ИСТИНА

The distribution of variation in a genome is the result of an intricate interplay between mutation, recombination, selection, and demography and is influenced by the reproductive system and ecological constraints. Sequencing the Arabidopsis thaliana model plant genome facilitate studies of plant evolution over the past decade. Important observations have emerged from the analyses of several loci that have been subjected to comparative sequencing in this cruciferous weed: (1) a number of genes have alleles that fall into two distinct classes (allelic dimorphism), and (2) there is more recombination than might be expected, given that A. thaliana is a selfer. The AtPrx53 gene of Arabidopsis thaliana encoded for highly active anionic peroxidase demonstrated both of these features. The DNA polymorphism in the AtPrx53 gene which encodes anionic peroxidase was analyzed in 20 Arabidopsis thaliana accessions. There was two divergent sequence types (Col and Dj-like haplotypes) in the AtPrx53 gene that differ by 2 indel and 16 non-singleton nucleotide polymorphisms including 5 nucleotide polymorphic sites responsible for 4 deduced amino acid replacements. Two of the amino acid substitutions (Phe/Ser180and Asp/Asn270) could be responsible for the difference in electrophoretic mobility of AtPrx53 allozymes. (Kupriyanova et al., 2006). Two distinct haplotypes were present at intermediate frequencies, resulting in positive Tajima’s D and Fu and Li’s D* values. The directions of the Tajima’s D and Fu and Li’s D* tests statistics, as well as the results of the linkage disequilibrium analysis show deviations from the neutral-equilibrium model. In contrast to previously noted evidence of moderate reciprocal recombination in Adh (Innan et al., 1996) and other Arabidopsis genes (Kuittinen et al., 2000), the estimate of Rm (minimum number of recombination events) for Atprx 53 is zero. The revealed pattern of polymorphism at the AtPrx53 peroxidase and the strong haplotype structure in the absence of detectable recombination across AtPrx53 may be interpreted as evidence of balancing selection on this locus. It is suggested that the Col and Dj haplotypes may have originated from locally adapted ancestral populations and may be currently maintained by local selection on alternative alleles of AtPrx53 or nearby locus despite the widespread dispersal of this species. In contrast to these data, a molecular mapping analysis provided evidence that the AtPrx53 represents recombination "hot spot", where recombination rates per kb are 500 times higher than the genome average. It was not clear how the fact of high recombination rate could be combined with strict allelic dimorphism of the Atprx53 gene. In fact, for the majority of other genes existence of reciprocal recombinants between two haplotypes is shown. Depending on the way the Holliday Junction is resolved, recombination may result either in reciprocal recombination or in gene conversion. Reciprocal recombination affects a series of homologous loci downstream of the recombination break point. Gene conversion, on the other hand, leads to the alteration of single segments only. The cloning and DNA sequencing of recombinant chromosomes has shown that the abnormal high frequency of recombination is the result of conversion mechanisms. The recombinant alleles have mosaic structure. More over, mitotic gene conversion that results in the creation of genetically altered somatic cells has been documented. The analysis of codominant DNA marker’s segregation demonstrated that conversion events is conduct to elimination of heterozygote state of hybrid individuals, the share of homozygotes for maternal alleles was increased. The hypothesis about a role of gene conversion in maintenance of allelic dimorphism of Atprx53 gene is developed.