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«Simulation of transgenic pollen dispersal by use of different grain colour maize A dissertation submitted to the SWISS FEDERAL INSTITUTE OF ...»

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Diss. ETH No. 16508

Simulation of transgenic pollen dispersal

by use of different grain colour maize

A dissertation submitted to the


For the degree of

Doctor of Natural Sciences

Presented by

Michael Bannert

Dipl. Ing. Agr.

born June 13, 1973

citizen of Bischofswiesen (Germany)

Accepted on the recommendation of

Prof. Dr. Peter Stamp, examiner

Prof. Dr. Klaus Ammann, co-examiner

Dedicated to Dr. Alberto Soldati *22.3.1945 +17.3. 2004 Alberto Soldati was born in 1945 as son of a farmer in Lugano - Tessin and found his way to the Federal Institute of Technology Zurich. There, he became the well-known representative of the ETH Field Research Station in Eschikon-Lindau, which he helped to plan from the modest beginnings and which was efficiently managing until the sudden end of his career.

Due to his scientific expertise and thanks to his friendly personality he always succeeded in a smooth and efficient working environment. Alberto Soldati initiated the maize crosspollination research of the ETH Agronomy and Plant Breeding group in the year 2002 that resulted in the present thesis.

Alberto Soldati was my supervisor and supported me in scientific and personal questions.

In 17. March 2004 his sudden death forced me to finish our project without him. But his working part should be never forgotten like also his person.

Summary Cross-pollination in maize was a topic that has been researched in former years by plant breeders in order to guarantee seed purity. Nowadays, the cultivation of transgenic maize varieties has rapidly increased worldwide; therefore, cross-pollination research has experienced an equally rapid comeback, this time in the context of managing coexistence of conventional and transgenic maize. This differs somewhat to the present plant breeding situation, where female plants get detasseled and are highly receptive to both the pollen from the male parent but also to adventitious pollen from other neighbouring fields. Maize is a wind pollinated crop that produces high amounts of pollen and favours cross-pollination by protandry of flowers and has therefore biologically the potential to cross-pollinate by pollen dispersal into neighbouring fields.

Some studies about cross-pollination in maize have been published in recent years across the world; but so far no experiments have been carried out for the Swiss alpine conditions that are different in constellations of potentially important influence factors like weather conditions and topography. In the years 2003 and 2004 22 field experiments were conducted, mainly in two different regions that are typical for Switzerland: In an alpine region of Kanton Uri and in the Swiss Midlands of Kanton Zurich. Cross-pollination was not measured directly by use of transgenic maize varieties. Instead of this a seemingly simple approach was used by simulating transgenic cross-pollination by maize varieties of different grain colours. In this visual marker system yellow grain maize is assumed to be transgenic and its cross-pollination of a white grain receptor maize can be immediately detected by xenia effect, in this case the transfer of yellow grain colour shows upon the white grain plant for every successful fertilisation. An experimental white grain test hybrid, DSP17007, was identified as comparable to modern hybrids, which was proven in pilot experiments. The method of measuring cross-pollination by counting the number of yellow grains was time- and costeffective and enabled high sample rates, which made it possible to investigate complex crosspollination patterns in high detail in order to optimize the sampling procedures as well as in order to analyze cross pollination events even at very low levels of incidences.

Cross-pollination was investigated at long distances of 50 m up to 4500 m in the alpine region of Kanton Uri. Thirteen white grain fields were arranged at different wind orientations to the yellow grain pollen donor. Cross-pollination rate of total fields was always below 0.02 %.

Cross-pollination pattern were in most cases like randomly dispersed, mainly single crosspollination events, with an average cross-pollination frequency of 1.8% of the sampled ears.

In four fields, located in distances of 50 m up to 370 m in the main wind direction, a low but marked cross-pollination at the field border was found; this indicated pollen dispersal by horizontal winds. The potential pollen dispersal distance was calculated according to the measured wind conditions in relation to settling height and speed; hereby it could be confirmed that most 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 vertical above the field, due to special events like thermals or gusts; these results help to explain the low rates of long distance pollen dispersal. Some 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 than of extreme or special atmospheric events. This should be carefully taken in account for any cross-pollination experiment.

Short distance cross-pollination experiments in the Swiss Midlands of the Kanton Zurich focused on the situation that maize field are adjacent to each other; such situations may arise when a farmer intends to produce different maize types on the same field or when two neighbouring farmers are planting their maize crops without another separation crop in between. Moreover, such a situation is of scientific interest as an extreme situation.

With different field designs the variability of the cross-pollination rate was checked by different constellation of influence factors on cross-pollination rate in order to define “worstcase” situations of maximum cross-pollination. In a few wind exposed locations a clear effect of the main wind direction was found, in most other cases the wind effect was probably minimized by the varied topography of the Swiss Midlands, characterized by hills, scattered woods and settlings. An important effect of the size ratios between pollen donor and pollen receptor fields was hypothesized. These size ratios varied from about 4:1 to 1:8, however there was no impact visible on the cross-pollination gradients in distance to the pollen donor;

only a small portion of the pollen anyhow dispersed beyond distances of 10 m. A strong effect was exercised 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 below 0.9% even in the neighbouring row at 1 m distance, which is in accordance of observed wilted (already fertilized) silks around five days after emergence. But it could be proven that flowering asynchrony can be strongly modified by the synchrony or homogeneity of flowering within a receptor field. One pollen donor field was seven days later in flowering than the mid silk emergence of 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 probably the white grain pollen availability had become already quite low in relation to the inflow from the yellow grain donor. Although variations of cross-pollination were high at close distance to the pollen donor the rates decreased rapidly with distance and were in all experiments beyond 15 m below 0.9 %, except for some rare spots with higher rates.

The results of this Swiss study supported and complemented results of international studies in a detailed and reliable manner; 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 cross-pollination values were calculated to the hemizygous case of transgene varieties at present: Cross-pollination data were halved as yellow varieties were homozygous to the transferred yellow grain colour marker.

Zusammenfassung Schon vor langer Zeit untersuchten Züchter die Fremdpollenbefruchtung von Mais um die Reinheit von Saatgut garantieren zu können. Heute, bei einem raschen weltweiten Anstieg der Kultivierung von genetisch modifizierten Maissorten, ist die Fremdpollenbefruchtung von Mais erneut aktuell, um die Koexistenz von gentechnisch veränderten und konventionellen Mais zu regeln. Die Koexistenz Thematik unterscheidet sich deutlich von der Saatgutproduktion, bei der von den Mutterpflanzen die männlichen Blütenstände entfernt werden und diese dann sowohl für den Pollen der Vaterpflanzen als auch für Pollen von ausserhalb der Anlage sehr empfänglich sind. Mais ist eine windbestäubte Kulturpflanze und einige biologische Merkmale der Blüte, wie die Produktion von hohen Pollenmengen und die Protandrie (Vormännlichkeit), begünstigen Fremdpollenbefruchtung.

In den letzten Jahren wurden weltweit einige Studien über die Fremdpollenbefruchtung von Mais veröffentlicht. Aber bis jetzt gab es noch keine Untersuchungen unter den alpinen Bedingungen der Schweiz, bei der wichtige mögliche Einflussfaktoren wie Wetterbedingungen und Topographie anders sind. In den Jahren 2003 und 2004 wurden hauptsächlich an zwei unterschiedlichen Standorten, die typisch für Schweizer Verhältnisse sind, 22 Feldversuche durchgeführt: In der alpinen Region des Kantons Uri und dem Schweizer Mittelland von Kanton Zürich. Die Fremdpollenbefruchtung wurde nicht direkt mit transgenen Maissorten untersucht sondern durch die Verwendung von Maissorten mit unterschiedlicher Kornfarbe simuliert. Bei diesem visuellen Marker System simuliert gelbkörniger Mais die transgene Sorte und weisskörniger Mais die konventionelle Sorte.

Befruchtet Pollen von gelbkörnigen Sorten in weisskörnigen Mais entsteht dort aufgrund des Xenien Effekts mit jeder erfolgreichen Befruchtung ein gelbes Korn. Als weisskörnige Sorte wurde die Test-Hybride DSP17007, die sich in Pilotversuchen als geeignet und vergleichbar mit heutigen Maissorten gezeigt hatte, verwendet. Diese Methode, die Fremdpollenbefruchtung durch einfaches Zählen von gelben Körnern auf der weisskörnigen Sorte zu ermitteln, ist bezüglich Zeit- und Kostenaufwand effektiv und ermöglichte hohe Stichprobenzahlen, um detaillierte Verbreitungsmuster zu untersuchen und auch seltene Fremdpollenbefruchtungen noch erkennen zu können.

In den alpinen Regionen des Kantons Uri wurde die Fremdpollenbefruchtung über lange Strecken zwischen 50 m und 4500 m untersucht. In beiden Versuchsjahren wurden zusammen dreizehn weisskörnige Versuchsfelder in verschiedenen Abständen und Wind Orientierungen zu den gelbkörnigen Feldern angelegt. Die Fremdpollenbefruchtung des gesamten Feldes blieb immer unter 0.02%. Die Verbreitungsmuster der Einkreuzungen waren hauptsächlich durch zufällige über das gesamte Feld verstreute Einkreuzungen, meist einzelne Einkreuzungen auf durchschnittlich 1.8% der untersuchten Kolben, charakterisiert. Bei vier Feldern, die in Abständen von 50 bis 370 m in Hauptwindrichtung eines gelbkörnigen Feldes orientiert waren, konnten an dem zugewandten Feldrand in der Randreihe etwas höhere Einkreuzraten gefunden werden, die vermutlich den Polleneintrag durch horizontale Windverfrachtung zeigen. Die durchschnittliche mögliche Pollenverfrachtungs Distanz wurde aufgrund der Windmessungen in Bezug zur Sinkgeschwindigkeit des Pollens errechnet. Damit konnte gezeigt werden, dass der meiste Pollen innerhalb von 50 m Reichweite zu Boden geht.

Ein Versuch zur vertikalen Verfrachtung des Pollens zeigte dass nur eine sehr kleine Menge des ausgeschütteten Pollens durch Windböen oder thermische Aufwinde in 4 m Höhe über das Maisfeld gelangt. Diese Ergebnisse bestätigen die gefundenen niedrigen Auskreuzungsraten jenseits 50 m Distanz. Einige „hot-spots“, Bereiche mit abweichend hoher Einkreuzung, wurden gefunden. In den Fällen, in denen die nähere Umgebung dieser „hot-spots“ genauer untersucht wurde, konnten gelbkörnige Verunreinigungspflanzen als Ursache festgestellt werden. Die auch von anderen Experimentatoren beobachteten „hot-spots“ sind daher vermutlich eher auf Saatgutverunreinigungen als auf extreme Wetterereignisse zurückzuführen.

Schwerpunkt am Versuchsstandort im Schweizer Mittelland im Kanton Zürich waren Versuche zur Fremdpollenbefruchtung über kurze Distanz bis 50 m mit dem Fall eines Anbaus von gelbkörnigem Mais direkt neben weisskörnigem Mais. Solche Situationen treten auf wenn ein Landwirt verschiedene Maissorten kultiviert oder wenn zwei benachbarte Landwirte ihre Maisfelder ohne einer anderen Feldfrucht dazwischen anlegen. Besonders aber ist diese Situation von wissenschaftlichem Interesse weil so die maximal mögliche Fremdpollenbefruchtung nachgewiesen werden kann.

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