Origins of Eukaryotic Sexual Reproduction
LECA=last eukaryotic common ancestor as distinct
from LUCA (last universal com. anc.)
Probably had the ability to reproduce
sexually…how do we know this??
Transition to a sexual LECA involved 4 and we will
focus on 3 “transitions” (I think I cover 3 here..?)
These things had to evolve…
• Cells needed the ability to fuse and then go
through meiosis.
• There were likely some “who to mate” with
evolutionary changes going on (mating types
first, isogamy=gametes of equal size and then the
evolution of anisogamy=gametes of different
sizes).
• Solve issues with transmission of organelle
genomes.
“This era featured numerous sexual experiments,
most of which failed but some of which were
incorporated, integrated and modified.”
These “innovations most likely coevolved in a
parallel and disjointed fashion.”
Once in place “the evolution of eukaryotic sex has
featured countless prezygotic and postzygotic
variations, the outcome being the segregation of
panmictic (very mixed-everyone mating with
everyone else) populations into distinct species..”
Ex of prezygotic and postzygotic reproductive
isolating mechanisms?
Cells needed the ability to fuse or one cell to be
engulfed by another. Then need ability to go through
meiosis (can’t always be combining genomes)!
Phagocytosis (a complex process) probably set the
stage for cell fusion. (Is some discussion of the role of
transposons in promoting cell fusion…but..)
Another possibility is endomitosis..what is that?
If engulfed have problem with internal membrane
may be digested?
If fused-fate of membranes-are both kept?
“Increases in ploidy confer indubitable benefits”
Why?
• “novel sequences/functions arise in duplicate
genes without compromising existing
pathways”
• “recessive non-adaptive alleles, masked but
carried through time, may prove to be adaptive
in future contexts.”
• Hybrid fitness (having both of two alleles might
make you tolerate a greater temp rangecodominance).
Problems with all these copies of entire genomes??
• Organizing a successful mitosis/meiosis
• Regulating gene expression
Yeast species that are haploid and diploid tested
under diverse conditions, which did better and had
higher fitness?
Some did better in haploid state…others in diploid so
maybe ability to move between states (depending on
environmental conditions) was selected for? Maybe
whole sex reprod aspect (mixing up genes) is not the
key point that we usually think it to be.
So to review….although we think meiosis had to
evolve as a way to combine your genome with
another (importance of variation and sexual
recombination) it might have been an advantage
in and off itself (you might do better in a haploid
state sometimes-ability to toggle between two
states an advantage).
Parasexuality
Before you can run you must walk…
Aspergillus-fungus
Haploid hyphae fuse to make diploid and then
randomly loses chromosomes to return to haploid
state.
Candida-fungus
Mating occurs between diploids
Get cell fusion and are tetraploid.
Then adverse conditions result in random
chromosome loss
“These patterns may be a derived state….but one
can envision analogous earlier versions of genetic
exchange involving cell-cell fusion followed by
parasexual ploidy change from which true sexual
cycles evolved via the invention of meiosis.”
(also interesting…In Candida there is a little
recombination that goes on and a gene that is a
central player in meiosis plays a role…)
Then there are some other crazy things Candida or
yeasts can do….which we are not going to go into.
Meiosis
Maybe the ability to reduce the number of
chromsomes back to haploid evolved for a very
different reason (and these weird tossing of
chromosomes is irrelevant)
Maybe meiosis evolved solely as a way to purge
deleterious recessive mutations from the genome
because it reveals recessive alleles in haploid
progeny of heterozygous individuals.
Maybe you had meiosis evolving and then
“surviving” haploid products went through
endomitosis?
An interesting aside…
Spo11 is a gene shared with prokaryotic forebearsDNA manipulating enzyme (is in almost all
eukaryotes)
It introduces the DNA double strand breaks that
provoke meiotic recombination (it is from an
ancestral Archaeal topoisomerase).
Im other words it is a topoisomerase that lost its
ability to re-ligate DNA.
What does meiosis do so beautifully?
• Reduces ploidy number (tossing chromosomes
like the fungi do does this too but….)
• Purges alleles and unmasks advantageous but
recessive alleles
• Generates complete sets of chromosomes
rather than incomplete or chaotic sets
• Generates recombinant offspring via
independent assortment and crossing over
And what about…circular to linear
chromosomes…???
Probably occurred hand in hand with evolution of
meiosis because crossing over between two
circular chromosomes generates a circular
dicentric that is unstable.
During Meiosis…Remember 2N goes to 4N which
goes to 2N and then 1N.
For Meiosis to evolve what do you need?
How is it different from mitosis?
Need….
Recombination to occur between homologous
chromosomes and not just between sister
chromatids (a chromosome and its copy).
Replication of genetic material must occur during
Meiosis I BUT not Meiosis II
Need sister chromatids that hang on to each other
through Meiosis I and into Meiosis II
(This material is on part of page 5 and most of 6)
More on Fusion
Haploid cells have to fuse! (but cannot just go
around fusing randomly all the time=chaos)
“Indiscriminate cell-cell fusions generate
disparate chromosome complements that are
toxic to a successful meiosis.”
So need to recognize other haploid cells and
fuse with them selectively….
We know that prokaryotes recognize self and
others readily…
Biofilms, quorum sensing, secrete identifying
extracellular matrix materials and small molecules
Receptor mediated perception then triggers signal
cascades that modulate growth and metabolism.
So probably like-like haploid cell adherence triggered
intracellular signals that elicited the conditions for
cell-cell fusion.
Pre-zygotic sexual recognition systems
existed!Pheromones or cell surface molecules
stimulate fusion
Skip to page 12!
Transmission of Genomes
“The stages in the domestication of a
proteobacterium into a mitochondrion ….are not
known.”
But there is this cool Amoeba (Paulinella) that has
a “cyanelle” that is sort of a protochloroplast.
• Been there 60 million years
• 75% reduction in size of genome and 1% of the
amoeba’s genes are from the endosymbiont
Already in place in this system is the mechanism
to coordinate replication of the cyanelle’s DNA
with the replication of the amoeba’s DNA.
Mitosis yields two daughter cells each of which
has 2 cyanelles.
Wild cyanobacteria of this species reproduce
really quickly (6-12 hrs doubling time).
Amoebas of this species only reproduce every 1-3
days.
SO CONTROL of growth must have evolved early!
How many endosymbiont cells (how many
mitochondria for ex) does a typical eukaryote
have??
Varies widely but tends to be consistent in a
particular organism or cell type.
Sexual reproduction then becomes not just a “how
do you manage copies of your own genome (N,
2N,4N)” problem but a “what do you do with the
endosymbionts” problem.
In eukaryotes almost universally UP….meaning?
• Often organelles from one parent are tagged and
disposed of.
• “In most modern egg/sperm systems, the egg can
have millions of organelle genomes, whereas
sperm are either stripped of organelles or their
organelles are destroyed by the zygote.”
• This guards against heteroplasmy the presence of
two or more different organelle genomes in the
same organism.
• When mice were manipulated to have two
different mitochondrial genomes they showed
developmental and cognitive defects!
There are some exceptions in yeasts (mitochondria)
and a kind of algae (chloroplasts) sometimes have
do receive organelles from both parents but they
end up segregating them into different cells.
BUT there are a couple species that have biparental
inheritance of organelles…clover, passionflowers
and mussels….do this??
Once in place “the evolution of eukaryotic sex
has featured countless prezygotic and
postzygotic variations, the outcome being the
segregation of panmictic populations into
distinct species..”