Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

The genus Larrea dominates the warm deserts of North and South America. Larrea includes four species in South America and one species in North America. The North-South disjunction has interested scientists since the 1800’s, but there are also many other unique features in Larrea that give it a selective advantage in deserts. Larrea species are extremely drought tolerant, very long-lived, allelopathic, clonal and exhibit hybridization and polyploidy.

The origin and timing of the disjunction of Larrea have been inseparable issues, since the time of the disjunction relates to theories about the origin of Larrea. The use of packrat middens to date the distributions of Larrea in the North American deserts during the Late Pleistocene and Holocene suggest a South American origin. The purpose of this research was to examine the evolutionary and biogeographic history of the genus Larrea. This was accomplished by investigating the patterns of genetic variation in chloroplast DNA and by analyzing temporal and spatial distributions of the three geographically distinct polyploids of L. tridentata.

The phylogenetic relationships, based on chloroplast DNA restriction site analysis, were congruent with morphological studies. Cytoplasmic gene flow was postulated to account for identical haplotypes shared between L. nitida and L. cuneifolia from South America. L. nitida is thought to be the putative maternal donor of the chloroplast. Two distinct chloroplast haplotypes were found in North American L. tridentata, and one of those haplotypes was also found in South American L. divaricata. Low levels of genetic variation in chloroplast DNA were detected throughout the genus. This low variation could be a result either of extremely long generation times or of relatively recent diversification.

Three morphologically cryptic ploidy levels have been identified by Yang in L. tridentata (creosote bush): diploid (2n=26) in the Chihuahuan Desert, tetraploid (2n=52) in the Sonoran Desert, and hexaploid (2n=78) in the Mojave Desert. We determined ploidy levels within the southwestern deserts using guard cell size, based on work of J. Masterson. Mean guard cell sizes (± 1 SE) in the central Chihuahuan Desert were 225±9 µm2, in the Sonoran Desert 376±15 µm², and in the Mojave Desert 490±13 µm². Mean guard cell area was therefore significantly different among populations in the three deserts.

Polyploidy changes across the range of L. tridentata from the end of the glacial maximum to the present were inferred from measurements of guard cells of Larrea leaves preserved in pack rat middens. In the warm deserts of North America. Diploids and tetraploids were both present in the lower Colorado River Valley 10,000 to 21,000 years before present (B.P.), and were replaced by tetraploids and possibly hexaploids before 8,100 yr B.P. Hexaploids were present in the Mojave Desert by 6,500 yr B.P.