GENETIC-VARIATION WITHIN AND AMONG POPULATIONS OF THE MARSHALLIA-GRAMINIFOLIA COMPLEX (ASTERACEAE)

The Marshallia graminifolia complex has been variously treated as two distinct species (M. graminifolia and M. tenuifolia), and as two subspecies under M. graminifolia. Both are diploid and occupy the same bog-like habitats along the Gulf and Atlantic coastal plains of the southeastern United States. Enzyme electrophoresis was used to test various hypotheses of speciational modes and mechanisms in the Graminifolia Complex, including a progenitor-derivative relationship and gradual geographic isolation. Nine enzymes encoded by 16 loci were resolved, 13 of which were fixed for the same allele in all populations examined. Levels of heterozygosity were low. The mean genetic identity between the two taxa was high (l=0.99). There was no evidence for a progenitor-derivative relationship, and no pattern of geographic partitioning of genetic variation among populations. Morphological variation between the two taxa probably results from either simple polymorphism, phenotypic plasticity, and/or clinal variation. Because other congeners of Marshallia share similarly low levels of genetic variation, we believe that its species may have arisen from genetically depauperate ancestors.