Different genes code for two different forms of stickleback fish. Click on image for full size (306 Kb) Courtesy of Zina Deretsky, National Science Foundation

The stickleback fish, Gasterosteus aculeatus, is one of the most thoroughly studied organisms in the wild, and has been a particularly useful model for understanding variation in physiology, behavior, life history and morphology caused by different ecological situations in the wild.

On biological levels from molecular and genetic to developmental and morphological, and finally ending with the population level, it has proven far more complex than even imagined.
Studies of stickleback have provided us with a much better understanding of how organisms cope with new environmental conditions, first through acclimation over an individual's lifespan, and subsequently through adaptation of population via changes in gene form (allele) frequencies.

Given the rapidly changing global environment, this research not only provides insight into evolutionary processes, but is of practical importance in understanding how organisms will adapt to a changing world.

There are two forms of the stickleback: the oceanic and the freshwater type. The oceanic form lives in the ocean and comes into shallow estuarine or freshwater rivers and streams to breed, and has repeatedly given rise to a freshwater form that lives its entire life isolated in freshwater habitats.

Oceanic stickleback are protected by a complete set of bony lateral plates along the sides and dorsal and pelvic spines on the top and bottom of the fish. These structures help the fish survive attacks by birds and other fish-eating predators. The lateral plates develop first at the front of the fish, near the spines, and then are gradually added towards the tail until the entire side of the stickleback is covered.

Freshwater stickleback almost always evolve the loss of lateral plates, and sometimes the spines, as shown in the figure. This evolutionary change can occur very rapidly, sometimes in only dozens of years. An explanation for the loss of the bony plates is that energy is shunted away from bone formation and toward growth and reproduction instead, especially since the freshwater environment is stressful to the fish. In contrast to the ocean, freshwater lakes (especially in the far north) become iced over, limiting the prey items available to stickleback throughout most of the winter.

Coding for the lateral plates was initially determined to have a relatively simple genetic basis with one gene identified as a major contributor, Ectodysplasin-A (Eda). However subsequent mapping showed that in addition to the region of the genome surrounding Eda, two additional blocks of the same chromosome were also tightly linked to each other and the lateral plate trait. Genetic mapping work on Alaskan stickleback was conducted by William Cresko at the University of Oregon and supported by the National Science Foundation.

Fish develop full lateral plates if they have at least one copy of the Eda complete version of the gene (heterozygous or homozygous for the Eda complete allele). The fish lack the full complement of plates if they are homozygous for the recessive gene--Eda low. From the laboratory mapping results, and the rapid loss of plates observed in nature, biologists hypothesized that selection would always be for the Eda low allele in freshwater.

An experimental test by Barrett et al. has shown surprisingly unexpected results in fitness of the fish. The fish's lifespan is approximately a year. Over the course of a year, researchers sampled a controlled population of stickleback. They found that early in life, fish with Eda low were not as successful.

However, midway through their life, the tables turned and the fish with a copy of Eda low were more successful at surviving. In retrospect, these data might not be so surprising given the results from Cresko on the additional linkage blocks. Selection is likely directly on the Eda alleles when the fish is older, but may be on the other linked genomic blocks when the fish is younger, leading to a correlated change in Eda alleles. A challenge now is to determine what these other genes are, and how they might affect traits and fitness.

Text above is courtesy of the National Science Foundation

Different genes code for two different forms of stickleback fish. Click on image for full size (306 Kb) Courtesy of Zina Deretsky, National Science Foundation

Scientists have been studying stickleback fish to learn more about how animals cope when their environment changes. Since the Earth is changing quickly because of global warming and other global changes, this research helps us understand how animals will adapt to a changing world. There are two main ways that the stickleback fish cope when their environment changes. First, individual fish can change the way they live their lives. Second, a whole population of animals can adapt over time through the process of microevolution by natural selection.

Some stickleback fish live in the ocean while others live in freshwater. The fish that live in the ocean have bony plates and spines for protection. These plates help the fish survive attacks by birds and other predators. Freshwater stickleback fish usually don’t have the plates, and sometimes the spines, as shown in the illustration at the left.

This change is due to the process of microevolution. For these fish, it can happen very rapidly, sometimes in only dozens of years. That’s fast for evolution. One hypothesis for why the freshwater fish no longer have the protective plates and spines is that they use their energy for growth and reproduction instead of plate production. The freshwater environment is stressful for the fish. Lakes become iced over, limiting the prey items available to stickleback throughout most of the winter, so growth and reproduction are probably better ways to survive.

Of course, it is not up to an individual fish whether it grows plates and spines.  It is coded in a fish’s genes. Scientists have been studying the genes of the stickleback fish to learn which genes allow fish to develop lateral plates. They found the gene that codes for plate development. They called the gene Eda. However more gene mapping showed that in addition to Eda, two other blocks of the same chromosome were also linked to the growth of plates. Fish that had Eda and the other genes grew plates. Fish that had the recessive gene instead of Eda did not grow plates.

Different genes code for two different forms of stickleback fish. Click on image for full size (306 Kb) Courtesy of Zina Deretsky, National Science Foundation

There are two main ways that the stickleback fish and other animals cope when their environment changes. The fish in the new environment can change the way they live their lives. There is another way they change too. Over many years and many generations of fish, the whole population of fish can adapt to the new conditions through the process of microevolution. This changes the amounts of genes in the gene pool. And genes are what make the fish look like fish.

Oceanic stickleback fish have bony plates and spines for protection. These plates help the fish survive attacks by birds and other predators. Freshwater stickleback fish usually don’t have the plates, and sometimes the spines. This difference is due to microevolution. It can happen very rapidly, sometimes in only dozens of years. That’s fast for evolution.

Of course, it is not up to an individual fish whether it grows plates and spines.  It is coded in a fish’s genes. Scientists have been studying the genes of the stickleback fish to learn which genes allow fish to develop plates. They have found that fish that had certain genes grew plates. Fish that had different genes instead did not grow plates.