1. Why Sex?

2. What is Sex? Fusion of receptive nuclei
Requires that cells find each other and then fuse (compatible mating types)
Requires some method of reduction division in which homologs associate

4. Types of Eukaryote Lifecycles

5. Rotifer Life Cycle

6. Haplontic and Diplontic

7. Isomorphic Alternation of Generation

8. Heteromorphic Alternation of Generation

9. Life Cycle of Aspergillus
Has asexual, sexual, and parasexual cycles

10. Reproduction by Cloning
Rapid and effective increase in cell number and genome

11. Reproduction by Sex
Half genes transferred to offspring. Cells fuse (2 make 1)

12. Whose Fitness?
Charles Darwin: the importance of sexual reproduction is hybrid vigor (1859 & 1871)
Thus, the individual benefits
Product of natural selection
August Weisman: the importance of sexual reproduction is to eliminate deleterious mutations (1905)
Benefits populations

13. Muller’s Ratchet
Accumulation of deleterious mutations in a clonal population decreases fitness (H. J. Muller)

14. Red Queen Hypothesis
Evolutionary arms race that clonal populations cannot win (William Hamilton)

15. Cloning like purchasing 100 lottery tickets, all with the same number
Not different from parents
Unable to respond to changes in environment (including parasites)
Recombination allows sexual reproduction to be like purchasing 50 lottery tickets, each with a different number
Always different from parents
Elimination of deleterious mutations, inactivated genes caused by jumping genes, and ineffective chromosomal ensemble

16. Recombination
All daughter chromosomes are unique

17. All eukaryotes have genes for meiosis
Thus, the Last Common Eukaryotic Ancestor must have been meiotic with the potential for sexual reproduction

18. Origin of Meiosis
Initially, strong pressure to maintain a functioning genome.
That required the elimination of excess chromosomes from aneuploidy and polyploidy
Being diploid is the only way a cell can have the minimal number of complementary chromosomes.
Meiosis evolved as a sorting division in which complementary chromosomes associate

19. Origin of Recombination
Likely occurred by delaying the separation of homologs by failure to digest the cohesion proteins binding chromosomes (Tom Cavalier-Smith)
Thus, homologs held together for extended period
The binding proteins then fooled the cell mechanism into having a second division to separate the replicates chromosomes

20. Who Benefits? Nick Barton (Univ of Edinburgh)
Sarah Otto (Univ of British Columbia)
Models of evolution. Sex benefits BOTH individuals and populations