Saturday, May 10, 2008

New genetic study throws 'ōhi'a (Metrosideros who?) into identity crisis

There's nothing easy about the Hawaiian 'ōhi'a.

Researchers doing genetic analyses of the trees are finding that they are just as confusing inside (genetically) as they are outside (in their physical appearance).

(Image: The shiny red leaves of a young 'ōhi'a, whose genetic makeup is now in doubt.)

'Ōhi'a are the dominant trees in many native Hawaiian forests. And they are amazingly variable.

Many folks search the forests for alternative flower colors. The starburst blossoms are generally seen in shades of red, but rarely are found in orange, salmon and yellow colors.

The leaves can be round and stiff or lance-shaped and delicate. They can be hairy or smooth, dull or shiny. New leaves can be greenish with purple-red veins or entirely red. The 'ōhi'a can grow into a towering tree or remain a shrub. In bog habitats, mature flowering trees can be found just a few inches tall, natural bonsai.

It's no wonder that the dominant Hawaiian species is known as polymorpha, many forms.

Over the years, scientists have sought to classify the 'ōhi'a, a member of the myrtle family whose genus is Metrosideros, by its appearance. One problem was that in the same small grove, they would find many different-looking trees growing together and presumably interbreeding.

New research is suggesting that, with 'ōhi'a, you can't make species distinctions based on appearance. More on that later.

Previous genetic work has suggested that the tree crossed the Pacific from New Zealand, and that it may have made the jump to Hawai'i from the Marquesas, where botanists find the closest relatives of Hawaiian 'ōhi'a.

And scientists have long believed that the tree that now dominates much of the Hawaiian natural landscape was a comparatively late arrival—showing up as little as a million years ago.

New dating is being reported in a paper in the “Proceedings of the Royal Society B,” by Diana Percy, Adam Carver, Warren Wagner, Helen James, Scott Miller and Robert Fleischer, all of the Smithsonian Institution, and Clifford Cunningham of Duke University. The paper is entitled, “Progressive island colonization and ancient origin of Hawaiian Metrosideros (Myrtaceae).”

Their genetic research finds that 'ōhi'a has been in the Hawaiian Islands for about 3.9 million years, and perhaps a million or two more. At that time, Kaua'i and Ni'ihau were the only emerged islands among the current main Hawaiian Islands, meaning that 'ōhi'a must have channel-jumped to newer islands as they formed.

'Ōhi'a seeds are tiny, can be dispersed by wind, can survive a while in salt water, and the tree grows quite well on new lava flows. So the genus is clearly capable of making such journeys.

The researchers studied 97 'ōhi'a samples from five islands. Their samples included representatives of the five accepted Hawaiian Metrosideros species: macropus, polymorpha, rugosa, tremuloides and waialealae. They also looked at samples of Metrosideros collina from the Austral, Society and Marquesan islands, Metrosideros excelsa from New Zealand and Metrosideros nervulosa from Lord Howe Island.

Molecular dating work on the samples suggests Kaua'i was inhabited first by Metrosideros, and that from there the genus jumped to O'ahu. Moloka'i was populated on at least two different occasions from O'ahu, but also on one occasion directly from Kaua'i. Maui got its 'ōhi'a from Moloka'i, and the Hawai'i island got its from Maui about half a million years ago.

Metrosideros...appears to have diversified in the Hawaiian Islands following the geological succession of islands,” the authors write. “The chloroplast data present a distinct geographical pattern that supports a hypothesis of sequential colonization of progressively younger islands.”

The scientists were able to confirm that the Hawaiian 'ōhi'a are most closely related to the Marquesan forms than any other. They are, in fact, close enough that the authors argue that the Marquesan and Hawaiian plants ought to be part of the same genus. They suggest that the name Metrosideros polymorpha be used for both.

Wagner, in an email, said the research will require botanists to reconsider the way Hawaiian wildlife evolved, since it's now clear that the 'ōhi'a was a part of the Hawaiian botanical picture far earlier than anyone previously thought.

“An older origin of the genus in the Hawaiian islands significant implications for understanding the evolution of Hawaiian ecosystems of which 'ōhi'a is a major component and the evolution of endemic organisms that may have co-evolved with Metrosideros,” Wagner wrote.

The other thing that's new is that current understandings of how to differentiate the members of the genus are going to require rethinking.

Wagner said their work indicates that the physical appearance of the plant doesn't correlate with its DNA, and that physical characteristics don't accurately reflect differences at a molecular level. That means that if you separate the Hawaiian 'ōhi'a into different species and varieties based on their appearance, you'll run into trouble.

“This was both a surprise and not surprising. This Metrosideros has been a challenge to biologists trying to classify it for many decades with various systems proposed by each of the major botanists to have studied the Hawaiian flora,” Wagner said. And in that list of botanists, he includes himself.

Not only isn't the physical appearance an unreliable guide to how the plans are actually genetically related—it turns out it's no guide at all.

“To find a situation where we are not sure how dependable the characters are is not so surprising. That there was essentially no correlation was a surprise,” Wagner said.

There are a number of theories on how this could happen. One is a whole lot of interbreeding within the 'ōhi'a clan. Another is that different genetic lines could evolve into similar-looking plants. Or perhaps something else.

“We are embarking on additional studies to attempt to determine more definitive answers to this puzzle,” Wagner said.

© 2008 Jan W. TenBruggencate