Anyone who says evolution isn’t real should take pause at this new study (they won’t, but that is neither her nor there). A study published today in Science demonstrates that a common orchard pest, codling moths, has evolved extremely rapidly to be resistant to current methods of eradication. The resistant gene is sex-linked and grants the moths almost 100,000 fold resistance.

More details after the jump.

Bad apples

Codling moths are your stereotypical “worm in the apple” worm. They are a common pest problem in apple orchards, as well as pears, walnuts and other fruits orchards. The most effective method of control to date is actually not a chemical pesticide but rather a virus. The C. pomonella granulovirus (CpGV) is a highly virulent, highly specific virus used worldwide as a means to control codling moths. It is a very environmentally stable virus that can be sprayed onto orchard trees. CpGV is completely harmless to humans and animals. Because of its high virulence and lethality to codling moths, only sporadic cases of resistance have been seen, which usually clear up quickly.

Unfortunately, in the last three years orchards in France and Germany have been reporting increasing losses due to codling worms, presumably becaues of an increasing resistance to CpGV. The research team sampled codling moths from 13 affected orchards. Most of the orchards showed codling moths with a 1000-fold increase in resistance, while two showed a 100,000-fold increase in resistance. More moths were collected and bred in captivity to attempt to isolate the genetic source of this resistance. After some genetic breeding wizardry, populations of moths completely resistant to CpGV were isolated. The source of resistance, it turns out, is a single mutation. More worrying is that this gene is a sex-linked gene.

The key is sex (heh heh)

Human sex is determined by a Y chromosome. Males have one X and one Y chromosome (XY) while females have two X chromosomes (XX). Codling worms are similar but in reverse (and use different letters, because geneticists are strange). Males have two Z chromosomes (ZZ) and females have one Z and one W (ZW).

The resistance gene is located on the Z chromosome. It also appears to grant resistance even when there is only one copy of the gene. This means that all moths with the gene (even if males are only heterozygous for the mutation) are granted resistance. And since the gene is sex linked on the Z chromosome females that get a single resistance gene are capable of surviving and reproducing.

Furthermore, since CpGV is generally the only means of pest control used, it is the only active selecting agent deciding which moths live and which die. Even orchards with low concentrations of the resistance gene show fast explosion of resistance becaues of this selective uniformity. Lastly, the nature of the virus itself makes it the situation difficult. CpGV only contains one strand of viral DNA, instead of hundreds like other viruses. This means the genetic variation of the virus itself is fairly uniform, preventing viral mutations from nullifying the resistance gene.

Uh Oh

Clearly, the situation is not looking good for orchard owners, many of whom will be forced to start using other pest-control methods. Organic orchards especially are going to be in trouble as they do not use chemical pesticides (not that chemical pesticides are particularly effective against codling moths anyway).

This is a good lesson, however, on the effects of evolutionary biology in practice. To prevent something like this from happening again, viruses that contain multiple genetic lines and multiple methods of infection should be used. The single, uniform lethality factor used by the CpGV quickly selected a resistant mutation. The more the virus is used, the more quickly it selects resistant moths.

Unfortunate for orchard owners but pretty cool from a evolutionary point of view. One could guess that the mutation can’t be much older than 5 or 6 years, perhaps even less. Very cool

References

Asser-Kaiser S, Fritsch E, Undorf-Spahn K, Kienzle J, Eberle K, Gund A, Reineke A, Zebitz C, Heckel D, Huber J, Jehle J. Rapid Emergence of Baculovirus Resistance in Codling Moth Due to Dominant, Sex-Linked Inheritance. Science 2007; 317(5846):1916-1918. DOI:10.1126/science.1146542