CHLOROPLAST DNA RESTRICTION SITE VARIATION AND PHYLOGENETIC-RELATIONSHIPS IN THE GENUS THLASPI SENSU-LATO (BRASSICACEAE)

Abstract

The classification of the genus Thlaspi s.l. is difficult and controversial. All hypotheses have been based on morphological and anatomical data and no cladistic analyses have been performed. In the current study restriction site variation of chloroplast DNA among 45 populations of 22 taxa was employed to assess phylogenetic relationships in the genus Thlaspi. Although only 22 taxa have been analyzed herein (ca. 30% worldwide) these species represent all sections. One length mutation and 114 restriction site mutations were detected. Cladistic analysis of the chloroplast DNA data supported four groups that are congruent with four segregate genera (Thlaspi s. str., Microthlaspi, Noccaea, and Raparia), previously recognized by Meyer on the basis of seed anatomy. Sequence divergence between these groups was higher than usually found in intrageneric analyses and comparable to levels of divergence between related genera of other angiosperm families. The chloroplast DNA phylogeny suggests that the perennial taxa are not ancestral to the annual taxa and that the annual habit originated more than once in Thlaspi s.l. A paucity of restriction site mutations precludes resolving relationships among species of Noccaea, whereas the two intraspecific taxa of the T. perfoliatum polyploid complex (Microthlaspi) were separated by at least 17 restriction site mutations. Consideration of chloroplast DNA divergence within the clades Noccaea and Microthlaspi indicates that the rate of plastome evolution is uncoupled from rates of morphological/anatomical diversification. Chloroplast DNA polymorphisms were detected among eight populations of T. perfoliatum subsp. perfoliatum with different ploidy levels. There seems to be geographic partitioning of the two plastome types found, possibly warranting taxonomic recognition.