Transcript Lecture 28
Disadvantages of Asexual Reproduction continued • 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 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? • • • • • 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.