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Mapping blueberry genes.

Just as a road map shows a traveler the location and proximity of cities and towns, a genome map gives a scientist crucial information about the genes that control characteristics of a plant.

And now Lisa J. Rowland, a plant geneticist with the ARS Fruit Laboratory located at the Beltsville (Maryland) Agricultural Research Center, has developed an initial genome map for blueberries.

"A genome map shows a plant's chromosomes, along with the different genes or markers they contain," Rowland says. "The blueberry map currently comprises 12 linkage groups, which should represent the plant's 12 chromosomes."

To an untrained eye, the blueberry genome map looks like a chart with 12 different-length columns of numbers. The numbers are actually names" that have been given to the genetic markers thus far identified by DNA analysis.

Rowland analyzed DNA samples from blueberry plants with both low and high chilling requirements, using a system called RAPD--Random Amplified Polymorphic DNA--to identify various genetic markers. Markers can be an enzyme, a piece of DNA, or some other visual trait. Since the pieces of DNA analyzed were random samples, they could belong to any of the 12 blueberry chromosomes. But from those DNA bits, Rowland and colleague Amnon Levi found 70 genetic markers that naturally segregate as the 12 linkage groups of the current blueberry map.

With the chromosomes well covered with markers, Rowland says it will be possible to follow them in blueberry plants and eventually use them to tag genes of special importance. She is especially interested in identifying markers linked to--located near--the genes that control chilling requirement.

The blueberry and other woody perennials form flower buds in the summer and fall. As days grow shorter and night temperatures lower, perennials go dormant and stop growing. This is the time when they develop the cold hardiness that makes them able to withstand the much lower temperatures of winter. Apple, pear, and peach trees go through this same process.

Rowland says the blueberry could be a model for these other woody perennials. She uses blueberries because the plants are small, are easy to maintain in a greenhouse or cold room, and have separate leaf and flower buds.

Depending on the variety, blueberries need between 200 and 2,000 hours of exposure to temperatures from 32 [degrees] F to 45 [degrees] F--their chilling requirement--for growth resumption and budbreak in the spring. Therefore, blueberries with a high chilling requirement cannot be grown in areas like southern Georgia and Florida. And varieties that now grow in these regions can't be grown farther north.

"Planting blueberry plants with chilling requirements appropriate to the areas in which they grow best is vitally important for fruit to be produced," Rowland says. "If we can identify and follow the genes that control these traits, then we can perhaps develop better strategies for breeding blueberry plants that would thrive and be productive in different climates."

In another project, University of Maryland graduate student Mubarack Muthalif is helping Rowland identify the genes--not simply markers located near them--responsible for both chilling requirement and development of cold hardiness.

"So far, we've isolated three proteins from blueberry buds that seem to correlate with cold hardiness," Rowland says. "We know this is true because quantities of these proteins increase when the plants become more cold hardy. Then, when buds open and levels of cold hardiness drop, the protein levels drop as well."

Muthalif has purified the proteins and identified one as cyclophilin.

"We know that cyclophilin is present in every living organism examined," Rowland says. "And in humans, it binds to the drug cyclosporin (an immuno-suppressant) and suppresses the immune system. In blueberries, we don't yet know what this protein binds to or what its function is."

Scientists in this article are at the USDA-ARS Fruit Laboratory, Building 004, 10300 Baltimore Ave., Beltsville, MD 20705-2350; phone (301) 504-6654, fax (301) 504-5062.
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Author:Stanley, Doris
Publication:Agricultural Research
Date:Sep 1, 1993
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