Distribution and ecological consequences of ploidy variation in Artemisia sieberi in Iran

Abstract

Because of their high proportion in the plant kingdom polyploid taxa are considered to have had evolutionary advantages over their diploid ancestors. These advantages may have included new characteristics that enable polyploids to occupy a broader range of habitats. In this context, we assess the ecological range of Artemisia sieberi, a canopy dominant within an important vegetation type in Iran. We assess the extent to which ploidy covaries with geographical and ecological distribution and look for ecologically-significant differences in the functional characteristics of diploids and polyploids.Populations of A. sieberi were sampled widely in Iran and soil characteristics, climate and anatomical and phytochemical plant attributes were measured. Also, in parallel, an independent genetic assessment of populations was carried out using genetic fingerprinting.Two ploidy levels were identified: 75% of the 34 populations of A. sieberi populations sampled were tetraploid (2n = 4x = 36) with the remainder diploid (2n = 2x = 18). Plants of differing ploidy also differed anatomically, genetically and chemically. Tetraploid populations had larger cells and lower stomatal densities and a different essential oil composition. They also appear ecologically distinct, occupying more fertile, mesic habitats than diploids.Genetic fingerprinting revealed the existence of two genetically differentiated subgroups independent of ploidy but with some geographic and ecological pattern. We conclude that diploids and tetraploids have a different ecological distribution and that the absence of mixed diploid-tetraploid populations is a reflection of differing fitness in different habitats. We suspect that a key ecological difference between diploids and tetraploids is the increased stomatal size of tetraploids, possibly resulting from the increased genome and hence cell size following polyploidisation. Polyploid-formation may be constrained in arid habitats by problems of water-use efficiency associated with the larger stomata originally produced.