Differential ability of three bee species to move genes via pollen

Abstract

Since the release of genetically engineered (GE) crops, there has been increased concern about the introduction of GE genes into non-GE fields of a crop and their spread to feral or wild cross-compatible relatives. More recently, attention has been given to the differential impact of distinct pollinators on gene flow, with the goal of developing isolation distances associated with specific managed pollinators. To examine the differential impact of bee species on gene movement, we quantified the relationship between the probability of getting a GE seed in a pod, and the order in which a flower was visited, or the cumulative distance traveled by a bee in a foraging bout. We refer to these relationships as ‘seed curves’ and compare these seeds curves among three bee species. The experiments used Medicago sativa L. plants carrying three copies of the glyphosate resistance (GR) allele as pollen donors (M. sativa is a tetraploid), such that each pollen grain carried the GR allele, and conventional plants as pollen recipients. Different foraging metrics, including the number of GR seeds produced over a foraging bout, were also quantified and contrasted among bee species. The lowest number of GR seeds set per foraging bout, and the GR seeds set at the shortest distances, were produced following leafcutting bee visits. In contrast, GR seeds were found at the longest distances following bumble bee visits. Values for honey bees were intermediate. The ranking of bee species based on seed curves correlated well with field-based gene flow estimates. Thus, differential seed curves of bee species, which describe patterns of seed production within foraging bouts, translated into distinct abilities of bee species to move genes at a landscape level. Bee behavior at a local scale (foraging bout) helps predict gene flow and the spread of GE genes at the landscape scale.

Honey bees exhibit greater patch fidelity than bumble bees when foraging in a common environment

“…The higher patch fidelity of honey bees could limit pollen movement and gene flow among patches (Rasmussen & Brødsgaard, 1992). Lower gene flow by honey bees, relative to bumble bees, is supported by within foraging bout data indicating a lower probability for honey bees to move genes longer distances relative to bumble bees (Fragoso & Brunet, 2023b). In natural populations, higher gene flow lowers genetic differentiation and homogenizes the genetic diversity of plant populations (Ellstrand, 2014).…”

The tripping mechanism of flowers affects pollen transfer dynamics

“…Even in plants that are capable of selfpollination, pollinating insects can increase seed set (Lang & Danka 2001;Abrol 2007) and fruit production (Klein et al 2003). Pollen transfer dynamics affects how far pollen and the genes they carry are moved (Castellanos et al 2003;Santa-Martinez et al 2021;Fragoso & Brunet 2023), and these processes influence the genetic structure of plant populations (Slatkin 1987). In agriculture, these processes also affect the potential for escape of genetically engineered cultivar genes (Kershen & McHughen 2005).…”

“…During the tripping process, the stigma brushes the pollinator's body as the anthers simultaneously deliver pollen onto the pollinator. Differences in tripping rates among pollinator species can affect the distance traveled by pollen (pollen dispersal), and subsequent gene flow, with bee species with higher tripping rates moving pollen shorter distances relative to bee species with lower tripping rates therefore limiting gene flow (Santa-Martinez et al 2021;Fragoso & Brunet 2023). However, less is known about the fundamental processes of pollen transfer dynamics, such as pollen accumulation on a bee's body and pollen deposition on stigmas, for plants with a tripping mechanism.…”