Summer 1999 Volume 8, Number 3
Inbreeding Trees Bring Costs
by Helen Fosgate, University of Georgia -Journal of Forestry
Illegal and socially taboo among humans, inbreeding is com-mon, even natural, between trees. Still, it isn't without consequences: Inbred trees grow and develop slowly, they're often deformed, and many die suddenly and inexplicably before reaching maturity.
A team of scientists at the University of Georgia and the New Zealand Forest Research Institute have discovered why. Using inbred Monterey pines as a model, they identified seven genes that can cause the pine to die, for more than they suspected. It's the first time researchers have isolated lethal genes in any tree species.
The research was funded by the New Zealand Fund for Public Good Science, the USA Collaborative Science program, the New Zealand Lottery Board and George McIntire-Stennis funds. Findings were published last year in the journal Theoretical and Applied Genetics (Volume 96, Issue 5).The article may be downloaded as a PDF file from the journal's website at http://link.springer.de/link/service/journals/00122/bibs/8096005/80960640.htm .
Death is one of the most common results of inbreeding, said Bruce Bongarten, a forest geneticist in the University of Georgia's Warnell School of Forest Resources. "In selectively breeding trees, the idea is to cross closely related family members to increase the frequency of desirable traits, like fast growth, disease resistance, or high yields. Unfortunately, it also increases the effects of harmful and lethal genes."
Mating between close relatives increases the chances that the some bad genes will be provided by both parents and expressed in the offspring. Hemophilia is a well-known example in humans. Few inbred trees survive to reproduce in a natural forest setting. In selective breeding programs, it's just too costly to breed, plant, and nurture trees that grow poorly or ultimately die.
Identifying the genes is the first step in purging them from a breeding populat
ion. It
sounds simple, but Bongarten said it's complicated by the fact that not all lethal genes
cause death 100 percent of the time, In this study, the team found only one gene that
always kills and six others, called partials or sublethals, that kill sometimes but not
always.
"The discovery of so many sublethals will make it for more difficult to get rid of them," said Bongarten. 'These genes are recessive or hidden. We'll have to use molecular markers to identify them. Even then, it will be a time-consuming procedure."
The driving force behind tree breeding program is the greater profitability of tall straight fast growing trees for timber and engineered wood products. The Monterey pine, native to California and a few Islands off Mexico is one of the most economically important trees in the world. Bongarten said breeding programs have been ongoing for nearly 50 years, and genetically improved varieties are now used to establish all commercial stands.
"Ninety-five percent of the planted trees in New Zealand are Monterey pines" said Bongarten. "It's also grown in South Africa, Australia, more recently in South America primarily in Chile. It tends to grow best in Mediterranean climates." Scientists chose the Monterey pine for this study because of the need for selfed seed-seed mated with itself. Their long running breeding program meant New Zealand scientists already had selfed seed as well as a partial molecular map for this species. Both were essential in isolating the lethal genes. Bongarten's PhD student, Hanhul Kuang, spent a year in New Zealand, isolating the Monterey pine DNA and analyzing the data.
We just now have the molecular tools to study this, Bongarten said. "molecular markers allow us to by-pass a lot of the steps of traditional breeding programs", but his study shows us we still have a long way
For more information, contact Daniel Morkewitz, WarnellSchool of Forest Resources, University of Georgia, Athens, GA30602; (706) 542-0133; e-mail: dmorke@smokeyforestry.uga.edu.