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Evolution of Malacothrix and relatives (Cichorieae; Compositae), with special reference to taxa of California's Channel Islands

Davis, W. S.1; Lee, Joongku2; Baldwin, Bruce G.3

Published Jun 18, 2026 on Dryad. https://doi.org/10.5061/dryad.mkkwh71dj

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Abstract

Biosystematic, cytogenetic, and morphological data and observations bring additional evolutionary context to an expanded molecular phylogenetic study of the primarily western North American Malacothrix and relatives (Cichoreae; Compositae) based on nuclear ribosomal (nr)DNA sequences. One of the two major clades resolved (Malacothrix 1) includes a subclade corresponding to the M. foliosa complex (M. incana, M. indecora, and all subspecies of M. foliosa), with all recognized taxa native to the California Channel Islands and all but one (M. incana) endemic there or to the California Islands more generally. The insular diploids of the M. foliosa complex mostly retain high interfertility and appear to be isolated geographically or ecologically, although recent introduction of M. incana to San Nicolas Island has resulted in extensive hybridization with the single-island endemic M. foliosa subsp. polycephala. Based on experimental cytogenetic, morphological, and molecular phylogenetic results, the recently described tetraploid M. junakii, endemic to Middle Anacapa Island, is evidently an allopolyploid from hybridization between diploid ancestors in the M. foliosa and M. clevelandii complexes, which are resolved as sister groups in the nrDNA tree. The tetraploid M. squalida, endemic to Anacapa and Santa Cruz islands, and the putative polyploid M. insularis, endemic to Las Islas Coronado, Mexico, belong to another subclade (Malacolepis) of the Malacothrix 1 clade that also includes the widespread diploid M. coulteri, which represents one of two subgenomes in M. squalida based on experimental cytogenetics and morphological considerations. The other major clade in Malacothrix (Malacothrix 2) includes the perennial M. saxatilis complex of highly interfertile, allopatric or putatively ecologically divergent diploids, wherein the endemic Channel Islands taxon M. saxatilis var. implicata is evidently sister to the other varieties, including the mainland and Santa Catalina Island native var. tenuifolia. Insular endemics across subclades of Malacothrix and their close mainland relatives are self-compatible, at least in part, in keeping with Baker’s Rule. These island endemics also generally have cypselae with characteristics associated with the insular syndrome of reduced dispersability, either lacking outer pappus (M. foliosa complex), unlike all mainland taxa except M. floccifera, or having outer pappus with no persistent bristles (M. saxatilis complex), unlike most mainland taxa of Malacothrix, including those inferred to have been bird-dispersed from North America to South America (M. clevelandii and M. coulteri). An expanded circumscription of Malacothrix to result in generic monophyly requires reinstating M. platyphylla and the new combinations Malacothrix acaulis (Torr. & A. Gray) B. G. Baldwin & Joongku Lee, Malacothrix parryi (A. Gray) B. G. Baldwin & Joongku Lee, and Malacothrix wrightii (A. Gray) B. G. Baldwin & Joongku Lee for all taxa previously treated in Atrichoseris, Anisocoma, and Calycoseris.