Sunday, February 3, 2013

Asian Artemisia makes touchdown on Hawaiian soil and heads for the hills


Above the surf, sand, and mai-tai toting tourists, the Hawaiian Islands soar to heights that would be... uncomfortable for bikini and board short-clad beach-goers. Yet, these cold and windy high elevation environments are home to some of Hawaii’s greatest treasures; botanical absurdities reside on top of the Haleakala, Mauna Kea and Mauna Loa volcanoes that grow nowhere else in the world. But, just 6 million years ago the southeastern Hawaiian islands were nothing more than an expanse of open ocean. Where did the high-elevation flora of Hawaii come from? Are the species related to plants from boreal and temperate regions that came to the islands already adapted to a cold climate? Or, are they relatives of plants living in the lower elevation tropical climates that managed to dramatically adapt to new conditions? For the famous silverswords and woody violets, most evidence supports the former explanation, but a new study in the Journal of Biogeography has found a group of plants that seem to buck this trend.

Artemisia mauiensis grows only on the slopes of the Haleakala volcano on the island of Maui.  There are a couple other species of Artemisia that also grow only in Hawaii, but they live in the warmer lower elevations. Have these endemic Artemisias all evolved from one ancestor that originally colonized Hawaii? Given that there are 350-500 species of Artemisia distributed in a wide variety of habitats all around the world, it’s possible that the high-elevation species is a descendent of a colonizer from northern latitudes, while the lowland species came to the islands by way of a more tropical ancestor.

Using DNA from 114 species of Artemisia from around the world and an estimate of the date when the first Artemisia evolved based on 31 million-year-old fossil pollen, two researchers at the University of California Berkeley created a phylogeny of species relationships that included Artemisia mauiensis and two other Hawaiian endemics. This ‘family tree’ showed that all three of the Hawaiian species were each others’ closest relatives and descendants of an ancestor that probably lived 1½ million years ago. That ancestor likely came from Asia, since the Hawaiian species’ next closest relative, Artemisia chinensis, lives on islands stretching across the Pacific from Taiwan to the Bonin Islands. Since chinensis is a tropical plant that can’t handle the cold, it is likely that the high-elevation mauiensis evolved in a relatively short amount of time from a tropical ancestor to a plant well-adapted to cold temperatures and harsh conditions.

But how did the tropical Asian ancestor get to Hawaii? Birds, most likely. The researchers discovered that two of the Artemisia species, chinensis and the oldest of the hawaiian endemics, Artemisia kauaiensis, both have long curved hooks on the ends of their fruits that could easily get caught in a migratory bird’s feathers and accidentally carried half-way across the Pacific. The Hawaiian species that evolved more recently don’t have these hooks- which parallels other instances where plants living on islands lose characteristics that make dispersal easier.

This isn’t the first time that a warmer-weather Artemisia has changed tactics and headed for colder climes. In their paper the researchers bring up several other examples, such as how Artemisia species living in the Arctic evolved several different times from ancestors who were originally more southerly. The worldwide success of Artemisia raises the intriguing idea that species whose genes allow them to easily adapt to different environments diversify more rapidly. Such evolutionary flexibility may just be the key to life’s diversity.

You can find this article at:
Christopher R. Hobbs, & Bruce G. Baldwin (2013). Asian origin and upslope migration of Hawaiian Artemisia (Compositae-Anthemidea) Journal of Biogeography : 10.1111/jbi.12046