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Daphnia Generations. RE: Live Foods Digest V4 #34

Hi Bill and All,

You pose an interesting question regarding the number of generations and how
it relates to adaptivity.  The following is from memory based on studies I
did long ago.  If anyone knows of exceptions, please chime in.

        Sexual generations occur only after males are produced which then
mate with females to produce zygotic eggs.  These become ephippia, the
hard-shelled eggs that can dry out repeatedly and will last in the
environment for at least a year until the climate cycle produces water that
is of the right character to simulate the ephippia to hatch.  These all
hatch into females which represent the new (sexual) generation.

        In the simplest case, the whether cycle is annual and there is only
one sexual generation per year.  The ephippia hatch in the spring as new
females each with their own unique set of genes.  These females are the
matriarchs of new clone lines.  They produce females just like themselves
via asexual reproduction.  The process is repeated many times and the clones
build up to large numbers.  As the pond dries up or some other change occurs
the females produce special eggs that develop into males.  These males mate
with the females and start another sexual generation.  Typically this occurs
in the late summer or fall and the entire population spends the winter in
the ephippial stage, the adults having all died because the pond dried up or
in some other way not suitable for the adult state.

A yearly cycle might produce 25-50 asexual generations and only 1 sexual
generation.  Some clones would be better adapted than others and would be
present in greater numbers when the sexual cycle begins and would contribute
more to the new generation than the poorly adapted clones.