Transcript Why Sex? - Susquehanna University

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