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RE: apogamy

  • Subject: RE: [ferns] apogamy
  • From: "Winter, Wim de" Wim.dewinter@wur.nl
  • Date: Sun, 9 Jan 2005 22:45:54 +0100
  • Content-class: urn:content-classes:message
  • Thread-index: AcT2X48ClejO4i5NSWyhVxm8dzo5dgAMYx8L
  • Thread-topic: [ferns] apogamy

Betty,

You're right about hybrid origin species that can have arisen more than once.
This may explain the present varieties in (alloploid) Dryopteris filix-mas,
and why not so the various forms of Pteris cretica. I had not yet thought of
that.

The beech fern is Phegopteris connectilis, which is triploid in Europe and
America. A sexual diploid has been reported from Japan, but I don't know about
the rest of Asia. It's very uniform throughout its range so there's no
indication at first sight for multiple origins. But I do not understand the
hybrid mentiond in the Flora of North America under P. hexagonoptera (2n):

"G. A. Mulligan and W. J. Cody (1979) reported hybrids between Phegopteris
hexagonoptera and P. connectilis from a few localities in Quebec, New
Brunswick, and Nova Scotia. These hybrids are apogamous and have a chromosome
number of 2n = 120."

Mulligan, G.A., and Cody, W.J. 1979. Chromosome numbers in Canadian
Phegopteris. Can. J. Bot. 57:1815-1819.

I will get a copy and see what explanation they have.

Wim



-----Original Message-----
From:	owner-ferns@hort.net on behalf of Betty Hamilton
Sent:	Sun 1/9/2005 4:09 PM
To:	ferns@hort.net
Cc:
Subject:	Re: [ferns] apogamy
Wim!

	Thanks for the correction!   The talk I heard years ago (WOW does that
  make me feel old!) must have focused on induced apogamy!

	Which beech fern are you describing?  And are you refering to European
populations, or American, or worldwide?   As a triploid, it must have
arisen through a chance hybridization between a diploid and triploid
species, and unless it can undergo a spontaneous doubling, it has
reached an evolutionary dead end, though there is still the possibility
of dominant mutations arising.    It is worth remembering that chance
hybridizations may not be exceedingly rare in nature where parental
populations are growing together under favorable conditions.  Every
time a successful hybrid offspring occurs, you would get a somewhat
different combination of genes.   For every such hybrid that then
becomes apogamous and susccessfully reproduces, variation would exist
between the resulting populations.  Further, more such populations
increases the chance that a spontaneous doubling will occur, creating a
fertile hexaploid, and thus the potential for a successful, sexually
reproducing species.   But that is a lot of if's.

	Another possible source of variation would be plasmids ("jumping
genes").  Chlorophyll color patterns in corn leaves lead to the
original discovery of plasmids.


Betty, in South Bend IN, USA
where winter is relenting a bit, before the next cold blast arrives



On Jan 7, 2005, at 3:01 PM, Winter, Wim de wrote:

> 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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