The independent gametophytic stage of Trichomanes speciosum Willd. (Hymenophyllaceae), the Killarney Fern and its distribution in the British Isles

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Walsonia 22: 1-19 (1998) The independent gametophytic stage of Trichomanes speciosum Willd. (Hymenophyllaceae), the Killarney Fern and its distribution in the British Isles F. J. RUMSEY, A. C. JERMY Department
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Walsonia 22: 1-19 (1998) The independent gametophytic stage of Trichomanes speciosum Willd. (Hymenophyllaceae), the Killarney Fern and its distribution in the British Isles F. J. RUMSEY, A. C. JERMY Department of Botany, the Natural History Museum, Cromwell Road, London, SW7 SBD and E. SHEFFIELD School of Biological Sciences, University of Manchester, Oxford Road, Manchester, M13 9PT ABSTRACT Uniquely amongst European ferns Trichomanes speciosum Willd., the Killarney Fern, has perennial, gemmiferous gametophytes which may grow and persist in the absence of the sporophyte generation. The presence of widespread independent colonies of the gametophyte generation and their habitats is briefly described and their distribution in the British Isles documented. The conservation implications of this unique situation are discussed. KEYWORDS: gametophyte, sporophyte, pteridophyte, Atlantic cryptogamic community, ecology, conservation biology. INTRODUCTION HISTORICAL Trichomanes speciosum Willd., the Killarney Fern or Bristle Fern has been described as one of the rarest and most celebrated species in the British flora (Ratcliffe et al. 1993). The species' great rarity, beauty and image of tropical incongruity have acted to generate a potent mythology, only paralleled by that of some orchids. It is one of a large genus of filmy-ferns showing their greatest diversity in tropical montane rain forests. Trichomanes speciosum was originally described from plants collected in the Canary Islands and is restricted now to those and other islands of the Macaronesian archipelago and a few relict sites in Europe. In Britain considerable and understandable secrecy has surrounded the location of this fern, following the wholesale depredation of localities to adorn the drawing rooms of the Victorian upper classes (Alien 1969). Trichomanes speciosum has a long history of records, a resume of which is given below. The first ever collection of T. speciosum in the British Isles was that by Dr Richard Richardson in 1724 (Ray 1724), by a spring head on Bell Bank, Bingley, Yorkshire, voucher material of which can be seen in the Sloane Herbarium at BM - (H.S. 145, f. 9; H.S. 302, f. 66). Within 50 years its original site had been destroyed and T. speciosum was thought to have died out but was refound in the early 1780s (Bolton 1785; Teesdale 1800) and again brought into cultivation. Unfortunately this was probably responsible for the plant's demise, as from about 1785 T. speciosum was not recorded as being seen again and was, once again, considered extinct. The botanical exploration ofireland in the first years of the 19th century, however, revealed this species to be more widespread and achieving abundance at some sites in the extreme south-west, most notably around the Killarney area of Co. Kerry (v.c. Hl-2). This fact is commemorated in its current vernacular name, although to many Victorian botanists it was the Irish Fern . Its potential for specialised culture, already developed for plants demanding similar high humidity brought in from New Zealand and elsewhere, was quickly seen, and plants were sold through professional outlets in Britain, and by rural entrepreneurs in Ireland. 2 F. J. RUMSEY, A. C. JERMY AND E. SHEFFIELD As the nineteenth century advanced, the advent of the Wardian case enabled the elite of London to have Killarney ferns in their drawing rooms. Almost from its discovery, Trichomanes speciosum was under threat in Britain and Ireland. Collection and subtle but crucial disturbance to micro habitats through visitation still arguably pose the greatest threats to this species' survival (Wigginton, in press). The necessary caution surrounding the plant's localities has, however, made it difficult to establish losses and gains, as the history of the species within its few sites, e.g. in terms of extent, performance and fertility is, where known, unpublished, or so vaguely identified that its worth is regrettably limited. Even past and present distribution in broader terms is difficult to establish accurately, posing further complications for those seeking to understand the environmental factors acting to limit the species' distribution and explaining its dispersal. This is of increased importance given recent discoveries which have revealed a disparity in the distributional extent and amount of the two phases of the life cycle. THE GAMETOPHYTIC STAGE OF THE LIFE CYCLE Trichomanes speciosum is u'nique amongst European ferns in that its gametophytic generation, the sexual or gamete-bearing phase of the life cycle, is not only perennial but produces specialized structures for its vegetative propagation (gemmae), allowing the potential development of extensive stands of this usually overlooked generation. Originally described and illustrated from cultivated material over a century ago (GoebeI1888), the distinctive filamentous gametophyte of this species appears to have been completely overlooked in the field prior to the finds reviewed here. The fact that fern gametophytes can establish and reproduce themselves independently of the sporophyte has been known for some time, and the, now classic, examples from Eastern U.S.A. of gametophytes of Hymenophyllaceae, and other ferns in the families Grammitidaceae and Vittariaceae that have in some instances spread several hundreds of kilometres beyond the ranges of the sporophyte or persist in their complete absence, have been well-documented (Farrar 1967; Farrar et al. 1983). Farrar (1985) has shown that populations of the gametophyte of N. American Trichomanes boschianum Sturm, the United States endemic equivalent of T. speciosum, exist up to 40 km distance from the nearest sporophytes. In addition, a further more widespread and numerous Trichomanes taxon, found in the eastern United States and initially thought to be the gametophyte of T. petersii A. Gray, has been shown (by comparison of enzymes) to be a new species, T. intricatum Farrar, currently known only as its gametophyte generation (Farrar 1992). Preliminary investigation elsewhere suggested independent Trichomanes gametophyte populations may be a widespread phenomenon in temperate areas (Rumsey & Sheffield 1996). During a sabbatical visit to the U.K. late in 1989, Farrar discovered gametophytes he recognized to be those of a Trichomanes species at two sites in the English Lake District (Rumsey et al. 1990). These were compared isozymically with a range of Trichomanes species and gave banding patterns identical to, and characteristic of, T. speciosum (Rumsey et al. 1993; Rumsey 1994). As records increased of this inconspicuous, albeit mostly non-sexual gametophytic stage of a taxon that was rare, and listed as a critically endangered and protected species, its presence posed profound implications for those drafting and enforcing conservation legislation. The gametophyte has been located within the known sporophytic distribution from Tenerife, Canary Islands; Madeira; the Azores; Algeciras, S. Spain; Asturias, N. Spain (Viane, pers. comm., 1992) ; Douro Valley, Portugal; Brittany (Jermy & Viane in Ripley 1990; Prelli, pers. comm., 1993) ; and Apuane Alpes, N. Italy; all sites recorded by A.C.J. and/or F.J.R. unless otherwise stated. However, research has revealed that gametophyte populations extend into continental Europe far beyond the known sporophytic range of the species and closely parallel the known past distribution of Hymenophyllum tunbrigense (L.) Srn (cf. Richards & Evans 1972). The gametophyte generation is now known from several areas in central Europe. It is most widespread and relatively abundant in the Wasgau, the southern Pfalzerwald of Germany (C. Stark, pers. comm., 1996) and the adjacent N. Vosges of France (Jerome et al. 1994). It is also reported from the sandstone massif shared by Luxembourg and Germany in the southern Eifel (Rasbach et al. 1993, 1995; Bujnoch & Kottke 1994; Reichling & Thorn 1997), and is known to be present as small scattered populations in the Elbsandsteingebirge, straddling eastern Germany and the Czech Republic (Vogel et al. 1993), and the Zittauer Gebirge (J essen, pers. comm., 1997) - its easternmost known locality. It would appear to be thinly scattered in the intervening areas, with records from the northern Black Forest, east of Heidelberg in the Neckar valley, from the Spessart in the Main valley north-west of Wiirzburg, from DISTRIBUTION OF THE KILLARNEY FERN GAMETOPHYTE 3 the Wupper valley near Solingen and from the northern Eifel near Monschau (Bennert et al. 1994; Kirsch & Bennert 1996). Over the past six years, an extensive survey by F.J.R. and A.C.J. and local recording by others in the British Isles has shown the gametophyte to be widespread, far beyond the present range of the sporophyte. Many suitable areas remain to be investigated but a summary of the position to early 1997 is presented here with the hope that it will stimulate additional recording. The wider gametophyte distribution reported here poses many questions, not least of which are: how is the disjunction between the generations perpetuated? (an issue addressed by Rumsey et al. 1992; and further discussed in Rumsey & Sheffield (1996» and, when was the current range achieved and by what means? (the topic of an on-going research project at the Natural History Museum, London). TRICHOMANES SPEClOSUM IN THE BRITISH ISLES Crucial to the understanding of the distribution of this species is the elucidation of the ecological differences between the two phases of the life-cycle, an aspect which may play a major role in perpetuating independence of either generation (Rumsey & Sheffield 1996). Much can be inferred from a comparison of the overall distribution of the two generations, the broad extent of which are given in Fig. 1. The distribution of the sporophyte within Britain has been outlined by Ratcliffe et al. (in Wigginton, in press). They report the past presence of 24 separate colonies, in 17 localities, occurring in a total of eleven widely scattered vice-counties. Only 16 colonies in ten localities are known to be extant. In Ireland only ten sites (in six vice-counties), out of the 43 once recorded were reported to be extant by Curtis & McGough (1988). The species is, however, clearly underrecorded, especially in the hill country of Counties Kerry and Cork where Ratcliffe et al. (1993) report the presence of 26 of the 30 Irish colonies known to them over the past three decades. The importance of these rare sporophyte colonies as potential sources, by means of spore dispersal, of the wider gametophyte distribution, remains to be resolved and is currently under investigation. The history of the discovery of the sporophyte in various areas of the British Isles is not elaborated further here but is discussed in part by Roberts (1979) and Church (1990). Since the discovery of the distinctive filamentous gametophyte generation and with growing awareness of its habitat preferences, wider surveys have revealed it to be remarkably widespread, if often extremely localized. We must assume that a combination of a morphology not readily assignable to anyone cryptogamic group, coupled with growth in a poorly investigated and often inaccessible environment has resulted in its being overlooked for so long. Given this oversight a brief description of the habitats in which the gametophyte may be found is given below. MORPHOLOGY OF THE GAMETOPHYTE The morphology of the gametophyte has been described and illustrated (Rumsey et al. 1990; 1993) but is described again here with the hope that a wider audience will come to recognize it. Trichomanes speciosum gametophytes consist of branched filaments, the individual cells of which are c urn wide and um long, that grow interwoven into tufts or mats with an open, felt-like appearance (Fig. 2). These are of a clear bright glowing green when well hydrated, taking on a somewhat bluish-black metallic cast as the filaments crumple on drying. The gametophyte colonies can vary in overall size, from occurring as scattered filaments among bryophytes, to more or less pure patches covering several square metres to a depth of about one centimetre. The majority of sites, however, support small tufts ranging from thumbnail-sized patches to up to c. 10 cm. The combination of colour, shape and restriction to particular niches within habitats makes field recognition of the gametophyte relatively easy in the majority of cases. The filaments maintain a rigidity, giving a distinctive wool-like resilience, when lightly touched, and by which an experienced worker can identify the colony or mat. They are distinguished from bryophyte protonemata by their larger diameter filaments, the cells of which are without oblique end-walls, and from filamentous green and yellow-green algae by their pale brownish rhizoids and the presence of characteristically-shaped gemmifers, gemmae and gametangia (sex organs), when present. The 4 F. J. RUMSEY, A. C. JERMY AND E. SHEFFIELD 60 / I,.--:;::; FIGURE 1. World distribution of Trichomanes speciosum Willd.:. both generations; 0 gametophyte only. All known records mapped on the Atlas florae Europaeae base map (Jalas & Suominen 1972) amended to include the Canary Islands and Madeira. Dots indicate the presence of the species at any time within 50 km squares of the UTM grid map. DISTRIBUTION OF THE KILLARNEY FERN GAMETOPHYTE 5 \ -, '.' './ / \. '\1',,/( T' / \ J J.',' ~\' ., :r J -,/1(',. ~ \,/1 1,. '' , ~. \ '! '.. '~~.-',I -_~4 I;, f. ') / 'J...,\, 11 ~ ;---~- 2-,~ 7/ ' \', /'-~_I ~/, - 1 '~' ~/ , \ 1 .' /'- \. , ) l,vj '. , I I -, _ _,.' ,;:. \... ~~' i '.,, ~ -' ---,c' /# ~' ' 1- \,.' ~',~./ f:;i' I '/ ',...,..,~,/ I ( I', ',, .-. \--' ;/',,'...-L- I _...' ~.,4 './ ~ I I \,. 2 (1 k I.)~, (1 2 5 k :.,.:~. 4 (1 (1 J!,,4 7' ::: 4 FIGURE 2. Upper. Typical gametophyte habitat under millstone grit boulders at Bell Bank, Yorkshire, 1989, Don Farrar, who first drew attention to the gametophyte in Europe, is holding the torch, (Photograph, A. C. Jermy.) Lower, Scanning electron micrograph of a gametophyte mat (x 30). (Micrograph, Natural History Museum,) 6 F. J. RUMSEY, A. C. JERMY AND E. SHEFFIELD most superficially similar algae grow in wet, well illuminated positions, in which T. speciosum gametophytes have never been found. Discrimination from other ferns is straightforward as no other native European fern genera produce filamentous gametophytes. As the gametophytes of all taxa within Trichomanes Section Lacosteopsis Prantl (= Vandenboschia Copeland) are so morphologically similar as to be effectively indistinguishable, certainly in the field, the possibility that one or more independent gametophyte taxa (i.e. lacking a known sporophyte) may occur in Europe, as in N. America, must be considered. This is particularly likely to be the case given that the diploid progenitors of the tetraploid T. speciosum are unknown. Available molecular evidence would suggest, however, that all material examined to date is of one, admittedly variable, taxon. Confusion following the spread of exotic species is unlikely but cannot entirely be ruled out (e.g. Rumsey et al. 1993). The gametangia are very similar in structure to those of other Trichomanes sensu lato (Stokey 1948; Yoroi 1972) and are obviously of a Filicalean form (see Fig. 3). The archegonia are produced on a specialised structure, the archegoniophore which is borne on a short broader filament and is produced just above the substrate, deep in the gametophyte tufts. They are apparently rarely produced, being found in less than 10% of gametophytes collected from the field, and later development in material grown in the laboratory has made no significant difference to this figure. In contrast, the antheridia, which may be found on the same filament, are more readily produced; c. 25 % of gametophytes had at least one when collected and following cultivation nearly 75 % had produced them (Rumsey & Sheffield 1996). Antheridial dehiscence with functional (i.e. motile) antherozoids has, however, rarely been observed. Thus the potential for gametophyte colonies to generate sporophytes cannot be assumed. DISTRIBUTION OF THE GAMETOPHYTE IN THE BRITISH ISLES All gametophyte records have been lodged with the Biological Records Centre, Monks Wood. While details of the gametophyte's distribution remain to be established, especially in Scotland and Ireland, the provisional map (Fig. 4) probably represents an accurate picture of its distribution and general regional abundance. Up to February 1997, gametophytes have been recorded in 38 British vice-counties: 1, 2, 3, 4, 14, 34, 35, 36, 39,41,42, 43,44,45, 46, 47, 48, 49,52, 57,62, 63,64, 67,69, 70, 88, 95,98, 100, 101,102, 103,104,105,107,108 and 109 and in 120 lo-km grid squares (hectajs). In three ofthese, both those in v.c. 1 (West Cornwall), and one in v.c. 2 (E. Cornwall) it is present in grottoes and artificial features in gardens, and in two cases closely associated with sporophytes which are assumed to have been deliberately introduced. It is currently recorded from 13 Irish vicecounties: HI, 2, 3, 6, 8,10,13,16,20,26,27,33,35 and 221O-km squares but has not been searched for as exhaustively as in some other places. Some local botanists have been encouraged by finding both topography and geology in their area suitable for the gametophyte and have quickly come to recognise both its habitat and morphology. We believe that as more become acquainted with the gametophyte, much of the appropriate country where sandstone and the coarser volcanic rocks predominate will be shown to house this stage of the Killarney Fern. It should be noted, however, that although widespread, the gametophytic generation is by no means common. More than 75% of the grid squares mapped contain only single populations, in many cases restricted to a single microtopographical feature. Furthermore, we must stress (and see below) that the species in its entirety is protected under laws in the European Union, the U.K. and the Republic of Ireland. After many years of observation of the sporophyte generation Ratcliffe et at. (1993) concluded its distribution ... is puzzling in that it is absent from a great many apparently suitable habitats within its climatic range . The problem is that too little is currently known of the environmental constraints on either generation's growth and survival. The apparently anomalous distribution of this species, as with many other rare Atlantic cryptogams which show great individual longevity but very little or no current dispersive ability, can be arguably best explained as the product of rare climatic and stochastic events. Given no, or very limited, ability to recolonize once lost, the occurrence of these species implies a local continuity of sitelhabitat suitability, where an absence merely suggests that conditions have become unsuitable, if only once in the last few hundred years. It is thus not surprising that attempts to match this species' distribution to climatic factors, often expressed as means, have met with only limited success. DISTRIBUTION OF THE KlLLARNEY FERN GAMETOPHYTE 7 A c o E F G FIGURE 3. Trichomanes speciosum gametophytes. A. Germinating spore (r = rhizoid) ; B. Gametophyte with archegoniophores ( jl); C. Gametophyte with antheridium (d') and gemma; D. Gemma; E. Gemmifers; F. Archegoniophore with archegonia; G. Antheridia. Scale bar = 100/lm. 8 F. 1. RUMSEY, A. C. lermy AND E. SHEFFIELD 9 2J------f J- ~-_.!~.~~~~~~ ~--~8 1~----f---~ KM :, NIL ES :, 0l[], i 6. ; 0.9 I ) FIGURE 4. Distribution of the Trichomanes speciosum WilId. gametophyte generation in the British Isles ( ). GAMETOPHYTE SITE RECORDS First records only for each tetrad are listed below. Vice-county names follow Dandy (1969) for those in Britain, and in Ireland they are as adopted by Scannell & Synnott (1987). In order to protect the location of extant sporophyte sites in
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