Abstract
Woody perennial plants on islands have repeatedly evolved from herbaceous mainland ancestors. Although the majority of species in Euphorbia subgenus Chamaesyce section Anisophyllum (Euphorbiaceae) are small and herbaceous, a clade of 16 woody species diversified on the Hawaiian Islands. They are found in a broad range of habitats, including the only known C4 plants adapted to wet forest understories. We investigate the history of island colonization and habitat shift in this group. We sampled 153 individuals in 15 of the 16 native species of Hawaiian Euphorbia on six major Hawaiian Islands, plus 11 New World close relatives, to elucidate the biogeographic movement of this lineage within the Hawaiian island chain. We used a concatenated chloroplast DNA data set of more than eight kilobases in aligned length and applied maximum likelihood and Bayesian inference for phylogenetic reconstruction. Age and phylogeographic patterns were co‐estimated using BEAST. In addition, we used nuclear ribosomal ITS and the low‐copy genes LEAFY and G3pdhC to investigate the reticulate relationships within this radiation. Hawaiian Euphorbia first arrived on Kaua`i or Ni`ihau ca. 5 million years ago and subsequently diverged into 16 named species with extensive reticulation. During this process Hawaiian Euphorbia dispersed from older to younger islands through open vegetation that is disturbance‐prone. Species that occur under closed vegetation evolved in situ from open vegetation of the same island and are only found on the two oldest islands of Kaua`i and O`ahu. The biogeographic history of Hawaiian Euphorbia supports a progression rule with within‐island shifts from open to closed vegetation.
Highlights
Woody perennial plants have repeatedly evolved from herbaceous ancestors in isolated situations, such as islands and mountaintops (Bohle, Hilger, & Martin, 1996; Carlquist, 1974)
The remaining 16 named species form a clade within Euphorbia subgenus Chamaesyce section Anisophyllum, hereafter referred to as Hawaiian Euphorbia (Yang & Berry, 2011)
A prior phylogenetic study with taxon sampling throughout section Anisophyllum suggested that Hawaiian Euphorbia originated following allopolyploidy, with their closest relatives being small herbs occurring in dry, warm, and exposed habitats in southern United States, northern Mexico, and the Caribbean, including E. cinerascens, E. leucantha, E. mendezii, E. stictospora, and E. velleriflora (Figure 1f; Yang & Berry, 2011)
Summary
Woody perennial plants have repeatedly evolved from herbaceous ancestors in isolated situations, such as islands and mountaintops (Bohle, Hilger, & Martin, 1996; Carlquist, 1974). A prior phylogenetic study with taxon sampling throughout section Anisophyllum suggested that Hawaiian Euphorbia originated following allopolyploidy, with their closest relatives being small herbs occurring in dry, warm, and exposed habitats in southern United States, northern Mexico, and the Caribbean, including E. cinerascens, E. leucantha, E. mendezii, E. stictospora, and E. velleriflora (Figure 1f; Yang & Berry, 2011). The long- distance dispersal most likely occurred via the tiny seeds (typically 1–2 mm long) that adhere to birds with their mucilaginous seed coat (Carlquist, 1966, 1980; Price & Wagner, 2004) Following their arrival on the Hawaiian Islands, Hawaiian Euphorbia became woody, and some species lost the mucilaginous seed coat and developed larger seeds (Carlquist, 1966). We tested whether Hawaiian Euphorbia moved into forest understory a single time and dispersed among islands, or if they moved into forest understory independently on different islands
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