Unraveling the variation, phylogeny, and taxonomy of Lactuca spp. for germplasm management and breeding

This review describes the taxonomy, phylogeny, and diversity of wild Lactuca spp. and their relationship to cultivated lettuce (Lactuca sativa). The value of the different species as sources of useful variation for a range of economically important traits is detailed and the case is made for the conservation of more accessions in global genebanks. The integration of knowledge from research using the wild and domesticated species is highlighted. Lettuce (Lactuca sativa L.) is an annual species in the family Asteraceae (formerly Compositae). It is a widely cultivated leafy vegetable and one of the oldest domesticated crops. In addition to their high nutritional value, lettuce plants are aesthetically pleasing. A range of lettuce morphotypes has been developed to suit various production systems, meet the demands of the processing industry, and to satisfy consumer demand. Disease- and pest-resistant cultivars are a priority, as pesticide application is often not possible because of potential pesticide-related health risks due to lettuce's relatively short cultivation period. Wild Lactuca species closely related to lettuce were first utilized in breeding programs in the early twentieth century in USA and later in Europe. They continue to play a vital role in modern breeding as sources of resistance to important diseases and pests, tolerance to abiotic stresses (e.g., salinity, drought, temperature), and sources of variation in leaf taste, structure, and secondary-metabolite composition. The importance of wild Lactuca species in lettuce crop improvement extends beyond the close relatives of the crop. A thorough understanding of the taxonomy and phylogenetic relationships within the genus Lactuca provides insight into the history of domestication of lettuce, the management of genebank collections, and the utilization of germplasm in breeding and genetic studies. This knowledge aids identification and functional analysis of key genes and development of molecular markers for lettuce improvement. Current research supports the classification of L. serriola L. as the direct progenitor of cultivated lettuce, with the Middle East and Egypt identified as two centers of domestication. Lactuca virosa L. and L. saligna L. have also been utilized in lettuce breeding over the past 20-30 years and there has been an increased interest in exploiting other Lactuca species in recent years. Despite their importance, wild Lactuca species remain underrepresented in public-sector genebanks, with only a limited number of accessions available. Challenges, such as incomplete biogeographic and ecological data and frequent miss-identification of accessions, hinder the effective management and use of the collections in breeding programs. Recent studies have advanced our understanding of relationships within the subtribe Lactucinae, but the taxonomy of this lineage remains unresolved. The current taxonomic classification recognizes 73 Lactuca spp.; however, further research is needed to confirm species delineation and refine their classification. This paper proposes two potential approaches to address the unresolved taxonomy of Lactucinae: either recognizing each monophyletic unit as a distinct genus or adopting a broader definition of the genus Lactuca to encompass its diverse lineages.