Biosystematic and Phenetic Analysis of Marshallia (Asteraceae)

Publisher Website
Google Scholar
Abstract
A phenetic analysis based on morphological features of Marshallia revealed four major clusters of OTU''s. These clusters only roughly correspond to the previously recognized species complexes. The most distinct species is M. trinervia, segregated from the remaining species by ovate leaf shape. It is in the same cluster as M. grandiflora, a diploid species closely related to tetraploid M. mohrii. However, M. mohrii also clusters with the Graminifolia-complex, which is composed of M. graminifolia and M. tenuifolia. These latter two species are diploid, morphologically indistinguishable, and occupy the same bog-like habitat along the Atlantic and Gulf Coastal Plains; consequently M. tenuifolia is treated as M. graminifolia subsp. tenuifolia (Raf.) Watson, comb. et stat. nov. A third cluster is composed of M. obovata and M. caespitosa. The fourth cluster, adjacent to the Graminifoliacomplex, includes only M. ramosa. This species was previously thought to be more closely related to M. caespitosa. Morphological variation is continuous among the species, with few abrupt boundaries. They all occupy specific habitats. In addition, the Graminifolia-complex has diverged in seasonal flowering phenology from the others. All species were cross-compatible at approximately equivalent levels (averaging 15% crossability) with the exception of M. ramosa, a diploid species that possesses a number of chromosomal translocations. Little morphological differentiation has occurred, with ecological divergence and chromosomal evolution more apparent among these species in Marshallia. The extensive intergradation of morphological features among the species may be a result of either: 1) recent isolation with accumulation of few genetic barriers to crossing; or 2) a more ancient divergence with low genetic variation present when the species were initially evolving, followed by little morphological divergence.