Transcript Lecture 28

Disadvantages of Asexual Reproduction
• Disadvantages of asexual reproduction continued
• 3. Müller’s Ratchet (an hypothesis)
• 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
• 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
A. marmorata
Would it be easy to overlook a hybrid?
10 hybrids
A numerical
advantage based
on micro-habitat
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?
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