High gene flow maintains genetic diversity following selection for high EPSPS copy number in Kochia scoparia

Citation

Martin S.L., Benedict, L., Wei, W., Sauder, C. A., Beckie, H.J., and L. M. Hall (submitted) High gene flow maintains genetic diversity following selection for high EPSPS copy number in Kochia

Plain language summary

The major weed species kochia has successfully evolved resistances to four different herbicide modes of action. These resistances appear to spread quickly through kochia populations and could result in genetic bottlenecks, reduced standing genetic variation, and a reduced ability of populations to adapt further. Here we used double digest restriction enzyme associated sequencing to determine the level of gene flow among kochia populations, their levels of genetic diversity, and whether selection for glyphosate resistance resulting from an increase in copy number of the gene encoding the 5-enolpyruvylshikimate-3-phosphate synthase enzyme (EPSPS) is associated with a reduction in genetic variation. We determined that populations from the Canadian Prairies show little to no genetic differentiation and no correlation between genetic and geographic distance. This indicates that there is high gene flow among populations and no population structure. There is some evidence that kochia populations are somewhat genetically depauperate in comparison to other weed species, but genetic diversity did not differ between populations and individuals with high or low EPSPS copy number. While kochia is expected to be predominately outcrossing, the inbreeding coefficients suggest there fewer heterozygotes in these populations than expected, indicating that self-pollination rates may range up to 59%. We conclude any alleles for herbicide resistance will be able to spread quickly through the species’ populations and that the spread of these alleles appears unlikely to reduce the overall genetic variation of these populations. However, given this level of gene flow among populations, populations are unlikely to adapt to local conditions or form ecotypes without extremely strong selection pressure.

Abstract

1. The major weed species kochia has evolved resistances to four different herbicide modes of action. These resistances appear to spread quickly through kochia populations and could result in genetic bottlenecks and diminished standing genetic variation reducing the ability of populations to adapt further.
2. We used double digest restriction enzyme associated sequencing to determine the level of gene flow among kochia populations, levels of genetic diversity, and whether selection for glyphosate resistance resulting from an increase in copy number for the gene encoding the 5-enolpyruvylshikimate-3-phosphate synthase enzyme is associated with a reduction in genetic variation.
3. Populations from the Canadian Prairies show little to no genetic differentiation (FST = 0.01) and no correlation between genetic and geographic distance (r2 = -0.02 p = 0.56) indicating that there is high gene flow among populations and no population structure. There is some evidence that kochia populations are somewhat genetically depauperate in comparison to other weed species, but genetic diversity did not differ between populations or individuals with high or low 5-enolpyruvylshikimate-3-phosphate synthase copy number. While kochia is expected to be predominately outcrossing, the inbreeding coefficients suggest there are 23% fewer heterozygotes in these populations than expected and no variation was found to be associated with the chloroplast.
4. Synthesis and applications. These results indicate that any alleles for herbicide resistance will spread quickly through kochia’s populations, but that this spread is unlikely to reduce overall genetic variation in or limit the species’ ability to respond to further selection pressure.

Publication date

2018-03-01

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