Karyomorphological studies in Oserya, Vanroyenella and Tristicha (Podostemaceae sensu lato)

Karyotype analyses of three Podostemaceae genera are reported for the first time. Somatic chromosome numbers were 2n = 28 (x = 14) for Oserya coulteriana Tul. and Vanroyenella plumosa Novelo and Philbrick (subfamily Podostemoideae). Their karyotypes were similar (13m + 1 sm chromosome pairs). The pantropical Tristicha trifaria (Bory ex Willdenow) Sprengel (subfamily Tristichoideae), with 2n = 20 (x = 10), had a karyotype of 10 m chromosome pairs. Although the karyotypes were homogeneous in the species studied, intergeneric variation of their genomes was evident in chromosome size and genome length. The largest chromosomes and genome lengths were found in O. coulteriana, while chromosomes of intermediate size occurred in V. plumosa. Tristicha trifaria had the smallest chromosomes. New data presented herein, combined with data from the published literature, indicates that Tristichoideae is uniform with x = 10 and that polyploidy is the principal mechanism of chromosome evolution. In contrast, in the Podostemoideae the main mechanism of evolution is aneuploidy. Podostemoideae possess a range of base numbers: x = 10, 13-15 and 17. Disploidy may be the underlying reason for such variation. Cytotaxonomic evidence supports the recognition of Podostemaceae s.s. and Tristichaceae as distinct families. (C) 2002 Elsevier Science B.V. All rights reserved.