EVOLUTION OF PERICARP SURFACE STRUCTURE IN NEPETA S. S. (LAMIACEAE) AS INFERRED FROM ANALYSIS OF ITS DATA

Abstract

Nutlet pericarp structure is important in the taxonomy of Lamiaceae (Labiatae) at different taxonomic levels. Within the family it has also been found that variation in pericarp structure is strongly correlated with the phylogenic results obtained from molecular DNA analyses. The genus Nepeta L., with more than 200 species mainly centred in SW Asia, is one of the taxonomically most complex genera within the family. Traditional taxonomic treatments of Nepeta are mainly based on gross morphology. As in other groups of Lamiaceae, pericarp structure provides some of the diagnostic characters in this genus. In order to investigate patterns of pericarp evolution within Nepeta, we used scanning electron microscopy to examine nutlet surfaces and pericarp cross sections and explored variation of these characters against a molecular phylogeny based on ITS sequences. Based on this phylogenetic analysis, Nepeta in its present circumscription is not monophyletic. Evolutionary trends in structure of nutlet pericarps are apparent although they require confirmation with more robust phylogenies. In particular, nutlets with tuberculate/thorny-like pericarp may have evolved once within this genus, in the common ancestor of five of the six subclades identified within Nepeta, and have been lost independently several times. We also show that evolution of tubercules in Nepeta is not related to plant life span. Our results also indicate that more genetic markers (both plastid and nuclear) are necessary to reconstruct a reliable and robust organismal phylogeny.