Re: 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]

---------------------------------------------------------------------
Support hort.net -- join the hort.net fund drive!
http://www.hort.net/funds/
---------------------------------------------------------------------
Support hort.net -- join the hort.net fund drive!
http://www.hort.net/funds/

[demime 1.01d removed an attachment of type application/ms-tnef which had a name of winmail.dat]

---------------------------------------------------------------------
Support hort.net -- join the hort.net fund drive!
http://www.hort.net/funds/
---------------------------------------------------------------------
Support hort.net -- join the hort.net fund drive!
http://www.hort.net/funds/
Other Mailing lists | Author Index | Date Index | Subject Index | Thread Index