Creosote bush (Larrea tridentata) ploidy history along its diploid-tetraploid boundary in southeastern Arizona-southwestern New Mexico, USA

Creosote bush (Larrea tridentata) is a dominant shrub in the warm deserts of North America and also a classic example of an autopolyploid complex. We determined ploidy levels for creosote leaves preserved in ancient packrat middens from the Peloncillo Mountains, AZ to better understand the history of ploidy race distribution along its diploid-tetraploid boundary. We also measured modem creosote ploidy level at several sites spanning the AZ-NM border to augment sampling in this large geographic area. Modem plants were mostly diploids, with tetraploids only observed in our northernmost sites. Ancient creosote from the Peloncillo middens (3170-145 cal yr BP) were all diploids. Modem creosote at the Peloncillo site is also diploid, but with significantly larger guard cells areas that may be a response to increasingly hot, dry conditions. The lack of tetraploids in the midden fossils suggests the arrival of tetraploids at their eastern margin may have occurred only recently, mirroring the late arrivals of other Sonoran Desert plants (e.g., Simmondsia chinensis) along the AZ-NM border.