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RE: apogamy

  • Subject: RE: [ferns] apogamy
  • From: "Winter, Wim de" Wim.dewinter@wur.nl
  • Date: Fri, 7 Jan 2005 21:01:29 +0100
  • Content-class: urn:content-classes:message
  • Thread-index: AcT04Emrj7b8yWu/SkCVt102VfBAyAAEKFDF
  • Thread-topic: [ferns] apogamy

Betty,

Apogamy normally starts with skipping the meiosis during sporogenesis. So the
spores and consequently, the gametophytes, are diploid. No chromosome doubling
takes place. The process you're describing is known as "induced  apogamy" and
was demonstrated by Manton to result in haploid sporopyhtes (or diploid when
the parent was tetraploid).

The mechanism, of course, doesn't contradict your assumption that it allows
xerophyte species to expand the ecological range in which ferns can grow.
Sounds to me as the most parsimonous explanation in those conditions.

It doesn't explain though the success of e.g. the beech fern. Triploid and
apogamous, it thrives in situations whit loads of other plants. How did it
manage to do so, without natural selection? Or do other mechanisms exist that
induce genetic variety without recombination (someone  mentioned the many
varieties of Pteris)?

Wim


-----Original Message-----
From:	owner-ferns@hort.net on behalf of Betty Hamilton
Sent:	Fri 1/7/2005 6:30 PM
To:	ferns@hort.net
Cc:
Subject:	Re: [ferns] apogamy
Wim posed this question:

> why are they doing so well? Apogamous species with a Tertiary
> distrubution must be rather old, no (e.g. Phegopteris connectilis)?

The way I understand it, apogamy occurs when the egg cell has been too
long without a sperm (ignoring the whole issue hybrids).   The egg cell
"gets old" and undergoes an internal duplication of its chromosome
compliment to reach 2N, and then proceeds to develop as if it had been
fertilized.   So apogamy provides a mechanism to overcome lack of free
water or very low spore numbers, but does not replace normal sexual
reproduction.  Perhaps the very old apogamous species are doing so well
because they can reproduce in a wider range of habitats and conditions
than non-apogamous species.  The way I see, it, more individuals in
more habitats means more opportunities for genetic variation in
response to environmental change as long as apogamy is facultative, not
obligate.




On Jan 7, 2005, at 6:54 AM, Winter, Wim de wrote:

>> Apogamy is a quicker easier more reliable way to reproduce.
> Well, is that generally true? It is the solution to overcome two very
> different problems:
>
> 1. it makes the gametophyte independent of free water necessary for
> fertilization, thus enabling the apogamous species to reproduce in
> dryer
> environments, helped by the somewhat faster development;
>
> 2. it bypasses the impossible meiosis in species with a genome that
> cannot be
> divided in two equal sets of homologous chromosomes (normally hybrids
> that
> have not polyploidised).
>
> The cost are high: no more sex. An organism without sex is like a
> manager that
> doesn't listen to his subordinates: though he can survive on his
> existing
> knowledge he will not get any new ideas for business expansion or
> adaptation
> to a changing market. Even organisms that are normally parthenogenetic
> ;like
> bacteria go at length to exchange some body fluids now and then. The
> whole
> idea of evolution is largely based on modification and mixing.
>
> It is true, however, that when you're a fern growing happily at a
> suitable
> location, you'd rather have your self multiplied than have you sprores
> blown
> away and go all the way through the vulnerable gamo stage en
> fertilization.
> Many ferns therefore have found that some vegetative reproduction in
> addition
> to spores is a useful novelty to acquire.
>
> So generally it appears to me that apogamy is no advantage, though an
> emergency fix in extreme situations (unsuitable environment, wrong
> chromosomes). But the question remains: why are they doing so well?
> Apogamous
> species with a Tertiary distrubution must be rather old, no (e.g.
> Phegopteris
> connectilis)?
>
> Wim
>
> [demime 1.01d removed an attachment of type application/ms-tnef which
> had a name of winmail.dat]
>
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