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 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
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: s 2 = 0.519 (N =10*) C. tigris:s 2 = 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 loads.