Award Date


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Biological Science

Number of Pages



The evolutionary and biogeographic history of the genus Larrea was assessed by determining patterns of genetic variation in chloroplast DNA, nuclear ribosomal internal transcribed spacer (ITS1) DNA, and temporal and spatial distributions of the three geographically distinct polyploids of Larrea tridentata. The phylogenetic relationships, based on the chloroplast DNA (cpDNA) restriction site analysis, were congruent with morphological studies. Cytoplasmic gene flow was postulated to account for identical haplotypes being shared between L. nitida and L. cuneifolia from South America, with L. nitida as the putative maternal donor. Two distinct cpDNA haplotypes were found in North American L. tridentata, and one of these haplotypes was also found in South American L. divaricata. Low levels of variation were detected throughout the genus, perhaps as a result of extremely long generation times or recent diversification. ITS1 sequences within the genus Larrea are highly variable within individuals as well as species, indicating that concerted evolution has failed to homogenize repeats within an individual. However, intermediacy of concerted evolution was present within the genus where some phylogenetic relationships were supported. This intermediacy may be a result of rapid diversification, polyploidy, introgression and/or long generation time. Finally, ploidy levels within the southwestern deserts were inferred using guard cell size and mean guard cell area was significantly different among all three deserts, but the boundaries of the ploidy levels were not distinct. Polyploidy changes across the range of Larrea tridentata since the end of the glacial maximum to present were inferred from measurements of guard cells of leaves preserved in pack rat middens in the warm deserts of North America. Diploids and tetraploids were present in the lower Colorado River Valley 10,000 to 18,000 years before present (yr B.P.), and were replaced with tetraploids 8,100 yr B.P. Hexaploids were present in the Mojave Desert by 6,500 yr B.P.


Analysis; Genus; Larrea; Middens; Molecular; Pack Rat Middens; Packrat; Phylogenetic; Polyploidy; Systematics; Pack Rat Middens; Phylogenetic

Controlled Subject

Botany; Genetics; Ecology; Paleobotany

File Format


File Size

2590.72 KB

Degree Grantor

University of Nevada, Las Vegas




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