Cooperative social clusters are not destroyed by dispersal in a ciliate

<p>Abstract</p> <p>Background</p> <p>The evolution of social cooperation is favored by aggregative behavior to facilitate stable social structure and proximity among kin. High dispersal rates reduce group stability and kin cohesion, so it is generally assumed that there is a fundamental trade-off between cooperation and dispersal. However, empirical tests of this relationship are rare. We tested this assumption experimentally using ten genetically isolated strains of a ciliate, <it>Tetrahymena thermophila</it>.</p> <p>Results</p> <p>The propensity for social aggregation was greater in strains with reduced cell quality and lower growth performance. While we found a trade-off between costly aggregation and local dispersal in phenotypic analyses, aggregative strains showed a dispersal polymorphism by producing either highly sedentary or long-distance dispersive cells, in contrast to less aggregative strains whose cells were monomorphic local dispersers.</p> <p>Conclusion</p> <p>High dispersal among aggregative strains may not destroy group stability in <it>T. thermophila </it>because the dispersal polymorphism allows social strains to more readily escape kin groups than less aggregative strains, yet still benefit from stable group membership among sedentary morphs. Such dispersal polymorphisms should be common in other social organisms, serving to alter the nature of the negative impact of dispersal on social evolution.</p

Evolution of dispersal and life history strategies – Tetrahymena ciliates

Background: Considerable attention has focused on how selection on dispersal and other core life-history strategies (reproductive effort, survival ability, colonization capacity) may lead to so-called dispersal syndromes. Studies on genetic variation in these syndromes within species could importantly increase our understanding of their evolution, by revealing whether traits co-vary across genetic lineages in the manner predicted by theoretical models, and by stimulating further hypotheses for experimental testing. Yet such studies remain scarce. Here we studied the ciliated protist Tetrahymena thermophila, a particularly interesting organism due to cells being able to transform into morphs differing dramatically in swim-speed. We investigated dispersal, morphological responses, reproductive performance, and survival in ten different clonal strains. Then, we examined whether life history traits co-varied in the manner classically predicted for ruderal species, examined the investment of different strains into short- and putative long-distance dispersal, while considering also the likely impact of semi-sociality (cell aggregation, secretion of 'growth factors') on dispersal strategies. Results: Very significant among-strain differences were found with regard to dispersal rate, morphological commitment and plasticity, and almost all core life-history traits (e. g. survival, growth performance and strategy), with most of these traits being significantly intercorrelated. Some strains showed high short- distance dispersal rates, high colonization capacity, bigger cell size, elevated growth performance, and good survival abilities. These well performing strains, however, produced fewer fast-swimming dispersal morphs when subjected to environmental degradation than did philopatric strains performing poorly under normal conditions. Conclusion: Strong evidence was found for a genetic covariation between dispersal strategies and core life history traits in T. thermophila, with a fair fit of observed trait associations with classic colonizer models. However, the well performing strains with high colonization success and short- distance dispersal likely suffered under a long-distance dispersal disadvantage, due to producing fewer fast-swimming dispersal morphs than did philopatric strains. The smaller cell size at carrying capacity of the latter strains and their poor capacity to colonize as individual cells suggest that they may be adapted to greater levels of dependency on clone-mates (stronger sociality). In summary, differential exposure to selection on competitive and cooperative abilities, in conjunction with selective factors targeting specifically dispersal distance, likely contributed importantly to shaping T. thermophila dispersal and life history evolution

The evolution of worker caste diversity in social insects

Morphological diversification of workers is predicted to improve the division of labor within social insect colonies, yet many species have monomorphic workers. Individual‐level selection on the reproductive capacities of workers may counter colony‐level selection for diversification, and life‐history differences between species (timing of caste determination, colony size, genetic variation available) may mediate the strength of this selection. We tested this through phylogenetically independent contrast analyses on a new data set for 35 ant species. Evidence was found that early divergence of queen‐worker developmental pathways may facilitate the evolution of worker diversity because queen‐worker dimorphism was strongly positively associated with diversity. By contrast, risks for colonies that invest in specialized workers and colony size effects on costs of worker reproduction seem unlikely to strongly affect the evolution of worker diversity because there was no significant association between colony size and diversity when controlling statistically for queen‐worker dimorphism. Finally, worker diversity was greater in species with multiple lineages per colony, and it was negatively associated with relatedness in monogynous species. This could be due to high intracolonial genetic variance favoring the expression and evolution of great worker diversity or to diversity evolving more easily when there is selection for repression of worker reproduction (worker policing)

Polyandry in social Hymenoptera — disunity in diversity?

Multiple mating by queens occurs in many species of social Hymenoptera despite its likely costs. Hypotheses to explain multiple mating include a need for more sperm than provided by a single male, the convergence of queen and worker sex-allocation optima and various genetic diversity hypotheses. For some species the sperm need hypothesis fails since queens retain only a single male’s worth of sperm. In other cases, sperm store does increase with the number of matings. Similarly for the sex-allocation and genetic diversity hypotheses, data from some species are in support, those from others are not. Comparative analysis reveals a negative correlation between level of genetic diversity (of which queen mate number is an important determinant) and parasite load; findings in the best-studied case are complex: monandry and higher levels of polyandry are each selectively favored over moderate polyandry. Out of 14 identifiable hypotheses five are judged most useful for future work. Unfortunately, the search for a simple unitary model to explain all cases seems futile. A model encompassing all of these factors is desirable for studies on single species, but would be complex. Comparative analyses remain desirable, but should encompass the likelihood that different factors predominate in different groups

A) Typical digital picture of cells used to extract quantitative variables

<p><b>Copyright information:</b></p><p>Taken from "Evolution of dispersal and life history strategies – ciliates"</p><p>http://www.biomedcentral.com/1471-2148/7/133</p><p>BMC Evolutionary Biology 2007;7():133-133.</p><p>Published online 6 Aug 2007</p><p>PMCID:PMC1997130.</p><p></p> B) Cells (D2 strain) in normal growth condition (with nutrients) showing large size and a largely rounded shape. C) Cells (20 strain) eight hours after nutrient removal, showing reduced size and (middle cell) the typical elongated shape of the fast-swimming dispersal morph described by Nelsen and Debault [69]

Summary principal component plot representing the correlations between the seven variables summarizing the four experiments

<p><b>Copyright information:</b></p><p>Taken from "Evolution of dispersal and life history strategies – ciliates"</p><p>http://www.biomedcentral.com/1471-2148/7/133</p><p>BMC Evolutionary Biology 2007;7():133-133.</p><p>Published online 6 Aug 2007</p><p>PMCID:PMC1997130.</p><p></p> Graph displays means of five random associations to illustrate within strain variation; see text for detail

Principal component plot representing correlations between cell morphology and survival variables under starvation for the ten strains

<p><b>Copyright information:</b></p><p>Taken from "Evolution of dispersal and life history strategies – ciliates"</p><p>http://www.biomedcentral.com/1471-2148/7/133</p><p>BMC Evolutionary Biology 2007;7():133-133.</p><p>Published online 6 Aug 2007</p><p>PMCID:PMC1997130.</p><p></p> PC1is linked to overall survival and elongation capabilities. PC2represented the cell elongation strategy, opposing strains where all cells elongates similarly for a long time to strains where some cells elongate more than others, up to becoming dispersal morphs

Principal component plots expressing correlations between cell morphology and growth variables in presence of nutrients for the ten strains studied

<p><b>Copyright information:</b></p><p>Taken from "Evolution of dispersal and life history strategies – ciliates"</p><p>http://www.biomedcentral.com/1471-2148/7/133</p><p>BMC Evolutionary Biology 2007;7():133-133.</p><p>Published online 6 Aug 2007</p><p>PMCID:PMC1997130.</p><p></p> See text for details. PC1expressed the opposition between strains with big round small elongated cells at carrying capacity. PC2illustrated the strategy of growth, while PC3represented overall growth performance

Correlation of dispersal rate and cell elongation in the dispersal experiment in presence of nutrients for the ten strains studied

<p><b>Copyright information:</b></p><p>Taken from "Evolution of dispersal and life history strategies – ciliates"</p><p>http://www.biomedcentral.com/1471-2148/7/133</p><p>BMC Evolutionary Biology 2007;7():133-133.</p><p>Published online 6 Aug 2007</p><p>PMCID:PMC1997130.</p><p></p