Larrea tridentata (Creosote Bush) is the dominant shrub of warm deserts of North America. In contrast to the other dominant plants in these deserts Larrea is a relatively recent arrival from South America. Larrea tridentata was probably derived from an ancestral population of L. divaricata Cav., a relatively widespread species occurring in Argentina and Peru, via long-distance dispersal to the Chihuahuan Desert as recently as 30,000 years ago. The expansion of creosote bush to the northern extension of the Chihauhuan Desert and eastward in the Sonoran Desert may be as recent as the last 5000 years. In order to establish a baseline study of genetic diversity and structure in L. tridentata, we carried out a detailed isozyme investigation along the north-south distributional axis of diploid populations in the Chihuahuan Desert from New Mexico to Mexico. Seventeen populations containing a total of 518 individuals were sampled. Twelve enzyme loci were examined electrophoretically. All loci were polymorphic in at least one population and no locus exhibited less than 5 alleles. Values of genetic diversity at the species and population level in L. tridentata are among the highest recorded for any angiosperm. The value for species level gene diversity (He) is 0.353 and mean number of alleles per locus is 6.417. However, F statistic analysis shows significant inbreeding. The coancestry coefficient shows a low level of among population differentiation. The high degree of genetic variability is maintained within populations. There is no north-south geographic pattern in partitioning of genetic variation. We will discuss possible mechanisms for the maintenance of such high levels of genetic diversity in the face of low but significant inbreeding.

Key words: Chihuahauan Desert, diploid, genetic diversity, isozymes, Larrea tridentata