Recent Evolution - That Old Chestnut

Horse chestnut leaves infested with Cameraria ohridella lavae.

If you live in Southern England, for an example of very recent evolution you probably only need walk to your nearest horse chestnut tree. I took this photograph near Sunningwell, Oxfordshire earlier on Saturday when we went out for a last distant look at Didcot Power Station, which is probably England's best-loved 'eyesore' but which is due to disappear in a cloud of dust in a few hours time. The chances are that by now your nearest horse chestnut tree will be becoming infested with the leaf-miner caterpillars of a moth which was unknown before 1985 when the first outbreak was recorded in Macedonia, Greece.

Since then, Cameraria ohridella has spread at an average of 60 Km per year across western Europe reaching England in 2002, where it is now widespread, causing the leaves of horse chestnut trees (Aesculus hippocastanum) to turn brown, wither and fall off by late summer. Devastating though this attack appears, the general health of the trees seems not to be unduly affected since they come back into leaf and grown normally the following spring.

Although this moth was a newly-discovered species in 1985, specimens of it were accidentally collected and pressed in botanical specimens as early as 1879 only to be rediscovered when a team of researchers carried out a systematic search of specimens stored in herbaria.

Abstract
Determining the native geographic range or origin of alien invasive species is crucial to developing invasive species management strategies. However, the necessary historical dimension is often lacking. The origin of the highly invasive horse-chestnut leaf-mining moth Cameraria ohridella has been controversial since the insect was first described in 1986 in Europe. Here, we reveal that herbarium collections across Europe indicate a Balkan origin for C. ohridella. We successfully amplified nuclear DNA and mitochondrial DNA barcode fragments from larvae pressed within leaves of herbarium samples collected as early as 1879. These archival sequences confirm an identity of C. ohridella and set back its history in Europe by more than a century. The herbarium samples uncovered previously unknown mitochondrial haplotypes and locally undocumented alleles, showing local outbreaks of C. ohridella back to at least 1961 and dynamic frequency changes that may be associated with road development. This case history demonstrates that herbaria are greatly underutilized in studies of insect–plant interactions, herbivore biodiversity, and invasive species' origins.

David C Lees, H Walter Lack, Rodolphe Rougerie, Antonio Hernandez-Lopez, Thomas Raus, Nikolaos D Avtzis, Sylvie Augustin, and Carlos Lopez-Vaamonde 2011.
Tracking origins of invasive herbivores through herbaria and archival DNA: the case of the horse-chestnut leaf miner.
Frontiers in Ecology and the Environment 9: 322–328. http://dx.doi.org/10.1890/100098

Spread of C. ohridella from the Balkans.

Examination of the DNA from these preserved specimens shows that there have been two or three significant changes which have almost certainly caused the sudden explosion of this moth from its probable homeland in the Balkans, one of them genetic and two environmental. The interesting thing about the genetic change is that it seems to have been in the mitochondrial DNA.

Of the thirty known mitochondrial haplotypes occurring in the Balkans, only three, known as A, B and C are known to have moved into the rest of Europe of which haplotype A is the dominant form. This probably represents a new haplotype which somehow has enabled C. ohridella to colonise new environments, something which may have been aided by a succession of years of above average temperature and below average rainfall across Europe. However, hot dry weather as such does not seem to suit the moth judging by its failure so far to invade the Iberian Peninsula.

The second environmental change which may have facilitated the initial expansion out of the Balkans was increased road building - the moths are known to be transported accidentally by road goods vehicles.

So we have an example of a genetic change, in the presence of fortuitous environmental change, leading to a massive increase in population and range of a species but seemingly restricted to just three of the thirty mitochondrial haplotypes and predominantly just one of them. This illustrates the founder effect, in which a small, and therefore genetically unrepresentative, founder population moves out of its normal range into new territory and then moves quickly out into the new territory by a process which increases the proportion of the alleles which facilitated the expansion in the first place, so, for example, the genetic profile of C. ohridella in, say, southern England, may be very different to that in the Balkans.

And so we have an example of evolution in progress as the allele frequency in different parts of the range changes and adapts to environmental change and opportunity, and Britain now faces late summers with an important amenity tree becoming disfigured and unsightly and losing most of its leaves by late August.

However, the explosion of C. ohridella into western Europe is also an environmental change which other species can exploit. It is now becoming a major food source for blue tits (Parus caeruleus), great tits (Parus major) and marsh tits (Parus palustris) which are learning to extract them from the leaves and are now believed to account for 2 to 4 percent of lavae.

It has also created a potential new resource for parasitic wasps to exploit. The prediction is that a generalised parasitic wasp will 'discover' this new resource and will become a major predator so bringing about a degree of population control of a species which currently appears to be limited only by the availability of horse chestnut trees.

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