From: "Kathie Hodge" <Kathie_Hodge at qmrelay.mail.cornell.edu>
>Thanks for the thought-provoking info on how rare tropical trees are MORE,
>rather than less likely to be dioeious. It seems counterintuitive! I wonder
>if anyone has speculated on why this should be the case.
Perhaps they are rare *because* they are dioecious. Much of the literature
about tropical tree breeding systems explores this point. Principal
investigators include James Hamrick at Georgia and Kamal Bawa at Boston U.
Of course, if dioecy is a cause of rarity, which is "bad", then it becomes
an even more interesting question why dioecy has evolved so many times
within monecious groups.
>I also appreciated your thoughts on how recombination might mess up attributes
>a pathogen needs to survive, like host behavior modification. However, I don't
>see why this argument should apply to these fungi more than it does to any
>other organism, the majority of which seem to have sex all the time without
>going extinct as a result of messing up some important function.
My point was that the relative advantages depend on the level of
opportunities to out-cross. From a group-selectionist point of view,
if you can't out-cross, you may be better off as an asexual species.
This argument has been made with respect to parthenogenic species of
freshwater fish in isolated stream systems.
>I have been unable to find anything substantive in the literature on why
>asexuality should be selected for in pathogens/parasites. If you know of
>anything interesting, please post or respond directly to me.
I have not been sufficiently interested in this question to collect
references, but I think you might find some good references in work
by Lynn(e) Margulis. Also, references have been provided here and/or
in bionet.plants in the past year, when the topic of the evolution of
sex came up. John Maynard Smith has published on the topic. Selection
for sex and for asexual reproduction are two sides of the same coin,
and there is certainly an enormous substantive literature on both.
Una Smith Biology Department smith-una at yale.edu
New Haven, CT 06511