MadSci Network: Evolution
Query:

Re: When and how did meiosis, sex, appear in evolutio

Date: Tue Apr 24 18:13:35 2001
Posted By: Brian Foley, Post-doc/Fellow Molecular Genetics
Area of science: Evolution
ID: 988141355.Ev
Message:

EXCELLENT question!!!

	Another question would be: Was meiosis and sex "invented" just
once?  Or did plants, animals, fungi and other organisms each start
out as haploids and invent sex independently?

	I am not an expert on this topic, but as far as I know, meiosis
and sex was invented just one time, and all organisms that use meiosis
today are descended from that one ancestor.  This would place the date
back some 2 billion years, before fungi, plants, protozoans and animals
split from a common ancestor.
	There are other types of "sex", or more correctly, other ways
for similar organisms to exchange genes.  For example, bacteria have
plasmids and phages that can carry genes from one bacterial lineage
to another, and influenza viruses have segmented genomes which allow
some recombination.
	An interesting fact that many people do not know about, is that
many sexual organisms which use haploid and diploid genomic states just
as mammals do, do not have just 2 sexes, nor do they spend the majority
of their life cycle in the diploid state.  Fungi, for example spend 
most of their time haploid, and only briefly enter the diploid state.
Some fungi have hundreds of different sexes rather than just 2.  Most
plants are diploid for the majority of the time, but many of the plants 
are both male and female.  Even within the animals which have male and
female sexes, their is a lot of variety in how the sex is determined.
Most animals do not use the XX = female and XY = male chromosome
system that primates use.  Aligators determine sex by temperature at
which their eggs incubate, for example, so both males and females 
have the same chromosomes.  Honeybees use different food given to 
the embryo to make drones, workers and queens.
	It seems quite likely that sex was one of the critical 
"inventions" that allowed the more rapid creation of the most 
diverse life forms on earth today.  Although bacteria and other
asexual mono-cellular life forms have more diversity in basic 
metabolism and biochemistry than the plants and animals and fungi, 
the multi-cellular organisms with sex have the greatest diversity of
morphologies and the most rapid changes in morphology over time.
But even within a single group of organisms, there can be huge
differences in those rates of change.  For example beetles and ants
are both insects, but the beetles have taken on much more diverse
morphologies than the ants.  All ants look pretty much alike except
for differences in size and color, whereas beetles have all sorts
of shapes and lifestyles as well as differences in size and color.
	Even if meiosis and sex was only "invented" once, it has
been "forgotten" many times.  Many organisms have lost the ability
to reproduce sexually and now reproduce only by cloning themselves.
It happens quite often in the plant kingdom for example.  

	If I am wrong about sex only being invented once, please 
write to me and let me know.  

Brian Foley
btf@t10.lanl.gov



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