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Sex is Extremely Interesting! Jeremy J. Kirchman, Ph.D. Curator of Birds, New York State Museum jkirchma@mail.nysed.gov. What is Sex?. What is sex according to the typical student?. What is sex according to the typical student?. What is sex according to the typical student?.
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Sex is Extremely Interesting!Jeremy J. Kirchman, Ph.D.Curator of Birds, New York State Museumjkirchma@mail.nysed.gov
Two Views of Evolution of Sex: Origin and Maintenance • Like all “Origin” questions, the origin of sex is controversial and very hard to study. • Maintenance and evolution of sexual reproduction is the subject of LOTS of research.
Competing Theories for the Origin of Sexual Reproduction 1. Viral Eukaryogenesis: Eukaryotic cells arose from a combination of a pox-like virus and a bacterium. The virus incorporated genetic material from the bacterium and took over the role of information storage for the cell. Meiosis arises because of selective pressure on the virus to spread horizontally throughout the population by cell-to-cell fusion. Two cells infected with related but different viruses fused because they recognised each other as uninfected. After the fusion of the two cells, incompatibilities between the two viruses result in a meiotic-like cell division.
Competing Theories for the Origin of Sexual Reproduction 2. Neomuran Revolution : The cellular machinery for recombination evolved in the ancestor of the archea and the eukaryotes. Later meiosis evolves in the eukarya when mechanics were adapted to the presence of nucleosomes. The archaea maintain simple recombination machinery while eukaryotes developed true cell fusion and ploidy cycles. Both bouts of mechanical evolution were motivated by similar selective forces: the need for accurate DNA replication without loss of viability.
Competing Theories for the Origin of Sexual Reproduction 3. God did it. “Sexual reproduction and recombination are part of God's plan, since they limit the expression of mutated Functional and physical units of heredity passed from parent to offspring. In addition, God created humans as males and females for the expressed purpose of marriage and unity within the body of Christ, as fellow servants and heirs in Him.”
Why Have Sex? Evolution and maintenance of sexual reproduction August Weismann, 1889, the purpose of sex was to generate genetic variation. Charles Darwin, 1889, the effects of hybrid vigor “is amply sufficient to account for the ... genesis of the two sexes.”
Each generation a female can produce 4 offspring. An asexual female would produce 2x as many grand children as a sexual female. What is the cost of Sex? • John Maynard Smith (1978) – Proposes a mathematical model for “the two-fold cost of producing males”. Maynard Smith hows that asexual reproduction would quickly replace sexual reproduction. Asexual populations grow twice as fast as sexual populations.
Long-term Fitness Benefits of Sex Short-term Fitness Benefits of Sex Cost of sex can’t be avoided. Benefits must outweigh the costs because sex is ubiquitous.
Long-term benefit: Muller’s Ratchet Over time, the population experiences a greater genetic load of accumulated deleterious mutations. Sex and recombination can help prevent Muller’s ratchet from turning, by allowing new combinations of alleles. The flip side of Muller’s Ratchet is that beneficial combinations are created as well.
Recent evidence for Muller’s RatchetAccelerated Mutation Accumulation in Asexual Lineages of a Freshwater SnailNeimana et al. 2009 Molecular Biology and EvolutionHigh levels of deleterious mutations in recently established asexual lineages of Potamopyrgus antipodarum, shows there may even be short term benefits of sex from the purging of bad genes.
Short term benefits of sex? • Are there any immediate, adaptive advantages of producing genetically variable offspring?
Short term benefits of sex? Are there any immediate, adaptive advantages of producing genetically variable offspring? Environmental unpredictablity - Sex is a bet-hedging strategy (George C. Williams 1975). Parents cannot anticipate which genotype will by most successful. All parents send many seeds, creating competition between siblings. Natural selection therefore favors parents which can produce a variety of offspring. Williams’ “Elm Forest”. Success of a seed in newly opened cell depends on its genotype.
Short term benefits of sex? Are there any immediate, adaptive advantages of producing genetically variable offspring? Parasites, sex and the Red Queen Hypothesis (W. D. Hamilton 1980). In the arms race between hosts and parasites, there is constant, strong selection for new gene combinations.
Who’s doing it and who’s not doing it? Twilight Vampires: NOT
Who’s doing it and who’s not doing it? Twilight Vampires: NOT Tiger: DOING IT
Doing it: Nearly all eukaryote species. Many plants have sexual and “vegetative” reproduction. Plant sexual systems are alternation of generations between sporophyte and gametophyte. Fungal lineages are based on the various ways they package spores. Many also reproduce “vegetatively”. Many invertebrates have very whacky reproductive systems that include production of sexual and asexual individuals.
Not doing it: Curious cases of asexual reproducers. All these examples support the theory that sex is costly and will be avoided under certain special circumstances.
Aphids and Daphnia: Cyclical Parthenogenosis (Greek: parthenos, "virgin", + genesis, "creation") Spring females give live birth to clones. Multiple generations of 10 days (Daphnia), 20-40 days (Aphids) leads to billions of descendants! In fall, change in photoperiod leads females to produce males and females. Males are genetically identical to their mothers but with one less sex chromosome. These sexual aphids lack wings and even mouthparts! In Daphnia the males are tiny. Fall sexuals mate and females lay eggs. Overwintering eggs all hatch in spring as females.
Family Teiidae, Genus Cnemidophorus Whiptail lizards 30% of the genus (20 of 60 sp.) are fully parthenogenetic; All females. Parthenogenetic species result from hybridization among species. Mating behaviors are still seen. Females mount each other, triggering sex hormones. Lizards who act out the courtship ritual have greater fecundity than those kept in isolation!
Phylum Rotifera, Class Bdelloidea Bdelloid Rotifers: Psuedocoelomate animals 0.1 - 3.0 mm long 450 freshwater species. All are asexual Desiccation tolerant up to nine years!
Bdelloid Rotifers are models for studying the evolution of sex • 50 million year fossil record of high “species” diversity. • How does a purely asexual lineage persist for millions of years in the face of changing environments and evolving parasites?
Science 2000. Genetic architechture in bdelliod rotifers is different.
Science 2007. Bdelliod rotifers adopt shreds of genetic material from other species in the same dried-up puddle.
Science 2010. Bdelliod rotifers avoid the parasite arms race by desiccation and dispersal.
Science 2010. Bdelliod rotifers avoid the parasite arms race by desiccation and dispersal. After 28 and 35 days, 85 and 90.5%, respectively, of rehydrated populations remained fungus-free
Science 2010. Bdelliod rotifers avoid the parasite arms race by desiccation and dispersal. After 7 days, 40% of wind-dispersed populations remained fungus-free compared to 0% of wet dispersed.
Final Thoughts • The evolution of sex is the proper framework for studying meiosis. • Sex is a good topic for discussing evolutionary costs and benefits; Even very costly structures or behaviors will evolve if the benefits are high enough. • Lots of tangent topics for students to research and explore: sexual selection, mating system evolution, conflict between males and females.
