Agriculture Reference
In-Depth Information
C. The Ping
Pong Strategy of Sex and Adapting to the Environment
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There are overwhelming advantages of recombination to the genera-
tion of genetic variability and to quality checking of the genome.
However, if these were the only driving forces for the evolution of
the haploid/diploid transitions, it would seem unnecessary for more
taxonomically primitive haploid and diploid forms to exist so often as
substantially different phenotypes. From an evolutionary perspective,
it is informative that these differing morphologies and tissue and organ
functions in haploid/diploid generations of primitive taxa are often
specialized for adaptation to different environments. Alternation of
generations has always been interpreted to have evolved to provide
alternating phenotypes, even in alternating homomorphic generations,
where alternating morphologies are essentially the same, but different
functional phenotypes can still be identi
ed. In fact, a change in
the environment often induces a change in generations. This would
allow the species to cope with very rapid (compared with evolution
timescale) and extreme changes in environments (e.g., very wet ver-
sus very dry) by simply alternating generations through meiosis or
fertilization.
Compared with the bene
ts of recombination, much less attention has
been paid to this ping
pong, or back and forth transition of epigenetic
states in haploid/diploid phenotypes that can provide adaptation with a
changing environment. In most locations the environment does not just
change randomly but also alternates (cycles) between distinct seasons.
On a longer timescale, dramatic climate changes have occurred also on a
cyclic basis, for example, Dryas periods of ice age phenomena (Tang
et al. 2009). The haploid/diploid phenotype alternations may have also
evolved to conform and cope with the cyclical environmental changes,
for example, wet versus dry and cold versus hot seasons. If the environ-
ment becomes more stabilized, one generation can reproduce asexually
tomaintain the more adapted form (Bell 1994). There are many examples
of this, especially in more evolutionarily primitive species such as
mosses or ferns, where usually the gametophyte can be adapted to or
even require very wet environments and the alternate sporophytic form
can thrive in much drier conditions as a result of several specialized
structures, for example, water conducting tissues and stomata (Renzaglia
et al. 2000; Ranker and Hau
-
er 2008). It is tempting to hypothesize that
slower-occurring spontaneous DNA sequence mutations may then allow
genetic assimilation that stabilizes one of these alternating phenotypes to
provide a major shift in
fitness to different environments (Costa and
Shaw 2006). Genetic assimilation can be seen in the same conceptual
