Research Projects:
GENETIC DIVERSITY WITHIN SARRACENIA LEUCOPHYLLA, THE WHITE-TOPPED PITCHER PLANT
Mary Jo W. Godt and J. L. Hamrick
The southeastern United States is the center of diversity for the fascinating carnivorous genus Sarracenia, the pitcher plants. Many of the southeastern Sarracenia species are rare and endangered, in part due to loss and alteration of wetlands. Fire suppression has also led to the demise of many pitcher plant populations, which require periodic burning to maintain the open sunny habitat favored by the plants.
Sarracenia leucophylla, the white-topped pitcher plant, is one of the showiest plants in the genus. Collection of the attractive pitchers for dried floral arrangements further endangers this species. The range of S. leucophylla includes the states of Georgia, Florida, Alabama and Mississippi. In Georgia the species was once found in five southern counties; today it is restricted to a single small population.
Rare plant species that have suffered population losses, and whose populations are declining in size often experience the random loss of genetic diversity, decreased gene exchange via pollen and seed, and inbreeding. Because genetic diversity provides the raw material that permits adaptation to changing environmental conditions, loss of genetic variation poses an extinction risk for species. Decreased gene exchange exacerbates the random loss of genetic diversity. Inbreeding, a common phenomenon in small populations, can also lead to a loss in fitness, particularly in species that usually outcross. Inbreeding depression (a fitness loss associated with mating between relatives) has previously been documented in Sarracenia species.
In Spring 2002, we initiated a genetic survey of S. leucophylla with the goal of describing genetic diversity and structure within the species. Under our supervision a young visiting scientist, Dr. Zhengfeng Wang, took the lead on this project. A conservation biologist from the South China Institute of Botany, Dr. Wang was supported by a fellowship from The Chinese Academy of Sciences, People's Republic of China, during his stay at the University of Georgia. The goal of Dr. Wang's visit was to enhance his technical genetic skills and to learn more about conservation genetics; this project was ideally suited.
For genetic analysis we collected 48 leaf samples from each of ten S. leucophylla populations (one in GA, five in FL, three in AL and one in MS) throughout its range. One ex situ population propagated for restoration in Georgia was also analyzed. We used a technique called "starch gel electrophoresis" to examine differences in protein structure that reflect differences in the DNA of plants.
Eighteen enzyme genes were used to describe genetic diversity within S. leucophylla. Of these 15 (83.3%) were variable, with a mean number of alleles = 3.33. Compared to species having similar life-history traits and to previously analyzed pitcher plant species, S. leucophylla displayed unexpectedly high genetic diversity. For example, genetic diversity within the species (Hes) was 0.224 and mean population genetic diversity (Hep) was 0.183. These compare to values of Hes = 0.189 and He = 0.051 for the purple pitcher plant, S. purpurea, the most widespread species in the genus. We found that S. leucophylla has a richer genetic base than four other previously analyzed Sarracenia species, which bodes well for its future.
However, small S. leucophylla populations maintained less genetic diversity than larger ones, suggesting that genetic variation was being lost in some populations. Loss of genetic diversity may accelerate if populations continue to decline in size.
Statistically significant genetic differentiation was found among populations, but it was comparable to that found for species with similar breeding systems and dispersal mechanisms. A significant correlation (r = 0.46) between genetic distance and geographic distance was found for the ten S. leucophylla populations, indicating that distant populations exchanged fewer genes. If populations go extinct, gene exchange is likely to decline further, potentially leading to increased genetic losses.
The ex situ population maintained genetic diversity similar to that found in natural populations. Its broad genetic base indicates that genetic diversity is not an initial concern for restoration efforts in Georgia.
In sum, S. leucophylla appears to be relatively healthy genetically, despite its threatened status. However, maintenance of populations will be required to avoid future genetic erosion. In most cases, this will require active fire management. Several sampled populations were heavily shaded by woody vegetation. These populations are declining in size because plants are not reproducing. In the absence of timely management such populations are likely to go extinct.