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_______________________________
Michael Marcotrigiano, Ph.D
Director of the Botanic Garden and Professor of Biological Sciences
Smith College
Lyman Conservatory, 15 College Lane
Northampton, MA 01063
email: mmarcotr@smith.edu
voice: 413-585-2741; fax: 413-585-2744
www.smith.edu/garden
www.science.smith.edu/~mmarcotr
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"Art is the unceasing effort to compete with
the beauty of flowers and never succeeding."
Marc Chagall
>>> gibernau@cict.fr 05/19/03 09:49AM >>>
Dear Julius, and Friends,
I have many comments on your last message.
You are right Julius, there are two phenomena :
self-pollination, that is the fertilization of ovules by its own pollen
that will consequently lead to the formation of seeds.
Apomixis that will lead to the formation of (viable) seeds without
any
fertilization (e.g. no pollen needed) by ?parthenogenesis/parthenocarpy?
of
some cells (you have about 10 kinds of apomixis according to the kind of
cells implied: egg-cell, endosperm,?).
In the first case, two genetic informations are mixed whereas in the
second
just one genetic information is duplicate.
As you mention for Urospatha and Xanthosoma acutum, may not present
self-pollination as you observe that protogyny is complete (no overlap).
Then apomixis is certainly present in these taxa. The best way to test
it
(without counting chromosomes) is to castrate (part of) inflorescences
to
see if just female flowers can produce seeds. In my recent study of
Montrichardia (for those interested see attached file), I observe that
bagged inflorescences produced seeds, but I was not able to distinguish
between self-pollination and apomixis. Such answer needs a specific
experiments.
I suspect that apomixis is common in horticultural aroids as it?s the
case
for many other families. Fig trees are a good example: about 700
varieties
of Ficus carica, the edible fig, created by men. Half of them are
parthenocarpic, they produce mature figs with (or not) seeds, but
without
the fig wasp pollinators (no pollen needed); the other half is more
?traditional? and needs to be pollinated by the fig wasps (carrying
pollen)
in order to get mature figs. Consequently parthenocarpic fig species can
be
cultivated in areas outside the natural range of the fig wasp.
I don?t know if it?s better to self-pollinated or to reproduce by
apomixis,
in the first case genetic recombination can occur and can create some
variability, but inbreeding can cause low seed set. Some apomictic Rubus
species which are self-fertile have developed in a second step
self-pollinating mechanisms.
Apomixis occurs in many taxa thus it must advantageous in some
situations.
In many cases, its origin may be from sexually sterile hybrid polyploids
(with high genetic diversity).
Advantages :
Assured reproduction (seeds formation) in absence of pollination (sexual
partners) or/and pollinators.
Clonal reproduction equivalent of vegetative reproduction
?Cost of meiosis? and the mother genetic contribution 100% (instead of
50%)
to her offspring
Fix and disseminate an extremely fit genotype.
Disadvantages :
Accumulate disadvantageous mutations
Inability to recombine novel mutants
A very narrow niche/habitat
No adaptation to stable habitats (often weeds in transitory habitats).
You thoughts were right except may be about which system is more
advantageous.
I don?t know if it helps!!!!
Sincerely yours,
Marc
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