Amphioxus (Branchiostoma floridae)

Amphioxus, also called the lancelet, is a small, fishlike filter-feeder most often found buried in the sand of shallow marine environments. This curious little transparent organism occupies a unique evolutionary position, and as such has been the subject of much scientific debate and study since the 18th century. Though invertebrates, lancelets are the closest living relatives to vertebrates, and have many vertebrate-like characteristics. Most notably, amphioxus have a notochord, the predecessor to the backbone.

Because of its unique blend of invertebrate and vertebrate characteristics, scientists have debated the taxonomy of amphioxus since first studying it. Originally identified as a mollusk, amphioxus was then classified as a jawless fish, but its taxonomy continued to be the subject of debate. Amphioxus was studied perhaps most passionately by Ernst Haeckel in the late 19th century. Haeckel proposed that amphioxus represents a lower ancestral class of vertebrates, and believed it was the key to understanding the origins of vertebrates.

Modern molecular techniques have shown that Haeckel was very close to the mark. It is now clear that amphioxus approximates the last common ancestor of all vertebrates. It is therefore an ideal organism in which to study the evolutionary origins of vertebrates, and its genome was selected for sequencing by the Joint Genome Institute.

The sequencing of the amphioxus genome was completed in 2006. At about 600 million base pairs, the genome is smaller than those of many vertebrates because it lacks the large-scale duplications characteristic of vertebrates. As such, it represents a more ancestral genomic condition, as opposed to the highly derived genomes of vertebrate model organisms.

Studies at CIG are currently focused on annotating the genome of amphioxus. Comparative genomics approaches will be used to identify genomic elements, both coding and noncoding, that are shared with vertebrates. Identification of these elements will aid in the understanding of the genomic determinants of vertebrate evolution.