Phylogenetic Studies of North American Callitriche (Callitrichaceae) Using Chloroplast DNA Restriction Fragment Analysis

Abstract

Phylogenetic study in the Callitrichaceae is hampered by reduced floral structure and environmentally mediated phenotypic variation of both vegetative and reproductive (fruit) char- acters. Yet a phylogenetic perspective is requisite to an understanding of ecological, morphological, and reproductive system evolution in the family. Cladistic analysis of chloroplast DNA (cpDNA) restriction fragment variation, combined with several morphological characters, was conducted on 11 taxa of Callitriche (nine indigenous to North America and two European). These studies provide the first explicit phylogenetic hypotheses for the Callitrichaceae and support the monophyly of several groups. Callitriche hermaphroditica, the single obligately submerged species examined, aligned basal to all other North American taxa. The remaining indigenous North American taxa formed two opposing clades: 1) the terrestrial C. nuttallii, C. peploides, C. terrestris grouped with the am- phibious C. marginata, and 2) the amphibious C. heterophylla var. heterophylla and var. bolanderi, C. trochlearis, and C. verna. Relationships among the taxa within these clades are unresolved. These analyses indicate that the internal geitonogamous breeding system is synapomorphic for the latter clade, and suggest that the terrestrial growth habit may be derived from the amphibious growth habit in the family. This study demonstrates the utility of using cpDNA restriction fragment variation to gain phylogenetic perspectives in aquatic angiosperms.