Assessing pollen viability among genotypes of the federally endangered Florida ziziphus (Ziziphus celata) Rhamnaceae

Background/Question/Methods

In addition to its mating system, many factors affect sexual reproduction in a flowering plant species. Florida ziziphus (Ziziphus celata) is a federally endangered clonal shrub narrowly endemic to the Lake Wales Ridge of central Florida. The 14 wild populations comprise fewer than 45 genetic individuals, about a third of which are represented by an ex situ population at Bok Tower Gardens in Lake Wales, FL. Florida ziziphus has a gametophytic self-incompatibility system (GSI) whereby plants sharing the same S-alleles are cross-incompatible. Half of the wild populations are uniclonal and therefore incapable of fruit production, but the multi-genotype Bok population produces a large fruit crop annually. We have previously shown that variable rates of pollen germination and ineffective pollen tube growth may also contribute to reproductive failure in this species. Our main goal in this study was to compare pollen viability among genotypes in the ex situ and wild populations. In 2008 and 2010, we collected flowers at anthesis from 98 plants representing 27 wild genotypes. Using lactophenol-aniline blue stain and a hemocytometer, we scored at least eight sub-samples from each plant for pollen viability. We compared percent viability within and among genotypes, between years, and between the ex situ and wild populations using nonparametic statistics.

Results/Conclusions

Pollen viability differed significantly among genotypes (p < 0.001), with individual plants varying from 1% - 98%. For 13 genotypes, percent viability exceeded 80%, while for seven genotypes it was < 50%. Overall, pollen viability was significantly greater in 2008 than in 2010 (p < 0.002), but there was no significant difference between the ex situ and wild populations in either year. These results are consistent with data from mating system experiments indicating that in some reciprocal crosses between compatible genotypes one partner is less successful as a pollen donor than as a pollen recipient. Thus, while low S-locus diversity and scarcity of compatible mates are the primary causes of sexual reproductive failure in Florida ziziphus, pollen sterility may also play a role. Knowledge of the pollen viability of wild genotypes is useful in creating genetically diverse and sexually compatible populations on publicly protected sites, the central goal of the recovery plan for this highly imperiled species.