Genetic diversity of Shaw's agave and soil associated microbes in Southern California preserve

Shaw’s Agave (Agave shawii ssp. shawii) is an endangered maritime
succulent growing along the coast of California and Northern Baja
California. The population inhabiting Point Loma Peninsula has a
complicated history of transplantation without documentation. The low
effective population size in California prompted agave transplanting from
the U.S. Naval base site (NB) to Cabrillo National Monument (CNM). Since
2008, there are no agave sprouts identified on the CNM site, and concerns
have been raised about the genetic diversity of this population. We
sequenced two barcoding loci, rbcL and matK, of 27 individual plants from
5 geographically distinct populations, including 12 individuals from
California (NB and CNM). Phylogenetic analysis revealed the three US and
two Mexican agave populations are closely related and have similar genetic
variation at the two barcoding regions, suggesting the Point Loma agave
population is not clonal. Agave-associated soil microbes used
significantly more carbon sources in CNM soil samples than in NB soil
likely due to higher pH and moisture content; meanwhile, soil type and
soil chemistry analysis including phosphorus, nitrate nitrogen, organic
matter, and metals revealed significant correlations between microbial
diversity and base saturation (p < 0.05, r2 = 0.3676), lime buffer
capacity (p < 0.01, r2 = 0.7055), equilibrium lime buffer capacity
(p < 0.01, r2 = 0.7142), and zinc (p < 0.01, r2 = 0.7136).
Soil microbiome analysis within the CNM population revealed overall
expected richness (H’ = 5.647 to 6.982) for Agave species, while the
diversity range (1 - D = 0.003392 to 0.014108) suggests relatively low
diversity marked by high individual variation. The most prominent
remaining US population of this rare species is not clonal and does not
seem to be threatened by a lack of genetic and microbial diversity. These
results prompt further efforts to investigate factors affecting
Agave's reproduction and fitness.