Canola myopia
This study provides a much-needed foundation for understanding the population genetics and demography of CFM. However, we still know little about the historical ecology and evolution of this species, or whether CFM is likely to be a significant risk to canola production in North America. Notably, the hypothesis that CFM is native, based on its widespread distribution as well as its mitochondrial DNA and parasitoid diversity (Mori et al. 2019), remains speculative. The disjunct distributions of highly differentiated population genomic units in canola producing regions may provide additional evidence for this speculation, and lines of reasoning for future research. Our surveying and sampling was limited to the canola production regions across the Prairie provinces. Given the short history of widespread canola production in Canada (ca. 40 years), if CFM is native then it must have some native (and/or naturalized) hosts within or outside of this geographic region. Alternative host associations have yet to be thoroughly evaluated for this species, although CFM larvae and galls were found on mustard (Brassica juncea va. Centennial Brown) grown in a small plot on an AAFC research farm in Melfort, Saskatchewan (BAM, unpublished). This locality is outside of the typical mustard growing region of southwestern Saskatchewan and inside the primary distribution of CFM. If alternative hosts do exist for this species, our canola-centric sampling may have anthropogenically biased our assessments of population structure in two ways: 1) these geographically disparate, differentiated populations at the edge of the canola production region may represent bleed-over genetic structure from an alternative and more geographically widespread host range, and 2) the lack of strong differentiation across the majority of our sampling region may reflect a relatively recent bottleneck onto the anthropogenic host. Thus, if CFM is native, as hypothesized, our current assessment of population structure may suffer from the presence of unsampled “ghost populations” (sensu Beerli 2004) present on alternative hosts outside of the canola production region. Failing to sample ghost populations can decrease confidence in population assignments of sampled individuals (Beerli 2004; Slatkin 2004), and may explain the substructure recovered in our clustering analyses (Fig. 1, Fig. 2), and the ambiguous support for an optimal value of K in Structure analyses (Fig. S1).
This is largely supposition at this point; however, given the lack of alternative hypotheses to explain the disjunct pattern of highly differentiated populations at the edges of the canola production region, we believe it deserves follow up research. Most importantly, it will be vital to increase survey efforts to other Brassicaceae both within and outside the canola production region. Pheromone monitoring tools (e.g. Mori et al. 2016) would greatly facilitate this survey effort. Expanded surveying to also include more diverse potential habitats will provide important information about the range and host preferences of this species, and facilitate comparisons of regional or host-associated population densities that may inform CFM risk assessments and monitoring. Temporal sampling throughout the growing season will also help to clarify the number of generations that CFM can produce each year, and elucidate the ecological and population dynamics of early versus late generations. Finally, if our hypothesis of alternative hosts is substantiated, CFM may provide a unique model system for studying the consequences of a contemporary host shift onto a major commercial crop species, thus informing both the evolution of insect-plant relationships as well as impacts on pest management (Chen 2016; Bernal & Medina 2018; Bernal et al. 2019).
DATA ACCESSIBILITY Raw NGS sequencing files and mitochondrial DNA sequences are in the process of being deposited on NCBI SRA and GenBank, respectively.
AUTHOR CONTRIBUTIONS
BAM, MAV, JH, and SH contributed to study design, field collection and DNA sequencing. EOC, JRD, and BAM conducted analyses and wrote the manuscript. All authors provided feedback on manuscript drafts.
ACKNOWLEDGEMENTS
We would like to thank members of Agriculture and Agri-Food Canada, Canola Council of Canada, and Alberta Agriculture and Forestry, in particular Scott Meers and Shelley Barkley, for their help in collecting larval samples. This study contains information licensed under version 2.0 of the Open Government License - Canada. We also wish to recognize computational support provided by WestGrid (www.westgrid.ca) and Compute Canada (www.computecanada.ca). Research funding was provided by the Alberta Canola Producers Commission (Alberta Canola) and the Saskatchewan Canola Development Commission (SaskCanola) as part of the Canola Agronomic Research Program (CARP Grant 2017.12) to MAV and BAM, and the University of Alberta to BAM.
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