Evolution and genetic population structure of prickly lettuce (Lactuca serriola) and its RGC2 resistance gene cluster

Genetic structure and diversity of natural populations of prickly lettuce (Lactuca serriola) were studied using AFLP markers and then compared with the diversity of the RGC2 disease resistance gene cluster. Screening of 696 accessions from 41 populations using 319 AFLP markers showed that eastern Turkish and Armenian populations were the most diverse populations and might be located in the origin and center of diversity of L. serriola. Screening 709 accessions using the microsatellite MSATE6 that is located in the coding region of most RGC2 homologs detected 366 different haplotypes. Again, the eastern Turkish and Armenian populations had the highest diversities at the RGC2 cluster. The diversities at the RGC2 cluster in different populations were significantly correlated with their genomewide diversities. There was significant variation of copy number of RGC2 homologs in different populations, ranging from 12 to 22 copies per genome. The nucleotide diversities of two conserved lineages (type II) of RGC2 genes (K and L) were not correlated with diversities calculated using the MSATE6 or AFLP data. We hypothesize that the high genomewide diversity and diversity of the RGC2 cluster in eastern Turkish and Armenian populations resulted from high abiotic and biotic stresses in the regions of origin of L. serriola.