A team of Texas A&M University researchers has documented the natural transfer of DNA between animals and plants, a unique and rarely documented phenomenon.
During the course of their study, the team found that a group of DNA sequences in pine trees, spruces and other conifers stemmed from insects that transferred them to one of their ancestors approximately 340 million years ago.
"Loblolly pine is an important economic resource across the southeastern U.S., including Texas," said Claudio Casola, a forest genomics assistant professor at Texas A&M University and senior author of the study. "We are just now starting to understand the different parts that form the very large genome of this pine tree and other conifers.
"We called these conifer DNA sequences 'Dryads' after the Greek mythological nymphs that inhabit trees," he added. "Dryads are one of the many groups of DNA sequences known as DNA repeats."
These DNA repeats - also referred to as transposable elements - are very adept at replicating themselves, which is the reason that they make up such a large portion of certain plant species such as conifers.
The high proportion of these elements makes them of particular interest to scientists, as they can greatly influence the genetics of the plants that possess them.
"You can think of transposable elements as 'genomic parasites,'" Casola said. "They spread into new genomes kind of like viruses spread between people. Unlike flu and other viral disease, these 'genomic infections' occur rarely, but once established, they can persist for millions of years."
In fact, many years ago, viruses such as HIV evolved from transposable elements. However, many of these elements - called endogenous retroviruses (ERVs) - act more as "DNA fossils" of these viruses back from a time when they wreaked havoc on mammals.
"No less than 8 percent of our own genome is made of ERVs," Casola said. "Retroviruses, ERVs and other DNA repeats such as Penelope-like sequences share a unique way to make new copies of themselves."
Casola and his team speculated that Dryads stemmed from Penelope-like retroelements that were introduced into the conifer genome years ago. In order to confirm their theory, they analyzed the genome sequences of 1,029 species that were neither animals or conifers.
"Some of these other species seemed to contain Penelope-like retroelements; however, after thorough inspections of these DNA sequences, we concluded that they were due to DNA contamination from either animals or conifers," he said.
Further analyses confirmed that the Dryads are likely derived from insect Penelope-like retroelements.
This unique DNA transfer between animals and plants likely altered the evolution of the conifer genome, although in what way is unclear as of now.
The findings were published in the March 31 issue of Genome Biology and Evolution.
