Regional and global biodiversity may be underestimated due to the presence of cryptic species: species that are morphologically similar, but genetically distinct. Here we focus on two cryptic duckweed species, Lemna minor and L. turionifera, which have overlapping geographic ranges and are easily mistaken for one another. We developed species-specific primers based on DNA barcoding sequences to facilitate the rapid identification of these two monomorphic duckweeds, allowing us to investigate their presence and distribution in Alberta, Canada. While current reports indicate the presence of L. turionifera (and the morphologically distinct L. trisulca) in Alberta, our data indicate that L. minor is also present, predominantly in the southern part of the province. Thus, this paper (1) contributes to the accuracy and completeness of a regional flora, and (2) provides useful and flexible tools for the rapid molecular identification of cryptic Lemna species, species of wide interest in such diverse fields as biotechnology, toxicology, bioremediation, and ecology.

Identifying the 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, Lemna minor L. and L. turionifera Landolt, which have overlapping ranges in our study region of Alberta, Canada, and elsewhere, and are easily mistaken for one another. We used genotyping-by-sequencing to determine the population genetic structure of both duckweed species. A total of 192 samples were sequenced and after filtering, >16,000 SNP 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. turionifera singly, we found no evidence of genetically distinct populations among 67 samples from 43 sites. In contrast, 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. 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.

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