At 7:16 AM -0000 2/27/97, John Hewitson wrote:
>Dear Plant-eds,
>>My students ask "What's the point of a triploid tissue?"
>>The thinking goes like this:-
>>* Triploid tissue is unique to angiosperms. True?
>>* Natural selection does not support a structure unless it confers some
>advantage. Agreed?
>>* The fact that triploid tissue exists suggests that there is some
>advantage. What is the advantage of a TRIPLOID endosperm?
>>* Does a study of angiosperms suggest that the most recent members use this
>tissue less, OR do they make greater use? What is the direction that
>evolution seems to be taking?
John,
Not every endosperm IS triploid; most are, but some are even more
polyploid. Books teach triploid because of its common status.
There are other embryo sac types.
There is an old hypothesis that polyploidy of certain tissues
diverts nutrients from diploid tissues. This helps explain the
accumulation of nutrients by endosperm tissue at the expense
of the maternal tissues without a concomittant investment in the
diploid embryo. Until the endosperm starts making enzymes to
degrade its own tissues at seed germination, or until the embryo
does the same to transfer nutrients to itself (in species with
limited/no endosperm) the endosperm holds these aggrandized
nutrients. I haven't read any of the supporting papers from the
old days, and so I'm not sure if this is much more than an hypothesis.
If you want a human analog, there is a parallel evolution of fetal
hemoglobin and a relationship between maternal hemoglobin, maternal
myoglobin, and fetal hemoglobin for oxygen. These are VERY different
solutions to a similar problem of nutrient balances.
There certainly is STRONG evidence that polyploidy of normal tissues
results in larger plants, flowers, and fruits. Use of colchicine to
induce polyploidy was one of the initial thrusts for crop yield. So
there is a very practical side to polyploidy. Parallel improvements
by natural means are also evident. Increased competitiveness would
certainly be adaptive.
Another interesting, related topic is allopolyploidy. This is both
a natural and artificial phenomenon. Among ferns and oaks, there
are huge hybrid swarms among natural populations. Many "natural"
species are, in fact, allopolyploid hybrids. These occur when
matings between "incompatible" species occur. Each gamete brings
a unique chromosome set to the syngamy. The unmatching sets would
render the resulting progeny sterile. By as yet incompletely understood
mechanisms, the progeny can double the chromosomes to generate an
allo-tetraploid. This organism has two double-sets of chromosomes
and therefore is usually fertile. If mating with other allotetraploids
of the same kind, the "species" reproduces stably which is why they
are sometimes listed as a species of their own. Cytogenetics though
reveals they are just allopolyploid hybrids. The point is: polyploidy
provides the advantage of complex hybridization of species, improving
the genetic diversity of the gene pools by making "impossible"
recombinations a reality.
Hope this gives you and your students some ideas to consider.
ross
_______________________________________________________________
Ross Koning | koning at ecsu.ctstateu.edu
Biology Department | http://koning.ecsu.ctstateu.edu/
Eastern CT State University | phone: 860-465-5327
Willimantic, CT 06226 USA | fax: 860-465-4479
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