Population Divergence and Evolution of the Endangered Sesbania tomentosa (Fabaceae)

Abstract

Sesbania tomentosa (Fabaceae) is an endemic flowering plant primarily adapted to coastal strand and dry lowland habitat in the Hawaiian Islands, now extant in relicts of its former range. Efforts have been made to delineate distinct taxa from among the remaining populations. In the most recent treatment of Hawaiian Fabaceae, however, S. tomentosa was recognized as a single variable species. In an attempt to address issues of taxonomy, the present study compared phylogenetic hypotheses of Hawaiian Sesbania determined by morphological markers with those determined by molecular analyses (DNA sequence and microsatellite marker variation) and assessed their relative level of congruence. A complete lack of variation between eight putative taxa from six islands at two nuclear DNA regions (1035 bp) contrasts with the highly differentiated population structure of the nine microsatellite loci sampled, while confidence in the relationships proposed in morphological phylogenies based on putative taxonomy was low. Instead, Bayesian genetic clustering assignments and associated private alleles occurred in a distinct phylogeographic pattern. As a result, populations from Nihoa, Kaua„i and O„ahu are distinguished as a separate subspecies of S. tomentosa, populations from Maui Nui and Hawai„i Island (respectively) form two additional subspecies, and a fourth subspecies endemic to SE Moloka„i distinguishes itself from the rest of Maui Nui. Naturally-occurring populations of Sesbania tomentosa plus a substantial number of outplanted individuals were analyzed for levels of allelic diversity, heterozygosity and inbreeding. Evidence of genetic bottlenecks in populations was also investigated, as well as an analysis of population sub-structuring. Natural ecological dynamics affecting population differentiation often leave lasting genetic signatures, and are addressed alongside contemporary impacts on plant habitat when discussing the divergence of plant population remnants. The molecular data can be interpreted to support the hypothesis that distinctive-appearing remnant populations of this highly variable species have diverged at an accelerated rate due to human induced habitat fragmentation within the larger context of the speciation process itself. This study also provides examples of increasing genetic diversity in outplantings when intentional mixing of populations to augment diversity was practiced, as well as in situations where the genepools of natural populations are dynamic over time.