Sequence of the Mitochondrial Genome of Lactuca virosa Suggests an Unexpected Role in Lactuca sativa’s Evolution

The involvement of the different Lactuca species in the domestication and diversification of cultivated lettuce is not totally understood. Lactuca serriola is considered as the direct ancestor and the closest relative to Lactuca sativa, while the other wild species that can be crossed with L. sativa, Lactuca virosa, and Lactuca saligna, would have just contributed to the latter diversification of cultivated typologies. To contribute to the study of Lactuca evolution, we assembled the mtDNA genomes of nine Lactuca spp. accessions, among them three from L. virosa, whose mtDNA had not been studied so far. Our results unveiled little to no intraspecies variation among Lactuca species, with the exception of L. serriola where the accessions we sequenced diverge significantly from the mtDNA of a L. serriola accession already reported. Furthermore, we found a remarkable phylogenetic closeness between the mtDNA of L. sativa and the mtDNA of L. virosa, contrasting to the L. serriola origin of the nuclear and plastidial genomes. These results suggest that a cross between L. virosa and the ancestor of cultivated lettuce is at the origin of the actual mitochondrial genome of L. sativa.

Robustness and accuracy of Maxent niche modelling for Lactuca species distributions in light of collecting expeditions

Niche modelling software can be used to assess the probability of detecting a population of a plant species at a certain location. In this study, we used the distribution of the wild relatives of lettuce (Lactuca spp.) to investigate the applicability of Maxent species distribution models for collecting missions. Geographic origin data of genebank and herbarium specimens and climatic data of the origin locations were used as input. For Lactuca saligna, we varied the input data by omitting the specimens from different parts of the known distribution area to assess the robustness of the predicted distributions. Furthermore, we examined the accuracy of the modelling by comparing the predicted probabilities of population presence against recent expedition data for the endemic Lactuca georgica and the cosmopolitan Lactuca serriola. We found Maxent to be quite robust in its predictions, although its usefulness was higher for endemic taxa than for more widespread species. The exclusion of occurrence data from the perceived range margins of the species can result in important information about local adaptation to distinct climatic conditions. We discuss the potential for enhanced use of Maxent in germplasm collecting planning.

Three Combined Quantitative Trait Loci from Nonhost Lactuca saligna Are Sufficient to Provide Complete Resistance of Lettuce Against Bremia lactucae

The nonhost resistance of wild lettuce (Lactuca saligna) to downy mildew (Bremia lactucae) is based on at least 15 quantitative trait loci (QTL), each effective at one or more plant developmental stages. We used QTL pyramiding (stacking) to determine how many of these QTL from L. saligna are sufficient to impart complete resistance towards B. lactucae to cultivated lettuce, L. sativa. The alleles of four of the most promising QTL, rbq4, rbq5, rbq6+11, and rbq7 are effective at both the young and adult plant stages. Lines with these four QTL in all possible combinations were generated by crossing the respective backcross inbred lines (BIL). Using the 11 resulting lines (combiBIL), we determined that combinations of three QTL, rbq4, rbq5, and rbq6+11, led to increased levels of resistance; however, one QTL, rbq7, did not add to the resistance level when combined with the other QTL. One line, tripleBIL268, which contains the three QTL rbq4, rbq5, and rbq6+11, was completely resistant to B. lactucae at the young plant stage. This suggests that these three QTL are sufficient to confer the complete resistance of the nonhost L. saligna and any additional QTL in L. saligna are redundant. Histological analysis of B. lactucae infection in L. saligna, the BIL, and the combiBIL 48 h after inoculation revealed different microscopical phenotypes of resistance. The QTL differed with respect to the stage of the infection process with which they interfered.

Potential Sources of Genetic Resistance in Lactuca spp. to the Lettuce Aphid, Nasanovia ribisnigri (Mosely) (Homoptera: Aphididae)

Lettuce aphid, Nasanovia ribisnigri (Mosely) (Homoptera: Aphididae), is an economically important pest of lettuce (Lactuca sativa L.). High-level resistance found in a wild relative, Lactuca virosa L. accession PIVT 280, is conditioned by the Nr gene, which has been transferred to European cultivars and is being commercially transferred to U.S.-adapted cultivars. New sources of resistance to guard against possible resistance-breaking strains of lettuce aphid were sought in a greenhouse screening of 1203 accessions of lettuce, which included 1047 accessions of L. sativa L., seven accessions of Lactuca perennis L., 18 accessions of Lactuca saligna L., 125 accessions of L. serriola L., and six accessions of L. virosa L. Two new and potentially unique sources of resistance to lettuce aphid were found in L. serriola accession PI 491093 and L. virosa accession PI 274378. The genetic relationship of resistance in PI 491093 and PI 274378 with that in PIVT 280 remains to be determined.

Analysis of wild Lactuca accessions: conservation and identification of redundancy

Germplasm accessions of wildLactucaspecies are maintained worldwide inex situcollections as gene reservoirs for quality and disease resistance traits for cultivated lettuce. Accessions of 12Lactucaspecies from 6 genebanks were compared via morphological characterization and AFLP (Amplified Fragment Length Polymorphism)-based profiling to estimate the extent of duplication. A method of assessing redundancy within very similar, but not identical accessions, is proposed, based on 352 polymorphic AFLP products. Seven duplication groups showed a high level of AFLP similarity, and one pair ofLactuca salignaaccessions displayed identical AFLP profiles. In several cases, the morphological assessment indicated that a taxonomic reclassification of accessions was necessary. Candidate duplicates were identified using population parameters and inter- and intra-accession variability. The implications of these findings on the conservation of wild species are discussed.

Leafminer Resistance in Lettuce

Leafminer (Liriomyza langei Frick) is a major insect pest of many important agricultural crops including lettuce (Lactuca sativa L.). The goals of this study were to evaluate lettuce genotypes for resistance to leafminer and to estimate heritabilities of three leafminer-resistant traits. Forty-six lettuce genotypes were evaluated in two tests in insect cages. Wild species (Lactuca serriola L., Lactuca saligna L., and Lactuca virosa L.) had significantly fewer leafminer stings than cultivated lettuce (L. sativa) in both tests. PI 509525 (L. saligna) had few leafminer stings and no flies emerged. Leaf (leaf and romaine) lettuce also showed significantly less stings than head (crisphead and butterhead) types, while differences between leaf and romaine lettuces, and between crisphead and butterhead types were nonsignificant. Broad-sense heritability for number of stings per unit leaf area was relatively high, averaging 65% over the two tests. Heritabilities for egg-hatching period and flies per plant were 10% and 15%, respectively. Stings per unit leaf area from the two tests were highly correlated (r = 0.828), suggesting that resistance was stable over different plant ages and against different pressures of leafminer. These results suggest that genetic improvement of cultivated lettuce for leafminer resistance is feasible.