ATHENS, Ga. -- Illegal and socially taboo among humans, inbreeding is common, 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, far 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 Programme, the New Zealand Lottery Board and by Georgia McIntire-Stennis funds. Findings were published earlier this year in the journal Theoretical and Applied Genetics.
"Death is one of the most common results of inbreeding," said Bruce Bongarten, a forest geneticist in UGA'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 same "bad" genes will be provided by both parents and expressed in the offspring. Hemophilia in the British royal family 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 population. 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 sub-lethals, that sometimes kill but not always.
"The discovery of so many sub-lethals will make it far 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 programs is the skyrocketing value 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 commercially in South Africa, Australia and, 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 Ph.D. student, Hanhui 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," said Bongarten. "Molecular markers allow us to by-pass a lot of the steps of traditional breeding programs, but this study shows us we still have a long way to go."