Hawaiian Vaccinium

Hawaiian ''Vaccinium'' ( blueberries) is a monophyletic group (a clade including all extant species and their common ancestor) comprising three species endemic to the archipelago of Hawaii: ''Vaccinium reticulatum, Vaccinium dentatum and Vaccinium calycinum'', all commonly known in Hawaii as ʻōhelo.

While ''Vaccinium'' as a larger group is characterized by an inferior ovary and brightly-colored berries that are indehiscent, the Hawaiian group has traditionally been distinguished as having uniquely well-developed calyx lobes and longer calyx tube depth, more cylindrical corolla shape (as compared to urceolate-globose), reduced or absent staminal awns (as opposed to well-developed), longer pedicel length, and — compared with temperate relatives — much longer leaf persistence. They are terrestrial or epiphytic shrubs, typically 1 - 6 feet in height, occasionally up to 10 feet, ranging widely throughout the Hawaiian islands over relatively high elevation (500 - 3,700 m). The three species thrive in many plant communities, except for ''Vaccinium reticulatum'', which tends to thrive around lava flows, yet is not limited to them.

Within the group, distinct taxa vary in berry color (red, yellow, black, blue), bloom color (white, red, pink and green), foliage shape and size, and pedicel length. ''Vaccinium reticulatum'' and ''Vaccinium dentatum'' are evergreen, while '' Vaccinium calycinum'' is deciduous. All three species tend to fruit and flower throughout the year, but maximum flower and fruit production generally occurs during May - July. Outcrossing between all three species has been successful, and many hybrids have been described. All three species are also capable of selfing, but resulting seed viability differs throughout the species complex.

Evolutionary history and taxonomy

This group is thought to be derived from within section ''Myrtillus'' of ''Vaccinium'' proper, and is thought to have a North American origin of dispersal. However, confidence in the existing molecular evidence for this hypothesis is low, and therefore the status of the sister group to the Hawaiian clade is still unknown. Hawaii is known as a historical hotspot for adaptive radiation because of immense biological opportunity over small, isolated areas, especially advantageous for plants that colonized the islands when they were first formed by volcanic activity. Endemic Hawaiian plant lineages that have undergone adaptive radiation exhibit patterns associated with a loss of dispersal capacity: small populations, isolated usually to one island, if not one small area of one island, exhibiting "explosive" diversity in a small space, reflecting probable "rapid speciation" or an accelerated rate of evolution. However, there is much debate and controversy surrounding the definition and characterization of adaptive radiation.

All three species of Hawaiian ''Vaccinium'' show the opposite pattern of adaptive radiation: they are widespread throughout the Hawaiian islands, and have retained their dispersal capacity, thus suggesting, among other hypotheses, a relatively recent dispersal to the archipelago. However, the extent of Hawaiian Vaccinium’s diversification at population levels is not well known. Another characteristic typical of lineages that have undergone adaptive radiation is the ability to self-fertilize. Selfing is said to be rare elsewhere in ''Vaccinium'', but is well established in, Hawaiian ''Vaccinium''. Seed viability among self-fertilized individuals varies, however, between the three species. Controlled experiments found that while selfing is very successful in '' Vaccinium calycinum, Vaccinium reticulatum'' and '' Vaccinium dentatum'' show much poorer (62%) seed viability on average, probably due to morphological conditions in the calyx. Researchers have hypothesized that the self-compatible gene is not yet fixed in entire populations of ''Vaccinium calycinum'' and ''Vaccinium reticulatum''.

The evolutionary history of the larger group ''Vaccinium'' has long been complete mystery for plant systematists and evolutionary biologists: species that have been found to be genetically related to not fall into groups traditionally described by morphological similarity, nor do they follow geographic pattern. What is certain is that the plant species traditionally understood to form the genus ''Vaccinium'' do not form a monophyly. Given this information, it is difficult to speculate with confidence upon the evolutionary history of Hawaiian ''Vaccinium'', though there is some confidence that there is a single common ancestor of the group.

Section ''Macropelma''

Hawaiian ''Vaccinium'' was originally placed in a section named ''Macropelma'', which traditionally included the three Hawaiian species and a mysterious South Pacific Island species known as ''Vaccinium cereum.'' ''Vaccinium cereum'' was originally described by Sleumer as the type specimen for section '' Macropelma''. There is much ongoing debate as to the taxonomic placement of these four species as more information about their genetic relationships becomes available. The key distinctive morphological feature separating ''Vaccinium cereum'' from the Hawaiian taxa is the pseudo-10-locular ovary, which is similar to ovary structure common of Asian ''Vaccinium'' species, as opposed to the strictly 5-locular present in most New World species and the Hawaiian taxa. It was long believed that this pseudo-10-locular ovary was the plesiomorphic condition of the ancestor of Hawaiian ''Vaccinium'', and the three Hawaiian taxa proliferated from ''V. cereum''. However, as noted above, the Hawaiian taxa are hypothesized with moderate confidence to belong in the ''Myrtillus'' section, which is primarily North American. Combined evidence including molecular work done by Kron and Powell, together with Sam Vander Kloet’s detailed examination of morphological variation throughout the four species has concluded that ''Vaccinium cereum'' is probably a hybrid species, with origins shared between ''Vaccinium calycinum'', a member of the Hawaiian taxa and '' Vaccinium fragile'', a taxon of East Asian origin in section Eococcus. If Hawaiian ''Vaccinium'' is confirmed to be derived from section Myrtillus, this may mean that ''Vaccinium cereum'' represents an entity of union between new world and old world ''Vaccinium.'' However, these are simply exciting postulations for now, as there is much uncertainty surrounding the evolutionary history South Pacific ''Vaccinium'' in general. For instance, though there is confidence that Hawaiian taxa are close in relation to primarily North American section ''Myrtillus'', some taxonomic treatments based on molecular data of ''Myrtillus'' have included Japanese species '' Vaccinium yatabei.'' These findings support that a Japanese species is just as likely to be closest in genetic relation to the Hawaiian taxa than the North American members. Such a situation would place Japan as the likely origin of dispersal.

''Vaccinium cereum'': a South Pacific hybrid

It should be noted first that ''V. cereum'' is not Hawaiian. It ranges throughout islands in the South Pacific including the Cook Islands, Tahiti, the Marquesas Islands, the Society Islands, at high elevations 838 - 1430 feet. Vander Kloet noted that ''Vaccinium cereum'' uniquely has a pseudo-10-locular ovary and a complex floriferous