PubMed Health⌕ Search

Biomedical subjects

M Milinski

Publications and source records attributed to M Milinski.

15 recordsLinked to original sources
PubMed2006

Schistocephalus cotti n. sp. (Cestoda: Pseudophyllidea) plerocercoids from bullheads Cottus gobio L. in an Arctic river in Finland, with a key to the plerocercoids of the Palaearctic species of the genus.

We compared plerocercoids of Schistocephalus Creplin, 1829 from Cottus gobio (n = 57) and Gasterosteus aculeatus f. semiarmatus (n = 45) from the River Utsjoki, Finland, taken only from single worm infections. Segment numbers in the two populations were distinct (G. aculeatus range 55-107, average 74 (SE 1.66), median 73; C. gobio range 122-189, average 146 (SE 1.78); median 144). The mean difference between populations, 71.47, t = 28.76 with 100 degrees of freedom, two-tailed p value <0.001, was considered extremely significant. Amplification of microsatellite loci that were originally designed for Schistocephalus from G. aculeatus was positive for all larvae from G. aculeatus (n = 20), whereas in no plerocercoids from C. gobio (n = 20) were any of the six microsatellites amplified, indicating that plerocercoids from G. aculeatus and C. gobio were two distinct genetic populations of Schistocephalus. The material from C. gobio is described as S. cotti n. sp. Plerocercoids of the Palaearctic species of Schistocephalus are identified as follows: S. nemachili Dubinina, 1959 with 228-235 or more segments, specific to Barbatula spp. (Balitoridae); S. pungitii Dubinina, 1959 with 62-92 (usually 70-80) segments, specific to Pungitius pungitius; S. solidus (Müller, 1776) in two forms, one in G. aculeatus f. leiurus and f. semiarmatus, with 48-100 (usually 65-75) segments, and the other in G. aculeatus f. trachurus, with 99-138 (usually 112-122) segments; and S. cotti n. sp. with 103-189 (usually 130-159) segments, probably specific to cottids. Nearctic Schistocephalus were not considered owing to the uncertain status of some North American records. Some other species of Schistocephalus of highly doubtful status were briefly noted. Cross-infection experiments and molecular studies are recommended to further elucidate the interrelationships between the various species of Schistocephalus.

Animals↗
PubMed2004

Modulation of granulocyte responses in three-spined sticklebacks Gasterosteus aculeatus infected with the tapeworm Schistocephalus solidus.

Leukocytes isolated from the head kidney and peripheral blood of 3-spined sticklebacks Gasterosteus aculeatus L. were analysed by means of flow cytometry during infection with the tapeworm Schistocephalus solidus (Müller, 1776). Although parasites increased their body weight continuously throughout the observation period (98 d), proportions of granulocytes increased in blood and head kidney only up to Day 63 post-infection (p.i.). Thereafter, declining proportions of granulocytes were observed in both organs. Thus the relative decrease in granulocyte number was not correlated to a decline in the parasitic load of the fish. To investigate a possible modulatory impact of S. solidus on granulocyte function, head kidney leukocytes were isolated at times before Day 63 p.i. and tested in vitro for their capacity to produce reactive oxygen species (ROS). Head kidney leukocytes from S. solidus-infected fish, analysed immediately after isolation (ex vivo, Day 40 p.i.), exhibited a higher ROS production when stimulated with phorbol myristate acetate (PMA), than leukocytes from naive, sham-treated control fish and fish that had resisted or cleared the infection (exposed but not infected). The latter showed an increased spontaneous ROS production that was not correlated to the numbers of granulocytes present in the head kidney isolates. In infected sticklebacks, spontaneous and PMA-induced ROS production was significantly correlated with numbers of granulocytes present in the head kidney isolates, suggesting that elevated ROS production was due to higher numbers of responding cells rather than an increased capacity of single cells. In vitro, after cultivation for 4 d in the presence of pokeweed mitogen (PWM) or extracts from S. solidus, head kidney leukocytes from control fish showed an increased ROS production and phagocytic activity compared with non-stimulated control cultures. In contrast, head kidney leukocytes from infected fish isolated on Days 48 and 44 p.i., failed to respond to S. solidus antigens in vitro. During S. solidus infection, granulocyte mobilisation resulted in elevated numbers of these cells in head kidneys, but the lack of an in vitro response to S. solidus antigens indicates an in vivo priming of granulocytes by the parasite. These observations may reflect the ability of S. solidus to impair the host's immune response once the parasite is developing in the body cavity of G. aculeatus.

Analysis of Variance↗
PubMed2003

The consequences of self-fertilization and outcrossing of the cestode Schistocephalus solidus in its second intermediate host.

Many hermaphroditic parasites reproduce by both cross-fertilization and self-fertilization. To understand the maintenance of such mixed mating systems it is necessary to compare the fitness consequences of the two reproductive modes. This has, however, almost never been done in the context of host-parasite coevolution. Here we show the consequences of outcrossing and selfing in an advanced life-stage of the cestode Schistocephalus solidus, i.e. in its second intermediate host, the three-spined stickleback (Gasterosteus aculeatus). Each juvenile stickleback was simultaneously exposed to 2 experimentally infected copepods, one harbouring outcrossed the other selfed parasites. At 60 days p.i. parasites were removed from the fish's body cavity and, with microsatellite markers, assigned to either outcrossed or selfed origin. Prevalence was not significantly higher in outcrossed parasites. However, those fish that were infected contained significantly more out-crossed than selfed parasites. Thus the probability of a selfed parasite to progress in the life-cycle is reduced in the second intermediate host. Furthermore, we found that even the multiply infected fish increased in weight during the experiment. Nevertheless, total worm weight in multiply infected fish was significantly lower than in singly infected ones, which thus might be a parasite life-history strategy.

Animals↗
PubMed2003

Optimal growth strategies of larval helminths in their intermediate hosts.

We consider optimal growth of larval stages in complex parasite life cycles where there is no constraint because of host immune responses. Our model predicts an individual's asymptotic size in its intermediate host, with and without competition from conspecific larvae. We match observed variations in larval growth patterns in pseudophyllid cestodes with theoretical predictions of our model. If survival of the host is vital for transmission, larvae should reduce asymptotic size as intensity increases, to avoid killing the host. The life history strategy (LHS) model predicts a size reduction <1/intensity, thus increasing the parasite burden on the host. We discuss whether body size of competing parasites is an evolved LHS or simply reflects resource constraints (RC) on growth fixed by the host, leading to a constant total burden with intensity. Growth under competition appears comparable with "the tragedy of the commons", much analysed in social sciences. Our LHS prediction suggests that evolution generates a solution that seems cooperative but is actually selfish.

Animals↗
PubMed2003

Simultaneous hermaphrodites reproducing in pairs self-fertilize some of their eggs: an experimental test of predictions of mixed-mating and Hermaphrodite's Dilemma theory.

Theory predicts (1) that mixed-mating systems (i.e. reproduction through both selfing and outcrossing) should usually not evolve and (2) that reproducing simultaneous hermaphrodites should be in a conflict over the preferred sexual role (The Hermaphrodite's Dilemma). In an in vitro system with the endoparasitic cestode Schistocephalus solidus, a simultaneous hermaphrodite, we tested predictions of both the mixed-mating and the Hermaphrodite's Dilemma theory. Using microsatellite markers, we measured the proportion of selfed offspring and the total reproductive output of each worm within pairs varying in mean weight and weight difference. Worms produced more outbred offspring not only with increasing total weight of the pair, but also with decreasing weight difference between the two paired worms. These results suggest: (1) that this parasite species reproduces by mixed-mating, which may be maintained by stochastic density fluctuations in the definitive host and hence unpredictability of self reproduction and (2) reproductive conflict may prevent worm pairs from achieving an optimal intermediate selfing rate.

Animals↗
PubMed2001

Cooperation through indirect reciprocity: image scoring or standing strategy?

Theorists have only recently shown that cooperation through indirect reciprocity can evolve. The first modelling approach favoured a mechanism called image scoring. Helping someone increases one's image score, whereas refusing to help reduces it. The evolutionary outcome was a discriminator image scoring strategy that helps everybody who has, for example, a positive image score. Two experimental studies with humans found results that were compatible with discriminator image scoring. However, a new analysis of other theorists, based on another population structure, has cast doubts on the evolutionary stability of strategies using the recipient's score as a sole basis for decision. The new theoretical study confirmed that a strategy aiming at "good standing" has superior properties and easily beats image scoring. An individual loses good standing by failing to help a recipient in good standing, whereas failing to help recipients who lack good standing does not damage the standing of a potential donor (but would reduce his image score). The present empirical study with 23 groups of seven human subjects each was designed for distinguishing between the two proposed mechanisms experimentally. The results differed strongly from standing strategies, which might demand too much working memory capacity, but were compatible with image scoring or a similar strategy to a large extent. Furthermore, donors of constant "NO players" compensated for their refusing to help these players by being more generous to others.

Cooperative Behavior↗
PubMed2001

Female sticklebacks count alleles in a strategy of sexual selection explaining MHC polymorphism.

The origin and maintenance of polymorphism in major histocompatibility complex (MHC) genes in natural populations is still unresolved. Sexual selection, frequency-dependent selection by parasites and pathogens, and heterozygote advantage have been suggested to explain the maintenance of high allele diversity at MHC genes. Here we argue that there are two (non-exclusive) strategies for MHC-related sexual selection, representing solutions to two different problems: inbreeding avoidance and parasite resistance. In species prone to inadvertent inbreeding, partners should prefer dissimilar MHC genotypes to similar ones. But if the goal is to maximize the resistance of offspring towards potential infections, the choosing sex should prefer mates with a higher diversity of MHC alleles. This latter strategy should apply when there are several MHC loci, as is the case in most vertebrates. We tested the relative importance of an 'allele counting' strategy compared to a disassortative mating strategy using wild-caught three-spined sticklebacks (Gasterosteus aculeatus) from an interconnected system of lakes. Here we show that gravid female fish preferred the odour of males with a large number of MHC class-IIB alleles to that of males with fewer alleles. Females did not prefer male genotypes dissimilar to their own.

Alleles↗
PubMed2000

Cooperation through image scoring in humans.

The "tragedy of the commons," that is, the selfish exploitation of resources in the public domain, is a reason for many of our everyday social conflicts. However, humans are often more helpful to others than evolutionary theory would predict, unless indirect reciprocity takes place and is based on image scoring (which reflects the way an individual is viewed by a group), as recently shown by game theorists. We tested this idea under conditions that control for confounding factors. Donations were more frequent to receivers who had been generous to others in earlier interactions. This shows that image scoring promotes cooperative behavior in situations where direct reciprocity is unlikely.

Altruism↗
PubMed1998

Working memory constrains human cooperation in the Prisoner's Dilemma.

Many problems in human society reflect the inability of selfish parties to cooperate. The "Iterated Prisoner's Dilemma" has been used widely as a model for the evolution of cooperation in societies. Axelrod's computer tournaments and the extensive simulations of evolution by Nowak and Sigmund and others have shown that natural selection can favor cooperative strategies in the Prisoner's Dilemma. Rigorous empirical tests, however, lag behind the progress made by theorists. Clear predictions differ depending on the players' capacity to remember previous rounds of the game. To test whether humans use the kind of cooperative strategies predicted, we asked students to play the iterated Prisoner's Dilemma game either continuously or interrupted after each round by a secondary memory task (i.e., playing the game "Memory") that constrained the students' working-memory capacity. When playing without interruption, most students used "Pavlovian" strategies, as predicted, for greater memory capacity, and the rest used "generous tit-for-tat" strategies. The proportion of generous tit-for-tat strategies increased when games of Memory interfered with the subjects' working memory, as predicted. Students who continued to use complex Pavlovian strategies were less successful in the Memory game, but more successful in the Prisoner's Dilemma, which indicates a trade-off in memory capacity for the two tasks. Our results suggest that the set of strategies predicted by game theorists approximates human reality.

Biological Evolution↗
PubMed1996

Human cooperation in the simultaneous and the alternating Prisoner's Dilemma: Pavlov versus Generous Tit-for-Tat.

The iterated Prisoner's Dilemma has become the paradigm for the evolution of cooperation among egoists. Since Axelrod's classic computer tournaments and Nowak and Sigmund's extensive simulations of evolution, we know that natural selection can favor cooperative strategies in the Prisoner's Dilemma. According to recent developments of theory the last champion strategy of "win--stay, lose--shift" ("Pavlov") is the winner only if the players act simultaneously. In the more natural situation of players alternating the roles of donor and recipient a strategy of "Generous Tit-for-Tat" wins computer simulations of short-term memory strategies. We show here by experiments with humans that cooperation dominated in both the simultaneous and the alternating Prisoner's Dilemma. Subjects were consistent in their strategies: 30% adopted a Generous Tit-for-Tat-like strategy, whereas 70% used a Pavlovian strategy in both the alternating and the simultaneous game. As predicted for unconditional strategies, Pavlovian players appeared to be more successful in the simultaneous game whereas Generous Tit-for-Tat-like players achieved higher payoffs in the alternating game. However, the Pavlovian players were smarter than predicted: they suffered less from defectors and exploited cooperators more readily. Humans appear to cooperate either with a Generous Tit-for-Tat-like strategy or with a strategy that appreciates Pavlov's advantages but minimizes its handicaps.

Journal Article↗
PubMed1995

Boldness and predator deterrence: a critique of Godin & Davis.

Godin & Davis (1995) claimed to have provided experimental evidence for a predator pursuit deterrence function of predator inspection in the guppy. We doubt that they have shown that inspection behaviour per se deters attack because their evidence is actually correlational and thus inherently ambiguous; furthermore we think their methods are flawed.

Animals↗
PubMed1975

[Selection of strays from the group by predators during prey].

Equally sated three-spined sticklebacks (Gasterosteus aculeatus) that had a free view of stray individuals and of a swarm of Daphnia magna preferentially preyed upon the strays; the resulting risk to the strays increased with swarm density (a leads to c leads to e in Fig. 1). This applied also to situations with constant swarm numbers but varied density (a leads to b, d leads to e). The results from two experiments with equal swarm density (b/e, c/d) suggest selection to be affected also by swarm number and/or volume.

Animals↗
PubMedDate not supplied

TIT FOR TAT in sticklebacks and the evolution of cooperation.

The problems of achieving mutual cooperation can be formalized in a game called the Prisoner's Dilemma in which selfish defection is always more rewarding than cooperation. If the two protagonists have a certain minimum probability of meeting again a strategy called TIT FOR TAT is very successful. In TIT FOR TAT the player cooperates on the first move and thereafter does whatever the opponent did on the previous move. I have studied the behaviour of fish when confronting a potential predator, because conflicts can arise within pairs of fish in these circumstances which I argue resemble a series of games of Prisoner's Dilemma. Using a system of mirrors, single three-spined sticklebacks (Gasterosteus aculeatus) approaching a live predator were provided with either a simulated cooperating companion or a simulated defecting one. In both cases the test fish behaved according to TIT FOR TAT supporting the hypothesis that cooperation can evolve among egoists.

Animals↗