Hawaiian Vaccinium

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.^[3] 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.^[6][7] 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.^[1] However, there is much debate and controversy surrounding the definition and characterization of adaptive radiation.^[7]

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,^[6][1] 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.^[1][6] Selfing is said to be rare elsewhere in Vaccinium,^[5] 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, V. reticulatum and V. dentatum show much poorer (62%) seed viability on average, probably due to morphological conditions in the calyx.^[5] Researchers have hypothesized that the self-compatible gene is not yet fixed in entire populations of Vaccinium calycinum and Vaccinium reticulatum.^[5]

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.^[3] What is certain is that the plant species traditionally understood to form the genus Vaccinium do not form a monophyly.^[3] 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.^[3]

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.^[8] Vaccinium cereum was originally described by Sleumer^[6][5] as the type specimen for section Macropelma. There is much ongoing debate^[5][3] 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,^[3] together with Sam Vander Kloet’s^[5] detailed examination of morphological variation throughout the four species has concluded that V. cereum is probably a hybrid species, with origins shared between V. calycinum, a member of the Hawaiian taxa and V. 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 V. 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.^[3] Such a situation would place Japan as the likely origin of dispersal.

It should be noted first that V. cereum is not Hawaiian. It ranges throughout islands in the South Pacific including the Cook Islands, the Marquesas Islands, and Tahiti in the Society Islands, at high elevations of 255–436 m (838–1,430 ft).^[3][9] Vander Kloet^[5] noted that Vaccinium cereum uniquely has a pseudo-10-locular ovary and a complex floriferous shoot, both characters associated with East Asian species of Vaccinium and not Hawaiian Vaccinium, which are strictly 5-locular in ovary structure.^[5] Vaccinium cereum is said to be, on average, more similar to Hawaiian taxa in other reproductive and vegetative characters than Eastern Asian species, but persistently retains the pseudo-10-locular ovary, characteristic of Eastern Asian species.^[3] Morphological variation throughout Vaccinium cereum’s range is enormous: pubescence, glaucescence, fruit and flower color all vary widely from island to island, sometimes from population to population on the same island, and, miraculously, from individual to individual within populations. Some become more uniform on larger islands where populations seem to be more stable,^[5] but the norm seems to be outlandish. Vander Kloet, a researcher very experienced with Vaccinium, once found a single individual that exhibited all types of inflorescences he had ever seen on any Vaccinium throughout the world, all on a single plant.^[5]