Download Origin of Sexual Reproduction

Origin of Sexual Reproduction
Lukas Schärer
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Evolutionary Biology
Zoological Institute
University of Basel
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12.10.2016
Advanced-level Evolutionary Biology
What is sexual reproduction?
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from various websites
What is sexual reproduction?
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the life cycle of the green algae Chlamydomonas
Summary
• what is sexual reproduction?
• the historical sequence
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• bacterial ‘mitosis’
• bacterial recombination
• mitosis
• syngamy
• meiosis
• mating types
• anisogamy
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The historical sequence
• sexual reproduction involves many different phenomena
• the historical sequence of evolutionary innovations probably is
• asexual reproduction (binary cell division, mitosis)
• limited recombination in bacteria
• fusion between genetically dissimilar cells (syngamy)
• meiosis (with segregation and recombination)
• mating types
• anisogamy and gender
• when we think of sexual reproduction we usually think of organisms
that have gone through this entire sequence
• but this does not help us to understand what the origin of sexual
reproduction is
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Bacterial ‘mitosis’
• cell division by bacterial ‘mitosis’ in prokaryotes
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from Fogel & Waldor 2006
Bacterial recombination
• plasmids can be transferred from one bacterium to another during
bacterial conjugation
• but this ability is often a function of the exchanged material itself
• could this process be of parasitic origin?
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from Stearns & Hoekstra 2005
Mitosis
• cell division by mitosis in diploid eukaryotes
mitosis
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from Stearns & Hoekstra 2005
Syngamy
• during syngamy two cells fuse completely
• this brings genetic material of (usually) different individuals together
• eventually requires a process that allows this information to be
separated again in a fair way
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from Stearns & Hoekstra 2005
Meiosis
• a comparison between meiosis and mitosis
meiosis
mitosis
??
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from Stearns & Hoekstra 2005
Did syngamy or meiosis evolve first?
• endomitosis and one-step meiosis
• occurs in some single-celled eukaryotes
• e.g. dinoflagellates and sporozoans
• advantageous in a variable environment
• diploidy may protect against DNA damage
(or rather, it may facilitate DNA repair)
• haploidy may allow faster growth
• recombination (crossing-over) probably
offers no advantage
• unknown if it occurs in extant systems
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from Maynard Smith & Szathmáry 1995
DNA repair and recombination
• gene conversion is involved
in DNA repair and it uses
cellular mechanisms that are
similar to recombination
• so recombination could have
originated from DNA repair
mechanisms
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from Devoret in Michod & Levin 1988
endomitosis
one-step meiosis
>1 endomitoses
followed by >1
meioses
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from Keeling & al. 2005
Did syngamy or meiosis evolve first?
• endomitosis and one-step meiosis
• occurs in some single-celled eukaryotes
• e.g. dinoflagellates and sporozoa
• advantageous in a variable environment
• diploidy may protect against DNA damage
(or rather, it may facilitate DNA repair)
• haploidy may allow faster growth
• recombination (crossing-over) probably
offers no advantage
• unknown if it occurs in extant systems
• syngamy evolves
• due to an advantage of heterosis (hybrid
vigour) or possibly selfish genetic elements
• now recombination may offer an advantage
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from Maynard Smith & Szathmáry 1995
Selfish genetic elements and the evolution of sex
• in an asexual species a selfish genetic element can only spread
within the genome of its host
• so it would benefit from inducing its host to fuse with another cell
• and to spread to the uninfected genome
• this is true whether the gene has a positive, neutral of negative effect on the host
• the resulting diploidy may be advantageous for the fused cells
because it allows DNA repair and can lead to heterosis
• but the selfish genetic element only benefits if the cells divide again
• the division may allow the host to get rid of mutations
• via segregation and recombination
• such selfish genetic elements may only have been important at the
origin of sexual reproduction and are probably not responsible for
its current maintenance
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following Hickey & Rose in Michod & Levin 1988
Meiosis
• segregation and recombination
• segregation increases with chromosome number
• chromosome number varies dramatically
• n=1 in nematode (Parascaris)
• n=127 in hermit crab (Eupagurus)
• taxa with uniform chromosome number • n=13 in most dragonflies
• n=18 in most snakes
• taxa with variable chromosome number
• 2n=7 in males and 2n=6 in females of Muntiacus
muntjac vaginalis
• 2n=46 in Muntiacus reevesi
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from Stearns & Hoekstra 2005
Did syngamy or meiosis evolve first?
• endomitosis and one-step meiosis
• occurs in some single-celled eukaryotes
• e.g. dinoflagellates and sporozoa
• advantageous in a variable environment
• diploidy may protect against DNA damage
(or rather, it may facilitate DNA repair)
• haploidy may allow faster growth
• recombination (crossing-over) probably
offers no advantage
• unknown if it occurs in extant systems
• syngamy evolves
• due to an advantage of heterosis (hybrid
vigour) or possibly selfish genetic elements
• now recombination may offer an advantage
• two-step meiosis evolves
• may limit the spread of sister-killer alleles
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Maynard Smith & Szathmáry 1995
2-step meiosis
Sister-killer alleles
1-step meiosis
or
reductional division
(no recombination)
kill here
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equational division
(with recombination)
kill here
following Haig & Grafen 1991
2-step meiosis
Sister-killer alleles
1-step meiosis
or
reductional division
(no recombination)
equational division
(with recombination)
kill here
or kill here
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following Haig & Grafen 1991
So much has already happened and still no real ‘sex’
• mitosis
• two-step meiosis
• segregation and recombination
• syngamy
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Mating types
• initially gametes were probably isogamous and
could have fused with all other conspecific gametes
(including self gametes)
⊕
⊖
• but if the aim is to exchange genes, then selfing is not helpful
• this may have selected for an incompatibility system
• avoid fusing with yourself
• more mating types would allow fusing with gametes
from more partners
• surprisingly, we often find that there are only two
mating types (usually called plus and minus)
⊖
⊕
• this means that every second cell one encounters is a
unsuitable fusion partner
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modified from http://biodidac.bio.uottawa.ca
Mating types
• genomic conflict with symbionts
(e.g. mitochondria and chloroplasts)
may explain why there are often
only two mating types
• a possible solution is uniparental
inheritance of cytoplasmic elements
• usually only from one mating type
• in Chlamydomonas mitochondria are
inherited by the ⊖ and chloroplasts
are inherited by the ⊕ mating type
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Mating types
• how to evolve two mating types?
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from Maynard Smith and Szathmary 1995
Mating types
• hypotrich ciliates have two types of
sexual processes
• conjugation with multiple mating
types, but without the exchange of
any cytoplasm
• complete fusion of cells with only
two mating types
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Anisogamy: the origin of the male and female sex
• sexual reproduction involves the fusion
between two haploid germ cells to
form a diploid zygote
• sex can occur via isogamy or anisogamy
• from iso ‘equal’ and gamos ‘marriage’
• most organisms we are familiar with
are anisogamous
• but both types exist even today within,
for example, the green algae
• isogamy is very likely the ancestral condition
isogamous and anisogamous gametes
in the green algae
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modified from http://biodidac.bio.uottawa.ca
Anisogamy: the origin of the male and female sex
• the male sex makes the small germ cells and female sex makes the
large germ cells
• the ‘male’ and ‘female’ sexes only exist in anisogamous species
• sexual reproduction is possible without the ‘sexes’
bird eggs and sperm
eggs and sperm of Drosophila bifurca
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from Bjork & Pitnick 2006
Anisogamy: the origin of the male and female sex
• evolution of anisogamy
• disruptive selection can lead to the
evolution of a cell type that specialises
in swimming (small) and another that
specialises in providing energy (large)
• selection against fusion between two
small cells (not viable)
• fusion between large cells rare (and
also disadvantageous)
• the small cells (or the individuals
making them) compete for access
to the large cells (or the
individuals making them)
• ➙ sexual selection
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after Parker 1982
Anisogamy: the origin of the male and female sex
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from Lessels et al. 2009
Anisogamy: the origin of the male and female sex
• the male sex can be seen as a parasite of
the investment by the female sex
• the evolution of anisogamy involves a primordial
sexual conflict
• the origin of the twofold cost of sex
• the ‘main’ cost of sexual reproduction only
appeared after evolution of anisogamy, so sex
initially may not have needed a big advantage
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from http://biodidac.bio.uottawa.ca
Summary
• what is sexual reproduction?
• the historical sequence
!
• bacterial ‘mitosis’
• bacterial recombination
• mitosis
• syngamy
• meiosis
• mating types
• anisogamy
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Literature
• Mandatory Reading
• Pages 148-162 in Chapter 9 on ‘The origin of sex and the nature of species’ in
Maynard Smith & Szathmáry (1995). The Major Transitions in Evolution. Freeman.
• Suggested Reading
• Pages 177-186 of Chapter 8 on ‘The evolution of sex’ in Stearns & Hoekstra
(2005). Evolution: An Introduction. 2nd Edition. Oxford University Press
• Chapter 10 by Hickey & Rose on ‘ The role of gene transfer in the evolution of
eukaryotic sex’ in Michod & Levin (1988). The Evolution of Sex. Sinauer.
• Books
• Michod & Levin (1988). The Evolution of Sex. Sinauer.
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