Population genomics of Monadenia (Gastropoda: Stylommatophora: Xanthonychidae) land snails reveals structuring but gene-flow across distinct species and morphotypes

Establishing conservation priorities requires an understanding of the diversity within and among taxa. Land snails in the subgenus Monadenia consist of six species, three of which are recognized in Oregon, M. fidelis, M. chaceana and a recently discovered M. infumata found north of its presumed range limits in Northern California. Further, M. fidelis is composed of at least six named subspecies, one of which is a candidate for federal listing, and extensive sampling and expert assessments of shell morphology have uncovered even more distinct forms. Still, it is unknown if these morphological variants are truly distinctive units of diversity or rather reflect environmentally driven plasticity. Here we investigate whether there are multiple, structured units of diversity in Washington, Oregon, and California Monadenia. We used COI, a mitochondrial barcode gene, a ddRAD-based genome-wide SNP dataset, along with expert-assessment of morphology, to better delimit diversity of Monadenia in Oregon and in parts of Washington and California to inform conservation strategies for this group. Morphological analyses confirm the existence of distinctive morphotypes, but genomic data show widespread admixture, even at the species level. Still, we do find limited geographic structuring between samples collected in the northern versus southern portions of the study area and possibly weak structuring between populations in the Coast Ranges compared to the Cascade Mountains. Genetic differentiation was similar among the morphotypes in the north-south grouping, but pairwise estimates of differentiation were much greater among some morphotypes and species. Our finding of admixture and gene flow across Monadenia, even at the species level, complicates assessment of individuated units of diversity critical for establishing conservation prioritization. Further work is still necessary, such as including more Monadenia taxa, in order to evaluate species limits and investigate mechanisms underlying the morphological diversity in this group.

Automated summary

This research investigates the diversity of Monadenia in Washington, Oregon, and California, revealing structured units of diversity. Genomic data show widespread admixture and significant differences in genetic differentiation among different morphotypes and species. Further work is needed to evaluate species limits and mechanisms underlying morphological diversity in this group.