Lecture 28

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Transcript Lecture 28

Disadvantages of Asexual Reproduction
continued
• Disadvantages of asexual reproduction continued
• 3. Müller’s Ratchet (an hypothesis)
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• Asexual reproduction is like a ratchet mechanism.
• Most mutations are deleterious
• These should accumulate and result in a one-way decline
in fitness.
• Counter argument: diploidy (and especially triploidy) in
asexuals would reduce the effects of deleterious mutations.
• 4. Destabilizing hybridization.
• 4. The establishment of new parthenogenetic species will
be difficult.
• Parthenogenetic females can mate with male sexual
ancestors.
• This destabilizes a functional genome.
• Sterile hybrids are the usual product.
• Distributions of parthenogens should be allopatric to those
of gonochoristic congeners.
• But this is not the case.
• Destabilizing hybridization is presently taking place
between A. tesselata (P) and A. marmorata (G) at Arroyo
del Macho, Chaves County, NM.
• What are the effects?
Testing the hypothesis from a serendipitous discovery
Genus Cnemidophorus
Became genus
Aspidoscelis in 2002
5 entity guild
Aspidoscelis exsanguis
A. inornata
Aspidoscelis tesselata
Hybrid
A. marmorata
Would it be easy to overlook a hybrid?
10 hybrids
examined
electrophoretically
A numerical
advantage based
on micro-habitat
utilization
Product of nine
separate hybridization
A reproductive advantage
The gonochoristic species is morphologically more variable
• GAB, COS, LSG (scale count characters)
• Variation of all three included in a composite
character: the 1st principal component from a
PCA (68% of variation).
• C. tesselatus: s2 = 0.519 (N =10*)
• C. tigris:
s2 = 1.211 (N = 9+)
• * those with genetic data (34 loci)
• + females
• The Frozen niche hypothesis
• Like its genotype, the niche of a parthenogen should be
“frozen” by the hybridization event.
• Parthenogenesis should be disadvantageous.
• But at Macho Draw the parthenogen appears to have a
greater advantage.
• General-purpose genotype hypothesis (Lynch)
• Genomes from two ecologically successful sexual species
give parthenogens greater ecological breadth than either
progenitor species. Seen in breadth of habitat use.
• General purpose genotypes would be conserved by
parthenogenesis (not broken up by recombination).
Why is sex so prevalent?
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Linkage disequilibrium is established under conditions of
strong selection, drift, and asexual reproduction.
Tends to counter linkage equilibrium
The “Red queen” hypothesis
Host/parasite relationships
Available evidence: genotypic variability reduces parasitic
loads.