Geographical Distribution of Diploid and Tetraploid Cytotypes of Thymus sect. Mastichina (Lamiaceae) in the Iberian Peninsula, Genome Size and Evolutionary Implications

The occurrence of diploid and tetraploid cytotypes in Thymus section Mastichina (Lamiaceae) was investigated by chromosome counting and flow cytometric measurements of DNA content. To cover the entire native distribution of the section across the Iberian Peninsula, plant material was sampled from 29 populations, representing all three taxa (Thymus albicans, T. mastichina subsp. mastichina and T. mastichina subsp. donyanae). We provide first estimates of genome size for taxa of this section. Analyses revealed the existence of new cytotypes in T. mastichina subsp. mastichina (2n = 2x = 28, 30) and confirmed the existence of previous ones (2n = 4x = 56, 58, 60). We also confirmed the presence of exclusively diploid cytotypes (2n = 2x = 30) in the southwestern Iberian endemics T. albicans and T. mastichina subsp. donyanae. We conclude that the southwestern Iberian Peninsula acted as a Pleistocene glacial refugium that may have permitted speciation processes within this section. Several hypotheses concerning these processes and the origin and distribution of the studied taxa are discussed.

This work was supported by projects P06-RNM-01675 (8474) of the Andalusian Government (Consejería de Economía, Innovación y Ciencia), CGL2007-64839-C02-01 and 02/BOS and CGL2010-22234-C02-01 and 02/BOS of the Spanish government, and grant projects 2009SGR00439 “Grup de Recerca Consolidat” from the Generalitat de Catalunya and A52 “Grupo de investigación consolidado” Bioflora from the Aragon Government and the European Social Fund. V. G. was granted a research contract from the Junta de Andalucía.

Peer Reviewed