Source: Eidgenössische Technische Hochschule Zürich (ETH Zürich)

Simulation of transgenic pollen dispersal by use of different grain colour maize
by Michael Bannert
Diss. ETH No. 16508

SUMMARY

Cross-pollination of maize was studied by plant breeders in former years in order to guarantee seed purity. Today, the cultivation of transgenic maize varieties has world-wide increased rapidly. As a result, cross-pollination research has made a comeback in the context of managing the coexistence of conventional and transgenic maize. This situation is different to the plant breeding situation, where female plants get detasseled and are highly receptive both to the pollen from the male parent but also to adventitious pollen from neighbouring fields. Maize is a wind-pollinated crop that produces large amounts of pollen and favours cross-pollination by protandry of flowers. Therefore maize has a biological potential to cross-pollinate by pollen dispersal into neighbouring fields.

Studies about cross-pollination in maize have been published in recent years in many countries of the world. However, so far no experiments have been carried out under the Swiss alpine conditions, which differ in constellations of potentially important influence factors like weather conditions and topography. Twenty-two field experiments were conducted in 2003 and 2004, mainly in two different regions that are typical of Switzerland: In an alpine region of canton Uri and in the Swiss midlands of canton Zurich. Cross-pollination was not directly measured by use of transgenic maize varieties. Instead of this an alternative approach was used by simulating transgenic cross-pollination with maize varieties of different grain colours. In this visual marker system, yellow grain maize is assumed to be transgenic and white grain maize as conventional. Because of the xenia effect the dominant yellow grain colour can be transfered by pollen to white grain maize and such cross-pollinations are visible as yellow grains on the white grain maize ears. The experimental white grain hybrid, DSP17007, was tested in pilot experiments and identified as being comparable to modern hybrids. The method of measuring cross-pollination by counting the number of yellow grains on white grain ears was time and cost effective and enabled high sample rates. This made it possible to investigate complex cross-pollination patterns in high detail in order to optimize sampling procedures as well as to analyse cross-pollination events at very low levels of incidences.

Cross-pollination in long distance was investigated at distances of 50 to 4500 m in the alpine region of canton Uri. Thirteen fields of white grain maize were arranged in different wind orientations to the yellow grain pollen donor. The rate of cross-pollination of total fields was always below 0.02 %. Cross-pollination patterns were in most cases like randomly dispersed, with mainly single cross-pollination events and an average cross-pollination frequency of 1.8% of the sampled ears. In four fields, located in distances of 50 to 370 m in the main wind direction to the yellow grain pollen donor field, a low but marked cross-pollination occurred at the field border indicating pollen dispersal by horizontal winds. The potential distance of pollen dispersal was calculated according to the measured wind conditions in relation to settling height and speed. Thereby it was confirmed that most of the shed pollen will not reach distances beyond 50 m. A pollen take-off experiment corroborated that only a very small portion of pollen will move vertically above the field, due to special events like thermals or gusts. These results help to explain the low rates of long distance pollen dispersal. A few “hot-spot” areas with higher cross-pollination rates existed. But whenever the surrounding plants were checked in detail, pure yellow grain contamination plants were found. Therefore “hot-spots” may be more often an effect of seed contamination rather than of extreme or special atmospheric events. This should be carefully taken in account for any cross-pollination experiment.

Cross-pollination experiments in short distance were focused in the Swiss midlands of the canton Zurich on the situation that maize fields are adjacent to each other. Such situations may arise when a farmer cultivates different maize types on the same field or when neighbouring farmers plant their maize crops without another separation crop between them. Moreover, such a situation is of scientific interest as an extreme situation. With different field experiment settings different constellations of influence factors were checked in order to investigate the variability of the cross-pollination rate and to define “worst-case” situations of maximum cross-pollination. In a wind exposed location there was a clear effect of the main wind direction; in most of the other cases the wind effect was probably minimized by the variable topography of the Swiss midlands, which are characterized by a small scale mix of hills, scattered woods and settlements. An important effect of the size ratios of pollen-donor and pollen-receptor fields was hypothesized. The investigated size ratios varied from about 4 : 1 to 1 : 8. However there was no visible impact on the cross-pollination gradients. Probably because of the high settling speed of the pollen only a small portion of pollen will be dispersed beyond distances of 10 m and therefore different field size ratios will not modify the cross-pollination rate significantly. A strong effect was shown by flower asynchrony. When the pollen-donor field sheds pollen five days later than the emergence of silks in the receptor field the cross-pollination rate was lower than 0.9%, even in the neighbouring row at a distance of 1 m, which is in accordance of observed wilted (already fertilized) silks around five days after emergence. It was proven that asynchronous flowering can strongly be modified by the synchrony or homogeneity of flowering within a receptor field. One pollen donor field flowered seven days later than the mid silk emergence of the receptor field. Therefore, little or no cross-pollination should have occurred. A surprisingly high cross-pollination rate was due to a high number of yellow grains on small weak ears that were late in flowering when the availability of white grain pollen was probably already quite low in relation to the inflow from the yellow grain donor. In all field experiments variation in cross-pollination was high at close distance to the pollen donor but the rates decreased rapidly with distance and beyond 15 m they were more or less below 0.9 % in all experiments.

The results of this Swiss study supported and complemented the results of international studies. In general cross-pollination rates were lower, which might be a feature of special Swiss conditions for climate, topography and landscape patterns or the special flower biology of the used maize varieties.

All values for cross-pollination were calculated for the heterozygous case of transgenic varieties at present. Therefore the cross-pollination values were halved as yellow varieties were homozygous for the transferred yellow grain colour marker.

Full report in PDF format: http://www.agrisite.de/doc/ge_img/pollen-swiss.pdf

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