Discovery of New Populations of Florida Ziziphus
Carl W. Weekley, Plant Ecology Lab
Archbold Biological Station

The recent discovery of two large new populations of Florida ziziphus on a site near Lake Wales is the most important development for the recovery of this critically endangered Lake Wales Ridge endemic since its initial rediscovery almost 20 years ago.
Florida ziziphus (Ziziphus celata), one of the rarest plants in Florida, is state and federally listed as Endangered. It was named and described for science in 1984 by Walter Judd and David Hall from a 40-year old herbarium specimen. At the time, it was thought to be extinct, but over the next decade half-a-dozen populations were found along the Ridge between Lake Wales and Avon Park.
Most populations occur on pasture sites that once supported endemic, rich, longleaf pine-wiregrass sandhills. Most remnant populations comprise a single genetic individual, although it may be represented by up to several dozen separate plants. Altogether, fewer than a dozen wild genotypes were known prior to 2007.

Florida ziziphus is a thorny, multi-stemmed shrub to 6 feet in height. Mature plants flower profusely. The tiny flowers--four would fit neatly on the face of a nickel—are bisexual, with both male and female reproductive structures. However, Florida ziziphus has a self-incompatibility system whereby individuals can only mate with plants belonging to a different mating type.
The number of mating types is genetically determined by the number of self-incompatibility (S-) alleles. To date, only three S-alleles have been confirmed for Florida ziziphus, the minimum number a species can have and still be capable of sexual reproduction. Because most populations comprise a single genetic individual, they are incapable of producing viable seeds. The isolation of remnant populations and their lack of genetic diversity are major obstacles to the recovery of Florida ziziphus.
A key attribute of Florida ziziphus’ mating system is that for each new S-allele the number of mating types increases dramatically. For example, if the number of S-alleles doubles from the currently known three alleles to six, the number of mating types will quintuple from three to 15, thereby increasing the genetic variability of resulting offspring. Increased genetic variability provides a significant hedge against extinction. The discovery of populations representing novel genotypes and mating types is thus critical to the recovery of Florida ziziphus.
In 2007, five new populations of Florida ziziphus were discovered. The most important are two large pasture populations near Lake Wales, comprising approximately 350 plants and at least 14 new genotypes. These two populations almost double the number of plants--and, more importantly, the number of genotypes--known from the wild. The likelihood that these large, genetically diverse populations contain one or more new mating types dramatically enhances the recoverability of one of Florida’s rarest and most imperiled species.
Plant conservationists from several institutions (including Archbold Biological Station, Historic Bok Sanctuary, and the Florida Museum of Natural History’s Laboratory of Molecular Systematics and Evolutionary Genetics) are working to assess and preserve the genetic variability represented by the newly discovered populations.
Acquisition of the land that holds the two large, multi-genotype Lake Wales pasture populations would constitute a major advance in the struggle to save Florida ziziphus.