Alternative reproductive strategies in the progenetic trematode Coitocaecum parvum: comparison of selfing and mating worms

The progenetic opecoelid trematode Coitocaecum parvum can reproduce either precociously by selfing in its second intermediate amphipod host or by mating in its normal definitive fish host. In this study, we describe and compare the infection parameters and some life history traits of both egg-producing worms and non-egg producing worms in both their second intermediate and definitive hosts. We showed that 58% of worms start to produce eggs while still in the amphipod. The relative abundance of progenetic worms increased with amphipod size, and egg-producing worms achieved greater size in amphipods than in fish. These 2 findings support the reproductive insurance hypothesis. No difference in size was revealed between eggs produced in the amphipods and those produced in the fish. Although more information is needed to thoroughly assess the respective costs and benefits of selfing and mating in this species, our conclusion is that adopting progenesis may have few, if any, long-term negative consequences for the parasite.     

Microsatellite loci for the progenetic trematode,Coitocaecum parvum (Opecoelidae)

Nine polymorphic microsatellite loci were characterized from the freshwater trematode Coitocaecum parvum. This parasite can either reproduce sexually in the definitive host or produce eggs by selfing inside its second intermediate host. Two to 11 alleles per locus were detected in 24 trematode sporocysts and observed heterozygosities ranged from 0.04 to 0.96. These loci will be useful for identifying parasite genetic clones within hosts and testing for effects of relatedness on parasite life history strategy.     

Optimal life-history strategies in seasonal consumer-resource dynamics

The interplay between individual adaptive life histories and populations dynamics is an important issue in ecology. In this context, we considered a seasonal consumer-resource model with nonoverlapping generations. We focused on the consumers decision-making process through which they maximize their reproductive output via a differential investment into foraging for resources or reproducing. Our model takes a semi-discrete form, and is composed of a continuous time within-season part, similar to a dynamic model of energy allocation, and of a discrete time part, depicting the between seasons reproduction and mortality processes. We showed that the optimal foraging-reproduction strategies of the consumers may be "determinate" or "indeterminate" depending on the season length. More surprisingly, it depended on the consumers population density as well, with large densities promoting indeterminacy. A bifurcation analysis showed that the long-term dynamics produced by this model were quite rich, ranging from both populations' extinction, coexistence at some season-to-season equilibrium or on (quasi)-periodic motions, to initial condition-dependent dynamics. Interestingly, we observed that any long-term sustainable situation corresponds to indeterminate consumers' strategies. Finally, a comparison with a model involving typical nonoptimal consumers highlighted the stabilizing effects of the optimal life histories of the consumers.     

Geographic variation in life cycle strategies of a progenetic trematode

Numerous parasite species have evolved complex life cycles with multiple, subsequent hosts. In trematodes, each transmission event in multi-host life cycles selects for various adaptations, one of which is facultative life cycle abbreviation. This typically occurs through progenesis, i.e., precocious maturity and reproduction via self-fertilization within the second intermediate host. Progenesis eliminates the need for the definitive host and facilitates life cycle completion. Adopting a progenetic cycle may be a conditional strategy in response to environmental cues related to low probability of transmission to the definitive host. Here, the effects of environmental factors on the reproductive strategy of the progenetic trematode Stegodexamene anguillae were investigated using comparisons among populations. In the 3-host life cycle, S. anguillae sexually reproduces within eel definitive hosts, whereas in the progenetic life cycle, S. anguillae reproduces by selfing within the metacercaria cyst in tissues of small fish intermediate hosts. Geographic variation was found in the frequency of progenesis, independent of eel abundance. Progenesis was affected by abundance and length of the second intermediate fish host as well as encystment site within the host. The present study is the first to compare life cycle strategies among parasite populations, providing insight into the often unrecognized plasticity in parasite developmental strategies and transmission.     

Evolution of thermotolerance in seasonal environments: the effects of annual temperature variation and life-history timing in Wyeomyia smithii

In organisms with complex life cycles, the adaptive value of thermotolerance depends on life-history timing and seasonal temperature profiles. We illustrate this concept by examining variation in annual thermal environments and thermal acclimation among four geographic populations of the pitcher plant mosquito. Only diapausing larvae experience winter, whereas both postdiapause and nondiapause adults occur only during the growing season. Thus, adults experience transient cold stress primarily during the spring. We show that adult cold tolerance (chill coma recovery) is enhanced in spring-like conditions via thermal acclimation but is unaffected by diapause state. Moreover, adult mosquitoes from northern populations were more cold tolerant than those from southern populations largely because acclimation responses were steeper in the north. In contrast to cold tolerance, there was no significant acclimation of heat tolerance (heat knockdown), and no significant differences in heat tolerance between northern and southern populations. Field temperature data show that because of evolved differences in diapause timing, adult exposure to cold stress is remarkably consistent across geography. This suggests that geographic variation in cold tolerance may not be the result of direct selection on adults. Our results illustrate the importance of the interplay between phenological and thermal adaptation for understanding variation along climatic gradients.     

Bacteria, granulocytomas, and trematode metacercariae in the digestive gland of Mytilus edulis: seasonal and interpopulation variation.

During the period October 1983-September 1984, mussels were sampled at monthly intervals from three populations in Denmark. The mussels were prepared for histopathological studies and 20 individuals were randomly chosen from each month and each population for a study of the prevalence and variation of the histological changes. The aim of the study was to test the hypothesis that the prevalence of parasites, bacteria, and granulocytoma differs among blue mussel populations during a year, due to the reproductive cycle and the environment. The only bacterial infection found was Chlamydia sp. and this was rare. Neither sex, month, nor population was an important determinant of the prevalence of bacteria. Metacercariae of the trematode Renicola roscovita had a prevalence of almost 54% in a population from Lilla Baelt but only 4% in a population from Isefjorden. A statistically significant interaction between month and population indicated that the prevalence varied over months but the pattern of variation was different among populations. No other parasites were found. The prevalence of granulocytomas within populations was positively correlated with the degree of anthropogenic contamination of the localities, which might suggest a functional connection. Neither month nor sex had a significant effect.     

A trematode infection with no effect on reproductive success of a sand-bubbler crab

The infection effects of the parasitic digenean trematode on the body weight and reproductive success of the sand-bubbler crab were examined. Gynaecotyla squatarolae (Trematoda: Microphallidae) infects the body cavity of Scopimera globosa (Decapoda: Scopimeridae) and uses the crab as its second intermediate host. The parasites infected all reproductive crabs examined to varying degrees. Larger crabs of both sexes had more parasites than smaller ones, probably because body size reflects age, and older crabs had a longer period of exposure to infection. Males had more parasites than females, probably because of sexual difference in acting time on the surface. Ovigerous females stay in closed burrows and do not act on the surface during incubation, and so have less chance of infection than males. The quantity of infecting parasites did not explain variations in either body weight or reproductive success of individual crabs in a field experiment. The life history of this parasite, relative body size of the crabs, and cost and the possible benefit of manipulation for the parasite may explain these results.     

Temperature and the life-history strategies of sea turtles

1. 1. Sea turtles have a high fecundity, high mortality, great longevity life history strategy.

2. 2. With the exception of the leatherback, turtle distribution is constrained by the 20°C surface isotherm.

3. 3. All sea turtles exhibit temperature-dependent sex determination (TSD) with pivotal temperatures close to 29°C.

4. 4. It is suggested that hatchling sex ratio will vary chaotically because of TSD.

5. 5. Because of TSD and natal homing, sea turtles are likely to be adversely affected by global warming.

6. 6. TSD and global warming have implications for conservation/management of sea turtles.

     

Host availability and the evolution of parasite life-history strategies

Parasites exploit an inherently patchy resource, their hosts, which are discrete entities that may only be available for infection within a relatively short time window. However, there has been little consideration of how heterogeneities in host availability may affect the phenotypic or genotypic composition of parasite populations or how parasites may evolve to cope with them. Here we conduct a selection experiment involving an entomopathogenic nematode (Steinernema feltiae) and show for the first time that the infection rate of a parasite can evolve rapidly to maximize the chances of infecting within an environment characterized by the rate of host availability. Furthermore, we show that the parasite's infection rate trades off with other fitness traits, such as fecundity and survival. Crucially, the outcome of competition between strains with different infection strategies depends on the rate of host availability; frequently available hosts favor "fast" infecting nematodes, whereas infrequently available hosts favor "slow" infecting nematodes. A simple evolutionarily stable strategy (ESS) analysis based on classic epidemiological models fails to capture this behavior, predicting instead that the fastest infecting phenotype should always dominate. However, a novel model incorporating more realistic, discrete bouts of host availability shows that strain coexistence is highly likely. Our results demonstrate that heterogeneities in host availability play a key role in the evolution of parasite life-history traits and in the maintenance of phenotypic variability. Parasite life-history strategies are likely to evolve rapidly in response to changes in host availability induced by disease management programs or by natural dynamics in host abundance. Incorporating parasite evolution in response to host availability would therefore enhance the predictive ability of current epidemiological models of infectious disease.     

Gradients of life-history variation: an intercontinental comparison of fishes

Multivariate analysis identified atwo-dimensional continuum of life-historyvariation among 301 fish species from Europe,North America, South America and the Atlanticand Pacific coasts of North America. The firstaxis was associated with larger body size,higher fecundity, delayed maturation, fewerreproductive events, and shorter breedingseason on one end and small size, lowfecundity, early maturity, multiplereproductive events per year, and prolongedbreeding season on the other. The second axiscontrasted fishes having larger eggs and moreparental care against fishes with the oppositesuite of traits.Phylogenetic affiliations of species wereapparent in the general patterns of ordinationof species within orders, indicatingevolutionary divergences in life-historypatterns. In fact, partitioning the variance oflife-history traits showed that taxonomic orderand latitude were the most important factorsand geographic region and habitat the least.Despite phylogenetic constraints, basiclife-history patterns showed consistencybetween distantly geographical regions,latitudinal ranges and basic adult habitats,indicating convergences in life-historypatterns. Although the basic life-historypatterns seemed repeatable among distantlyrelated taxa, geographical and latitudinalaffiliations were apparent. Species from SouthAmerica are skewed toward the opportunisticendpoint, whereas North American marine speciesare skewed toward the periodic endpoint of thetrilateral continuum model. Most of the fishspecies from South American data set came fromfluctuating environments, so an opportunisticstrategy of early maturation and continuousspawning permits efficient recolonization ofhabitats over small spatial scales. Incontrast, most species in the North Americanand European data sets came from seasonalhabitats that are nonetheless more hydrologicalstable, so a periodic strategy of delayingmaturation to attain large clutches enhancesadult survivorship during suboptimalenvironmental conditions and recruitment whenearly life stages encounter suitableenvironmental conditions. Similarly,latitudinal affiliations were also observed:opportunistic strategists more common intropical latitudes and periodic strategistsmore common in temperate and Arctic latitudes.     

Timing of first feeding and life-history strategies in salmon: genetic data

To investigate the relationship between genetic composition and some physiological traits of interest (age at smoltification, precocious maturation in male parr) in early and late first feeding Atlantic salmon, genetic variation at 6 isozyme and 8 microsatellite loci was examined. Early and late first feeding salmon showed different allelic and genotypic distributions at both isozyme and microsatellite loci. A positive relation between enzymatic loci heterozygosity and precocious active alimentation and earlier smoltification (S1 cf. S2) were also found.     

Geographic variation and environmental correlates of apparent survival rates in adult tree swallows Tachycineta bicolor

Determining demographic rates in wild animal populations and understanding why rates vary are important challenges in population ecology and conservation. Whereas reproductive success is reported frequently for many songbird species, there are relatively few corresponding estimates of annual survival for widespread populations of the same migratory species. We incorporated mark–recapture data into Cormack–Jolly–Seber models to estimate annual apparent survival and recapture rates of adult male and female tree swallows Tachycineta bicolor in eight local breeding populations across North America for periods of 7–33 yr. We found strong site‐specific and annual variation in apparent survival rates of adult swallows, and evidence of higher survival or site fidelity among males than females. There were no strong associations between putative overwintering region and survival. Strength and patterns of winter climate‐apparent survival relationships varied across four sites monitored for >15 yr; at one site, spring pond conditions, local spring precipitation and, to a lesser extent, winter North Atlantic Oscillation Index were credible predictors of annual apparent survival. Further work is needed to evaluate how survival is related to environmental conditions throughout the annual cycle and how these factors affect population dynamics of swallows and related species of conservation concern.     

Ecogeographic analysis of morphological and life-history variation in the Italian treefrog

We describe the spatial pattern of variation in body size, body shape, and adult population age structure of the Italian treefrog (Hyla intermedia). By means of a non-parametric test of matrix association (the Partial Mantel Test), we investigate the role of climate and geography in explaining the patterns of phenotypic variation. Body size is the most important source of morphometric variation, both at intra- and inter-populational levels. Body shape, despite its secondary role in explaining variation, still maintains high discriminative power among populations. Age structure differs significantly among populations, since body size correlates positively to age, these differences partly explain geographic variation of body size. The patterns of variation in both age and body size are congruent with climate variation, suggesting causal relationship. However, we find no evidence for adaptive explanations and suggest that climate-induced demographically differential survivorship might be the proximate cause responsible for the associations observed. Unlike body size, body shape varies congruently with the pattern of between-population geographic distances, which we show to be highly correlated with their genetic distances, thus suggesting isolation by distance as the proximate cause of the association.