Colony of young aphids.

Aphids could be considered the “mosquitoes” of the plant world, depending on the “blood” of plants to survive. They live in symbiosis with bacteria that pass from one generation to the next, producing essential amino acids. Aphids with the same genotype can be wingless or winged. In different seasons, they develop as asexual females who produce offspring with identical genes through parthenogenesis. When temperatures drop, they can give birth to males who then fertilize the eggs laid by females.

The genome of the pea aphid, sequenced by the International Aphid Genomics Consortium, reflects these unusual characteristics and more, said Dr. Stephen Richards, assistant professor in the Baylor College of Medicine Human Genome Sequencing Center and leader of the sequencing effort. The consortium released the 464 megabyte draft genome of the pea aphid (Acyrthosiphon pisum) in the current issue of PLoS Biology.

“Because this is a different kind of insect — not a fruit fly, not a beetle, not a hymenoptera (butterfly and moth) — we are seeing things that people have not seen in other projects,” Richards said.

Dr. David Stern, professor of ecology and evolutionary biology at Princeton University, said that even though he pushed hard to get the aphid genome sequenced, “it turned out to be far more interesting than I was expecting.”

He agreed with Richards that the aphid presents a special case.

“Look at this little insect, sitting on a plant and sucking plant juices. You don’t realize that it is involved in a historic battle with plants for access to its life blood. All its genes have evolved to allow it to exploit its feeding relationship,” said Stern.

“We found a lot of genes — 35,000 compared to 15,000 to 20,000 in other insects and 25,000 in humans,” said Richards, a corresponding author of the paper.

“Thus it seems that pea aphids (one among the 4,500 other aphid species on the planet) have duplicated some of their genes,” said Dr. Denis Tagu, senior scientist with the French National Institute for Agricultural Research. “What does this mean? It means that the pea aphid probably did a kind of ‘back-up’ of its genetic material. One hypothesis is that one copy of this back-up is kept unchanged and used for the functioning of the cells and the organism, and that the second set can allow modifications by mutations.”

“Most of the mutations are probably neutral or negative for the genes, with no effect on the biology of the organism. But some rare mutations might produce new functions for some of the genes that might help, in this case, the pea aphid adapt to its environment.”

“Another possibility is that maybe aphids require extra copies of genes because they have such complex life cycles,” Stern said.

“They have multiple forms to adapt to different environments. There are winged forms, forms without wings. Some produce asexually but give birth to life offspring. When the environment becomes more hostile, as in the fall, they give birth to males whose only purpose is to mate with females, who then lay eggs that hatch later on,” Stern said.

“Maybe the aphids need all these to regulate all parts of their life cycles. This genome has generated far more exciting questions than we could have anticipated. There is more mystery in this genome than anyone would have expected,” he said.

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