The genus Lathyrus L. (Fabaceae) is represented in Belgium by several native and alien species, amongst which the Lathyrus sylvestris–latifolius complex has long posed taxonomic difficulties. Since the 1940s, plants with intermediate morphological characters between L. sylvestris and L. latifolius have been observed in Belgium, combining the smaller corollas of the former with the broader, rounded leaflets of the latter. To clarify their identity, we conducted phylogenetic analyses of four chloroplast markers (trnS-G, trnL-F, rbcL, trnH-psbA) and the nuclear rDNA internal transcribed spacer (ITS), complemented by genome-wide SNP data obtained through DArTseq genotyping for 20 individuals representing L. latifolius, L. heterophyllus, L. sylvestris and the enigmatic Belgian specimens.
Phylogenetic analyses, based on the five markers, together with analysis of the SNP data (Principal Coordinates Analysis, Maximum Likelihood phylogenetic reconstruction and multispecies coalescent species delimitation), all revealed four well-supported genetic lineages corresponding to these taxa. The enigmatic specimens form a monophyletic clade sister to L. sylvestris, genetically distinct from both L. heterophyllus (including var. unijugus) and L. latifolius. These results exclude hybrid origin and support recognition of these plants as an independent evolutionary lineage, which can, thus, be regarded as a distinct species.
In light of both morphological and molecular evidence, we reinstate the name Lathyrus platyphyllos (Retz.) W.D.J.Koch for this taxon. Lathyrus platyphyllos differs from L. sylvestris by its broader leaflets with rounded apices and its wider stipules and stem wings. The species is non-native in Belgium, first recorded in 1943 and now locally naturalised along railway lines and other disturbed sites. Recognition of L. platyphyllos restores taxonomic clarity within the L. sylvestris–latifolius complex and highlights the need for further study of its native range, introduction pathways and potential invasiveness.