Phylogenetic trends in the evolution of inflorescence odours in Amorphophallus

Abstract

The chemical composition of inflorescence odours of 80 species of Amorphophallus (Araceae) were determined by headspace-thermal desorption GC-MS. When compared to published molecular phylogenies of the genus, the data reveal evidence both of phylogenetic constraint and plasticity of odours. Dimethyl oligosulphides were found as common constituents of Amorphophallus odours and were the most abundant components in almost half of the species studied. Odours composed mainly of dimethyl oligosulphides, and perceived as being ‘gaseous’, were only found among Asian species, and some of these species clustered in certain clades in molecular phylogenies; e.g. in two clades in Amorphophallus subgenus Metandrium. However, some species with gaseous odours were found to be closely related to species producing odours more reminiscent of rotting meat in which various minor components accompany the dominant dimethyl oligosulphides. These two broad types of odours have co-evolved with other inflorescence characteristics such as colour, with species with rotting meat odours having darker inflorescences. Species producing pleasant odours characterised by benzenoid compounds constitute two broad groups that are not related in published phylogenies. Species having fruity odours containing 1-phenylethanol derivatives mainly occur in a clade in subgenus Metandrium while those with anise odours composed almost solely of the 2-phenylethanol derivative 4-methoxyphenethyl alcohol are restricted to a clade in subgenus Scutandrium. Phylogenetic mapping of odours also indicates that the evolution of some odour types is likely to have been influenced by ecological factors. For example, species producing fishy odours dominated by trimethylamine and occurring in N and NE Borneo are not all closely related. Conversely, two sister species, A. mossambicensis and A. abyssinicus, which are morphologically very similar and have overlapping geographical distribution, produce odours which are very different chemically. The pressure of pollinator resource has therefore been a factor influencing the evolution of odours in Amorphophallus, driving both the divergence of odour types in some taxa and the convergence of odour types in others.