Linda Holland, a research biologist at Scripps Oceanography, and her
colleagues from the United States, Europe and Asia, have deciphered and
analyzed fundamental elements of the genetic makeup of a small,
worm-like marine animal called amphioxus, also known as a lancelet.
Amphioxus is not widely known to the general public, but is gaining
interest in scientific circles because of its position as one of the
closest living invertebrate relatives of vertebrates. Although
amphioxus split from vertebrates more than 520 million years ago, its
genome holds tantalizing clues about evolution.
Scripps research biologist Linda Holland
The research led by Holland is published in the July issue of the journal Genome Research
. A corresponding research paper is published in the June 19 issue of Nature
Holland and her colleagues studied the genes of the amphioxus species Branchiostoma floridae
through samples obtained in recent years during field work off Tampa, Fla.
Because amphioxus is evolving slowly-its body plan remains similar to
that of fossils from the Cambrian time-the animal serves as an
intriguing comparison point for tracing how vertebrates have evolved
and adapted. This includes new information about how vertebrates have
employed old genes for new functions.
"We are finding that today's complicated vertebrate has not invented a
lot of new genes to become complicated," said Holland, of the Marine
Biology Research Division at Scripps Oceanography. "Amphioxus shows us
that vertebrates have taken old genes and recombined them, changed
their regulation and perhaps changed the gene function."
Originally discovered in the 1700s, amphioxus appears fish-like with a
small tail fin and medial fins, but no paired ones. They spend most of
their time burrowed in sand, with their snouts extended for filter
Linda Holland's amphioxus field work is conducted in Tampa, Florida. Shown are Aubrie O'Rourke (left) and Laura Beaster-Jones.
The human genome has only about 25 percent more genes than the
amphioxus genome, according to Holland. During evolution, humans have
duplicated genes for different functions. Such duplication has given
humans and other vertebrates a much larger "toolkit" for making various
structures that are absent in amphioxus, including cells for pigment
and collagen type II-based cartilage, for example.
In the new research, Holland and her colleagues describe success in
probing the roots of important functions such as immunity. While
vertebrates have two types of immune systems-innate, which is a general
first line of defense against pathogens, and adaptive, involving
antibodies specific for particular pathogens-invertebrates like
amphioxus have only innate immune systems. In amphioxus, several of
these innate immune genes have been independently duplicated many times
over. It may be that with a second line of defense, vertebrates,
compared with invertebrates like amphioxus, are less reliant on innate
immunity to ward off infection.
The neural crest cells of vertebrates are an excellent example of 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. It can be
compared with a chef who takes basic leftovers in a refrigerator and
whips up a fine gourmet dish.
Spawning amphioxus. Image (and top) courtesy of Dr. Simona Candiani.
"The take-home message from this sequencing is that the human and amphioxus genomes are very much alike," said Holland.
A collaborative effort of some 30 laboratories around the world solved the sequence of the amphioxus genome.
Further, deeper analyses between the amphioxus and human genomes in the
years ahead will provide even more important clues about genetic
"All of this is just the tip of the iceberg," said Holland. "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."