Morphological characterization of diploid and triploidAcorus calamus(Acoraceae) from southern Western Siberia, parthenocarpy in sterile plants and occurrence of aneuploidy

Abstract

Finding morphological differences between cytotypes that are stable throughout their geographical range is important for understanding evolution of polyploid complexes. The ancient monocot lineage Acorus includes two groups, of which A. calamus s.l., an important medicinal plant, is a polyploid complex with a centre of diversity in Asia. European plants are sterile triploids introduced by humans. An early study suggested that plants from temperate Asia are tetraploids, but subsequent work revealed diploids and triploids rather than tetraploids in Asiatic Russia; however, cytotype diversity in Western Siberia is insufficiently known. We document the occurrence of diploids and triploids in Western Siberia. Triploids that do not differ in genome size from European Acorus are abundant in the valley of the river Ob where the ability for extensive vegetative propagation provides ecological advantages. An isolated population of aneuploid triploids with 33 chromosomes is found outside the Ob valley. Flow cytometry provides an efficient tool for identification of aneuploid plants in Acorus. All triploids are sterile, but their flowers develop uniform parthenocarpic fruits. Fruits of diploids usually vary in size within a spadix depending on the number of developing seeds. In contrast to North America, where the native diploid plants differ from the introduced triploids by the absence of a secondary midrib of the ensiform leaf blade, Siberian diploids are similar to triploids in possessing a secondary midrib. We confirm that diploids differ from triploids in the size of air lacunae in leaves, which is determined by cell number rather than cell size in septa of aerenchyma. A combination of spathe width and spadix length measured after the male stage of anthesis shows different (slightly overlapping) patterns of variation between diploids and triploids in our material.

Inference of Ploidy Level in 19th-Century Historical Herbarium Specimens Reveals the Identity of Five Acorus Species Described by Schott

“…Röst [10] studied leaf anatomy in four diploid, 22 triploid, and 14 tetraploid accessions of A. calamus (see [44] for a list of accessions used in [10]) and revealed that the size of air canals in leaf aerenchyma allows precise distinguishing of diploids, triploids, and tetraploids of A. calamus in a common garden experiment. Our follow-up studies [36,37] refined the protocol provided by Röst [10]. Leaf anatomy of specimens not cultivated in controlled conditions still provided information useful for inferring ploidy levels, though certain character overlapping between triploids and tetraploids was found [36,37].…”

“…Our follow-up studies [36,37] refined the protocol provided by Röst [10]. Leaf anatomy of specimens not cultivated in controlled conditions still provided information useful for inferring ploidy levels, though certain character overlapping between triploids and tetraploids was found [36,37]. Triploids, however, can be identified based on their sterility.…”

“…The protocol for the present study and the interpretation of leaf anatomy were the same as in our earlier works [36,37]. Dry material of leaf lamina was rehydrated in hot water for two days and transferred into 70% ethanol.…”

“…(2) Mean number of cells in the septa between the air canals visible in cross section of the leaf blade. See [36,37], for more details of the protocol.…”

“…The only way of resolving the taxonomic identity of Schott's species names is through the identification of type specimens among historical herbarium collections, by inferring characters missing in Schott's description from this material. Fortunately, earlier studies revealed clear correlations between plant fertility, leaf anatomy, and ploidy levels in Acorus [10,18,32,[35][36][37]. The use of these characters should provide a link between cytotypes and Schott's species names.…”