why sex?. what is sex? fusion of receptive nuclei requires that cells find each other and then fuse...
TRANSCRIPT
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
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)
• 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
• 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