Microsatellites that violate Chargaff's second parity rule have base order-dependent asymmetries in the folding energies of complementary DNA strands and may not drive speciation

Abstract

Models for meiotic recombination based on Crick's “unpairing postulate” require symmetrical extrusion of stem–loop structures from homologous DNA duplexes. The potential for such extrusion is abundant in many species and, for a given single-strand segment, can be quantitated as the “folding of natural sequence” (FONS) energy value. This, in turn, can be decomposed into base order-dependent and base composition-dependent components. The FONS values of top and bottom strands in most Caenorhabditis elegans segments are close, as are the corresponding base order-dependent and base composition-dependent components; any discrepancies are in the base composition-dependent component. This suggests that the strands would extrude with similar kinetics. However, interspersed among these segments and at the ends of chromosomes (telomeres) are segments containing short tandem repeats (microsatellites) which, by virtue of their high variability, have been postulated to inhibit the pairing of homologous chromosomes and hence drive speciation. In these segments, there are usually wide discrepancies between the FONS values of top and bottom strands, mainly attributable to differences in base order-dependent components. Analyses of artificial microsatellites of different unit sizes and base compositions show that this asymmetrical distribution of folding potential is greatest for microsatellites when the units are short and violate Chargaff's second parity rule. It is proposed that when there is folding asymmetry, recombination proceeds by special, strand-biased, somatic mechanisms analogous to those operating with Chi sequences in Escherichia coli. If meiotic recombination in the germ-line requires extrusion symmetry, then a general inhibitory influence of microsatellite-containing segments could mask the antirecombinational influence of their variability. Thus, microsatellites may not have driven speciation.