Patterns of clonal diversity in Dicranopteris linearis on Mauna Loa, Hawaii

The clonal mat-forming fern, Dicranopteris linearis (N. L. Burm.) Underw., dominates vast areas of rainforests on the windward slopes of Mauna Loa Vol cano on the island of Hawai'i. Because clone size has important ecological and evolutionary consequences in such a dominant species, we used isozyme a nalysis to investigate clone size and other aspects of genetic diversity an d reproduction over a broad range of environmental conditions on primary su ccessional sites (pahoehoe lava substrates). Isozyme analysis provided a measure of the upper limit of clonal size in th is int interdigitating clonal species. Each 0.5-ha primary successional sit e on Mauna Loa was comprised of a minimum of two to four clones. Genetic di versity in Dicranopteris was low; of 32 purative loci investigated, only 4 were polymorphic, with 2 or 3 alleles/locus. Over the 17 study locations on Mauna Loa and Kilauea Volcanoes, we identified nine multilocus genotypes b ased on unique combinations of allozymes. Seven of the nine genotypes were heterozygous for at least one locus, evidence of an intergametophytic matin g system. Highly dispersible spores, coupled with intergametophytic mating should promote higher genetic diversity We propose that the Following facto rs contributed to low genetic diversity: founder effects; extreme isolation from mainland gene pools; high potential for mating among different gameto phytes produced from the same sporophyte; relatively low numbers of safe si tes for gametophyte establishment over space and time; and long-term relian ce on vegetative growth. Leaf phenotypes were associated with genotype, bur also with environmental conditions. Enough variability within a genotype e xisted to support the current treatment of Hawaiian Dicranopteris as one sp ecies. Vegetative growth was the primary means by which Dicranopteris covered the landscape. Nevertheless, spore production, gametophyte establishment, and s exual reproduction were absolutely essential for colonization of the few fa vorable microsites available on pahoehoe lava substrates of Mauna Loa follo wing lava eruptions, dieback, and similar landscape-level disturbances.