Population genetic structure of two cryptic duckweed species (Lemna minor & L. turionifera) in Alberta using a genotyping-by-sequencing approach

Identifying population genetic structure is important for the development of species-specific management plans. Investigating the population genetics of cryptic species is even more critical. Here we focus on two cryptic duckweed species easily mistaken for one another, Lemna minor L. and L. turionifera Landolt, which have overlapping ranges in our study region of Alberta, Canada, and elsewhere. We used genotyping-by-sequencing to determine the population genetic structure of both duckweed species. A total of 192 samples was sequenced. After filtering, 16,007 single nucleotide polymorphisms were used to examine patterns of genetic diversity between and within L. minor and L. turionifera. The two species showed clear differentiation. When examining L. minor singly, we discovered at least three genetically distinct populations among the 30 samples from eight sites, even though these were from a small geographic area. In contrast, when examining L. turionifera singly, we found no evidence of genetically distinct populations among 67 samples from 43 sites. We also examined the relationship between surface water quality variables and the distribution of the two Lemna species. The sites containing L. turionifera had a wider range of water chemistry variables suggesting they are more tolerant of different environmental conditions. In contrast, each of the three genetically distinct L. minor groups had different water chemistry profiles. Large differences between L. minor and L. turionifera in their regional distributions and degrees of genetic differentiation highlight the importance of documentation and careful monitoring of Lemna species within Alberta, and in other regions where they co-occur.