why do botanists find them difficult?
Part 2 – continuation of an article in British Wildlife, Volume ??, Number ??, month 2016
GENEALOGY & EVOLUTION
The ancestry and breeding behaviour of British male ferns, as understood at present, are complicated and incompletely understood, but the science reveals much that (I think) makes male fern biology a fascinating, if demanding study.
In the absence of apomixis, reproduction of hybrid plants is rare, but if chromosome doubling coincides with hybridisation, reproduction may be reinstated. The cell division mechanism that produces the spores (meiosis) is, by and large, disabled by the non-combination of incompatible chromosomes from separate species, but it can snap back into action when polyploidy creates matching sets.
That is how we got Polypodium interjectum (a hexaploid: 222 chromosomes) from the sterile hybrid of P. cambricum (fertile diploid: 74) and P. vulgare (fertile tetraploid: 148) known, when we find it today, as P. x font-queri (sterile triploid: 111), followed by doubling (fertile hexaploid: 222).
Doubling during hybridisation is also how the common male fern D. filix-mas became a successful, widespread species. It is a fertile tetraploid (164 chromosomes), having twice the chromosomes of each of its diploid parents (82 x 2 = 164). The hybrid, like the parents, would have been sterile had not something happened during meiosis that duplicated its chromosomes and made what we consider to be a distinct species.
D. oreades is reckoned to be the most ancient of our five male ferns, a diploid British species whose ancestry is unknown, but it was probably involved in the origins of all of our male ferns. Its ancestry can be defined with a genome formula: OO.
D. filix-mas (OOCC) emerged from the coalition of D. oreades (OO) and another species no longer found in this vicinity, probably D. caucasica (CC) of eastern Europe and western Asia.
Each of the three scaly male ferns has a different genealogy from the others. It is thought that they are variously descended from hybridisation events long ago, involving our own D. oreades (OO), D. wallichiana (WW), found wild in the Himalayas, and D. caucasica (CC) (Fraser-Jenkins, 1996; 2007). Both Hugh Corley (1996, pers. comm.) and Clive Jermy (1999, pers. comm.) considered that WW could have been either D. wallichiana or D. crassirhizoma. Further research is required before we will know for certain, but we have enough information to enable us to reach some reasonably convincing conclusions.
The ancestor of D. affinis is thought to have been the hybrid D. oreades x D. wallichiana (OW); that of D. borreri is thought to have been D. oreades x D. caucasica x D. wallichiana (OCW); and D. cambrensis is probably D. oreades x D. affinis (OOW). Note that D. cambrensis is, therefore, not only the descendant of an ancient hybrid, but it could also be found, created de novo, in modern Britain. The recent hybrid might or might not be all over the country, but of course, we wouldn’t be able to tell because it would be indistinguishable from D. cambrensis.
It is well worth getting to know the foreign ancestral male ferns, familiarity with which will refine your identifications of the British species. D. wallichiana and D. crassirhizoma are striking plants that can be obtained from plant nurseries and many garden centres, but being rather plain and almost indistinguishable from D. filix-mas, D. caucasica is all but unobtainable.
‘Seeing’ ancestry in male ferns can help enormously in identification. The toughest, shiniest, yellowest, scaliest of the three, D. affinis, looks just like a mixture of stout, angular, golden, glossy, scaly D. wallichiana and soft, green, rounded D. oreades. The next in robustness, D. cambrensis, we can imagine being D. affinis diluted by a second dose of D. oreades genes: less glossy and tough than D. affinis with distinctly lobed pinnules. The least robust, D. borreri, looks most like D. filix-mas (influenced by its share of D. caucasica genes) but has some visually evident genetic influences (dilute relative to D. affinis) provided by D. wallichiana or D. crassirhizoma that show up particularly in its angular pinnules.
It was not until I had sorted this all out for my own interest that this background biology became clear and I was able to make progress in my own ability to recognise the core five male ferns. I was and am still baffled by some plants that are either intermediate, perhaps simply because of fine genetic variation or – and total misfits do turn up occasionally – because some individuals really are different and refuse to slot into this scheme. We should not worry unduly about anomalies, but be content to recognise the five common species, until specialists publish convincing descriptions of as yet unknown taxa playing an important role in British ecology, whilst keeping our eyes and minds open for unfamiliar forms to which we can’t apply a name.
BREEDING & HYBRIDISATION
D. oreades and D. filix-mas both reproduce sexually: their prothalli have both male and female organs. The three scaly male ferns lack archegonia (female), but since they have antheridia (male) and can produce antherozoids (sperm), they can cross sexually with the two ‘hermaphrodite’ species, D. oreades and D. filix-mas, creating hybrids (one way, as male parent).
What follows is a brief précis of what may be read in completeness in Fraser-Jenkins, 2007 and Trewren, 2014. If convoluted taxonomy causes your brain to shut down, reading the next two paragraphs is not recommended.
Hybrids of D. filix-mas and each of the scaly male ferns turn up reasonably frequently. Thus we find D. x complexa (D. filix-mas x D. affinis) which has been found to occur with both of the commoner subspecies of D. affinis. We also find D. x convoluta (D. filix-mas x D. cambrensis) and D. x critica (D. filix-mas x D. borreri). The last seems to occur more or less anywhere the parents cohabit, which they frequently do.
Hybrids of D. oreades are difficult to identify and seem to be much rarer. If recognised, only one, D. oreades x D. cambrensis, is likely to stand out as markedly different from the rest because D. oreades x D. affinis is the same as D. cambrensis and D. oreades x D. borreri would wind up with the same combination of genes, reshuffled, as D. x complexa. However, a plant tentatively named “D. x protocambrensis” (D. oreades x D. affinis) has been found on Arran and another provisionally called "D. x ogwenensis” (D. oreades x D. borreri) in North Wales. I dare not say any more.
A male fern hybrid is likely to be sterile (producing dysfunctional spores) so that these plants are unlikely to be able to procreate by sexual means. However, occasionally a few spores are viable and the ability to reproduce asexually by apomixis is then capable of expression, so clonal propagation becomes a possibility. This must be a rare occurrence because if not, every hybrid could give rise to a population of a novel male fern (a species), each with the potential for longevity and geographical expansion. It is likely that we do not have the situation faced by experts dealing with dandelions, hawkweeds and other tricky apomictic plant complexes, in which there are not a mere nine similar types, but hundreds! If that were happening, as yet unnoticed, it would be another burdensome reason why male ferns are difficult.
IN CONCLUSION I will paraphrase Chris Page’s ‘Symphony of Characters’ advice quoted above: Only by considering and comparing a full character set can hope to identify any of these ferns in the field or Male ferns can’t be separated properly by casually observing just one or two differences.
References (parts 1 & 2)
Fraser-Jenkins, C.R. (1996). A reaffirmation of the taxonomic treatment of Dryopteris affinis (Dryopteridaceae: Pteridophyta). The Fern Gazette. 15:3, 77-81.*
Fraser-Jenkins, C.R. (2007). The species and subspecies of the Dryopteris affinis group. The Fern Gazette. 18:1, 1-26.*
Merryweather, J.W. (2002). British Male Ferns Pteridologist. 4:1, 12-14.*
Merryweather, J.W. (2005). A Key to Common Ferns (folding identification chart). Field Studies Council, Shrewsbury.
Merryweather, J.W. (2007a). Male Ferns 2007. Pteridologist. 4:6, 173-177.*
Merryweather J.W. (2007b). The Fern Guide. 3rd edition. Field Studies Council, Shrewsbury.
Merryweather, J.W. (2007c). spreadsheet: Male Ferns. Download at www.slef.org.uk/user-friendly-field-guides.asp.
Merryweather, J.W. (2012). British Ferns, Clubmosses, Quillworts and Horsetails Illustrated. Free DVD from firstname.lastname@example.org; ask for the ‘Highland Nature Guides’ DVD.
Page, C.N. (1997). The Ferns of Britain and Ireland. 2nd ed. Cambridge University Press.
Trewren, K. (2014). Some taxa within the Dryopteris affinis complex. British Pteridological Society, special publication No. 13.
* May be downloaded at www.ebps.org.uk