Molecular diversity at the major cluster of disease resistance genes in cultivated and wild Lactuca spp.

Abstract

Diversity was analyzed in wild and cultivated Lactuca germplasm using molecular markers derived from resistance genes of the NBS-LRR type. Three molecular markers, one microsatellite marker and two SCAR markers that amplified LRR-encoding regions, were developed from sequences of resistance gene homologs at the main resistance gene cluster in lettuce. Variation for these markers were assessed in germplasm including accessions of cultivated lettuce, Lactuca sativa L. and three wild Lactuca spp., L. serriola L., L. saligna and L. virosa L. Diversity was also studied within and between natural populations of L. serriola from Israel and California; the former is close to the center of diversity for Lactuca spp. while the latter is an area of more recent colonization. Large numbers of haplotypes were detected indicating the presence of numerous resistance genes in wild species. The diversity in haplotypes provided evidence for gene duplication and unequal crossing-over during the evolution of this cluster of resistance genes. However, there was no evidence for duplications and deletions within the LRR-encoding regions studied. The three markers were highly correlated with resistance phenotypes in L. sativa. They were able to discriminate between accessions that had previously been shown to be resistant to all known isolates of Bremia lactucae. Therefore, these markers will be highly informative for the establishment of core collections and marker-aided selection. A hierarchical analysis of the population structure of L. serriola showed that countries, as well as locations, were significantly differentiated. These differences may reflect local founder effects and/or divergent selection.

Evolution and Genetic Population Structure of Prickly Lettuce (Lactuca serriola) and Its RGC2 Resistance Gene Cluster

“…Plant materials: Seeds from 993 individuals (accessions) of L. serriola populations were collected during the summers of 1993-1998 from 49 populations from 11 countries, with the majority included in previous studies (supplemental Table 1 at http:/ /www.genetics.org/supplemental/; Sicard et al 1999;Kuang et al 2006). Each collection was made from a single individual and subsequently treated as an independent accession.…”

“…Five to 10 seedlings from each accession were pooled to extract DNA representing the progenitor's genotype. DNA was extracted following the method of Sicard et al (1999). Each individual is considered homozygous because L. serriola is predominantly self-pollinated (Prince and Carter 1977).…”

“…The RGC2 cluster varies considerably in copy number, ranging from 10 to .30 copies in a haplotype (Kuang et al 2004). The RGC2 cluster in natural populations of L. serriola is very diverse, as shown by the numerous haplotypes identified by a microsatellite marker located in intron 3 of many RGC2 homologs (Sicard et al 1999).…”

Description of morphological characters of lettuce (Lactuca sativa L.) genetic resources

“…The application of molecular genotyping methods: RFLP (Restriction Fragment Length Polymorphism) (Kesseli et al 1991), RAPD (Random Amplified Polymorphic DNA) (Yamamoto et al 1994), AFLP (Amplified Fragment Length Polymorphism) (Hill et al 1996;Johnson et al 2000;Jeuken et al 2001;Jeuken, Lindhout 2004;Kitner et al 2008;Rajicic, Dehmer 2008), TRAP (Target Region Amplification Polymorphism) (Hu et al 2005), minisatellites and microsatellite fingerprinting or SSR (Simple Sequence Repeat) (Witsenboer et al 1997;Sicard et al 1999;Van de Wiel et al 1999) has contributed to the elucidation of various aspects of the taxonomy, variability and biodiversity of the genus. SSRs and AFLPs have also been used to characterize the entire lettuce collection of the Centre for Genetic Resources (CGN, Wageningen) (van Hintum et al 2003;Jansen et al 2006;Jansen, van Hintum 2007).…”

Marker-assisted optimization of an expert-based strategy for the acquisition of modern lettuce varieties to improve a genebank collection

“…For this purpose, the crop related wild gene pool is considered a rich source of novel resistance characters that may be introduced into a cultivated background (e.g. Bonnier et al 1992;Sicard et al 1999).…”