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(BAM, unpublished).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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