PRJNA666395
Shotgun sequencing of Delphinium exaltatum
Around 75 Eastern North American plant species have geographic ranges exhibiting a wide disjunction (around 650 km) between the Ozark and Appalachian Regions. The biogeographic forces proposed to have caused this disjunction include vicariance from an expansion in the Mississippi River during Pleistocene interglaciation, occupation of multiple Pleistocene refugia, or more recent long-distance dispersal. Here, we investigated the biogeographic forces giving rise to an Appalachian-Ozark disjunction in Delphinium exaltatum, an insect-pollinated perennial herb. We hypothesized that the oldest and greatest genetic divergence in D. exaltatum would occur between the Ozarks and the Appalachian regions, corresponding to the Mississippi River discontinuity. We sampled and genotyped populations of D. exaltatum from five localities in the Missouri Ozarks and seven in the Appalachians/Eastern United States. We analyzed patterns of genetic structure and tested the order and timing of divergence using DIYABC. We conducted niche reconstructions by calibrating ecological niche models using contemporary data and hind casting them to 21 Kybp. Analyses of genetic structure split the populations into four clusters: 1) a highland population in the central Appalachians; and three lowland populations comprising: 2) Missouri and eastern Tennessee, 3) eastern North Carolina, and 4) Ohio. The highland and lowland populations diverged first around 5600 generations ago, and the lowland lineages diverged from each other nearly simultaneously around 3500 generations ago. Niche reconstructions showed that areas with suitable climate for the central Appalachians populations, which occupy drier sites, became available between 8,000-16,500 years ago, and expanded northward since the end of the Pleistocene. Contrary to expectations, the oldest divergence occurred between the central Appalachian and lowland populations when suitable climate became available 8,000-16,000 YBP, and the disjunction in the range of the species likely resulted from northward range expansion as the climate changed in the Holocene.