Endosomatium, Wollaston, 1877

Maddison, David R, Sproul, John S & Mendel, Howard, 2020, Origin and adaptive radiation of the exceptional and threatened bembidiine beetle fauna of St Helena (Coleoptera: Carabidae), Zoological Journal of the Linnean Society 189 (4), pp. 1155-1175 : 1165-1166

Endosomatium and Pseudophilochthus were unrelated,

each representing a separate dispersal event onto St Helena by different lineages of bembidiines. However, Pseudophilochthus + Apteromimus + Endosomatium form a clade ( Figs 7 , 8 ; Table 3). A unique common ancestor of the Peaks Bembidion is supported by individual analyses of five of the seven genes examined and is strongly supported by the concatenated analyses ( Table 3). Thus, it appears that the extraordinary diversity of body forms of St Helena Peaks Bembidion arose from a single ancestor that arrived within the last 14 Myr and diversified into the 12 endemic species known today.

In spite of their morphological disparities, the genetic divergence of the four species whose DNA was sequenced is not at all remarkable, showing no more diversity ( Figs 7 , 9 ) than within the morphologically fairly uniform subgenus, Omotaphus ( Fig. 5A–C ). This pattern of extreme phenotypic divergence but limited genetic divergence matches that in some other island radiations. For example, the plant genus Bidens ( Asteraceae ) shows more morphological diversity among the 19 species in Hawai’i than it does throughout the entirety of the Americas, and yet the total genetic diversity in Hawai’i is comparable to that found within individual species of mainland Bidens ( Helenurm & Ganders, 1985) .

The St Helena Peaks Bembidion appears to have undergone a classical adaptive radiation ( Schluter, 2000; Gavrilets & Losos, 2009). The ecological opportunity provided by arrival on a depauperate island with few small predators, and perhaps none

Figures

Figure 5. A sampling of Bembidion, showing morphologically typical representatives from around the world and across the phylogeny.A–C, members of the African subgenus Omotaphus. D, another member of the Bembidion complex.E–G, other members of the Bembidion series. H–L, Bembidion outside the Bembidion series. A, Bembidion (Omotaphus) mellissii. B, Bembidion (Omotaphus) scotti. C, Bembidion (Omotaphus) gorilla. D, Bembidion (Bembidion) crurale. E, Bembidion (Notaphus) flohri. F, Bembidion (Ecuadion) walterrossii. G, Bembidion (Neobembidion) constricticolle. H, Bembidion (Taiwanobembidion) aliense. I, Bembidion (Liocosmius) darlingtonielum. J, Bembidion (Hydrium) nitidum. K, Bembidion (Plataphus) lividulum. L, Bembidion (Ocydromus complex) plagiatum. Images copyright David R. Maddison 2019, released under a Creative Commons Attribution 4.0

Figure 7. Maximum likelihood tree for the concatenated, seven-gene matrix. Branch length is shown proportional to relative divergence, as estimated by IQ-TREE; scale bar (to the left of Bembidion pedicellatum LeConte) indicates 0.01 units.

Figure 8. Majority rule consensus tree of 500 maximum likelihood bootstrap replicates for the concatenated, seven-gene matrix.Black dotsindicatethosebranches betweentherootof Bembidion andthe StHelena +Bembidion alsium +Omotaphus clade that are supported by bootstrap percentages> 90 for the concatenated matrix.

Figure 9. Maximum likelihood trees each of the seven studied genes, showing only the clade containing the subgenus Omotaphus (green), in addition to the species from St Helena and La Réunion (blue). Branch length is shown proportional to relative divergence, as estimated by IQ-TREE; scale bars indicate 0.01 units.