Novel microsatellite loci reveal high genetic diversity yet low population structure for alfalfa leafcutting bees in North America

Authors: Strange, James; DELANEY, DEBORAH - University Of Delaware; TARPY, DAVID - North Carolina State University; James, Rosalind

Submitted to: Conservation Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2017
Publication Date: 2/17/2017
Citation: Strange, J.P., Delaney, D.A., Tarpy, D.R., James, R.R. 2017. Novel microsatellite loci reveal high genetic diversity yet low population structure for alfalfa leafcutting bees in North America. Conservation Genetics. 18(3):679-687. https://doi.org/10.1007/s10592-017-0943-9.
DOI: https://doi.org/10.1007/s10592-017-0943-9

Interpretive Summary: Alfalfa is an important forage crop for livestock in North America, and the forage industry relies on the production of quality seed for establishing fields. Seed production by alfalfa is dependent on bee pollinators and chief among them is the alfalfa leafcutting bee. Annually, alfalfa leafcutting bees are commercially produced in Canada and shipped to the western United States where they are deployed for alfalfa seed production. Because the alfalfa leafcutting bee is non-native, having been accidentally introduced from Europe, we predicted that we would find a low level of genetic diversity in the North American populations and little local genetic adaptation due to the continual movement of bees from north to south. We developed sixteen DNA markers to compare the genetic diversity in North American populations to two European populations to assess the genetic diversity and population structure of the bees. We found that the North American populations had a surprisingly high level of genetic diversity, comparable to the levels seen in Europe. However, as we predicted, the North American populations exhibited no geographic population structuring, whereas the European samples were genetically distinct from each other and from the North American bees. These results suggest that the high level of genetic diversity observed in North America could be the basis for stock selection to increase pollinator health.

Technical Abstract: The alfalfa leafcutting bee, Megachile rotundata, is an economically important pollinator necessary for seed production of the critical forage crop alfalfa, Medicago sativa. At issue is how the large-scale commercial movement of this bee affects the genetic structure of local populations in the North American seed growing system, which is fundamentally important to understanding the population dynamics of this highly managed pollinator. We derived 18 novel microsatellites from the M. rotundata genome, then sampled M. rotundata from across North America, as well as its native range in Europe to serve as out-groups. Using 16 microsatellite loci, we describe the population structure of this bee. We found that M. rotundata collected from alfalfa seed farms has a surprising degree of genetic variability, similar to native European populations. Considering that the species was accidentally introduced into North America, we expected to find more of a founder effect. Despite the high genetic variability, we found little, if any, genetic structuring across North America, other than that the North American populations were distinct from the Old World populations sampled. While Bayesian methods detected some sub-structure in North American populations, the structuring was without geographic pattern, perhaps the result of the intense management and movement of these bees. The human trade and movement of these bees has created a nearly panmictic M. rotundata population, yet retained a high degree of gene diversity across the continent. This could have implications relevant to the preservation and conservation of other bee pollinators.