Extreme environmental conditions and phylogenetic inheritance: systematics of Myopordon and Oligochaeta (Asteraceae, Cardueae-Centaureinae)

Abstract

The genus Myopordon, with six perennial species from Iran, Lebanon, Syria and Turkey, had until recently been placed in the Carduinae subtribe. Oligochaeta, composed of four annual species from the Caucasus, India, the Near East and Pakistan, was considered related to the genus Rhaponticum in the Centaureinae subtribe. The two genera have apparently not much in common; however, evidence that both are closely related within the Rhaponticum group was provided by recent molecular phylogenetic reconstructions. New and previously published cytogenetical (fluorescent in situ hybridization of rDNA genes, fluorochrome banding of AT- and GC-rich regions, silver nitrate staining of active ribosomal genes, genome size assessment), karyological, molecular phylogenetic and palynological data are evaluated in the light of the recent evidence concerning the relationships of Myopordon and Oligochaeta. Myopordon presents a constant chromosome number and a homogeneous pattern in physical mapping of rDNA genes, contrasting with a strong variability in chromosome banding, genome size and pollen-type. Such tendencies could be related to the environmental pressures in the high mountains where species of Myopordon occur. This also accords well with the phylogeny and the geographic distribution of the genus. Phylogenetic reconstructions resolve Oligochaeta as being nested in the clade of Myopordon species. The exceptional cytogenetical, karyological, as well as palynological characters of Oligochaeta are apparently derived from a Myopordon-like ancestor.