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hardy  Aroids)" <ARISAEMA-L at NIC.SURFNET.NL> Aroids)" <ARISAEMA-L at NIC.SURFNET.NL>
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From: Susanne Renner <Renner at UMSL.EDU>
Subject: Arisaema sexual strategies; selfing A. tortuosum
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Dear Pascal,

Thank you for the information that A. tortuosum is indeed capable
of selfing and that many more species have strictly male and female years=,
without any simultaneously bisexual stage.  Everybody who agrees that
nutrition plays a big role in whether an arisaema becomes female, and thi=s
has been  demonstrated experimentally by several workers, starting with
Atkinson in 1898.  You say that because of this nutrition-depency, you
don't think sex change is an important diagnostic tool.  I wasn't thinkin=g
of sexual strategy as a taxonomic character though, but am interested in
its evolution

What is difficult to understand (and exceedingly rare among
flowering plants) is why a plant would switch off male sexual function wh=en
it is genetically capable of producing male gametes.  Giving up female
function makes sense when you're nutrionally stressed.  But male function
is cheap, so why give it up once you're big and healthy?  Among flowering
plants, sex change (= gender choice) is confined to Arisaema, a few
Cucurbitaceae, a few palms (Panax, Elaeis), and a three genera of orchids.
Spinach and maples occasionally do it, too.  Sex change requires the
evolution of a system that allows an ariseama to assess its nutritional
status and then produce female and/or male gametes accordingly.  Why are
all Arisaema capable of doing this but no other aroid, not even arisaema'=s
closest relatives? Any ideas?

Susanne