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 subsp ecies under M. graminifolia. Both are diploid and occupy the same bog- like habitats along the Gulf and Atlantic coastal plains of the southe astern United States. Enzyme electrophoresis was used to test various hypotheses of speciational modes and mechanisms in the Graminifolia Co mplex, including a progenitor-derivative relationship and gradual geog raphic isolation. Nine enzymes encoded by 16 loci were resolved, 13 of which were fixed for the same allele in all populations examined. Lev els of heterozygosity were low. The mean genetic identity between the two taxa was high (l=0.99). There was no evidence for a progenitor-der ivative relationship, and no pattern of geographic partitioning of gen etic 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 Marsha llia share similarly low levels of genetic variation, we believe that its species may have arisen from genetically depauperate ancestors.