Subject: Re: Voyagers on the Ark of Noah Actually, at least in multicellular organisms (i.

From:    Matthew P Wiener                       
Subject: Re: Voyagers on the Ark of Noah

>Actually, at least in multicellular organisms (i.e. slime molds), I
>would make a distinction between the several (13!) mating types and
>actual genders.

One species has 13.  The number can vary.

>               This is because of one of my pet recent ideas, put
>forth by Lawrence Hurst in two papers in the Royal Society journal
>last yr (the titles were:  Intragenomic conflict as an evolutionary
>force (by Hurst); and Cytoplasmic fusion and the nature of sexes (by
>Hurst & W.D. Hamilton).  I don't have the complete refs on me, but can
>post or email them to anyone interested).

PROC ROY SOC LOND B247, 189-194 and B248, 135-140 (1992).

>                                         Anyways, the idea is that
>incompatibility types can be "superimposed" on top of gender (and are
>in many circumstances), and are different because, if Hurst is right,
>genders are best defined by who donates organelles rather than merely
>compatibility.

This stuff is worth understanding in more detail.

"Incompatibility types" are just a segregation of organisms within a
species into groupings that don't mate with each other, for whatever
reason.  It's generally believed to be a protection against incest.
(I have no idea if there are `homosexual' protozoa out there.  They
may, for all I know, try to conjugate with each within a type, but
nothing comes of it.)

Gender is assigned by looking at what happens closely when gametes
fuse.  The organelles have their own DNA, and would normally be in
competition with each other.  Rather than risk cytoplasmic warfare,
one of the gametes restricts its contributions to nuclear DNA only.
In most organisms, this defines the male of the species.  However,
there are certain creatures that have a chain of genders.  You can
think of them as being 1 < 2 < 3 < .... < 9, say, with gender 1 being
the most "macho" and gender 9 being the most "femme".  So if a 4 mates
with a 1,2,3, only 4's organelles will contribute.  If a 4 mates with
a 5,6,7,8,9, 4's organelles will not contribute.  The child gender can
be anything.

Hurst and Hamilton realized that this situation is unstable whenever
there are more than 2 genders.  The wrong mutation will encourage
cytoplasmic cheating.  A 4's organelles might decide to not give in
so easily and cause troubles with the more femme.  Yet rather than
being a self-destructive mutation, it can be passed on to future
generations by mating with the more macho types.  This option is
not available with 2 genders, and such mutations are good for one
generation only.  So the theory is, basically, that multiple sexes
are possible, but they would usually evolve to pairs.

So I still would like to know.  How did _Physarum polycephalum_ go
to the ark?  13 by 13, or what?
-- 
-Matthew P Wiener (weemba@sagi.wistar.upenn.edu)