The taxonomy of the Ramalina siliquosa species aggregate (lichenized Ascomycetes)

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Abstract
The taxonomy of this western European, maritime, cliff-inhabiting species complex has in recent years been based primarily on chemistry. Five of the species are characterized by a replacement series of medullary depsidones and a sixth is acid deficient. The taxonomy of this group has been reinvestigated using modern methods of thin-layer chromatography and numerical taxonomy based on individuals collected from three well separated regions in western Britain.The discovery that low concentrations of protocetraric acid usually accompany salazinic acid leads to the supposition that it may be a precursor compound in the same way that norstictic acid is widely accepted as being the precursor of stictic acid. The chemical structures of the compounds also support such a view. In addition, hypoprotocetraric acid is structurally very similar to protocetraric acid so that two biosynthetic pathways can be hypothesized for the group. These are as follows: (1) hypoprotocetraric → protocetraric → salazinic acid and (2) norstictic → stictic acid.Three types of numerical classification were undertaken to test for the presence of a correlation between morphological attributes (39) and chemical attributes (variable numbers). These are classifications in which (1) chemical attributes were excluded. (2) medullary constituents were given equal weight, and (3) putative biosynthetic pathways were given greater weight than their constituent substances. The three methods each produced two virtually identical clusters corresponding to individuals possessing one or other of the two biosynthetic pathways, a result which established a close correlation between morphology and chemistry within the group. Cophenetic correlation coefficients between the classifications were in excess of r = 0.900. Acid-deficient individuals were found associated with both clusters.Comparison of the within- and between-group sums of squares showed that the two clusters represent taxa which in view of the strong correlation between morphology and biosynthetic pathways, should be recognized at the species level. Principal components analysis of a data set derived from that used in type 3 classification also produced the same two clusters. This analysis also yielded two subgroups within each cluster which are to be the subject of further studies. The correct names for the two species corresponding to individuals with the salazinic and stictic acid pathways are Ramalina siliquosa (Huds.) A.L.Sm. and R. cuspidata (Ach.) Nyl., respectively.