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

Establishing conservation priorities requires an understanding of the taxonomic limits of a group. 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. However, further work is still necessary, including more Monadenia taxa, in order to evaluate species limits and investigate mechanisms underlying the morphological diversity in this group.

确立保护优先级，需先明确类群的分类学边界。单齿蜗牛亚属（Monadenia）的陆生蜗牛共计6个物种，其中俄勒冈州已记录3种：费氏单齿蜗牛（M. fidelis）、蔡氏单齿蜗牛（M. chaceana）以及在其推测分布范围以北的北加利福尼亚地区新发现的M. infumata。此外，费氏单齿蜗牛（M. fidelis）至少包含6个已命名亚种，其中1个为联邦保护候选物种；通过大规模采样与外壳形态学专家评估，还发现了更多独特类群。但目前仍不清楚这些形态变异究竟是真正独特的多样性单元，还是环境诱导的表型可塑性结果。

本研究旨在探究华盛顿州、俄勒冈州与加利福尼亚州境内的单齿蜗牛亚属（Monadenia）类群是否存在多个具有结构化特征的多样性单元。本研究采用线粒体条形码基因COI、基于ddRAD（双酶切位点相关DNA测序）的全基因组SNP（单核苷酸多态性）数据集，结合形态学专家评估，以更精准地界定俄勒冈州、华盛顿州部分区域以及加利福尼亚州境内单齿蜗牛亚属（Monadenia）的多样性边界，为该类群的保护策略制定提供依据。

形态学分析证实了独特形态型的存在，但基因组数据显示类群间存在广泛的基因混合（基因渐渗），甚至在物种层面亦是如此。不过本研究确实发现，研究区域北部与南部采集的样本间存在有限的地理结构；与喀斯喀特山脉种群相比，海岸山脉种群间的结构分化程度可能较弱。南北分组的形态型间遗传分化程度相似，但部分形态型与物种间的两两分化估计值则显著更高。

本研究发现单齿蜗牛亚属（Monadenia）类群间存在基因混合与基因流，甚至在物种层面亦是如此，这使得确立保护优先级所需的个体化多样性单元评估变得更为复杂。不过，仍需开展更多研究（包括纳入更多单齿蜗牛亚属类群），以评估该类群的物种边界并探究其形态多样性背后的机制。