Origin of the invasive Arundo donax (Poaceae): a trans-Asian expedition in herbaria

Abstract

Background and aims: The hypothesis of an ancient introduction, i.e. archaeophyte origin, is one of the most challenging questions in phylogeography. Arundo donax (Poaceae) is currently considered to be one of the worst invasive species globally, but it has also been widely utilzed by man across Eurasia for millennia. Despite a lack of phylogenetic data, recent literature has often speculated on its introduction to the Mediterranean region.

Methods: This study tests the hypothesis of its ancient introduction from Asia to the Mediterranean by using plastid DNA sequencing and morphometric analysis on 127 herbarium specimens collected across sub-tropical Eurasia. In addition, a bioclimatic species distribution model calibrated on 1221 Mediterranean localities was used to identify similar ecological niches in Asia.

Key results: Despite analysis of several plastid DNA hypervariable sites and the identification of 13 haplotypes, A. donax was represented by a single haplotype from the Mediterranean to the Middle East. This haplotype is shared with invasive samples worldwide, and its nearest phylogenetic relatives are located in the Middle East. Morphometric data characterized this invasive clone by a robust morphotype distinguishable from all other Asian samples. The ecological niche modelling designated the southern Caspian Sea, southern Iran and the Indus Valley as the most suitable regions of origin in Asia for the invasive clone of A. donax.

Conclusions: Using an integrative approach, an ancient dispersion of this robust, polyploid and non-fruiting clone is hypothesized from the Middle East to the west, leading to its invasion throughout the Mediterranean Basin.

Keywords: Arundo donax; Asia; Mediterranean; Poaceae; archaeophyte; clonal species; crops; domesticated species; giant cane; giant reed; herbarium specimens; invasive species; morphometry; phylogeography.

Fig. 1.

(A) Combined plastid DNA network of the genus Arundo based on substitutions and mini-/microsatellites, and (B) on substitutions only. Haplotypes are colour coded; their character code corresponds to Supplementary Data Table S1; their size correspond to the number of individuals detected in the dataset. Small black circles indicate haplotypes not detected in the dataset.

Fig. 2.

(A) Geographical distribution of plastid DNA haplotypes and morphotypes. White circles, A. donax morphotype T1; grey circles, A. donax morphotype T2; black circles, A. formosana morphotype; *, seed occurrence. Coloured rings correspond to plastid DNA haplotypes (Fig. 1). (B) UPGMA tree based on morphological data. (C) Ecological niche modelling of A. donax calibrated on 1221 Mediterranean occurrences (black dots) and projected on sub-tropical Eurasia using MaxEnt.

Fig. 3.

Boxplots of morphological variables between A. donax T1 (A.d.T1), A. donax T2 (A.d.T2) and A. formosana (A.f.), i.e. number of flowers per spikelet (nbF), and length (mm) of lower glume (G1), upper glume (G2), lemma (L), palea (pa), lemma hair (pL) and awn (A), length of stomatal guard cells (X; μm), stoma density (dX; per 10⁴ μm²), number of rib prickles per μm (dP, per mm of rib line). Grey numbers, numbers of measures per variable per morphotype; bold lines, median values; dashed lines, entire variable range; R², coefficient of determination; ***Kruskal–Wallis P-value <0.0001.

Fig. 4.

Scanning electron micrographs of the limb abaxial epidermis and stoma of A. donax morphotype T1, with large stoma (X) and few indumentum elements (A and B, MDo1), and A. donax morphotype T2, with smaller stoma (X), numerous prickles (P) and long hairs (D and C, EDo6).