Abstract
Lettuce (Lactuca sativa L.) belongs to the genus Lactuca L. and is an important vegetable worldwide. Over the past decades, there have been many controversies about the phylogeny of Lactuca species due to their complex and diverse morphological characters and insufficient molecular sampling. In this study we provide the most extensive molecular phylogenetic reconstruction of Lactuca, including African wild species, using two chloroplast genes (ndhF and trnL-F). The sampling covers nearly 40 % of the total endemic African Lactuca species and 34 % of the total Lactuca species. DNA sequences from all the subfamilies of Asteraceae in Genebank and those generated from Lactuca herbarium samples were used to establish the affiliation of Lactuca within Asteracaeae. Based on the subfamily tree, we selected 33 ndhF sequences from 30 species and 79 trnL-F sequences from 48 species to infer relationships within the genus Lactuca using randomized axelerated maximum likelihood and Bayesian inference analyses. Biogeographical, chromosomal and morphological character states were reconstructed over the Bayesian tree topology. We conclude that Lactuca contains two distinct phylogenetic clades—the crop clade and the Pterocypsela clade. Other North American, Asian and widespread species either form smaller clades or mix with the Melanoseris species. The newly sampled African endemic species probably should be treated as a new genus.
Highlights
Domesticated lettuce (Lactuca sativa L.) is a member of the genus Lactuca L., which is grouped in the subtribe Lactucinae, tribe Cichorieae (Lactuceae), subfamily Cichorioideae of the family Asteraceae (Compositae; Judd et al 2007; Kadereit et al 2007)
The phylogenetic trees of 247 ndhF and 331 trnL-F gene sequences from different subfamilies using Randomized Axelerated Maximum Likelihood (RAxML) and Bayesian Inference (BI) analyses are shown in Fig. S1– S4
One ‘best ML tree’ for the concatenated sequences was inferred automatically from the RAxML analysis, which is generally congruent in topology with the BI 50 % majority rule consensus tree
Summary
Domesticated lettuce (Lactuca sativa L.) is a member of the genus Lactuca L., which is grouped in the subtribe Lactucinae, tribe Cichorieae (Lactuceae), subfamily Cichorioideae of the family Asteraceae (Compositae; Judd et al 2007; Kadereit et al 2007). There are a large number of lettuce cultivars within L. sativa. These cultivars can be divided in seven distinct cultivar groups: Butterhead Group, Crisphead Group, Cos Group, Cutting Group, Stalk Group, Latin Group and Oilseed Group (de Vries 1997). Tuisl (1968), Shih (1988a, b), and Kadereit et al (2007) established a narrow circumscription. In this concept, Shih and Kilian (2011) consider there to be between 50 and 70 Lactuca species. All these authors mentioned before only dealt mostly with regional Lactuca species and the genus has never been revised in its entirety
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