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It has been suggested that the capacity of fusion
with both kin and genetically unrelated conspecifics to form
chimeras (i.e. individuals with a mixture of genetically different
cells) is evolutionarily retained In several phyla because
the resulting organism obtains some selective advantages
over non-chimeric conspecifics. Many demosponges are
known to have fusible larvae that form young chimeric
sponges, but the ecological and evolutionary significance of
this phenomenon has seldom been investigated. It is reasoned
here that if chimeras have a selective advantage, their formation
will be expected to be favored by mechanisms promoting
larval encounters at settlement. By using sibling larvae of the
demosponge Tedania ignis in the laboratory, I tested the
hypothesis that larvae show a natural tendency to aggregate
and form chimeras In a 50 d field-transplantation experiment
using chimeric sponges obtained from the fusion of 2 sibling
larvae, 1 also tested the hypothesis that size and survival are
increased in chimeric individuals compared to non-chimeric
ones and investigated the permanent versus transitory character
of this chimerism. It was found that larvae did not show
any significant tendency to settle spontaneously near siblings,
and no fusion between siblings took place. When pairs of larvae
were forced to settle in contact, fusion was, however, the
outcome in all cases. It was also found that, although chimeric
sponges were stable and about twice as large as non-chimeric
sponges, they did not show increased survival. These results
disprove the common argument that the capacity of fusion
with conspecifics is evolutionarily retained in many sponges
because the gain in size after fusion improves the chances of
survival of the resulting chimeric individual. |
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