Fishing for similarities aids in research A new study has found that two genetically similar fish have also shown markedly different abilities to grow - a finding that could provide new ways to research previously disparate areas. The study in the November issue of the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, finds that the two fish - the giant danio and the zebrafish - share a great deal in common genetically. The giant danio, however, appears to have the ability to recruit new muscle throughout its life. Humans have the same ability before birth, but mostly lose it after birth - similar to the zebrafish. The finding could provide a new way to research seemingly unrelated areas such as muscle wasting disease and fish farming, according to the study. The zebrafish, while a staple of genetic research because of its ease to raise and maintain, has been found to be too small for physiological research. The giant danio, however, can grow to a size of six inches. Researchers looking for a close zebrafish relative to allow physiological research found what they needed in the giant danio - which also allows them to draw upon the zebrafish's already-mapped genome. Researchers saw the giant danio as the physiological model and began a series of experiments on differences in muscle growth between the fish. These findings can be used to aid researchers who want to investigate muscle wasting diseases such as muscular dystrophy, according to the study. A condition similar to muscular dystrophy can be induced in a zebrafish. Researchers now hope to induce that condition into a giant danio.

More on: Aquatic genetic research Giant Danio Scientific classification Kingdom: Animalia Phylum: Chordata Class: Actinopterygii Order: Cypriniformes Family: Cyprinidae Genus: Devario Species: D. aequipinnatus Binomial name Devario aequipinnatus (McClelland, 1839) Devario aequipinnatus Fish Mapper: Devario aequipinnatus Zebrafish Scientific classification Kingdom: Animalia Phylum: Chordata Class: Actinopterygii Order: Cypriniformes Family: Cyprinidae Genus: Danio Species: D. rerio Binomial name Danio rerio (Hamilton-Buchanan, 1822) Zebrafish Info Network Zebrafish resources Zebrafish FAQs Genome resources Zebrafish journal

In the first phase of the study, the researchers tracked the size of the fish from the time they hatched through the first four weeks of their larval stage. They found that the two species were the same size at hatch. By the end of week one, the giant danio was significantly larger and remained so. This result was not surprising, since the giant danio is so much larger at adulthood, about twice as large. But the study was the first to report the early larval growth of these species, the authors said.

The researchers also documented muscle changes over the first four weeks in the fleshy portion of the fish's midsection, known as the myotome. They measured growth in length and width and tracked the number and size of muscle fibers, and found that the fish employ different methods of growth.

"Muscle growth in vertebrates is defined as being either determinate or indeterminate," the authors wrote. "Animals such as mammals exhibit determinate growth, in which there is a finite size. In contrast, many fish species growth is indeterminate, in which there is no fixed size and some growth may continue throughout the life of the fish."

Indeterminate growers increase muscle mass both by recruiting new muscle fibers (hyperplasia) and by increasing the size of the existing muscle fibers (hypertrophy). Many fish species exhibit indeterminate (hyperplastic) growth.

In this first phase of the study, the researchers found that hyperplasia accounts for 67 percent of the giant danio's muscle growth, compared to the zebrafish in which hyperplasia accounts for only 47 percent of the growth.

In the study's second phase, the researchers applied growth hormone once every three weeks for 17 weeks to adults of both species. The adult danio continued to grow during the 17-week trial and analysis of muscle fibers showed hyperplastic muscle growth.

Adult zebrafish did not exhibit increased growth or hyperplasia in response to growth hormone, suggesting that zebrafish reach a growth plateau similar to mammals and hence exhibit determinate growth. The zebrafish exhibited little hyperplastic growth after the juvenile phase.

Study co-author Peggy R. Biga anticipates the giant danio will be able to handle the muscular dystrophy better because of its ability to increase muscle.

"What makes this intriguing is that human embryos exhibit hyperplasia, but then lose that ability after birth, with one exception," she said. "When humans injure a muscle, the muscle sends a signal to special cells attached to the muscle fibers, telling these cells to grow and join to adjacent muscle fiber to repair the injury."

As for the fish farming connection?

If scientists understood how fish grow, they could enhance the growth of farmed fish and decrease the time it takes for them to reach market size.

"The more we understand about growth, the more likely we can come up with acceptable ways to enhance it," Biga said.

The breakthrough comes at a time when Animal Lab Magazine is taking a closer look at aquatics. This month's magazine features Keith Bailey discussing laboratory design to support marine animals. In addition, Austin Bailey investigates biocontainment issues in aquatics facilities. As the use of aquatic animals for research increases, so does the need for standards. The practices and procedures prevalent in rodent facilities need to be adapted for aquatic facilities. Steve Aldrich and Chris Obenschain take a look at what needs to be considered in this area.