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dictyNews Volume 34 Number 10
dictyNews
Electronic Edition
Volume 34, number 10
March 19, 2010
Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to dicty@northwestern.edu
or by using the form at
http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit.
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Abstracts
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Cheating does not explain selective differences at high and low
relatedness in a social amoeba
Gerda Saxer, Debra A. Brock, David C. Queller and
Joan E. Strassmann
BMC Evolutionary Biology 2010, 10:76
Background
Altruism can be favored by high relatedness among interactants. We tested
the effect of relatedness in experimental populations of the social amoeba
Dictyostelium discoideum, where altruism occurs in a starvation-induced social
stage when some amoebae die to form a stalk that lifts the fertile spores above
the soil facilitating dispersal. The single cells that aggregate during the social
stage can be genetically diverse, which can lead to conflict over spore and
stalk allocation. We mixed eight genetically distinct wild isolates and
maintained twelve replicated populations at a high and a low relatedness
treatment. After one and ten social generations we assessed the strain
composition of the populations. We expected that some strains would be
out-competed in both treatments. In addition, we expected that low
relatedness might allow the persistence of social cheaters as it provides
opportunity to exploit other strains.
Results
We found that at high relatedness a single clone prevailed in all twelve
populations. At low relatedness three clones predominated in all twelve
populations. Interestingly, exploitation of some clones by others in the social
stage did not explain the results. When we mixed each winner against the
pool of five losers, the winner did not prevail in the spores because all
contributed fairly to the stalk and spores. Furthermore, the dominant clone
at high-relatedness was not cheated by the other two that persisted at low
relatedness. A combination of high spore production and short unicellular
stage most successfully explained the three successful clones at low
relatedness, but not why one of them fared better at high relatedness.
Differences in density did not account for the results, as the clones did not
differ in vegetative growth rates nor did they change the growth rates over
relevant densities.
Conclusions
These results suggest that social competition and something beyond solitary
growth differences occurs during the vegetative stage when amoebae eat
bacteria and divide by binary fission. The high degree of repeatability of our
results indicates that these effects are strong and points to the importance
of new approaches to studying interactions in D. discoideum.
Submitted by Gerda Saxer [gsaxer@rice.edu]
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A Ras signaling complex controls the RasC-TORC2 pathway and
directed cell migration
Pascale G. Charest, Zhouxin Shen, Ashley Lakoduk, Atsuo T. Sasaki,
Steven P. Briggs, and Richard A. Firtel
Developmental Cell, in press
Ras was found to regulate Dictyostelium chemotaxis, but the mechanisms
that spatially and temporally control Ras activity during chemotaxis remain
largely unknown. We report the discovery of a Ras signaling complex that
includes the Ras guanine exchange factor (RasGEF) Aimless, RasGEFH,
protein phosphatase 2A (PP2A), and a scaffold designated Sca1. The
Sca1/RasGEF/PP2A complex is recruited to the plasma membrane in a
chemoattractant- and F-actin-dependent manner and is enriched at the
leading edge of chemotaxing cells where it regulates F-actin dynamics
and signal relay by controlling the activation of RasC and the downstream
TORC2-Akt/protein kinase B (PKB) pathway. In addition, PKB and
PKB-related PKBR1 phosphorylate Sca1 and regulate the membrane
localization of the Sca1/RasGEF/PP2A complex, and thereby RasC activity,
in a negative feedback fashion. Thus, our study uncovered a molecular
mechanism whereby RasC activity and the spatiotemporal activation of
TORC2 are tightly controlled at the leading edge of chemotaxing cells.
Submitted by Rick Firtel [rafirtel@ucsd.edu]
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[End dictyNews, volume 34, number 10]
