LIPPIA ALBA (VERBENACEAE): A NEW TROPICAL AUTOPOLYPLOID COMPLEX?

Premise of the study: Tropical regions have high species diversity, and polyploidization is a major mechanism of speciation in plants. However, few cases of natural polyploidy have been reported in tropical regions. Lippia alba, is a tropical, aromatic shrub with a wide distribution, extensive morphological plasticity, and several chemotypes. The species has long been recognized as a diploid with 2 n = 30 chromosomes. Recently, two variations in chromosome number (2 n = 60; 2 n = 12-60) have been reported, suggesting the occurrence of polyploidy within the species. Methods: Flow cytometry was used to investigate the genome size in 106 accessions from 14 Brazilian States. Conventional and molecular cytogenetic techniques and pollen viability analysis were employed to characterize each chromosome number observed. Key results: The DNA 1C-value varied from 1.17 to 3.45 pg, showing a large variation in genome size. Five distinct chromosome numbers were observed (2 n = 30, 38, 45, 60, 90); three are cytogenetically described here for the first time. The 5S rDNA signals varied proportionally according to each chromosome number, but 45S rDNA sites did not. High rates of meiotic irregularity were observed, mainly in cytotypes with higher chromosome numbers. Conclusions: The data provide new support for the occurrence of a polyploid series in Lippia alba. We provide a hypothesis for how this complex may have arisen. Other cryptic polyploid complexes may remain undiscovered in tropical regions.