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hardy  Aroids)" <ARISAEMA-L at NIC.SURFNET.NL> Aroids)" <ARISAEMA-L at NIC.SURFNET.NL>
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From: "P.Bruggeman" <pbruggeman at WISH.NET>
Subject: Re: evolution of sex change doesn't require inbreeding depr=
ession 
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Susan,

>Rather, I suspect, that the selective factor for the evolution of sex
change may lie in extreme environmental heterogeneity from one year to th=e
next (This is known as the patchy environment hypothesis for the evolutio=n
of sex change). An alternative model is what's know as the size advantage
hypothesis.
>This hypothesis holds that an invidual should start out as the sex th=at
suffers the least pentaltity from smallness (normally this is the male se=x)

I sure hope you are still talking about plants.... (sorry, I couldn't res=ist
the temptation)

>and then switch to the sex that reaps the biggest benefit from size
(normally this is the female sex as it takes more resource to mature seed=s
than make pollen). .

But now on a serious note, just because it seems nutrition plays an
important role in the flower-type (male or female) of today's Arisaema, i=s
it correct to use this to explain something that might have happened in t=he
past?. Was it really the enviroment that prompted genetic change or was i=t
something else? Taking into consideration the number of Arisaema species,
the various habitats they grow in (some very fertile!), the variation in
general within the genus and the wide distribution of the genus, I find i=t
very hard to accept that nutrution-levels played such an important role i=n
the evolution of Arisaema while still only a few species are bisexual in =the
mature state. If it were true that poor growing conditions have caused th=is,
then only the ancestors of today's species (that might have started this
male only/female only behavior) must have suffered from this because many=of
the species we now know, live in very rich habitats and still flower male
one year and female another year, without any obvious nutritional reason.=I
would have expected more species to flower bisexual when mature than the
ones we know do. Could it be that Arisaema somehow lack the genetic abili=ty
to control self-pollination (a sort of genetic "deficit") in the "normal"
fashion and have solved the problem this way? And could it be that nutrit=ion
just emphasises this solution? If the resources are theoretically enough =to
produce female flowers and the plants still flowers male, could it be tha=t
Arisaema are not very good at using these resources and that only optimal
conditions produce a "predictable" sex-change?

Pascal