6. See text
The order Polypodiales encompasses the major lineages of polypod ferns, which comprise more than 80% of today's fern species. They are found in many parts of the world including tropical, semitropical and temperate areas.
Polypodiales are unique in bearing sporangia with a vertical annulus interrupted by the stalk and stomium. These sporangial characters were used by Johann Jakob Bernhardi to define a group of ferns he called the "Cathetogyratae"; the Pteridophyte Phylogeny Group has suggested reviving this name as the informal term cathetogyrates, to replace the ambiguously circumscribed term "polypods" when referring to the Polypodiales. The sporangia are born on stalks 1–3 cells thick and are often long-stalked. (In contrast, the Hymenophyllales have a stalk composed of four rows of cells.) The sporangia do not reach maturity simultaneously. Many groups in the order lack indusia, but when present, they are attached either along the edge of the indusium or in its center.
Both Polypodiales and Cyatheales differ from other ferns in having a photoreceptor called a neochrome, which allows them to perform photosynthesis better in low-light conditions, such as in the shadows on the forest floor. The common ancestor of the two groups appears to have derived the neochrome via horizontal gene transfer from a hornwort.
Historically, the ferns have undergone many different classifications (see review by Christenhusz and Chase, 2014). Smith et al. (2006) carried out the first higher-level pteridophyte classification published in the molecular phylogenetic era. Smith referred to the ferns as monilophytes, dividing them into four groups. The vast majority of ferns were placed in the Polypodiopsida, and that arrangement has persisted through all subsequent systems, despite some changes in nomenclature. Polypodiopsida is used in the strict sense (sensu stricto) by Smith et al. since it later came to be applied to all ferns (sensu lato), while this large group became known as Polypodiidae. This group is also informally known as the leptosporangiate ferns, while the remaining three groups (subclasses) ar referred to as eusporangiate ferns. The Polypodiidae have been divided into seven orders since that study, the largest of which is Polypodiales. The phylogenetic relationship between the orders of Polypodiidae is shown in this cladogram.
Smith et al. (2006) divided the Polypodiales into fifteen families, a practice continued in their 2008 revision. The Polypodiales were placed in the leptosporangiate ferns, class Polypodiopsida. Fifteen families, including members of the eupolypods, comprising two unranked clades, were recognized: Lindsaeaceae, Saccolomataceae, Dennstaedtiaceae, Pteridaceae, eupolypods I (Aspleniaceae, Thelypteridaceae, Woodsiaceae, Blechnaceae, and Onocleaceae), and eupolypods II (Dryopteridaceae, Lomariopsidaceae, Tectariaceae, Oleandraceae, Davalliaceae, and Polypodiaceae). While many of these families had previously been recognized with similar circumscriptions, the authors noted that Dryopteridaceae was more narrowly bounded than in historical circumscriptions, which had included their Tectariaceae, Onocleaceae, and Woodsiaceae. The circumscription of Lomariopsidaceae changed dramatically, most historical genera of that family (except Lomariopsis and Thysanosoria) being moved to Dryopteridaceae, while Cyclopeltis and Nephrolepis were added. Saccolomataceae were removed from the dennstaedtioids. Cystodium was tentatively placed in Lindsaeaceae, away from its historical position with the tree ferns. Woodsiaceae was acknowledged to be of uncertain circumscription and perhaps paraphyletic; the inclusion of Hypodematium, Didymochlaena, and Leucostegia perhaps also rendering Dryopteridaceae paraphyletic. The grammitids were included in Polypodiaceae to render that family monophyletic.
The linear sequence of Christenhusz et al. (2011), intended for compatibility with the classification of Chase and Reveal (2009) which placed all land plants in Equisetopsida, reclassified Smith's Polypodiopsida as subclass Polypodiidae and placed the Polypodiales there. This classification incorporated new phylogenetic evidence to make several changes at the familial level, resulting in an expansion to 21 families. Lonchitis and Cystodium were removed from the Lindsaeaceae and incorporated into new families, Lonchitidaceae and Cystodiaceae respectively. Within eupolypods I, Woodsiaceae proved to be paraphyletic and was reduced to the genera Cheilanthopsis, Hymenocystis, and Woodsia, while the remainder of its genera were removed to Cystopteridaceae, Diplaziopsidaceae, Rhachidosoraceae, Athyriaceae, and Hemidictyaceae. Within eupolypods II, Nephrolepis was placed in a new family, the Nephrolepidaceae, due to uncertainty in its phylogenetic placement, while Hypodematiaceae was split from Dryopteridaceae to contain the three problematic genera mentioned by Smith et al.
The classification of Christenhusz and Chase (2014) dramatically reduced the number of families recognized in this order to eight by "lumping", reducing many families to subfamilies and expanding the circumscription of Polypodiaceae and Aspleniaceae to encompass all of eupolypods I and eupolypods II, respectively. In the new Aspleniaceae, the former Cystopteridaceae, Rhachidosoraceae, Diplaziopsidaceae, Aspleniaceae, Thelypteridaceae, Woodsiaceae, Athyriaceae, and Blechnaceae became subfamilies Cystopteridoideae, Rhachidosoroideae, Diplaziopsidoideae, Asplenioideae, Thelypteridoideae, Woodsioideae, Athyrioideae, and Blechnoideae, respectively. The former Hemidictyaceae were included in the Asplenioideae, and the Onocleaceae in the Blechnoideae. In the new Polypodiaceae, the former Hypodematiaceae, Dryopteridaceae, Lomariopsidaceae, Tectariaceae, Oleandraceae, Davalliaceae, and Polypodiaceae became subfamilies Hypodematioideae, Dryopteridoideae, Lomariopsidoideae, Tectarioideae, Oleandroideae, Davallioideae, and Polypodioideae respectively. Didymochlaena was placed in its own subfamily, Didymochlaenoideae.
The PPG I classification (2016) used a process intermediate between the two previous approaches, by introducing a new rank, that of suborder, and organising 26 families within six suborders, largely returning to the families set out by Christenhusz et al. in 2011. In lieu of the expansion of Aspleniaceae and Polypodiaceae, eupolypods I and II were recognized and named as suborders:
- Saccolomatineae includes the single family Saccolomataceae.
- Lindsaeinae corresponds to the Lindseaceae of Smith et al., and includes the Cystodiaceae, Lindsaeaceae, and Lonchitidaceae. It is probably not monophyletic.
- Pteridineae includes the single family Pteridaceae.
- Dennstaedtiineae includes the single family Dennstaedtiaceae.
- Aspleniinae (formerly eupolypods I) includes the families Cystopteridaceae, Rhachidosoraceae, Diplaziopsidaceae, Desmophlebiaceae (containing only Desmophlebium), Hemidictyaceae, Aspleniaceae, Woodsiaceae, Onocleaceae, Blechnaceae, Athyriaceae, and Thelypteridaceae.
- Polypodiineae (formerly eupolypods II) includes the families Didymochlaenaceae (containing only Didymochlaena), Hypodematiaceae, Dryopteridaceae, Lomariopsidaceae, Nephrolepidaceae, Tectariaceae, Oleandraceae, Davalliaceae, and Polypodiaceae.
The classification of ferns is certainly not yet stable due to the narrow family concept presented by the PPG I classification. The larger family concepts of Christenhusz & Chase are likely to be more stable. Family classification is a matter of taste, preference and applicability, and only time will tell which classification will be followed by the general user community.
The following diagram shows the best current estimate of phylogenetic relationships between the families of the Polypodiales. The placement of the Saccolomataceae, Cystodiaceae, and the Lonchitidaceae-Lindsaeaceae clade relative to one another is not yet firmly established. Clades given different names in the two most modern classifications have been labeled to indicate names used by Christenhusz & Chase (C&C) and the Pteridophyte Phylogeny Group I (PPG).
Now-obsolete families of Polypodiales include:
- Drynariaceae - now in Polypodiaceae
- Grammitidaceae - now in Polypodiaceae
- Gymnogrammitidaceae - now in Polypodiaceae
- Loxogrammaceae - now in Polypodiaceae
- Platyceriaceae - now in Polypodiaceae
- Pleursoriopsidaceae - now in Polypodiaceae
- Vittariaceae - now in Pteridaceae
Polypodiales may be regarded as one of the most evolutionarily advanced orders of monilophytes (ferns), based on recent genetic analysis. They arose and diversified about 100 million years ago, probably subsequent to the diversification of the angiosperms.
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