Genetic diversity and differentiation of Taxodium in the south-eastern United States using cleaved amplified polymorphic sequences.

Two taxa of Taxodium, bald cypress and pond cypress, occur in the south-eastern United States. The ranges of these taxa overlap in the south-eastern Coastal Plain, with the range of the latter being more restricted. Although these taxa co-occur throughout a portion of the more expansive range of bald cypress (Taxodium distichum (L.) L. C. Rich), the habitats of the two taxa appear to differ. Consequently, considerable debate has occurred regarding the taxonomic status of pond cypress. Some authors recognize pond cypress as a distinct species (Taxodium ascendens Brongn.), whereas others recognize it as a variety/ecotype (Taxodium distichum var. imbricarium (Nutt.) Croom). In this study, the genetic diversity of these two taxa was investigated using 10 DNA markers based on sequences from cDNA clones of Cryptomeria japonica. Cryptomeria is a monospecific genus native to Japan, and is a close relative of Taxodium. These markers were codominant in Cryptomeria and were presumed to be codominant in Taxodium. DNA was extracted from leaf tissue collected from six populations of bald cypress and seven populations of pond cypress throughout Florida and Georgia. The average heterozygosities of bald cypress and pond cypress were 0.386 (SE 0.040) and 0.380 (SE 0.040), respectively. Most of the genetic variation (91.9%) was found within populations, 4.9% was found between populations and 3.2% between taxa. Results of DNA analysis using cleaved amplified polymorphic sequences (CAPS) in this study did not suggest that pond cypress was a species distinct from bald cypress. Our conclusion is that the two taxa of Taxodium should be given varietal status.