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Origin of Sexual Reproduction Lukas Schärer ! Evolutionary Biology Zoological Institute University of Basel 1 12.10.2016 Advanced-level Evolutionary Biology What is sexual reproduction? 2 from various websites What is sexual reproduction? 3 the life cycle of the green algae Chlamydomonas Summary • what is sexual reproduction? • the historical sequence ! • bacterial ‘mitosis’ • bacterial recombination • mitosis • syngamy • meiosis • mating types • anisogamy 4 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 5 Bacterial ‘mitosis’ • cell division by bacterial ‘mitosis’ in prokaryotes 6 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? 7 from Stearns & Hoekstra 2005 Mitosis • cell division by mitosis in diploid eukaryotes mitosis 8 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 9 from Stearns & Hoekstra 2005 Meiosis • a comparison between meiosis and mitosis meiosis mitosis ?? 10 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 11 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 12 from Devoret in Michod & Levin 1988 endomitosis one-step meiosis >1 endomitoses followed by >1 meioses 13 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 14 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 15 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 16 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 17 Maynard Smith & Szathmáry 1995 2-step meiosis Sister-killer alleles 1-step meiosis or reductional division (no recombination) kill here 18 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 19 following Haig & Grafen 1991 So much has already happened and still no real ‘sex’ • mitosis • two-step meiosis • segregation and recombination • syngamy 20 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 21 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 22 Mating types • how to evolve two mating types? 23 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 24 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 25 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 26 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 27 after Parker 1982 Anisogamy: the origin of the male and female sex 28 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 29 from http://biodidac.bio.uottawa.ca Summary • what is sexual reproduction? • the historical sequence ! • bacterial ‘mitosis’ • bacterial recombination • mitosis • syngamy • meiosis • mating types • anisogamy 30 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. 31