Ploidy race distributions since the Last Glacial Maximum in the North American desert shrub, Larrea tridentata

1 A classic biogeographic pattern is the alignment of diploid, tetraploid a nd hexaploid races of creosote bush (Larrea tridentata) across the Chihuahu an, Sonoran and Mohave Deserts of western North America. We used statistica lly robust differences in guard cell size of modern plants and fossil leave s from packrat middens to map current and past distributions of these ploid y races since the Last Glacial Maximum (LGM). 2 Glacial/early Holocene (26-10 C-14 kyr BP or thousands of radiocarbon yea rs before present) populations included diploids along the lower Rio Grande of west Texas, 650 km removed from sympatric diploids and tetraploids in t he lower Colorado River Basin of south-eastern California/ south-western Ar izona. Diploids migrated slowly from lower Rio Grande refugia with expansio n into the northern Chihuahuan Desert sites forestalled until after similar to4.0 C-14 kyr Bp. Tetraploids expanded from the lower Colorado River Basi n into the northern limits of the Sonoran Desert in central Arizona by 6.4 C-14 kyr BP. Hexaploids appeared by 8.5 C-14 kyr BP in the lower Colorado R iver Basin, reaching their northernmost limits (similar to 37 degreesN) in the Mohave Desert between 5.6 and 3.9 C-14 kyr BP. 3 Modern diploid isolates may have resulted from both vicariant and dispers al events. In central Baja California and the lower Colorado River Basin, m odern diploids probably originated from relict populations near glacial ref ugia. Founder events in the middle and late Holocene established diploid ou tposts on isolated limestone outcrops in areas of central and southern Ariz ona dominated by tetraploid populations. 4 Geographic alignment of the three ploidy races along the modern gradient of increasingly drier and hotter summers is clearly a postglacial phenomeno n, but evolution of both higher ploidy races must have happened before the Holocene. The exact timing and mechanism of polyploidy evolution in creosot e bush remains a matter of conjecture.