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Coral Family Tree Reorganized, Result of DNA Analyses

By Pippa Wysong, Access Excellence

La Jolla, CA (03/16/04)- The traditional classification system used for determining which families coral belong to is out, and DNA taxonomy is in. DNA analyses of dozens of corals in both the Atlantic and Pacific oceans show that many have been misclassified and the old system was wrong.Indeed, some corals found in both oceans thought to be closely related are actually distinct from each other, while others are more closely related than previously believed. These findings, by an international team of researchers, have turned coral taxonomy completely on its head.

Not only that but the study, published in Nature, shows that conservation efforts for coral need to focus on a wider number of reef areas if distinct families are to be protected. Traditionally, corals have been classified by looking at specific morphological features, but this is now inadequate, said Nancy Knowlton, PhD, from the Scripps Institute of Oceanography where she is director of the Center for Marine Biodiversity and Conservation. "Our results show that morphological studies, as conventionally done, give very misleading versions of the coral family tree," she said.

The study began as an attempt to determine the closest relatives of a few corals from the Caribbean that she and her colleagues were studying. But right from the start they found that DNA analysis showed lineage classifications that differed from what conventional taxonomy and morphology showed. The study was expanded to include corals from Brazil, Japan, Taiwan and Palau. Entire lineages had been misclassified.

Traditionally, features called septal teeth were used to classify some families of reef-building coral. Tiny creatures called polyps secrete a calcium compound that, over time, build not only the cup-shaped abodes the polyps live in but entire reefs. The septa are spoke-like calcareous structures that provide supports to the skeletal-cup the polyps live in.

This study shows, for the first time, that there is a family of corals unique to the Atlantic and that there is no pattern for septal tooth length in terms of determining which corals are related to which. It had been assumed that big-toothed coral in the Atlantic were related to big-toothed coral in the Pacific, and small with small. Researchers, however, also identified features other than septal teeth of the coral skeleton that show promise for helping identify families. This is important because it can help with classifications of fossil corals which lack DNA.

Researchers focused their studies on the Faviidae and Mussidae families which comprise a third of all reef-building coral in the Atlantic. Samples were taken from dozens of types of corals within those two families and submitted to detailed DNA analyses. DNA was taken from three genes: two from the mitochondria and one from the nucleus.

"It's important to have them from the nucleus as well as from the mitochondria because they're inherited separately. It gives you two completely independent sources of evidence for evolutionary relationships," Dr. Knowlton said. Researchers selected these specific genes for study because they were known to mutate at a pace that matches the evolutionary history of coral families.

"Some genes change really fast, almost too fast. They keep changing and changing, sort of overwriting the evolutionary signal. In some cases genes change so slowly that even things that have been separated for a long time haven't accumulated enough mutations to tell them apart. We looked long and hard for genes that were changing at the appropriate rate," Dr. Knowlton said.

The large number of samples studied provided an impressive amount of data to support the fact the existing family trees for coral were wrong. Another problem with using the septal teeth as a classification tool is that they are a feature that changes fairly rapidly.

"They keep getting invented over and over again," she said. Rapidly changing features make it difficult to do such detective work. The researchers estimate that the Atlantic and Pacific corals diverged about 34-million years ago.

An important consequence of this work ties in with conservation efforts of coral reefs. When threatened hotspots were identified and ranked a number of years ago by environmentalists, the rankings were based on the known coral species, ignoring family relationships. This new research shows that the patterns for priority need to be changed now that the family trees have been reorganized and new, unique members identified.

"There weren't supposed to be any major groups of coral that were only in the Atlantic. Our study showed that this assumption was wrong," she said. In order to preserve biodiversity, Atlantic corals need protecting too.


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