Dioon, Lindley, 1843

Hernández-Tapia, Jessica E., Jiménez-Ramírez, Jaime & Vovides, Andrew P., 2020, Taxonomic revision of the genus Dioon (Zamiaceae), Phytotaxa 442 (4), pp. 267-290 : 267-270

The genus Dioon

Dioon is one of the genus in the family Zamiaceae , order Cycadales (Christenhusz et al. 2011) . It is distributed in Mexico, and Dioon mejiae Standley & Williams (1950: 37) is endemic to Honduras and cultivated in Nicaragua ( Hill et al. 2004) ( Fig. 1 ).

The genus appeared approximately 56 million years ago, which makes it one of the earliest diverging genera within the subclass ( Condamine et al. 2015). The name Dioon comes from the greek word “δις” and “ωον”, which means “ two eggs ”. It was described for the first time by Lindley (1843) as “ Dion ”. However, Miquel (1848) corrected this name to “ Dioon ” on grammatical arguments. The spelling Dion was used in some other occasion ( De Luca et al. 1984), but the mistake was addressed formally, and currently Dioon is included in the list of conserved names ( Vovides & Moreno 1983; Brummitt (1987).

The total number of species is controversial, and it varies according to different authors. This situation results from several reasons. A major cause was the disagreement between taxonomists regarding the proper taxonomic category for some species or varieties ( Moretti et al. 1993, Jones 1993, Whitelock 2004), regardless the apparent consensus in the community of taxonomist of Cycads for avoiding “forma” and “variety” as proper ranks in favor of species category ( Walters & Osborne 2004). Several species were not validly published, because a formal description was not included in the publication were the name was made available (e.g. Moretti et al. 1993; Jones 1993; Gregory et al. 2005; Yañez 2006; González et al. 2008; Villaseñor 2016; Gutiérrez-Ortega et al. 2018), resulting in a myriad of invalid names that later on were spread because of its use in popular databases as Tropicos.org. Also, several monographic treatments are too narrow in geographical scope, and that result on an incomplete depiction of variation (e.g. Villaseñor 2016, who only includes the Mexican species, Table 1).

An interesting example of this situation is the delimitation of Dioon angustifolium Miquel (1847: 37) . This species exhibits a large morphological variation, which may vary even between localities ( Whitelock 2004). This large variability leads De Luca et al. (1982) to consider D. angustifolium as a variety of D. edule Lindley (1843:59) . Later, González-Astorga et al. (2003, 2005), confirmed D. angustifolium as an independent species, supported on analysis of population genetic and morphometric evidence. Several phylogenetic analyses have been conducted for this lineage ( Moretti et al. 1993; González et al. 2008; Gutiérrez-Ortega et al. 2017, 2018; Gutiérrez-Ortega et al. 2018; Dorsey et al. 2018). The most recent and comprehensive studies have consistently recovered four clades but differ in the naming of the discovered groups Gutiérrez-Ortega et al. (2017) first recovered the clades and named: Tomasellii, Purpusii, Edule and Spinulosum clades. Later, the same researches rename the discovered clades as Spinulosum, Eastern clade, Western clade and Southern clade (Gutiérrez-Ortega et al. 2018). Dorsey et al. (2018) propose the Edule clade, the Southern clade, the Western and Spinulosum clade.

Two main hypotheses regarding diversification in Dioon have been proposed. The first one considers that the genus had diversified in the Miocene promoted by the aridification processes (Gutiérrez-Ortega et al. 2018). Anatomical studies of the leaf have shown that anatomical traits vary depending on the climate, this may be related with the effect of the aridification on the diversification process ( Vovides et al. 2018; Gutiérrez-Ortega et al. 2018). The second hypothesis proposed that the diversification occurred in the middle or late Pleistocene, along the glacial cycles, which may represent the driven factor of diversification ( Dorsey et al. 2018).

We consider the second hypothesis has a better support, because the molecular markers used for the phylogenetic inference are much more precise and allow greater resolution in their phylogeny and in their phylogeographyc hypotheses. Additionally, the hypothesis proposed by Dorsey et al. (2018), it is also congruent with the morphology and the biogeography of the genus.

The arrangement of the vascular bundles of the leaf petioles is a diagnostic character in the order Cycadales , and is constant in all of the genus, but maybe the constriction-extension in the arrangement of the vascular bundles is important in the taxonomy of the group as well ( Tomlinson et al. 2018).

The main goal of the present revision is to present a comprehensive summary of the current knowledge in the genus, as well as to present an integrative analysis for the limit of species proposed for the genus.

Figures

FIGURE 1. Distribution of the genus Dioon.