LegumePlus member gets paper published in Science

Prof. Juha-Pekka Salminen (Turku, Finland) - 'Insects drive the evolution of plants'

Plant eating herbivores may affect plant evolution much more rapidly than has been previously assumed, reveals a new study by University of Turku. The results were published 5 October in Science.

For a long time it has been speculated that interactions between plants and plant-eating insects are responsible for the species diversity we face in nature. So far it has been difficult to verify this experimentally.

Professor Juha-Pekka Salminen from University of Turku (Finland) and his colleges from Cornell, Toronto and Montana Universities have shown that insect driven natural selection causes evolutionary changes in plant populations amazingly fast, even in a few years.

Five years ago they created 16 plant plots in Ithaca, New York. Into each plot they planted an identical composition of 18 genotypes of evening primrose (Oenothera biennis). Eight of the plots were treated biweekly with an insecticide and the other eight plots with water. Otherwise the plots were not manipulated at all.

Evening primrose defended itself chemically against insects
The plots started to evolve genetically even during one or two primrose generations, but the pattern became clearer towards the end of the 5-year experiment. The insect-driven natural selection favored genotypes that were better defended chemically.

– It was interesting to see that in the plots with natural levels of insects such genotypes became more common that were able to produce more complex tannins into the fruits. These tannins were shown to protect the fruits from seed-feeding insects, Salminen says.

At the same time these genotypes flowered later so that fruits and seeds were less accessible for the seed-feeding herbivores.
Researchers noticed that in the insect-free plots the situation was totally different. In the absence of insect stress primroses relaxed their defenses to produce more simple tannins and at the same time genotypes containing more complex tannins became less frequent.

Battle against dandelions changed the genotype composition
In the insect-free plots primroses had to fight against another type of stress.

– When we removed insects from the plots we also removed the natural herbivores of the dandelions. For this reason the number of dandelions was doubled while the number of primroses was cut to half due to increased competition with dandelions, says professor Salminen.
The competition with dandelions changed the primrose genotype composition in two ways: the genotypes with better competitive ability increased in frequency and at the same time they tried to secure their seed-production by flowering earlier.

– If we can say that plants behave rationally, then this type of evolution was rational in the absence of insects. Unfortunately the earlier flowering together with lower levels of chemical defenses will make primroses more prone to increased seed herbivory. In this sense the evolution did not proceed into correct direction in the insect-free plots, Salminen notes.

The direction of the evolution may change from year to year
This research shows why evening primrose populations typically contain several different genotypes. In nature plants face several simultaneous stress factors. The proportional strength of each of the stresses will then determine into which direction natural selection will drive the genotype composition of the populations.

If there is a competition of living space during one year and a strong insect stress during the subsequent year, the genotypes do not necessary evolve into the same direction. The direction of the evolution may rather change from year to year.

The weakest genotypes may still be extinct as happened to two genotypes during this experiment. The strongest genotypes may become more frequent. On the other hand, a long-term evolution into the same direction may eventually cause the birth of a brand new species.

Professori Juha-Pekka Salminen
p. 02 333 6753 / 0400 824452

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