Genetic Diversity In Chihuahuan Desert: Populations Of Creosotebush
Sean Medina
Biology 1615
Jack T. Later
November 15, 2013
Title: Genetic Diversity In Chihuahuan Desert: Populations Of Creosotebush
Introduction: Due to drought cycles and overgrazing over the past 200 years. It has opened up
the landscape of the Chihuahuan Desert into a woody shrub invasion into the once open
grasslands. The study helps conclude the genetic diversity found in the Larrea Tridentata and
the study of the disturbed habitat due to this pioneer plant. The Larrea Tridentata is the
dominant shrub of the deserts of North America. It ranges from California to Texas, from New
Mexico to Central Mexico. Spreading upwards from Mexico from the past 2 centuries and
originally found in Chile and Peru over 5000 years ago. The Larrea Tridentata has been
breeding rapidly with an unusual biogeographic pattern in chromosome numbers.
Reason for Research: To research the genetic variation in the desert species due to very low
amount of research having already been conducted. The research will include: 1. Genetic
variation in the diploid population, 2. Variation reduced the farther into North America, 3.
Inbreeding vs. outcrossing in mating system, 4. Genetic relatedness of population. We hope to
find more details on pioneering plants into new habitats.
Materials & Methods: 17 populations of the plant of 528 individuals were collected and
sampled from the whole of Chihuahuan desert and sampled at least 25 meters apart. Stored on
ice and transported to the lab. Through numerous extraction buffers, 6 leaves were ground up
with it and put in tubes. Filter paper wicks were soaked in more chemical and inserted into
starch gels for electrophoresis. 10 different enzymes were resolved in two different buffering
systems. The gels were electrophoresed overnight. The gels were cut up and incubated in more
enzymes at 37 degrees Celsius. Individuals from different populations were run on the same
gel to confirm consistency.
Results: The 10 enzyme systems revealed 12 putative loci that were resolved and reliably
scored. Banding patterns for all loci were all similar with those generally reported for diploid
vascular. All loci were polymorphic in at least the population results. No locus had fewer than
five alleles. Some loci were not scored for all individuals due to poor or inconsistent resolution.
Recently established populations in the northern area of the Chihuahuan desert did not show
decreased levels of genetic variation, Central Mexico had the lowest variation. The Las Cruces,
New Mexico population had the highest level of variation. Despite this overall significant level
of inbreeding, several individual loci showed very low levels of within population inbreeding.
The mantel test was conducted and found a moderate correlation between genetic and
geographic distances, but there was little correspondence between genetic and geographic
distance except that the central Mexico populations were close both genetically and
geographically.
Discussion: The Genetic Variation is very high compared to other similar plants. 100% of the
loci are Polymorphic, an average of six alleles per locus on the species level. On the population
level it is 95% of loci are polymorphic. The plant is nearly double the average of seed plants and
other L. Tridentata plants found in other deserts. The genetic variation is correlated to the
plants history and how woody perennials usually maintain a high genetic variation. With it
being the dominant shrub in its environment, the population is also high leading to a high
genetic variation. The Inbreeding in the L. Tridentata is found out from the deficient of
heterozygotes. This is not uncommon among a mixed mating species though. The results of this
just represent that the variation is among them instead of within, this is still not uncommon
among woody perennials. With most plants that can breed both ways, it depends on drought
weather, which increases breeding within. The Genetic structure of the L. Tridentata ranges
back over 20,000 years ago coming in waves in New Mexico, but finally settling in about 200
years ago. This is due to more human interaction and droughts. The strange part is the lack of
difference between much of the most northern plants and the most southern plants.
Conclusion: The Species is incredibly diverse and very impressive in long range breeding. It also
is impressive that as it moved northward and didn’t have a loss of variation. This is incredible
considering the soil types, climate regimes, and the land use histories. Future studies will help
understand the success in range expansion and survival in a newly colonized habitat.
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Bibliography:
http://www.ncbi.nlm.nih.gov/pubmed/21652451