R-3 Repository :: Browsing by Author "Strassmann, Joan E."

Browsing by Author "Strassmann, Joan E."

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Amino Acid Repeats Cause Extraordinary Coding Sequence Variation in the Social Amoeba Dictyostelium discoideum
(Public Library of Science, 2012-09) Scala, Clea; Tian, Xiangjun; Mehdiabadi, Natasha J.; Smith, Margaret H.; Saxer, Gerda; Stephens, Katie; Buzombo, Prince; Strassmann, Joan E.; Queller, David C.
Protein sequences are normally the most conserved elements of genomes owing to purifying selection to maintain their functions. We document an extraordinary amount of within-species protein sequence variation in the model eukaryote Dictyostelium discoideum stemming from triplet DNA repeats coding for long strings of single amino acids. D. discoideum has a very large number of such strings, many of which are polyglutamine repeats, the same sequence that causes various human neurological disorders in humans, like Huntington's disease. We show here that D. discoideum coding repeat loci are highly variable among individuals, making D. discoideum a candidate for the most variable proteome. The coding repeat loci are not significantly less variable than similar non-coding triplet repeats. This pattern is consistent with these amino-acid repeats being largely non-functional sequences evolving primarily by mutation and drift.
Collective worker control in the African social wasp, Polybioides tabidus
(2000) Henshaw, Michael Thomas; Strassmann, Joan E.; Queller, David C.
Social insect workers often sacrifice their own reproduction so that they may help relatives to reproduce more successfully. However, genetically dissimilar colony-mates may prefer to aid different kin, and this may result in disruptive conflicts. I developed polymorphic microsatellite genetic loci for the African swarm-founding wasp Polybioides tabidus to examine mechanisms to reduce such conflicts. Swarm-founding wasps have many reproductive queens in their colonies which should lower relatedness, increasing the potential for conflicts. I found that even though P. tabidus colonies contained many queens, relatedness was elevated because new queens were only produced after the number of old queens had been reduced to one, or nearly one. Queens were thus highly related, elevating relatedness in the colony as a whole, and promoting sociality. This unique pattern of queen production is consistent with a worker manipulation of the sex ratios known as cyclical oligogyny. Under cyclical oligogyny, new queens are produced when the colony has few queens, while males are produced when the colony has many queens. The males were indeed produced when queen number was higher, and I found evidence that the workers collectively controlled male production. Colonies which produced normal haploid males also produced diploid males, which have a diploid genome but are homozygous at the sex determining locus. P. tabidus does not appear to effectively distinguish between diploid and haploid males, and diploid males should have occurred in colonies without haploid males too. Their absence indicates that they were actively eliminated from colonies in which the workers did not favor male production. Workers also may have controlled who produced the males. Each worker should prefer to produce the males herself. However, I found that the queens produced the males. This may be explained by collective worker policing because the workers would be more highly related to queen-derived males than to the sons of other workers and should prevent reproduction by other workers. Alternatively, each worker might restrain herself if worker reproduction was costly to the success of the colony. The results of this study indicate that collective worker control is an important mechanism stabilizing cooperation.
Conflict and cooperation in the tropical wasp, Parachartergus colobopterus, and the chimeric multicellular organism, Dictyostelium discoideum
(2003) Platt, Thomas Gene; Strassmann, Joan E.; Queller, David C.
Several transitions to higher levels of biological organization have punctuated the history of life. These transitions occur when cooperative alliances lead to the integration of non-identical partners into more complex wholes. Yet there is inevitable conflict within these cooperative alliances. In the following chapters I investigate reproductive conflicts of interest between cooperators within insect societies and chimeric multicellular organisms. In the first chapter I show that in the tropical wasp, Parachartergus colobopterus, workers use aggression toward totipotent, emerging females to influence the reproductive future of the latter. By doing this workers resolve conflict over who reproduces in accord with their collective interests. In the second chapter I show that environmental heterogeneity can affect the outcome of conflict between co-aggregating clonal lineages of Dictyostelium discoideum. This helps account for the coexistence of cheaters and victims in natural populations of D. discoideum.
Cooperation, conflict, and experimental evolution in social amoebae
(2011) Kuzdzal, Jennie Jo Helene; Strassmann, Joan E.
Cooperation 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.
Evolution of trinucleotide microsatellite sequence variation across the Polistinae, a social wasp subfamily
(1998) Zhu, Yong; Strassmann, Joan E.; Queller, David C.
I examined sequences of 3 microsatellite loci that were derived from Polistes bellicosus investigated their evolution in 78 species from four tribes in the Polistinae subfamily of wasps. I found that the basic repeat structure of the microsatellite repeat region at these three loci was conserved across species at the subgenus level. Besides changes in the number of repeats, base substitutions were the dominant mutations in the repeat regions. The tribe from which the loci were isolated had longer repeat regions and more perfect repeats that the tribes at the same loci. At microsatellite flanking regions, I found that a high proportion of insertions duplicated 1-4 bases of their preceding sequences. Microsatellite repeats may originate from these such little insertions.
Evolutionary conflict in chimeras of the social amoeba, Dictyostelium discoideum
(2002) Fortunato, Angelo; Strassmann, Joan E.; Queller, David C.
Molecular biologists have extensively studied Dictyostelium discoideum and consider it a model organism for the study of cellular differentiation but paradoxically many aspects of the natural life and evolution of this organism are unknown. My study demonstrates there is high clonal diversity in nature and that different clones coexist in the same small soil sample. Thus, amoebae with different genotypes can join in the same multicellular organism making a genetic chimera in nature. I also show that a linear dominance hierarchy describes the interactions of genetically distinct clones in competition for survival in the multicellular stage. In fact, during this stage the amoebae differentiate into different types of cells allowing the pseudoplasmodium to form a fruiting body consisting of two principle cell types: the spore and the stalk cells. The formation of the fruiting body probably guarantees a better survival and dispersion of the spore but requires the death of 20% of the cells that initially comprise the pseudoplasmodium that differentiate into the stalk and the basal disc. Contributions of two clones in a chimera to spore and stalk are often unequal, with one clone taking advantage of the other's stalk contribution. To assess whether there was a hierarchy of exploitation among clones, I competed all possible pairs among seven clones. I found a clear linear hierarchy with one clone being most dominant, and the bottom clone losing in competition to all the others. The hierarchy at an earlier stage, evaluated with prespore and prestalk cells in the slug, was not as clear. These results suggest that there is a single principal mechanism for differential contribution to the spore and that it involves more than spore/stalk competition. My results legitimate the use of chimeric D. discoideum as a model organism for the investigation of issues relating to coexistence and conflict between cells during organismal development and thus as a simple model for the relationship between social organisms.
Evolutionary costs and benefits of a newly discovered symbiosis between the social amoeba Dictyostelium and bacteria
(2012) Brock, Debra Lynn Adams; Strassmann, Joan E.; Queller, David C.
Recent work has shown that microorganisms are surprisingly like animals in having sophisticated behaviours such as cooperation, communication, and recognition, as well as many kinds of symbioses. Here we show first that the social amoeba Dictyostelium discoideum has a primitive farming symbiosis that includes dispersal and prudent harvesting of the crop. About one-third of wild-collected clones engage in husbandry of bacteria. Instead of consuming all bacteria in their patch, they stop feeding early and incorporate bacteria into their fruiting bodies. They then carry bacteria during spore dispersal and can seed a new food crop, which is a major advantage if edible bacteria are lacking at the new site. However, if they arrive at sites already containing appropriate bacteria, the costs of early feeding cessation are not compensated, which may account for the dichotomous nature of this farming symbiosis. We also observed farmer Dictyostelium discoideum clones carry bacteria that they do not use as food. We hypothesized that these bacteria may play a defensive role against other D. discoideum clones. In our second study, we investigated the impact of these bacteria-carrying farmers on non-farming D. discoideum clones. We found that the presence of farming clones reduces spore production in non-farmers. Furthermore, this effect increases with frequency of farming clones, demonstrating the vulnerability of non-farming clones to farmers though in this experiment we had not separated the effects of the farmer clone and the bacteria they carry. In our third study we exposed non-farmers to a filtered supernatant from the most common non-food carried bacterium, Burkholderia xenovorans . This supernatant is likely to carry whatever the bacteria are producing. We treated Dictyostelium clones at the beginning of the social stage and found that the supernatant enhanced spore production of farming clones and hurt spore production of non-farming clones. This study shows that the effects of the bacteria can be restricted to a filtered supernatant alone. This discovery of symbiosis of D. discoideum with bacteria, and its impact on social interactions among D. discoideum clones will provide a fertile ground for further experiments on the evolution of sociality.
Evolutionary Genetics of Dictyostelids: Cryptic Species, Sociality and Sex
(2011) Kalla, Sara Edith; Strassmann, Joan E.
Dictyostelium discoideum serves as an ideal system to study social evolution because of the social stage of its lifecycle, where individuals aggregate to build a multicellular structure. However, much of its basic biology remains unknown and this limits its utility. I used three separate projects to fill these gaps. In my first project, I examined how speciation and genetic diversity affects kin discrimination using a related dictyostelid, Polysphondylium violaceum . I sequenced the ribosomal DNA of 90 clones of P. violaceum and found that P. violaceum is split into several morphologically identical groups. When allowed to cooperate in pairwise mixes, I found that some clones cooperated with others in their group, but in mixes between groups, clones did not cooperate. For my second project, I looked at whether D. discoideum has sex in natural populations. While sex has been observed in laboratory clones of D. discoideum , it is unclear whether sex occurs in natural populations, and sex can influence the evolution of traits. I used a dataset of microsatellites in 24 clones of D. discoideum to look for a decrease in linkage disequilibrium as a molecular sign of sex. Linkage disequilibrium is higher between physically close loci than between loci on different chromosomes. From this, I conclude that D. discoideum undergoes recombination in nature. Lastly, I used the genome sequence of D. discoideum to look at large scale patterns of evolution. Mutations tend to be biased towards A/T from G/C so, on average, mutations should lower the nucleotide content of sequences. The removal of these mutations, purifying selection, should preserve nucleotide content. I used the genomes of D. discoideum and Plasmodium falciparum identify classes of sequences that should be under different amounts of purifying selection and compared their nucleotide contents. In all cases, those sequences under more purifying selection had higher GC contents than sequences under less purifying selection. Looking at relative nucleotide content may thus serve as an indicator purifying selection. These three studies add insight on how cooperation works in dictyostelids as well as adding an understanding of how traits, social and otherwise, would evolve in this system.
Exploiting new terrain: An advantage to sociality in the slime mold Dictyostelium discoideum
(2006) Kuzdzal, Jennie J.; Strassmann, Joan E.; Queller, David C.
Identifying benefits to cooperation is important in studying social evolution. When the social amoebae of Dictyostelium discoideum starve, they aggregate to form multicellular slugs that migrate towards the soil surface to form fruiting bodies. Multicellularity in D. discoideum is thought to help provide protection from predators and aid in the dispersal of reproductive spores. Here we show an additional benefit of local dispersal. Utilizing D. discoideum's phototactic behavior, we induced slug migration over bacteria and show that this passage results in the removal of bacteria. Time-lapse video revealed that slugs do not dissociate on contact with bacteria, rather cells sloughed from the migrating slugs consume the prey and reproduce. Using an aggregation mutant, we show that the cells slugs shed are able to reach and colonize food sources solitary amoebae cannot. We propose that the exploitation of local food patches is an important selective benefit favoring multicellularity in D. discoideum .
Fitness consequences of genetic and phenotypic chimeras in the social amoeba, Dictyostelium discoideum
(2008) Castillo, David I.; Strassmann, Joan E.; Queller, David C.
Multicellular organisms regularly eliminate unfit or harmful cells in the process of normal development. This high level of cooperation is maintained because all cells within the organism are genetically identical and thus, have the same evolutionary interests. However, there are some multicellular organisms that develop not from a single cell but from many individuals. In the case of the social amoebae, Dictyostelium discoideum, the usually solitary amoebae aggregate with nearby cells when starving to form a motile, multicellular slug that migrates to the soil surface and forms a fruiting body consisting of a bail of spores held aloft by a stalk of dead cells. These aggregations may be a mix of genetically identical individuals or of multiple clones, called genetic chimeras. Within chimeras, a conflict may arise over which cells contribute to the reproductive spores versus the dead stalk. Previous work on D. discoideum has shown that uniclonal slugs migrate further than chimeric slugs of the same size across agar. Here we test whether this resuits in a fitness cost under more natural conditions. We examined migration of slugs across decaying leaves and soil as well as migration up through layers of these substrates, closely reflecting the natural migration of D. discoideum slugs in the wild. In most trials, chimeras performed worse than single clones. Thus, chimerism in D. discoideum should produce a fitness cost likely to be important in nature. Since D. discoideum readily mixes with multiple clones, the potential for conflict is high. Diverging evolutionary interests in chimeras could influence adaptive strategies for filling the spore and stalk roles. Here we explore the strategy for filling these roles in phenotypic chimeras, mixes of cells differing only in physical condition. We explore the fitness of D. discoideum spores and found that cells with better prospects tend to represent the subsequent generations. However, we also found that D. discoideum amoebae did not respond to the condition of cells with which they aggregate. Our results indicate that, within a genetically identical population, the differentiation of spore and stalk roles is a competition based on the condition of the individual cells.
Genetic structure of Polistes dominulus foundress associations
(1998) Zacchi, Francesca; Strassmann, Joan E.; Queller, David C.
Kin-selection theory states that individuals can increase their total fitness both through direct reproduction and through reproduction of relatives. The recently developed social contract theory asserts that dominant females should yield some direct reproduction to the subordinates in order to keep them in the colony. The theory predicts that a dominant will cede more reproduction to an unrelated subordinate than to a related subordinate since it will take more to keep her. I found that the social wasp, Polistes dominulus, is unusual in that foundresses regularly nest with non-relatives, even when relatives are available on other nests. This offers the opportunity to test one of the basic predictions of social contract theory, that reproductive skew increases as relatedness among co-foundresses increases. Subordinates of the collected colonies did get a small fraction of direct reproduction, but there was no difference in skew among colonies with different co-foundress relatednesses, contrary to skew theory predictions. Subordinates's relatedness to the queen does not affect colony efficiency. This study found no sign of either higher cooperation in colonies with more related females or greater conflicts in colonies founded by unrelated females.
Genotypic diversity and population structure in Dictyostelium discoideum
(2004) Smith, Margaret Harris; Strassmann, Joan E.; Queller, David C.
Current population genetics literature presumes a lack of population structure in microorganisms due to their large population sizes, ubiquitous dispersal, and absence of geographic barriers. Contrary to this, we found evidence of great population subdivision in the social amoeba, Dictyostelium discoideum. Pairwise values of Fst and Rst were significant between samples collected from sites separated by distances from 100m--10,000km (P < 0.05 after sequential Bonferroni). Three- and two-level analyses of molecular variance (AMOVA) confirmed the existence of population structure (Fst 0.30, Rst = 0.48, P < 0.001 for three-level; Fst = 0.297, Rst = 0.474, P < 0.001 for two-level). Further, linearized pairwise Fst values were significantly correlated with natural logarithm of geographic distance (r = 0.471, P < 0.012) indicating isolation by distance. Genotypic diversity of populations remained moderate (Nei (1987) average gene diversity ∼0.50) in spite of the decreased population diversity expected with such Fst and Rst values.
Grooming, aggression, and genetic relatedness in Parachartergus colobopterus, a neotropical swarm-founding wasp
(1995) Klingler, Ceal Jenifer; Strassmann, Joan E.
I 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.
Kin discrimination and possible cryptic species in the social amoeba Polysphondylium violaceum
(BioMed Central, 2011) Kalla, Sara E.; Queller, David C.; Lasagni, Andrea; Strassmann, Joan E.
Abstract Background The genetic diversity of many protists is unknown. The differences that result from this diversity can be important in interactions among individuals. The social amoeba Polysphondylium violaceum, which is a member of the Dictyostelia, has a social stage where individual amoebae aggregate together to form a multicellular fruiting body with dead stalk cells and live spores. Individuals can either cooperate with amoebae from the same clone, or sort to form clonal fruiting bodies. In this study we look at genetic diversity in P. violaceum and at how this diversity impacts social behavior. Results The phylogeny of the ribosomal DNA sequence (17S to 5.8S region) shows that P. violaceum is made up of at least two groups. Mating compatibility is more common between clones from the same phylogenetic group, though matings between clones from different phylogenetic groups sometimes occurred. P. violaceum clones are more likely to form clonal fruiting bodies when they are mixed with clones from a different group than when they are mixed with a clone of the same group. Conclusion Both the phylogenetic and mating analyses suggest the possibility of cryptic species in P. violaceum. The level of divergence found within P. violaceum is comparable to the divergence between sibling species in other dictyostelids. Both major groups A/B and C/D/E/F show kin discrimination, which elevates relatedness within fruiting bodies but not to the level of clonality. The diminished cooperation in mixes between groups suggests that the level of genetic variation between individuals influences the extent of their cooperation.
Kinship and the evolution of altruism in social amoebae and A model for the evolution of kin-limited interactions
(2011) Gilbert, Owen Michael; Strassmann, Joan E.
For 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.
Male production and worker policing in Parachartergus colobopterus, a neotropical, swarm-founding wasp
(1998) Henshaw, Michael Thomas; Strassmann, Joan E.
In 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.
Maternity and cyclical oligogyny in a colony of Parachartergus colobopterus
(1997) Quach, Steve Quan; Strassmann, Joan E.
Genetic 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.
Mechanisms of cheating behavior in the social amoeba Dictyostelium discoideum
(2007) Santorelli, Lorenzo Andrea; Strassmann, Joan E.
Dictyostelium 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.
Mutational processes in Dictyostelium discoideum: How mutations affect social behaviors and fitness
(2009) Middlemist, Sara A.; Strassmann, Joan E.
Part 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.
A new social gene in Dictyostelium discoideum, chtB
(2013) Santorelli, Lorenzo A.; Kuspa, Adam; Shaulsky, Gad; Queller, David C.; Strassmann, Joan E.; National Science Foundation; BioMed Central
Background: Competitive social interactions are ubiquitous in nature, but their genetic basis is difficult to determine. Much can be learned from single gene knockouts in a eukaryote microbe. The mutants can be competed with the parent to discern the social impact of that specific gene. Dictyostelium discoideum is a social amoeba that exhibits cooperative behavior in the construction of a multicellular fruiting body. It is a good model organism to study the genetic basis of cooperation since it has a sequenced genome and it is amenable to genetic manipulation. When two strains of D. discoideum are mixed, a cheater strain can exploit its social partner by differentiating more spore than its fair share relative to stalk cells. Cheater strains can be generated in the lab or found in the wild and genetic analyses have shown that cheating behavior can be achieved through many pathways. Results: We have characterized the knockout mutant chtB, which was isolated from a screen for cheater mutants that were also able to form normal fruiting bodies on their own. When mixed in equal proportions with parental strain cells, chtB mutants contributed almost 60% of the total number of spores. To do so, chtB cells inhibit wild type cells from becoming spores, as indicated by counts and by the wild type cells’ reduced expression of the prespore gene, cotB. We found no obvious fitness costs (morphology, doubling time in liquid medium, spore production, and germination efficiency) associated with the cheating ability of the chtB knockout. Conclusions: In this study we describe a new gene in D. discoideum, chtB, which when knocked out inhibits the parental strain from producing spores. Moreover, under lab conditions, we did not detect any fitness costs associated with this behavior.

Browsing by Author "Strassmann, Joan E."

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