1,721,069 research outputs found
Mutational processes in Dictyostelium discoideum: How mutations affect social behaviors and fitness
No full textPart I. Mutation is the most important biological force as it generates the variation that drives evolution and may play an important role in maintaining social structure in the social amoeba Dictyostelium discoideum. Using mutation accumulation lines of the social amoeba, I estimated the rate and degree of mutational effects on the social ability to form spores in chimeras by mixing equal proportions of cells of the ancestral clone with a mutated line and determining if the resultant spore proportion differs. Through the use of assays measuring growth, migration ability, and rates of spore germination, I assessed the fitness effects of mutation. In agreement with evidence that the majority of mutations are deleterious, I have found that the ability to get into the reproductive spores is diminished following mutation accumulation. Measuring growth rates on the selective medium revealed that approximately half of the lines showing a significant deviation from the ancestor have increased growth rates, possibly indicating the presence of beneficial mutations, while growth in a non-selective medium resulted in a loss of fitness. Additionally, spore germination decreased in lines with an abundance of mutations.
Part II. Restriction Enzyme Mediated Integration (REMI) is a method of transformation that generates tagged mutations. We employed the REMI mutants to select for cheaters by competing pools of mutants over many generations, allowing the lines to fruit each time. We plated out high densities of spores in order to facilitate the lines bypassing the vegetative cycle but still allowing the social cycle. This process was repeated 20 times. At the end of this process, the frequency of each line was assessed and each line was sequenced to identify the genes that were affected by REMI mutagenesis. Once we had obligate cheaters, we assessed fitness in a variety of ways: axenic growth and growth on bacteria, rate of spore germination, and distance traveled by migrating slugs. We then looked for a correlation between cheating and fitness. We expect to see a tradeoff between the ability to preferentially produce spores in chimeric mixtures and other aspects of fitness
Polyandry as a hedge against genetic incompatibility
No full textWhy do females across a wide range of taxa mate with more than one male? Here, I present the hypothesis that females engage in polyandry as a hedge against genetic incompatibility. I review evidence from the literature showing that the genomes of species are dynamic entities, constantly evolving as a consequence of genetic conflicts within and between the nucleus and the cytoplasm. Cellular endosymbionts, segregation distorter alleles, transposable elements and genomically-imprinted genes can all threaten female fitness by modifying maternal and paternal haplotypes in ways that render them incompatible within the developing embryo. I discuss the potential for polyandrous females to utilize postcopulatory mechanisms such as sperm competition, female choice of sperm, and reallocation of maternal resources from defective to viable embryos in order to minimize the risk and/or cost of fertilization by genetically-incompatible sperm. In a sperm precedence experiment carried out on the pseudoscorpion, Cordylochernes scorpioides, single-locus minisatellite DNA fingerprinting demonstrated strong last-male sperm precedence when females were mated to two males which broke down completely when females were mated to three males. This result indicates that the opportunity for postcopulatory sexual selection may be much greater in nature than is evident from standard, laboratory, two-male mating experiments. Polyandry in this pseudoscorpion is shown to be a deliberate strategy which increases reproductive success. In laboratory experiments, females restricted to mating with a single male experienced a higher rate of embryo failure and produced significantly fewer offspring than either females mated to more than one male in the laboratory or females naturally inseminated in the field. Previously proposed hypotheses such as forced copulation, insufficient sperm from a single mating, male nutrient donations, offspring genetic diversity and inherent male genetic quality cannot explain this higher reproductive success of polyandrous females. Observations of meiotic drive, highly-skewed sex ratios and paternal effects on sex ratio in this pseudoscorpion are consistent with the hypothesis that, by accumulating sperm from several males, C. scorpioides females reduce the number of embryos which fail as a consequence of genetic incompatibility between maternal and paternal genomes
Maternity and cyclical oligogyny in a colony of Parachartergus colobopterus
No full textGenetic data was obtained from a colony of Parachartergus colobopterus using DNA microsatellites. A colony cycle referred to as cyclical oligogyny is believed to account for the high relatedness in this polygynous species. The genetic data was analyzed to support the presence of cyclical oligogyny and determine some of the specific mechanisms behind it. Specifically, queen reduction, increasing reproductive dominance, sexual specialization by queens, and maternity of males (worker vs. queen) were examined. There was evidence for queen reduction occurring in this colony. To the contrary, no support was found for either increasing reproductive dominance or sexual specialization among the queens. It appears that the queens produced all of the males in this colony. This concurs with a worker preference for queen laying of the males that was seen in relatedness estimates
Grooming, aggression, and genetic relatedness in Parachartergus colobopterus, a neotropical swarm-founding wasp
No full textI examined biting, grooming, and inspection behavior in a pre-emergent colony of Parachartergus colobopterus, a neotropical swarm-founding wasp, to determine whether kin discrimination occurs between adult colony members. If within-colony kin discrimination exists, this is a likely place to find it. Low relatedness among colony members at some points in the colony cycle reduces advantages of nestmate discrimination and increases possible advantages of within-colony kin discrimination. I found no evidence of kin discrimination in length or frequency of biting and grooming interactions. Furthermore, interactants in biting, inspection, and grooming interactions were no more or less related than by chance. Results suggest that members of the species P. colobopterus do not identify and preferentially aid closer kin within a colony
Kinship and the evolution of altruism in social amoebae and A model for the evolution of kin-limited interactions
No full textFor decades, social amoebae have served as a model supporting broader
theories of social behavior. Owing to their peculiar aggregative life cycle, it
has seemed reasonable altruism in social amoebae is possible because of
adaptive mechanisms of kin discrimination, and kin discrimination evolves
to maintain this altruistic behavior. Nonetheless, these hypotheses have not
withstood critical tests in social amoebae or other organisms. As a result,
general theories of social evolution have rested on a few abstract theoretical
assumptions.
I here use social amoebae as a model system to examine these
assumptions through empirical study. First, I focus on the natural context of
social evolution in the social amoeba Dictyostelium discoideum. I establish
D. discoideum occurs frequently in a state of clonality during the social stage
and that obligate cheaters (non-altruists) are not present in nature (Chapter
1). I then show that kin discrimination in D. discoideum has only a weak
effect on genetic relatedness in the social stage (Chapter 2). Upon this
finding, I propose a hypothesis that kin recognition evolves in response to
facultative rather than obligate cheating (Chapter 2). I generalize this
argument in Chapter 3, where I propose a new "selfish genome" model of
kin recognition. This model is unique in that it accounts for the effects of
genome-wide relatedness between individuals on one another's fitness. This
model explains the adaptive basis of kin recognition-a trait thought to be
crucial for major evolutionary transitions.
I also describe two additional studies of social amoebae. In the first, I
report on the finding of a large clonal patch of a social amoebae. This is the
first example of such a phenomenon in a microorganisms (Chapter 4). In the
second, compare two forms of migration and development in social amoebae
(Chapter 5). This study shows social amoebae can be studied in a similar
way to animals, with a focus on the multicellular phenotype. I argue the
production of stalk during migration is an example of altruistic behavior
Cooperation, conflict, and experimental evolution in social amoebae
No full textCooperation and cheater control have helped shape life as we know it, but there is still much to learn. A eukaryote microbial model organism, like Dictyostelium discoideum , is an excellent system for advancing our understanding. When faced with starvation, multiple genetically distinct clones of D. discoideum aggregate together to form a chimeric fruiting body with a sterile stalk that holds aloft a sorus of hardy reproductive spores. One clone may be able to cheat and form disproportionately more spores, while forcing others to form more stalk. Here we discuss the impact of genetic relatedness on cooperation, and how social actions are temporally organized and can be affected by environmental conditions. First, we documented a potential strategy for facultative cheating within chimeras. We showed that the first cells to starve, and initiate the social stage, cheat cells that starved later. In another paper, we reviewed recent studies of social microbes, which demonstrate the importance of high relatedness in the evolution of cooperation and cheater resistance. In an experimental evolution study, we tested the hypothesis that de novo cheater mutants readily evolve under low relatedness conditions. We found that the majority of our lines evolved to cheat their ancestor. Further, we studied obligate cheaters, which pose a great threat to sociality. They gain a reproductive advantage in chimeras, but cannot cooperate clonally to form fruiting bodies. Wild obligate D. discoideum cheaters have never been documented, but we found that obligate cheaters readily evolved under low relatedness conditions in the laboratory. In another study, we looked at the effects of light level on spore production in D. discoideum and Dictyostelium citrinum . Overall, more spores were produced in the light than in the dark, probably because of reduced movement and cell loss during the motile multicellular slug stage. We found that these effects were species, clone, and environment dependent. Taken together, this work helps us understand how cooperation thrives in nature, despite the threat of cheaters
Social interactions in two species of social amoebae Dictyostelium discoideum and Dicyosteliuum purpureum
No full textThe core of sociality and one of the key forces behind the transition to multicellularity is cooperation. The study of social behavior in microorganisms has gained considerable attention in the last decade as researchers have discovered that many of the cooperative social interactions found in higher organisms can also be found in microbes. The dictyostelids are particularly amenable to the study of social evolution because of the potential for conflict and cooperation during multicellular formation. The formation of the multicellular fruiting body may lead to conflict because all nearby cells aggregate together, which may be distinct clones, each trying to increase its own fitness. I first explored how D. discoideum and D. purpureum interact and if either species looks to cheat the other when they interact. I found that both species prefer being clonal but cooperate with each other when it seems the benefits outweigh the costs. Cooperating amoebae are able to make larger fruiting bodies, which are advantageous for migration and dispersal, but both species suffer a cost in producing fewer spores per fruiting body. I next examined short-range social dispersal in the social amoebae, D. discoideum and D. purpureum. It appears that the evolutionary loss of stalked migration gives D. discoideum cells the advantage of delaying specialization and the ability to colonize more distant locations, but has significant costs due to migration distance, such as the fraction of cells that become fertile spores. In my final study, we examine the interaction of different clones of D. discoideum before and after migration. We show that chimerism and migration interact to produce fruiting bodies that have a proportionally higher spore allocation compared to clonal fruiting bodies after migration but were unable to determine whether the results that we see are an indication of clones defecting in a tragedy of the commons or more cooperation. With further study will be able to better explain the affects of cooperation on group dispersal and whether it can be used as a mechanism to reduce local competition
Male production and worker policing in Parachartergus colobopterus, a neotropical, swarm-founding wasp
No full textIn many social insects, workers are capable of producing males. Workers are more highly related to their own sons than to the sons of the queens, and so it is surprising that queens often monopolize male production even though they are outnumbered by the workers. A possible explanation is that workers prevent each other from reproducing when their relatedness to the sons of the queens is higher than their relatedness to the sons of other workers. Using microsatellite loci to assess genetic relatedness, we determined that workers of the wasp species Parachartergus colobopterus should prevent each other from reproducing. Analyses of the male genotypes showed that queens were producing the males in accord with our prediction. We did not find evidence for policing behavior however, so these results are also consistent with the hypothesis that worker reproduction has colony-level costs which have led to a conventional settlement
Mechanisms of cheating behavior in the social amoeba Dictyostelium discoideum
No full textDictyostelium discoideum is a eukaryotic micro-organism with a unique life cycle. The amoebae live as haploid, free-living cells in the soil feeding on bacteria and dividing asexually. Under starvation conditions, the cells aggregate and undergo a process of differentiation into spores and stalk cells. We speculate that the stalk cells are sacrificed to help raise the spores above the substrate and to improve their dispersal and survival. In the case of a mix between two or more genetically different clones, a conflict may arise over which cells become spores and survive and which become stalk and die. One that differentiates more spores than its fair share in chimera is called a "cheater" and the other a "loser". Dictyostelium discoideum is a useful organism for studying the complexity of social behavior in microorganisms. Molecular tools have been developed allowing the study of genetic mechanisms that underlie this social behavior.
To investigate the molecular basis of cooperation, several pools of knock-out mutants were generated using the REMI (R&barbelow;estriction E&barbelow;nzyme M&barbelow;ediated I&barbelow;ntegration) technique. To simulate evolutionary selection for cheaters, the different mutants were subjected to rounds of spore germination, growth and development in a mixed population. Only the spores were taken to the next generation. Real Time PCR confirmed that cheaters became over-represented in the evolving population because they contribute spores with a higher efficiency than the other mutants. Mutants expressing a normal phenotype were picked and isolated after 10 and 20 cycles of selection and mixed in pairwise experiments with the parental wild type. At least 35 mutants have been tested and 29 were cheaters. Analysis of the isolated genes suggested that several genetic pathways are involved in regulating or modulating the complex cooperation process in Dictyostelium discoideum.
Finally we characterized one cheater mutant, called chtB, which shows apparently normal phenotype when plated clonally. The mutant is lacking in the expression of the gene chtB. In chimeras, this causes the reduction of the expression of the prespore marker cotB in the wild type strain, enabling the cheater to differentiate more spores than the parental strain
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