Isolation by distance and strong population structure are potential indicators of nascent speciation, and such processes tend to leave genetic signatures detectable by reduced representation genomic screens. Understanding genetic relationships is especially important for effective management and conservation of threatened species. We describe the population structure of members of the Primula cusickiana species complex, a group of four closely related plants endemic to the Great Basin. Before being subsumed to varieties in 2001, each of these complex members was identified as its own species, largely due to their distinct populations primarily located in high-elevation glacial refugia separated by expansive low-elevation deserts and ranges. Our exploration into this species complex is motivated by prior research revealing significant genetic structure between the relatively proximate (~10 km) lower and upper Logan Canyon (Utah) populations of complex member P. cusickiana var. maguirei, which is listed as threatened. We collected leaf material from P. cusickiana complex member populations across the Great Basin and used a restriction site associated DNA sequencing (RADseq) approach to determine whether either population of P. cusickiana var. maguirei is more closely related to a different complex member than it is to the neighboring population within Logan Canyon.
Results/Conclusions
We found that the two Logan Canyon populations of P. cusickiana var. maguirei are more closely related to each other than to any populations of the other complex members, despite the level of genetic difference between them. We also found support for grouping of varieties nevadensis and domensis, as well as separation between different populations of variety cusickiana. These results have notable consequences for the taxonomy of the complex generally, as well as management of P. cusickiana var. maguirei specifically. While our taxonomic analysis largely upholds the groupings of each variety, we also find support for unique clades within variety cusickiana, characterized by populations experiencing isolation by distance. For variety maguirei, our findings illustrate the importance of managing both upper and lower Logan Canyon populations, and imply limited gene flow between the two populations. We discuss possible barriers to gene flow, including phenology and reproductive barriers caused by heterostyly.