Evolution Revelation

A primitive, worm-like marine animal that spends most of its time burrowed in sand is a hot commodity to marine biologists thanks to what its genome could tell them about vertebrate evolution.

Scientists at Scripps Institution of Oceanography at UC San Diego were part of a collaborative effort of some 30 laboratories around the world to sequence the genome of the invertebrate amphioxus.

Scripps research biologist Linda Holland led a study using the new information to draw fresh clues about a key area of the tree of life and the evolution of all vertebrates, including humans.

Amphioxus, also called a lancelet, is one of the closest living invertebrate relatives of vertebrates. Discovered in the 1700s, the fish-like amphioxus split from vertebrates more than 520 million years ago and now stands as a baseline indicator for scientists to compare historical changes and track evolutionary adaptations.

Holland and her colleagues studied the genes of the amphioxus species Branchiostoma floridae with samples obtained during fieldwork off Tampa, Fla.

According to Holland, the human genome only has about 25 percent more genes than the amphioxus genome. Humans, however, have duplicated genes for various functions. This duplication has given humans—and other vertebrates—a broad “toolkit” for building different structures that amphioxuses lack. These include, for example, cells for pigment and collagen type II-based cartilage.

In the new study, Holland and her coauthors describe success in probing the roots of vital functions such as immunity. While vertebrates have an innate immune system that provides a general first line of defense against pathogens, and an adaptive one with antibodies specific for particular pathogens, invertebrates like amphioxus have only innate immune systems. Several of these innate immune genes have been independently duplicated many times over in amphioxus. It may be that with a second line of defense vertebrates are less reliant on innate immunity to ward off infection.

Further, neural crest cells of vertebrates exemplify how “old” genes have acquired new functions. In all vertebrates, neural crest cells migrate from the developing neural tube throughout the body, giving rise to such structures as pigment cells, cartilage of the head, and a number of other cell types. Although amphioxus has a brain and spinal cord and makes them using the same genes in the same way as vertebrates, amphioxus has no neural crest cells. Even so, amphioxus has all of the genes necessary for generating migratory neural crest cells; vertebrates have just put them together in new ways. An analogy can be made to a chef who takes basic leftovers in a refrigerator and whips up a gourmet dish.

 “The take-home message from this sequencing is that the human and amphioxus genomes are very much alike,” said Holland. “All of this is just the tip of the iceberg. It will take a number of years for people to look in greater depth at the amphioxus and human genomes. In terms of figuring out what evolution has done and how it generally works, the amphioxus genome has really been a goldmine and will continue to be one in the years ahead.”

The new research was published by Holland and her colleagues in the journal Genome Research. A companion paper was published in the journal Nature.

-- Mario C. Aguilera