significantly with latitude. Studies reporting significant climatic trends through time tended to show more negative effects of temperature and more positive effects of precipitation upon populations than other studies, indicating climate change has already impacted many populations. Most studies of climate change impacts on biodiversity have focused on temperature and are from middle to high northern latitudes. Our results suggest their findings may be less applicable to low latitudes.     

Functional shifts in the use of parasitized hosts by a tephritid fly: the role of host quality

Superparasitism, a phenomenon in which parasitic insects layeggs in already-exploited hosts, provides a useful context inwhich to examine the dynamics of parental investment. This studyexplored conditions under which female Mediterranean fruit flies(Ceratitis capitata) shift from avoiding superparasitism ofhost fruit to preferring it, even placing eggs direcdy intoexisting egg-laying cavities. An a priori hypothesis of costsand benefits was used to predict how use and avoidance of parasitizedfruit would change in -response to changes in fruit size andripeness. We predicted that avoidance would decrease with increasingfruit size, while use would increase with decreasing ripeness.Using a field-cage assay, ripeness was held constant and thesize of host coffee berries manipulated. Avoidance of parasitizedberries was significandy less pronounced on large berries thanon small ones. In a second experiment, size was held constantand ripeness manipulated. On unripe berries, females depositedthe majority of clutches directly into existing egg-laying cavities.On ripe berries, by contrast, the same females deposited mostclutches in previously unparasitized fruit. Parallel patternsin the frequency of female-female contests were observed, supportingthe notion that a fruit's value is determined by an interactionbetween fruit size or ripeness, on one hand, and the prior occurrenceof eggs, on the other. Laboratory assays suggested that useof existing sites had advantages in terms of time savings; femalebehavior thus constitutes a relatively uncommon example of adaptivesuperparasitism in which parasitized hosts are actually preferredover unparasitized ones.[Behav Ecol 7: 235-242 (1996)]     

Geographical variation in habitat choice and host suitability in the parasitoidAsobara rufescens

In the Netherlands, Asobara rufescens (Förster) (Hymenoptera: Braconidae) is a parasitoid of drosophilid larvae in decaying plant material. In several places in the Mediterranean, parasitoids looking very similar to A. rufescens were collected on fermenting substrates. A hybridization experiment showed that the parasitoids were indeed A. rufescens. In an olfactometer Portuguese A. rufescens do not have a preference for either the odour of yeast or decaying leaves, while their Dutch conspecifics prefer the odour of decaying leaves. The survival probability of Portuguese A. rufescens in Drosophila melanogaster Meigen (Diptera: Drosophilidae), a species typical for fermenting substrates, is much higher than the survival probability of Dutch A. rufescens in this host species. It is hypothesized that decaying plant material may be unsuitable for drosophilid larvae during part of the year in the Mediterranean, forcing A. rufescens there to broaden its microhabitat choice. The use of fermenting substrates brings A. rufescens in contact with its sibling A. tabida Nees, a species typical for fermenting substrates in most of Europe. Portuguese A. rufescens appear to be genetically isolated from A. tabida. In the Netherlands, where the two species occupy different microhabitats, there is only a premating barrier.     

Spatial variation in population dynamics of juvenile Atlantic salmon: implications for conservation and management

Application of habitat models for predicting expected local densities of Atlantic Salmon Salmo salar in healthy populations has been hampered by a lack of generality in their fit to data from different systems. It is believed that this problem results at least in part from difficulties of effectively integrating factors that act across a range of spatial and temporal scales. Here, as an aid to developing more robust modelling and sampling methodologies, a simple process‐based model for local‐scale dynamics of Atlantic salmon juveniles is developed from first principles by integrating contemporary understanding of self‐thinning, density‐dependent growth and dispersal. The aim is to present a readily understood structure to illustrate the links between spawning and stocking strategies, habitat, migration and fish production. Based on this structure, contemporary understanding of the more complex biological processes that affect density, growth and habitat are discussed in relation to some of the key requirements of managers, including stocking for rehabilitation, assessment of predation impact and development of strategies for sampling populations effectively when deriving habitat‐production models. A major conclusion is that more structured, integrated research is required to provide the basic variables needed to model links between local and global scale habitat and fish production effectively. Nevertheless, application of the current understanding of the biology of Atlantic salmon should be of great benefit to managers in extracting key information from field surveys.     

Comparative sialomics between hard and soft ticks: implications for the evolution of blood-feeding behavior

Ticks evolved various mechanisms to modulate their host's hemostatic and immune defenses. Differences in the anti-hemostatic repertoires suggest that hard and soft ticks evolved anti-hemostatic mechanisms independently, but raise questions on the conservation of salivary gland proteins in the ancestral tick lineage. To address this issue, the sialome (salivary gland secretory proteome) from the soft tick, Argas monolakensis, was determined by proteomic analysis and cDNA library construction of salivary glands from fed and unfed adult female ticks. The sialome is composed of approximately 130 secretory proteins of which the most abundant protein folds are the lipocalin, BTSP, BPTI and metalloprotease families which also comprise the most abundant proteins found in the salivary glands. Comparative analysis indicates that the major protein families are conserved in hard and soft ticks. Phylogenetic analysis shows, however, that most gene duplications are lineage specific, indicating that the protein families analyzed possibly evolved most of their functions after divergence of the two major tick families. In conclusion, the ancestral tick may have possessed a simple (few members for each family), but diverse (many different protein families) salivary gland protein domain repertoire.