Karyotype variation, evolution and phylogeny in Borago (Boraginaceae), with emphasis on subgenus Buglossites in the Corso-Sardinian system.

Abstract

Karyological variation in the Mediterranean genus Borago and cytogeography of subgenus Buglossites in Corsica, Sardinia and the Tuscan Archipelago were investigated in combination with a molecular phylogenetic analysis aimed at elucidating relationships between subgenera and taxa with different chromosome features. Karyotype analysis was performed on population samples of B. pygmaea, B. morisiana, B. trabutii and B. officinalis. Phylogenetic analyses were based on ITS1 nrDNA and matK cpDNA sequences. Four base numbers were found, x = 6, 8, 9 and 15, and three ploidy levels based on x = 8. In subgenus Buglossites the Sardinian endemic B. morisiana is diploid with 2n = 18, while B. pygmaea includes three allopatric cytotypes with 2n = 30 (Sardinia), 2n = 32 (southern Corsica) and 2n = 48 (central northern Corsica and Capraia). In subgenus Borago, the Moroccan endemic B. trabutii and the widespread B. officinalis have 2n = 12 and 2n = 16, respectively. Molecular data support the monophyly of Borago, while relationships in subgenus Borago remain unclear. Borago trabutii appears as the earliest divergent lineage and is sister to a clade with B. officinalis, B. morisiana and B. pygmaea. Subgenus Buglossites is also monophyletic, but no correspondence between ITS1 phylogeny and B. pygmaea cytotypes occurs. Chromosome variation in Borago is wider than previously known. Two base numbers may represent the ancestral condition in this small genus, x = 6 or x = 8. An increase in chromosome number and karyotype asymmetry, a decrease in chromosome size and heterochromatin content, and the appearance of polyploidy are the most significant karyological changes associated with the divergence of the Buglossites clade. High ITS1 variation in the tetra- and hypotetraploid races of B. pygmaea suggests a multiple origin, while the lower polymorphism of the hexaploid race and its allopatric distribution in the northernmost part of the range is better explained with a single origin via union of unreduced and reduced gametes.