Reconstructing and Using Phylogenies

Schistosomiasis is a blood infection caused by a parasitic trematode flatworm, Schistosoma. More than 200 million people in South America, Africa, China, Japan, and Southeast Asia have this disease. During part of its life cycle, Schistosoma inhabits a freshwater snail; people become infected when they come into contact with water where infected snails live. Larval Schistosoma swim from a snail through the water and penetrate human skin. The flatworm matures and settles in the abdominal blood vessels. The disease is progressively debilitating, causing a slow death.

Until recently, only one trematode species, Schistosoma japonicum, was known to infect humans, and it was believed to be transmitted by a single species of snail in the genus Oncomelania. Then, in the 1970s, researchers discovered a different snail species that also transmitted Schistosoma to humans. This discovery stimulated anatomical, genetic, and geographic research on the trematode flatworms and snails of Asia.

Investigators found that S. japonicum was actually a cluster of at least six different species. They also discovered that the snails that host the various Schistosoma parasites are closely related to one another. Of the thirteen species of Oncomelania in Southeast Asia, only three can host Schistosoma. The other species have a genetic trait that allows them to resist infection by the parasite.

This information on the evolutionary relationships among snails is of great value in efforts to combat schistosomiasis. Scientists can now quickly determine whether or not a snail is likely to be a host for Schistosoma, and control efforts can be directed toward only those snails that transmit Schistosoma.

How did investigators infer the evolutionary relationships among the Oncomelania snails that are hosts of Schistosoma? Systematics, the scientific study of the diversity of organisms, provides answers to such questions. In this chapter, we will describe the methods systematists use to infer evolutionary relationships among organisms, and we will show how those evolutionary relationships are incorporated into classification systems. Taxonomy, which is a subdivision of systematics, is the theory and practice of classifying organisms.

Asian Snails Can Transmit Schistosomiasis Workers in the rice paddies of tropical Asia are at extreme risk of contracting schistosomiasis (known in some parts of the world as bilharzia).The disease is transmitted to humans via freshwater snails that thrive in the standing water of the paddies.

In this book, all trees show the most ancient ancestor at the left.

Red acquisition icons indicate the appearance of genes

Schistosoma to humans.

Modern

All present-day taxa descending from a common ancestor are shown at the right of the tree.

The distance between branches has no significance in these trees.

In this book, all trees show the most ancient ancestor at the left.

Red acquisition icons indicate the appearance of genes

Modern

All present-day taxa descending from a common ancestor are shown at the right of the tree.

The distance between branches has no significance in these trees.

Phylogenetic Tree Fish

Most -*r-ancient

Time

Present

Most -*r-ancient

Time

Present

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment