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December 20, 2001
Fish May Show How Nature Diversifies
A marine stickleback with skeletal structures shown in red. These fish have undergone a remarkable evolutionary radiation in thousands of coastal streams and lakes of the Northern Hemisphere. Freshwater
populations with different sizes, shapes, and numbers of spines, plates, and other characteristics are being interbred for genetic
studies of natural variation in recently evolved species.
Although the threespine stickleback fish has been celebrated on the
currency of the Netherlands and been a star of a pioneering 1928 French
documentary film, the fish has found its most receptive audience with
biologists, who have been studying it for more than 100 years. In what
may be its most important role yet, the stickleback is being used as a
model by researchers at the Howard Hughes Medical Institute (HHMI) at
Stanford University to track the genetic changes that define a species,
a puzzle that until now could not be tested experimentally in
vertebrate animals.
In the December 20, 2001, issue of the journal Nature, HHMI
investigator David
M. Kingsley, HHMI associate Catherine L. Peichel and their
colleagues at Stanford, the University of Wisconsin, Eau Claire, and
the University of British Columbia, report the creation of a genetic
map of the fish that will make it possible for Kingsley’s lab and
others to tie behavioral, ecological, morphological and physiological
differences among the various species of sticklebacks to changes in the
genome.
“We see this as our chance to find out how many genetic
changes it takes to evolve new traits,” said Kingsley.
“Using this method we can ask which genes or developmental
pathways nature uses to create a new species.”
The scientists were able to study the molecular evolution of the
threespine stickleback due to its recent evolution since the end of the
last Ice Age, which occurred 15,000 years ago. When the giant glaciers
melted, they created thousands of lakes and streams in North America,
Europe, and Asia. These waters were colonized by the
stickleback’s marine ancestors, which adapted to life in
freshwater. The spiny fish, which are one- to six-inches long, were
remarkably successful in adapting to various niches in their new
habitats.
“The fish have evolved so recently that it is still possible
to carry out crosses between the new species using artificial
fertilization,” said Kingsley. “This makes it possible to
use genetics to study the number and location of genetic changes that
are responsible for evolutionary change.”
The isolated pockets of sticklebacks have created thousands of
evolutionary experiments, he added. By studying the genetic variation
among various species, Kingsley said it should be possible to discover
how evolution generates new species adapted to life in different
environments.
As a first test of the new genetic map, Kingsley and his associates
crossed two species of sticklebacks that live in Priest Lake in British
Columbia. One species inhabits the grassy, murky lake bottom near
shore, while the other lives mainly in the open water. The two species
don’t interbreed in the wild and look dissimilar. The species
that lives near shore, for example, has less body armor and a thicker
body. The species that lives in open water more closely resembles the
ancestral form, which still lives in the open ocean, and has larger
eyes, a longer snout and jaw, and more numerous gill rakers for filter
feeding.
When the researchers looked at the spectrum of skeletal changes
between the species, they saw two phenomena. First, they discovered
that different chromosome regions control the development of different
parts of the fish skeleton. Even parts of the skeleton that are in
close proximity to each other are controlled by different regions of
the genome.
“This makes sense because when you think about the diversity
of size and shape among different animals, it is clear that vertebrates
have to be able to independently modify the size and shape of
individual skeletal features,” said Kingsley.
Second, the researchers discovered that the longest spine on the
fishes’ back and the single spine projecting from its belly,
whose lengths are highly correlated, are also linked genetically in a
single chromosome region.
“The fish use these spines for defense against
predators,” said Kingsley. “The total length of these
spines sets the cross-sectional diameter of the fish, which helps
determine whether or not they get eaten by predators like trout. Having
both spine lengths controlled by the same genetic region may help
explain how the fish achieve useful modifications of these functionally
related skeletal structures.”
The overall results point to many different chromosome regions that
affect specific aspects of skeletal anatomy in sticklebacks and reveal
a flexible genetic system for independent modification of the size and
number of different feeding and armor structures, the authors report.
But perhaps more important, said Kingsley, is the creation of a
resource that will help bring together a large body of ecological work
with the tools of modern genomics to create a new major model organism
for the study of evolution of species.
“There is a lot of interest right now in comparative
genomics,” said Kingsley. “But for most of the species
proposed to be studied, the timescale of evolutionary divergence is
enormous, making it difficult to sort out which genetic changes are
truly responsible for species differences. In contrast with the
sticklebacks, this genetic approach lets the organism tell us where the
relevant genes are. Rather than betting on a favorite gene being
important, we let the fish tell us which chromosome regions we should
pay attention to. Those regions can then be studied in detail to
identify the molecular basis of evolutionary changes in
vertebrates.”
Photo: Katie Peichel, Pam Colosimo, and David Kingsley (HHMI and Stanford University)
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Versión en español
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