2013 Annual Report
1a.Objectives (from AD-416):
Objective 1: Using an insect-pollinated crop system, elucidate principles and data requirements for better predictions of gene flow via pollen in insect-pollinated crops.
Objective 2: Using squash with transgenic resistance to viral pathogens as a model system, develop a methodology to assess the impact of this transgene recently introduced into the genome of a wild species.
1b.Approach (from AD-416):
The number, types and acreages planted to transgenic crops are increasing. Consequently, there is a need to predict the likelihood of gene escape for different crops and a need to develop methodology to determine the impact of a transgene as it introgresses into wild populations. Because many crops benefit from insect pollination, part of our research investigates how distinct insect pollinators disperse pollen from plant to plant and ultimately among populations (gene flow). A better understanding of the impact of pollinator type on pollen dispersal would help us evaluate the differential risk of gene escape for distinct insect-pollinated crops while increasing our ability to select alternative pollinators for specific crops in the event of a major honeybee decline. On the one hand we study the impact of pollinator group on pollen dispersal and gene flow using the blue columbine as a model system. Information developed using this system will later be applied to different crops. On the other hand we examine the consequences of a disease resistance transgene that confers resistance to three economically important squash viruses as it introgresses into wild populations. We determine both the direct effects of the transgene on the fitness of free-living Cucurbita pepo (wild squash) and the indirect effects on diabroticite beetles (the primary non-target herbivore) and bacterial wilt (the major disease that these beetles transmit). In addition we measure gene flow among wild squash populations and gather basic information on their pollination biology and mating system. These types of data are critical to the efficient evaluation by regulatory agencies of the potential risk of transgenes introduced into wild plant populations.
We have shown in a natural system that distinct pollinators can have different impacts on gene flow. In the Rocky Mountain columbine, Aquilegia coerulea, hawkmoths can move genes via pollen further than bumble bees. Moreover, gene flow by bumble bees appear more affected by features of the landscape relative to gene flow by hawkmoths. We have determined that such variation in the potential for gene flow and impact of the landscape on distinct pollinators also applies to agricultural systems. In alfalfa, honey bees have the potential to move genes via pollen further distances relative to bumble bees. In addition, the potential for gene flow was affected by plant density, an attribute of the landscape, for bumble bees but not for honey bees. Because gene flow via pollen in insect-pollinated crops depends in part on how pollinators move over the landscape, using alfalfa as a model system, we have started investigating how pollinator movements vary among types of pollinators. In particular, we are examining whether distances and directions moved between successive racemes and number of flowers visited per raceme differed among bumble bees, honey bees, and leaf cutting bees. While honey bees and leaf cutting bees are used as managed pollinators in alfalfa seed production fields, the bumble bee is a wild pollinator often observed foraging on alfalfa flowers. We will later use this information to develop a simulation model of pollinator movement to help predict gene flow by distinct insect pollinators.
We have found that the number of flowers visited per raceme differed among pollinators with bumble bees visiting more flowers per raceme relative to honey bees and leaf cutting bees. We will determine whether this impacts the outcrossing rate of plants visited by these distinct pollinators. In addition, when more flowers were visited on a raceme, a shorter distance was traveled between racemes. We have also established that while bumble bees and honey bees show directionality of movements within a foraging bout, leaf cutting bees do not. A foraging bout represents the flowers visited in succession within a patch. These data illustrate that when bumble bees and honey bees start moving in a given direction within a foraging bout they tend to keep moving in that direction while this is not the case for leaf cutting bees. All three pollinator types, however, showed no preference for a given direction when foraging bouts were grouped together. These data therefore suggest that, with respect to gene movement via pollen within a field, the movement of genes via pollen by insect pollinators will not occur preferentially in a given direction but is as likely to happen in any direction. Understanding how distinct pollinators move within alfalfa fields will help us develop a model of pollinator movements that can be used to help predict gene flow in alfalfa seed production fields. The information we gathered fulfilled Objective 1, using an insect-pollinated model system, elucidate principles and data requirements for better predictions of gene flow via pollen in insect-pollinated crops.
Pollinator movements in alfalfa fields: Differences among pollinators. With the increasing acreages planted to transgenic crops and the increasing number of transgenes inserted into some crops, it is important to develop methodology to predict and try to minimize gene flow or the movement of genes from such crops. An important step is to examine and contrast pollinator movements among different pollinators because distinct pollinators have been shown to differentially affect gene flow. ARS researchers in Madison, Wisconsin and collaborators contrasted the movements of three pollinator groups, honey bees, leaf cutting bees and bumble bees, foraging on alfalfa flowers. Honey bees and leaf cutting bees are used as managed pollinators in alfalfa seed production fields while bumble bees are wild pollinators often seen foraging on alfalfa flowers. The number of flowers visited per flower cluster differed among pollinators with bumble bees visiting more flowers per raceme relative to honey bees and leaf cutting bees. In addition, the more flowers visited per cluster, the shorter the distance traveled to the next cluster. These behaviors could impact the outcrossing rate of plants visited by the distinct pollinators. In insect-pollinated crops, knowledge and understanding of characteristics that affect the behavior of distinct pollinators will help better predict and minimize gene flow by these pollinators. This information will ultimately benefit farmers, the general public and regulatory agencies interested in scientific data on the risk of gene escape from genetically engineered (GE) crops.
Brunet, J., Zalapa, J.E., Pecori, F., Santini, A. 2013. Hybridization and introgression between the exotic Siberian elm, Ulmus pumila, and the native Field elm, U. minor, in Italy. Biological Invasions. Available: http://link.springer.com/article/10.1007%2Fs10530-013-0486-z.