Single-cell transcriptomics supports presence of cryptic species and reveals low levels of population genetic diversity in two testate amoebae morphospecies with large population sizes.

The enormous population sizes and wide biogeographical distribution of many microbial eukaryotes set the expectation of high levels of intraspecific genetic variation. However, studies investigating protist populations remain scarce, mostly due to limited 'omics data. Instead, most genetics studies of microeukaryotes have thus far relied on single loci, which can be misleading and do not easily allow for detection of recombination, a hallmark of sexual reproduction. Here, we analyze >40 genes from 72 single-cell transcriptomes from two morphospecies - Hyalosphenia papilio and Hyalosphenia elegans - of testate amoebae (Arcellinida, Amoebozoa) to assess genetic diversity in samples collected over four years from New England bogs. We confirm the existence of cryptic species based on our multi-locus dataset, which provides evidence of recombination within and high levels of divergence between the cryptic species. At the same time, total levels of genetic diversity within cryptic species are low, suggesting that these abundant organisms have small effective population sizes, perhaps due to extinction and repopulation events coupled with efficient modes of dispersal. This study is one of the first to investigate population genetics in uncultivable heterotrophic protists using transcriptomics data and contributes towards understanding cryptic species of non-model microeukaryotes.