Why Sex?

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

Types of Eukaryote Lifecycles

Rotifer Life Cycle

Haplontic and Diplontic

Isomorphic Alternation of Generation

Heteromorphic Alternation of Generation

Life Cycle of Aspergillus

Has asexual, sexual, and parasexual cycles

Reproduction by Cloning

Rapid and effective increase in cell number and genome

Reproduction by Sex

Half genes transferred to offspring. Cells fuse (2 make 1)

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

Muller’s Ratchet

• Accumulation of deleterious mutations in a clonal population decreases fitness (H. J. Muller)

Red Queen Hypothesis

• Evolutionary arms race that clonal populations cannot win (William Hamilton)

• 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

Recombination

All daughter chromosomes are unique

• All eukaryotes have genes for meiosis• Thus, the Last Common Eukaryotic Ancestor

must have had sexual reproduction

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 the minimal number of complementary chromosomes.

• Meiosis evolved as a sorting division in which complementary chromosomes associate

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

Who Benefits?

• Nick Barton (Univ of Edinburgh)

• Sarah Otto (Univ of British Columbia)

• Models of evolution. Sex benefits BOTH individuals and populations