The Nevada Primrose: A Rare Mountaintop Plant and Its Hidden Genetic Split

On the summit of Mount Washington, in Great Basin National Park, grows a rare plant with an unusual story. The Nevada primrose, Primula cusickiana var. nevadensis, is a perennial with showy purple petals, first described as a species of its own by Noel Holmgren in 1967.

One species, four varieties?

It belongs to the Primula cusickiana species complex, a group of related, similar-looking plants found across the Great Basin, currently treated as four varieties:

• var. maguirei, in the Bear River Range of Utah;
• var. cusickiana, across the Snake River Plain in Idaho, with one population in Jarbidge, Nevada;
• var. domensis, in the House Range of Utah, just across the state line from the park;
• var. nevadensis, in the Snake and Grant Ranges.

From 2018 to 2020, as a master's student at Utah State University supervised by Paul Wolf and Will Pearse, Austin Koontz carried out a genetic survey of the four varieties, to find out how genetically distinct they are and whether "varieties of one species" really describes them.

Where Primula cusickiana grows, with colours marking genetically distinct groups. NPS.

Looking alike, but not alike

The answer appears to be no. The study found very strong genetic divisions between the isolated populations, suggesting that each is undergoing cryptic speciation: populations in different places that look very much alike but are genetically very different.

The Great Basin's history may explain why. At the end of the Pleistocene, 11,700 years ago, the region was cool and fairly wet, with large lakes such as ancient Lake Lahontan and Lake Bonneville, and the ancestor of Primula cusickiana was probably widespread. Over the next 10,000 years, the Holocene, the Great Basin grew hotter and drier. Mountaintops stayed cool, and many organisms retreated to them — refuges scientists call refugia. The primrose, following the cool, moist soils it prefers, moved uphill, and its populations were cut off from one another. Isolated, they became more and more genetically distinct, producing the sharp divisions seen today.

The Mount Washington population

The research also shed light on the nevadensis population on Mount Washington. It appears to be a hybrid between the domensis population to the east, in the House Range, and another nevadensis population farther south, in the Grant Range. More work is needed to describe these populations, but the study suggests the Mount Washington primroses — and indeed every population — are highly distinctive, and Koontz hopes the work will lead to their protection. The study was published in Systematic Botany.

Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.

Sources

• Austin Koontz, researcher, "A Closer Look at Nevada Primrose," The Midden (Great Basin National Park), vol. 22, no. 2, winter 2022, National Park Service. https://www.nps.gov/articles/000/a-closer-look-at-nevada-primrose.htm
• The study: "Pronounced Genetic Separation among Varieties of the Primula cusickiana Species Complex," Systematic Botany. https://doi.org/10.1600/036364422X16573019348175
• The article's photographs of the four varieties, by the author, are not reproduced here.