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Australia: The Land Where Time Began |
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Slime Moulds – Co-existence of Diverse
Genotypes in Cellular Slime Moulds Can be Explained by Tradeoffs of
Fitness Between Spores and Nonaggregating Cells Cellular clime moulds, such as
Dictyostelium discoideum,
are amoebae that include single-cellular and multicellular stages in
their life cycle, the multicellular stage being achieved when they are
starving, at which time the individual amoebae aggregate. In the
aggregate they are not necessarily clonal; there is strong selection of
the individuals to be represented in the reproductive spores. A
reduction in overall genotypic diversity would result from this process,
which is not consistent with the wide range of diversity that is
observed in nature. Tarnita et al.
suggest that an additional component of fitness is provided by the cells
that fail to aggregate, in which case this can solve the inconsistency:
Strong selection for aggregation only occurs in environments where food
is replenished slowly. Otherwise unicellularity is strongly selected. Individual amoebae aggregate upon starvation
forming a fruiting body comprised of dead stalk cells and reproductive
spores. This process has been described in terms of altruism. Chimeric
fruiting bodies result from nonkin not being perfectly discriminated
against. Complex interactions among genotypes within chimeras have been
documented, which should theoretically reduce genetic diversity. This
is, however, not consistent with the wide diversity of genotypes
observed in nature. Recent work has shown that the loner cells that do
not participate in aggregation, which is a component of
Dictyostelium discoideum
fitness that has not been well studied, can be selected for, depending
on environmental conditions. Tarnita et
al. suggest that the
existence of loners in all cellular slime moulds could resolve the
apparent diversity paradox in 2 ways. The apparent genotypic skew in the
spores of chimeras could simply be the result of different investments
into spores verses loners. Alternatively, coexistence of multiple
genotypes can occur in ecosystems in which multiple local environments
differ in the characteristics of food recovery and are connected
globally by weak-to-moderate dispersal. Tarnita et
al. argue that the defining
of altruistic behaviour, winners and losers, is made impossible by the
presence of loners. Tarnita, C. E., A. Washburne, R. Martinez-Garcia,
A. E. Sgro and S. A. Levin (2015). "Fitness tradeoffs between spores and
nonaggregating cells can explain the coexistence of diverse genotypes in
cellular slime molds (slime mould) " Proceedings of the National
Academy of Sciences 112(9): 2776-2781.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |