Alternative Life-history Strategies Research Articles

Alternative life cycle strategies ofMegalodiscus temperatusin tadpoles and metamorphosed anurans

Megalodiscus temperatus (Stafford, 1905) is a common paramphistome trematode of North American amphibians with a two host life cycle and has been reported to infect frogs and rarely tadpoles. In this study we document the alternative life cycle strategy of M. temperatus in tadpoles and metamorphosed anurans. We show through field work and experimental infections that M. temperatus can establish in both anuran life stages and worms become gravid and release eggs in both tadpoles and metamorphosed frogs. However, worms exhibit differences in route of infection, development, egg production, and diet in tadpoles and metamorphosed anurans. These alternative life history strategies of M. temperatus suggest different selective pressures on the development and reproductive success of these worms in tadpoles and metamorphosed anurans, and we discuss the evolutionary avenues for and constraints on amphibian trematode life cycles presented by these two different anuran life stages.

Intra- and interspecific competition among helminth parasites: Effects on Coitocaecum parvum life history strategy, size and fecundity

Larval helminths often share intermediate hosts with other individuals of the same or different species. Competition for resources and/or conflicts over transmission routes are likely to influence both the association patterns between species and the life history strategies of each individual. Parasites sharing common intermediate hosts may have evolved ways to avoid or associate with other species depending on their definitive host. If not, individual parasites could develop alternative life history strategies in response to association with particular species. Three sympatric species of helminths exploit the amphipod Paracalliope fluviatilis as an intermediate host in New Zealand: the acanthocephalan Acanthocephalus galaxii, the trematode Microphallus sp. and the progenetic trematode Coitocaecum parvum. Adult A. galaxii and C. parvum are both fish parasites whereas Microphallus sp. infects birds. We found no association, either positive or negative, among the three parasite species. The effects of intra- and interspecific interactions were also measured in the trematode C. parvum. Both intra- and interspecific competition seemed to affect both the life history strategy and the size and fecundity of C. parvum. Firstly, the proportion of progenesis was higher in metacercariae sharing their host with Microphallus sp., the bird parasite, than in any other situation. Second, the intensity of intraspecific competition apparently constrained the ability of metacercariae to adopt progenesis and limited both the growth and egg production of progenetic individuals. These results show that the life history strategy adopted by a parasite may be influenced by other parasites sharing the same host.

Effect of metal stress on life history divergence and quantitative genetic architecture in a wolf spider

Effects and consequences of stress exposure on life history strategies and quantitative genetic variation in wild populations remain poorly understood. We here study whether long-term exposure to heavy metal pollution may result in alternative life history strategies and alter quantitative genetic properties in natural populations of the wolf spider Pirata piraticus. Offspring originating from a reference and a metal contaminated population and their reciprocal hybrid cross were bred in a half-sib mating scheme and subsequently reared in cadmium contaminated vs. clean environment. Results from this experiment provided evidence for a genetically based reduced growth rate and increased egg size in the contaminated population. Growth rate reduction in response to cadmium contamination was only observed for the reference population. Animal model analysis revealed that heritability for growth rate was large for the reference population under reference conditions, but much lower under metal stressed conditions, caused by a strong decrease in additive genetic variance. Heritability for growth of the metal contaminated population was very low, even under reference conditions. Initial size of the offspring was primarily determined by maternal effects, whereas egg size produced by the offspring was determined by both sire and dam effects, indicating that egg size determination is under control of the female genotype. In conclusion, these results show that metal stress can not only affect life history variation in natural populations, but also decreases the expression as well as the of the amount of genetic variation for particular life history traits.

Antagonistic pleiotropic effects reduce the potential adaptive value of the FRIGIDA locus.

Although the occurrence of epistasis and pleiotropy is widely accepted at the molecular level, its effect on the adaptive value of fitness-related genes is rarely investigated in plants. Knowledge of these features of a gene is critical to understand the molecular basis of adaptive evolution. Here we investigate the importance of pleiotropy and epistasis in determining the adaptive value of a candidate gene using the gene FRI (FRIGIDA), which is thought to be the major gene controlling flowering time variation in Arabidopsis thaliana. The effect of FRI on flowering time was analyzed in an outbred population created by randomly mating 19 natural accessions of A. thaliana. This unique population allows the estimation of FRI effects independent of any linkage association with other loci due to demographic processes or to coadapted genes. It also allows for the estimation of pleiotropic effects of FRI on fitness and inflorescence architecture. We found that FRI explains less variation in flowering time than previously observed among natural accessions, and interacts epistatically with the FLC locus. Although early flowering plants produce more fruits under spring conditions, and nonfunctional alleles of FRI were associated with early flowering, variation at FRI was not associated with fitness. We show that nonfunctional FRI alleles have negative pleiotropic effects on fitness by reducing the numbers of nodes and branches on the inflorescence. We propose that these antagonistic pleiotropic effects reduce the adaptive value of FRI, and helps explain the maintenance of alternative life history strategies across natural populations of A. thaliana.

Alternative antipredator responses of two coexisting Daphnia species to negative size selection by YOY perch

Our study showed for the first time in nature that two coexisting Daphnia adopted alternative life history and behavioural strategies to cope with negative size-selection predation by gape-limited young-of-the-year (YOY) perch. We evaluated the phenotypic plasticity in life history and behavioural traits of two coexisting Daphnia species, D. pulicaria (2 mm) and D. galeata mendotae (1.4 mm), in response to seasonal changes in predation by YOY yellow perch (Perca flavescens) in a mesotrophic lake. We expected that the large-sized D. pulicaria, the most likely subjected to sizeselective predation by YOY perch, will show stronger antipredator responses than the small-sized D. galeata mendotae. To test this hypothesis, we examined changes in life history and behavioural traits in juveniles and adults of both species during four YOY fish predation periods that were selected based on the presence of YOY perch in the pelagic zone and the relative abundance of Daphnia prey in their gut contents. Our study supports the scenario of negative size-selective predation by gape-limited YOY perch on both Daphnia species. The electivity index indicated that no daphnids with a body length .1.75 mm were predated by YOY yellow perch. Coexisting Daphnia exhibited phenotypic plasticity in their antipredator defenses based on their vulnerability to seasonal changes in size-selective predation of YOY perch. Juvenile Daphnia were the targeted prey and they responded by a decreased body length. Behavioural defenses were the dominant strategy used by both adult Daphnia populations to withstand high predation. A decreased size at maturity was not employed by Daphnia, except at the very end of the predation period. Behavioural defenses are short-term strategy adopted to avoid predation. Both antipredator defenses became unnecessary expenses and were no longer sustained after the predation period.

The predation cost of being a male: implications for sex‐specific rates of ageing

Evolutionary theory predicts that the rate of extrinsic (i.e. age‐ and condition‐independent) mortality should affect important life history traits such as the rate of ageing and maximum lifespan. Sex‐specific differences in mortality rates due to predation may therefore result in the evolution of important differences in life history traits between males and females. However, quantifying the role of predators as a factor of extrinsic mortality is notoriously difficult in natural populations. We took advantage of the unusual prey caching behaviour of the barn owl Tyto alba and the tawny owl Strix aluco to estimate the sex ratio of their five most common preys. For all prey species, there was a significant bias in the sex ratio of remains found in nests of both these owls. A survey of literature revealed that sex‐biased predation is a common phenomenon. These results demonstrate that predation, a chief source of extrinsic mortality, was strongly sex‐biased. This may select for alternate life history strategies between males and females, and account for a male life span being frequently lower than female lifespan in many animal species.

Alternative life-history and transmission strategies in a parasite: first come, first served?

Alternative transmission strategies are common in many parasitic organisms, often representing discrete phenotypes adopted in response to external cues. The facultative truncation of the normal 3-host life-cycle to a 2-host cycle in many trematodes provides an example: some individuals mature precociously, via progenesis, in their intermediate host and produce eggs without the need to reach a definitive host. The factors that determine how many and which individuals adopt the truncated life-cycle within a parasite population remain unknown. We investigated the occurrence of progenesis in the trematode Stegodexamene anguillae within its fish intermediate host. Location within the host was a key determinant of progenesis. Although the size and egg output of progenetic metacercariae encysted in host gonads did not differ from those of the few progenetic metacercariae in other host tissues, the likelihood of metacercariae becoming progenetic was much higher for those in the gonads than those elsewhere in the host. Progenetic parasites can only evacuate their eggs along with host eggs or sperm, providing a link between the parasite's transmission strategy and its location in the host. Host size and sex, and the presence of other parasite species in the host, did not affect the occurrence of progenesis in S. anguillae. However, the proportion of metacercariae in host gonads and the proportion of progenetic metacercariae both decreased with increasing numbers of S. anguillae per host. These results suggest that progenesis is adopted mostly by the parasites that successfully establish in host gonads. These are generally the first to infect a fish; subsequent arrivals settle in other tissues as the gonads quickly become saturated with parasites. In this system, the site of encystment within the fish host both promotes and constrains the adoption of a facultative, truncated life-cycle by the parasite.

Alternative reproductive strategies in the white-throated sparrow: behavioral and genetic evidence

Organisms exhibiting genetic polymorphism often also exhibit true alternative life-history strategies in which behavioral tactics are genetically fixed. Such systems are ideal for the study of the evolution of life histories because the consequences of selective episodes can be more easily identified. Here I report an interesting and classic example of a species exhibiting true alternative strategies. Due to a chromosomal inversion, male and female white-throated sparrows (Zonotrichia albicollis) occur as two distinct morphs, tan or white. Tan and white morphs mate disassortatively, and this mating pattern maintains the polymorphism in relatively equal proportions within the population. In comparison with tan males, white males are more aggressive, frequently intrude into neighboring territories, spend less time guarding their mates, occasionally attempt polygyny, and provide less parental care. White females are also more aggressive and solicit copulation from their mates twice as often as tan females. Multilocus DNA fingerprinting revealed that 31.8% of nestlings and 56.3% of broods from white male--tan female pairs were the result of extrapair fertilizations, whereas only 4.4% of nestlings and 6.3% of broods from tan male--white female pairs contained extrapair offspring. However, 4.4% of the nestlings (12.5% of broods) from tan male--white female pairs were attributable to conspecific brood parasitism, and brood parasitism was not a factor for white male--tan female pairs. For three excluded extrapair offspring, actual paternity was assigned to territorial white males, suggesting that, although white males are not preferred by females in previous mate choice studies, they do attempt to increase their reproductive success by securing additional matings via extrapair copulations. However, by doing so, white males also lose paternity at home. Behavioral and genetic evidence suggest that the two morphs use alternative strategies and that reproductive trade-offs may serve to maintain a competitive equilibrium between the morphs. Copyright 2003.

Sexually Different Growth Histories of the American Eel in Four Rivers in Maine

Growth histories of yellow-phase American eels Anguilla rostrata collected in four rivers in Maine, were back-calculated from sagittal otolith increments. Our objectives were to first determine whether sexually dimorphic growth rates exist and then compare the growth histories of American eels from four rivers within a geographic region. For female eels, the maximum growth rate was 31.9 ± 1.7 mm/year at age 8, decreasing to 25.1 ± 2.9 mm/year at age 14. Males attained a maximum of 29.8 ± 1.6 mm/year at age 3, decreasing to a minimum of 17.9 ± 1.3 mm/year at age 11. Females grew faster than males after age 4 and had a slower reduction in growth rate with age. These faster growth rates among females were similar in all four rivers. The observed growth rates are not consistent with current life history hypotheses and may indicate an alternative life history strategy. Because female eels benefit from a larger size (i.e., size refuge, increased fecundity, and greater niche breadth), they would benefit from a higher-risk growth strategy that increases growth rate during their earlier years and reduces the amount of time spent in an unfavorable size-class. The tradeoffs (i.e., mortality, developmental rate, pathogen resistance, and longevity) associated with this faster growth rate may not favor the males' life history requirements. Male eels do not achieve the size of females and therefore are not subject to the advantages associated with being larger. Therefore, they may use a risk-averse strategy that maintains submaximum growth rates to obtain the minimum size necessary to mature and complete the spawning migration while reducing the adverse affects of faster growth rates. We postulate that, in eels, intrinsic growth rates should be considered a life history trait that has evolved to meet the life history requirements of each sex.

To sea or not to sea: a brief review on salmon migration evolution

Since many salmonid species have the alternative life history strategies, they provide us a good opportunity to solve such a riddle. In a model, the anadromous life history of salmon may not be an evolutionarily stable state. If the gain in fitness from staying in a river exceeds the fitness from the sea, the fluvial life history can occur or vice versa. Some data may support these predictions. The relative reproductive success of migratory and fluvial females has latitudinal variation (so called geocline); the latter will be higher at the southern limits of the ranges, and conversely, the former will have higher fitness at the northern part, probably supporting the food availability hypothesis.

Mycetophagy in Filenchus misellus (Andrássy, 1958) Lownsbery & Lownsbery, 1985 (Nematoda: Tylenchidae), with notes on its morphology

AbstractPopulation growth rates on fungal colonies and morphological characteristics were investigated in a member of the genus Filenchus (Tylenchidae) collected from decomposing rice straw in Fukushima-city, northern Japan. The nematode was identified as F. misellus on the basis of the great similarity in morphological measurements to the known populations of the species. Population growth rates were measured 40 days after inoculating nematodes on to seven fungal species cultured in Petri dishes at 25oC. Growth rates were high on Chaetomium globosum and Coprinus cinereus, intermediate on Pleurotus ostreatus and Rhizoctonia solani and low on Agaricus bisporus, Fusarium oxysporum and Pythium ultimum. The ability of the nematode to feed on fungi in decomposing organic matter suggests the possibility for alternative life history strategies and greater ecological amplitude for members of Filenchus and other genera of Tylenchidae.

The influence of energetic requirements on the preferred temperature of overwintering juvenile Atlantic salmon (Salmo salar)

We used a horizontal temperature gradient to investigate the effect of alternate life history strategies and nutritional state on the preferred temperature of overwintering juvenile Atlantic salmon (Salmo salar L.). Contrary to our prediction, there was no significant difference in final preferred temperature between juvenile Atlantic salmon that will migrate to sea the following spring (early migrants) and those that show reduced growth and delay migration for at least another year (delayed migrants). Both migrant groups preferentially selected relatively low temperatures (<10°C), likely owing to their low appetite and growth rates. Food deprivation resulted in a significant increase, rather than our predicted decrease, in the final preferred temperature of the juvenile Atlantic salmon of approximately 2°C. We suggest that this is due to the need for an increased foraging effort to offset the projected energy deficit later in the winter. The final preferred temperature of delayed migrants increased from winter to spring, as predicted, coincident with increases in natural food availability and endogenous seasonal increases in appetite and growth rates. We conclude that the preferred temperature of overwintering juvenile Atlantic salmon may be influenced by future energetic requirements rather than the current level of energy reserves.

Models of alternative life‐history strategies, population structure and potential speciation in salmonid fish stocks

Summary High Arctic landlocked Arctic charr, Salvelinus alpinus L., are used as a reference system to examine the theory of alternative life history strategies in Salmonids and their implication for population structure and potential speciation. In these closed, autonomous systems, Arctic charr are the only fish species present and alternative life history strategies are represented by two distinct size modes, commonly known as ‘dwarf’ and ‘normal’ charr. Simple mathematical models are used to caricature the density‐dependent interactions within and between the dwarf and normal components of these bimodal populations. Two different assumptions concerning the genetic relationship between the two alternative life history strategies are investigated: complete reproductive isolation, or a single stock which chooses between the alternative strategies facultatively. In constant environmental conditions, the evolutionarily stable strategy (ESS) is not affected by the assumed genetic relationship, and the ESS proportions of dwarfs and normals are shown to depend on environmental parameters. The ESS proportions vary with these parameters along an evolutionarily stable continuum (ESC) and, in particular, a high proportion of normals is associated with high growth rate as a juvenile, and a ‘stunted’ or unimodal population with low growth rate. The predicted ESC for selection of the normal strategy is similar to that observed experimentally in the analogous case of smolting in Atlantic salmon, Salmo salar L. A difference in the ratio of fecundity to mortality rate between dwarf and normal sub‐populations is shown to be essential to maintaining bimodal populations, and it is argued that cannibalism is one mechanism which could achieve this difference. The model is extended to ask what ecological conditions would favour evolution toward the speciated or reproductively isolated state. It is concluded that short‐term environmental fluctuations select against speciation and it is speculated that most natural bimodal Arctic charr stocks will achieve maximum individual fitness within a ‘semispeciated’ configuration which is maintained by incomplete assortative mating.

Models of alternative life-history strategies, population structure and potential speciation in salmonid fish stocks

Models of alternative life-history strategies, population structure and potential speciation in salmonid fish stocks

Effect of periodical starvation on the life history of Brachionus plicatilis O.F. Müller (Rotifera): a possible strategy for population stability.

To estimate the changes in the life history of the rotifer Brachionus plicatilis O.F. Müller under starvation, we carried out an individual culture and determined the effects of periodical food deprivation on its asexual reproductive characteristics such as lifespan, reproductive period, age at first egg and offspring production, and lifetime fecundity (total number of offspring produced in her lifetime). Rotifers were fed for 1-3 h daily, and were then starved until the next day. Control animals were fed throughout their lifespan. Starved rotifers matured and produced their first offspring at an older age than the control animals. The periodical starvation resulted in a decrease in the lifetime fecundity to less than half that of the non-starved control. The reproductive period and lifespan were 2-3 times longer in the starved animals than in the control animals. The negative relationship between lifespan and lifetime fecundity is interpreted as a trade-off in an alternative life-history strategy of rotifers under starved conditions. The great decrease in fecundity and extension of lifespan enables rotifers to compensate to keep the population in equilibrium.

Evolution in stressful environments. I. Phenotypic variability, phenotypic selection, and response to selection in five distinct environmental stresses.

Considerable debate has accompanied efforts to integrate the selective impacts of environmental stresses into models of life-history evolution. This study was designed to determine if different environmental stresses have consistent phenotypic effects on life-history characters and whether selection under different stresses leads to consistent evolutionary responses. We created lineages of a wild mustard (Sinapis arvensis) that were selected for three generations under five stress regimes (high boron, high salt, low light, low water, or low nutrients) or under near-optimal conditions (control). Full-sibling families from the six selection histories were divided among the same six experimental treatments. In that test generation, lifetime plant fecundity and six phenotypic traits were measured for each plant. Throughout this greenhouse study, plants were grown individually and stresses were applied from the early seedling stage through senescence. Although all stresses consistently reduced lifetime fecundity and most size- and growth-related traits, different stresses had contrasting effects on flowering time. On average, stress delayed flowering compared to favorable conditions, although plants experiencing low nutrient stress flowered earliest and those experiencing low light flowered latest. Contrary to expectations of Grime's triangle model of life-history evolution, this ruderal species does not respond phenotypically to poor environments by flowering earlier. Most stresses enhanced the evolutionary potential of the study population. Compared with near-optimal conditions, stresses tended to increase the opportunity for selection as well as phenotypic variance, although both of these quantities were reduced in some stresses. Rather than favoring traits characteristic of stress tolerance, such as slow growth and delayed reproduction, phenotypic selection favored stress-avoidance traits: earlier flowering in all five stress regimes and faster seedling height growth in three stresses. Phenotypic correlations reinforced direct selection on these traits under stress, leading to predicted phenotypic change under stress, but no significant selection in the control environment. As a result of these factors, selection under stress resulted in an evolutionary shift toward earlier flowering. Environmental stresses may drive populations of ruderal plant species like S. arvensis toward a stress-avoidance strategy, rather than toward stress tolerance. Further studies will be needed to determine when selection in stressful environments leads to these alternative life-history strategies.

Effect of time of day, time of year, and life history strategy on time budgeting in juvenile Atlantic salmon, Salmo salar

Traditionally, behavioural studies on juvenile Atlantic salmon, Salmo salar, have been conducted during the day in summer. It is known that Atlantic salmon become nocturnal in winter, but very little is known about their behaviour at that time. Therefore, observations in a seminatural stream were carried out during the day and night, from February to June, comparing diel and seasonal differences in behaviour between fish adopting alternative life history strategies. The results showed a general trend for more activity in spring than in winter, and the fish were found to be foraging at surprisingly low light levels. There were differences in relative feeding rate between the life history strategies; the early migrant fish foraged mostly during the day whereas the delayed migrant fish did more foraging at night. There is some evidence that the early migrant fish made fewer feeding attempts over the winter, which is surprising, since they grow faster over that period. This suggests differences in foraging efficiency, which could contribute to the separation into these two life history strategies.

Maturation and spawning processes of anadromous and resident pond smelt in lake ogawara

Pond smelt,Hypomesus nipponensis McAllister, in Lake Ogawara demonstrate alternative life history strategies, as evidenced by the coexistence of anadromous and resident fish. However, it is unknown if anadromous and resident groups interbreed. In this study, maturation and spawning processes were examined and compared between anadromous and resident groups. Histological observations indicated negligible variation in the maturational stage composition of oocytes, the frequency of oocyte diameter being unimodal for all specimens at different maturational stages. Oocytes were absent in the ovaries of spent fish. Accordingly, the species can be considered a semelparous spawner with unimodal oocyte diameter distribution. Temporal changes in the proportion of spent fish were compared between anadromous and resident groups. Spawning of both groups began in late March and peaked over April 8–12. Although both groups did not differ significantly in the period of peak spawning, anadromous fish finished spawning earlier than resident ones. Anadromous fish were not able to spawn upon migration into Lake Ogawara, and quickly matured after immigration, contrasting with resident fish.

Sublethal toxic effects of cyanobacteria and nonylphenol on environmental sex determination and development in Daphnia

AbstractIn many cladoceran zooplankton, females divide their reproductive effort among females, males, and resting eggs depending on a suite of environmental conditions. Having alternative life‐history strategies allows cladocerans such as Daphnia to achieve high reproductive rates asexually when conditions are favorable and to produce offspring sexually for survival when the environment becomes unsuitable. Because of their rapid reproduction, Daphnia and other cladocerans are useful model organisms in aquatic toxicology. Bioassays using Daphnia serve two functions, to estimate the ecosystem level risk posed by specific chemicals and to quickly detect contaminated waters. Bioassays examine effects in terms of life history features such as growth, survival, and asexual fecundity. We studied the effects of toxic stress on Daphnia galeata mendotae grown under conditions in which it produces female, male, and ephippial offspring. Fecundities in terms of the three types of offspring showed different sensitivities to chemical stress. The stressors were a toxic strain of the cyanobacterium Microcystis aeruginosa and nonylphenol, a plasticizer, which is a surfactant that interferes with endocrine function in vertebrates. Production of resting eggs and female offspring were affected in both experiments, but the experiments showed different dose responses. Production of males was less sensitive. Exposure to nonylphenol also produced a characteristic developmental abnormality at environmentally relevant concentrations. Life table analysis showed that stress during the sexual phase of the life cycle of Daphnia reduces short‐term population growth and as well as affecting the stock of dormant individuals.

An alternative life history approach to the nutrition and feeding of Siluroidei larvae and early juveniles

Resume Successful commercial production of most cultured fish species has been facilitated by the intensification of larval rearing techniques. Siluroidei specics are no exception and early attempts at larval rearing in ponds were soon superseded by intensive hatchery production, at least for thosc species that are farmed on a commercial scale. The review focuses on alternative life history strategies and how these may providc clues to the early nutrition and fceding of siluroid fishes, as well as on the development and efficacy of practical feeds and feed application. The paper highlights several commonalities in terms of the nutritional and feeding requirements of the larvae of the various species cultured on a commercial and subsistence basis. The requirement for live feed for some species appears to be of short duration and al1 species can be successfully weaned ont0 dry feed at a relatively early stage. This is considered to be one of the reasons why the intensification of larval rearing of Siluroidei fishes has, in general, been highly successful. The review also comments on the live fooddry food debate and clcarly reveals that our knowledgc of Siluroidei larval nutrition and feeding is sorely lacking for many species, in comparison to othcr groups of fish. This emphasises the need for a concerted fundamental research effort.