Relict lineages with extreme ecology and physiology: metal hyperaccumulation on ultramafic substrates in New Caledonian Alseuosmineae (Asterales)

Background Relict lineages are an important component of biodiversity, but it is unclear under what circumstances these groups persist. A potential example of such a group is the Alseuosmineae (Asterales) of Oceania. This clade contains the three small families - Alseuosmiaceae, Argophyllaceae and Phellinaceae. The clade has highest diversity in New Caledonia, where there are extensive ultramafic substrates, creating an extreme edaphic environment. Aims Using several lines of evidence we aimed to show that Alseuosmineae qualify as a relict lineage, with specialised adaptations for ultramafic substrates. Methods Molecular phylogenetic and dating analyses were carried out on representatives from all Alseuosmineae genera. Metal concentration in 33 out of 44 Alseuosmineae species was measured in herbarium specimens with X-ray fluorescence spectrometry. Results Dating analyses indicated that Alseuosmineae began diversifying about 75-80 million years ago, and had much slower net diversification rates than other Asterales groups. One-third of the species occur on ultramafic substrates in New Caledonia. Six are categorised as nickel hyperaccumulators, and two as manganese hyperaccumulators. Conclusions Extinction was probably paramount in the history of Alseuosmineae, especially for continental species. We postulate that their adaptation to ultramafic substrates and metal accumulation may have contributed to their survival until today.