The early evolutionary and morphological diversity of Isoetes [...] - R.J. Hickey 1986

Systematic Botany(1986), 11(2): pp. 309-321 ? Copyright1986 by the American Society of Plant Taxonomists

The Early Evolutionary and Morphological Diversity of Isoetes, with Descriptions of Two New Neotropical Species R. JAMES HICKEY Botany Department, Miami University, Oxford, Ohio 45056 Two new Isoetes fromSouth America are described and contrastedwith morphologABSTRACT. ically similar and sympatricspecies. The firstof these species, I. baculata, representsa relictual morphotypewithin the Isoetaceae as indicated by the presence of completelylaminate leaves. The other species, I. eshbaughii, is unique in formingdesiccation resistantfoliar gemmae which are completelyenclosed by sclerifiedscales. Analyses on the distributionof alate leaves and scales, as well as peripheral fibrousbundles and sporangial pigmentation,show that thereare nested sets of species withinthe Isoetaceae which representmajorphylogeneticlineages. The patternof character nestingsuggestsan historicaltransitionfroma primitivelyaquatic habit to a seasonally amphibious one, with subsequent parallel reversionsback to aquatic zones. These habitatchanges have resulted in convergent morphologies which are in large part responsible for the previous confusion regarding the evolution of the genus and the dependence on megaspore morphology as the sole basis of infragenericclassification.A major advantage of the proposed phylogeny is that the demarcationof majorlineages now allows forthe establishmentof functionalingroupsand outgroups, and thus provides a frame of referencefromwhich additional phylogenetic hypotheses can be formulated.

The Isoetaceae are a small, cosmopolitan familyof aquatic, heterosporouspteridophytes which are related to the extantSelaginellaceae and Lycopodiaceae. The family is monotypic and comprises some 150 species (Tryon and Tryon 1982) which are classified primarilyon the basis of megaspore surface morphology (Pfeiffer1922; Fuchs-Eckert1982). The classification of such a large numberof species on the basis of variation in a single characteris intuitively suspect and several taxonomic studies indicate that this suspicion is warranted (Duthie 1929; Williams 1943). Beforethe establishment of an infragenericclassificationsystem 1922), based on megasporemorphology(Pfeiffer the significanceof megaspore charactersin Isoetestaxonomywas questioned by Eaton (1900).

A number of workershave regarded Pfeiffer's classificationas unreliable and unnatural (Williams 1943; Berthetand Lecocq 1977). Although megaspore surfacemorphologyis unreliable in delineating naturalsubdivisions within Isoetes, it does provide a ready means forspecies sorting and is of great importancein species identification(Boom 1982; Croft1980; Hickey 1981; Kott and Britton1983; Stolze and Hickey 1983; Taylor et al. 1975).

In recent years, a number of investigators (Reed 1965; Boom 1982; Hickey 1985), working within restrictedgeographical regions, have

adopted informalspecies alliances ratherthan use the currentclassificationsystem.Their reluctance to propose a new classificationis appropriatein light of the meager and often inconsistent data base currently available for Isoetes.

This paper is divided into four major sections. The firstintroduces and describes two new species of Isoetes fromSouth America.Each description is accompanied by an account of that species' distinctive features and each species is compared with morphologicallysimilar or sympatrictaxa. The second summarizes the variation and systematic distribution of vegetativereproductionin the genus, with particular referenceto a novel form of gemmae found in one of the newly described species.

The third section is concerned with the taxonomic and phylogeneticsignificanceof several unusual character states found in these new species. Correlations among these and other charactersand characterstatesfound in the genus are pointed out. In particular,I discuss the presence or absence of a subula, the development of peripheral fibrousbundles, the morphology and taxonomic distribution of scale leaves and phyllopodia, and the production of sporangial pigmentation.This suite of characters,when polarized and taken in concert,suggests a reinterpretationof the early evolution-

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ary historyof the genus. The finalsection presents discussions of the adaptative significance of these charactersand the significanceof the geographic distributionof the major lineages in Isoetes.

Isoetes baculata Hickey & Fuchs, sp. nov. (fig.

1).-TYPE: Brazil (Amazonas) Uaupes, im flussauf d. Colombianisches Seite, XI/28/ 1916, Luetzelburg23785 (holotype: M).

PARATYPE:Brazil (Amazonas) Uaupes, submersus auf colombianisch Ufer, XI/28/ 1918, Luetzelburg 23769 (M).

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Isoetesbaculatais readilyidentifiedby itslarge, densely baculate megaspores and by its long, flattened,flexuous leaves which are laminate, thoughnarrowlyso, fortheirentirelength. Isoetesbaculata,I. giganteaWeber,and I. bradeiWeber are the only known species of the genus to have entirely alate leaves and as such they closely resemble fossil Isoetitesin leaf morphology. The presence of completelylaminate leaves within the genus was firstdescribed by Weber (1922) but has been totallyignoredTsince.

The three extantspecies of Isoeteswith completely alate leaves are easily distinguished on the basis of megaspore surfacemorphology(table 1). Isoetesbaculatahas baculate megaspores, I. giganteahas saccate megaspores,and I. bradei has tuberculatemegaspores.The megasporesof I. baculataare unusual due to their propensity for electron charging when being photographed under an SEM. Despite numerous attempts with several differentand prolonged coating schedules, the spores continue to show this electroncharging.Isoetesbaculataand I. gigantea have large, widely ovate labia and all threealate species lack a well definedvelumcharacterstates which they share with I. panamensisMaxon and Morton and several other lowland, paleo- and neo-tropical species. The exact affinitiesof I. baculataare unknown because it does not share any obvious, uniquely derived characterswith otherspecies of the genus. Isoitesbaculata,I. gigantea,and I. bradeiconstitutea primitivegrade within Isoites.

Cormus globosus, bilobatus, 13-18 mm diametro; radicis ramosis dichotome e fossa singulari,circumbasaliexoriens.Folia 26-29, flexibiles, viridia, complanata, 600-650 mm longa, 5.0-8.0 mm lata basem, 0.6-1.0 mm lata medio; alis semipellucidis vel viridibus,chartaceisvel membranaceis,0.6-1.5 mm latis sporangium,ad apicem ascendens, apice attenuata; gemmis, stomatibuset squamellulis carentibus.Sporangium ellipticum, hyalinum, concolor, 5.5-7.0 mm longum, 2.5-4.5 mm latum,basale. Velum incompletum, tantummodo duae membranae angustae, ad 1.0 mm latae, utrinque sporangii consistente.Ligula hyalina, tenuis,pro residuo pulvini deltatovel late deltato,1.4 mm alto, 1.8 mm lato, exprimeni.Labium latissime ovatum vel depresse ovatum,integrumvel erosum,olivaceum, 0.7-1.6 mm altum, 1.3-1.6 mm latum.

Megasporae albidae, (470)580-730 (x = 667) ,um diametro,baculatae; cristaaequatoria strictavel undulata, distincta,2-3plo altiora quam latiora, Isoetes eshbaughii Hickey, sp. nov. (figs. 2cristisproximalibusstrictis, distinctis,altisquam 7). -TYPE: Bolivia,Dept. Cochabamba,Prov. latis. Microsporae cinerae, 35.0-43.0 (x = 39.0) Ayopaya,24 km in frommain rd at Quilla,umlongae, 27.0-34.0 (x = 31.0) ,umlatae, echincolla, plants amphibious in small muddy atae vel laevis, fibrillosae. pools on rock outcrop,4420 m, 27 March Etymology.Isoetesbaculata is named for its 1980, Hickey823 and Eshbaugh(holotype: distinctive,densely baculate megaspores. This GH; isotype: AAU, BM, MO, US).PARATYPE:Bolivia,Dept. Cochabamba,Prov. species name is co-authoredin considerationof the nearlysimultaneousrecognitionof it by H. Ayopaya, 24 km in frommain rd at QuilP. Fuchs-Eckert. lacolla, plants amphibious in small muddy Isoetesbaculatais a large, submerged aquatic pools on rock outcrop,4420 m, 27 March apparentlyconfined to the clear waters of the 1980, Hickey824 and Eshbaugh(GH, MU).

Vaupes River near the border of Colombia and Cormus globosus, bilobatus, 5-10 mm diaBrazil. The exact origin of these plants is in question and may be in either of these two metro; radicis dichotome ramosis e fossa sincountries. The apparently restricteddistribu- gulari circumbasali exorientibus. Folia 9-20, tion of this species may be an artifactof col- flexiles,60-150 mm longa, 6.0-7.0 mm bases lectingand it should be soughtalong otherclear lata, 0.5-1.0 mm medio lata; alis hyalinis,charwater tributariesof the Rio Negro in both Co- taceis,1.0-2.0 mm latis sporangium,4.0-8.0 mm lombia and Brazil. longis (15-19% per partes ascendentes), apici-

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microspore FIGS.1-4. Scanning electronmicrographsof Isoitesspores and gemmae. 1. Echinate-fibrillar of I. baculata(Luetzelburg 23785), bar = 20 ,m. 2-4. I. eshbaughii. 2. Leaf base with foliargemma (g) (Hickey 823 and Eshbaugh),bar = 2 mm. 3. Tuberculate megaspore (Hickey823 and Eshbaugh),bar = 200 ,m. 4. Echinate microspore(Hickey823 and Eshbaugh),bar = 20 jsm.

bus acutis; subulis teretis,viridis, apices attenuatis; stomatibus gemmisque foliorum praesentibus; squamellulis ad apicem cormi et circumgemmas foliarum;fasiculisfibrosisperiphericiscarentia.Sporangiumbasale, circulare vel ellipticum,2.5-5.0 mm longum,2.5-4.0 mm latum, hyalinum, concolor. Velum incompletum,in sporophyllisextimisdescendens ca. 0.2 mm,in sporophyllisintimisdescendens ca. 3.0 mm. Ligula hyalina, tenuis,pro ovato late pulvini residuo exprimeni,0.7-1.7 mm alto, 1.51.8 mm lato. Labium transverseanguste oblongum, integrum,0.2-0.4 mm altum, 1.0-1.2 mm latum,olivaceum. Megasporae albidae, 280-430 (x = 367) ,umdiametro,laeves vel tuberculatae, tuberculispersaepe confluentibuset megaspor-

ae rugulatae; cristis aequatoriis proximalibusque rectis,manifestis,altae latas aequantes.

Microsporae cinereae, 33.8-42.5 (x = 38.8) ,um longae, 25.0-31.3 (x = 27.5) ,umlatae, echinatae.

Chromosomatumnumerus 2n = 44.

Etymology.This species is dedicated to W. Hardy Eshbaugh of Miami University,Oxford, Ohio, foraid in the collection of this and other species of IsoetesthroughoutBolivia and Peru. is a small, delicate plant of Isoeteseshbaughii shallow, nutrientrich (llama dung) pools in the Cordillera de Cochabamba of west central Bolivia. The two known collections of this species were made a short distance fromone another.

Individuals of the type collection grew in the

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TABLE Character

1. Charactercomparison of extant,alate species of Isoetes.

I. baculata

Medial leaf width (mm) 3.0-4.0 Labium depressed ovate cordate

Sporangium position Megaspores Microspores

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1.3-1.6 mm wide 0.7-1.6 mm long basal baculate echinate-fibrillar

I. gigantea

0.6-1.5 widely to very widely ovate not cordate 5.5-7.0 mm wide 3.0-5.0 mm long elevated saccate papillate

deeper of the two pools and were completely submerged in 4 to 7 inches of water. The second collectionwas froma shallowerpool where many of the plants were completelyemergent along the muddy banks. The emergent habit plus the presence of scales on the corm and gemmae suggestthatthe pools are seasonal and that the plants are subjected to at least short periods of desiccation. is readilydistinguishedfrom Isoiteseshbaughii othercentralAndean species (table 2). It is differentiatedfromthe sympatricI. herzogiiWeber by its much more lax habit, tuberculatemegaspores (which vary toward sub-laevigate and sub-rugulate;fig.3), the presenceof scale leaves, and the production of foliar gemmae (figs. 2, 5-7). The lax, almost flaccid,nature of the sporophyllsis reminiscentof certaingrowthforms Weberand an undescribedspecies ofI. boliviensis fromTiclio Pass in Peru ("I. ticlioensis" H. P. Fuchs, nom. nud., Fuchs-Eckert1982; Hickey 1984). However, unlike this species, the lax habit of I. eshbaughiiis more extreme and appears to be genetically fixed. Collections of I. have retainedthislax habit fornearly eshbaughii fiveyears in the greenhouses at The University of Connecticut under a variety of culture regimes. When grown under identical conditions, I. herzogiiand the initially lax formsof "I. ticlioensis"become turgidand fastigiate.Isand "I. ticlioensis" can be furoites boliviensis ther differentiatedfrom I. eshbaughiiby their verrucate microspores; the microspores of I. are finelyechinate (fig. 3). eshbaughii

I. bradei

1.8-4.5 widely ovate not cordate ca. 2.0 mm wide ca. 2.2 mm long basal tuberculate papillate

either by apical dichotomies followed by fragmentation,or by means of gemmae. Apical dichotomies have been reported as "sports" by Motelay and Vendryes (1882), Solms-Laubach (1902), Eames (1936), and Karrfaltand Eggert (1977). As a consistentspecies specific character, there are three reports involving two species: Isoetesandicola(Amstutz) Gomez (and Rauh & Falk; Rauh the synonymousS. gemmifera and Falk 1959) and I. dichotomaMora & Hagemann (Mora-Osejo and Hagemann 1977). Plants distinguished as I. dichotomaappear to be no more than unusual variantsof the earlier pubFuchs and the two lished I. novo-granadensis names are considered to be synonymous(Hickey 1985). Thus, I. andicolais the only quillwort which commonlyreproduces by means of apical dichotomyand subsequent fragmentation.

Vegetative reproductionby means of gemmae was firstreported by A. Braun (1862) in his description of I. socia (=I. lechleriMett.).

These gemmae were reported to be foliar in origin. I have observed gemmae in numerous but all of these additionalcollectionsof I. lechleri arise fromcorticaltissues in the axils of leaves.

In 1879,Goebel described the structureand development of true foliar gemmae in I. lacustris L. These gemmae were embedded in the tissues of the leaf base and arose fromsporangial initials; their production was apparently a fixed genetic traitof thatpopulation (Longmer Lake in the Vosges Mountains, France). In 1950, Manton reported similar gemmae being produced by a population of I. lacustrisfromEngland. These plantletswere also reportedas foliar in origin but their development has not VEGETATIVE REPRODUCTION been studied. Gemmae formationin I. lacustris A survey of the literatureon Isoetes shows is, however, rare. Rauh and Falk (1959) prothat vegetative reproductionis rare,occurring posed a second species of Stylites,S. gemmifera,

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12mm

1'mm

mm

1

113mm

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FIGS. 5-12. Leaf bases, phyllopodia, and scale leaves of various Isoetesspp. 5. Leaf base of I. eshbaughii with two foliar gemmae, the gemma on the right with one normal leaf (Hickey 823 and Eshbaugh,spirit note aborted sporangia and ligular pits (Hickey823 material). 6-7. Scales fromcorm apex of I. eshbaughii, and Eshbaugh, spiritmaterial). 8. Phyllopodia of I. durieui(Sommerss.n., 1/I/1899-MU). 9. Phyllopodia of I. with partially degraded, nonsclerifiedalae histrix(Martellis.n., 16/V/1894-MU). 10. Scale of I. hieronymii showing partiallydegraded, nonsclerifiedalae (Howells.n.,1/VI/ 774-UC). 11. Scale of I. nuttallii (Hieronymus 1882-MU). 12. Sclerifiedleaf base tissue of I. melanopoda(Hall s.n.,1868-MU).

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TABLE Character

2. Comparison between I. eshbaughii and other centralAndean Isoites.

I. eshbaughiz

I. boliviensis

Habit

flaccid,lax

flaccid,lax

Velum-% sporangium coverage Megaspores

0-10 (40) tuberculate (sub-laevigatesub-rugulate) echinate foliar present

Microspores Gemmae Scales

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I. ticlioensis"

I. herzogu

60-80

flaccidto turgid, lax to stiff 10-25

(20) 70-90

laevigate

laevigate

rugulate (laevigate)

verrucate absent absent

verrucate cortical absent

echinate to ech-verrucate cortical absent

turgid,stiff

which was in part distinguished by the fre- reduced in size but are otherwisenormallydequent productionof corticalgemmae. It is now veloped (not sclerified). The inner leaves of known thatall of the plants referableto Stylites these gemmae and all of the leaves on the gemare facultativelygemmiferous (Hickey 1984) mae of more internal sporophylls are arrested and should be included as a single species early in development and are completelysclewithin the genus Isoetes. rified. These modified leaves are essentially With the publication of I. tegetiformans, Rury identical to the scales protectingthe cormapex (1978) described the firstknown species which and the seasonalityof scale productionin both consistentlyreproduces by means of cortical these organs is concurrent. The presence of gemmae. I have grown plants of I. tegetiformansscales on these gemmae indicatesthatin nature under an arrayof environmentalconditionsfor they functioneither as a desiccation-resistant the past ten yearsand have found gemmae pro- propagule or as a resting stage during unfaduction to be invariable. The most recent re- vorable conditions.The gemmae remain viable port of gemmae production was by Gomez forconsiderableperiods of timeand I have suc(1980) forI. storkiiPalmer. In this species, as in ceeded in germinatingthem from herbarium I. lechleri,gemmae production is cortical,orig- specimens up to four and one half years old. inatingin leaf axils. Isoetes storkii, however,only On a number of the specimens, gemmae are produces gemmae under environmentalstress. scatteredabout the bases of the plants among Cortical gemmae are also found in I. herzogii, old, partiallydegraded leaves. While mostveg"I. ticlioensis",I. savatieriFranchet,I. lechleri,I. etativelyreproducing Isoetes formdense cologrows as solitaryinglacialis Asplund, and in several new species nies or mats, I. eshbaughii (H. P. Fuchs, ined.; Fuchs-Eckert1982) fromEc- dividuals suggesting that the gemmae are uador (Hickey 1985). commonly dispersed over at least short disThe gemmae of I. eshbaughiiare unusual in tances before they become established. several respects.They are trulyfoliarin origin, MORPHOLOGICAL AND ECOLOGICAL DIVERSITY developing from de-differentiatedleaf tissue ratherthan fromsporangial initials as in I. laAla development.The Isoetes leaf is invaricustris(Goebel 1879). While the gemmae are ably described as awl-shaped with a proximal, distinctlyextra-sporangialin origin, they are reduced lamina (ala) and internalair chambers oftenfound within the fovea to either side or or lacunae. The presence of these lacunae has above the sporangium. Gemmae are on both been cited as evidence of a primitivelyaquatic mega- and microsporophyllsand are never as- habitat (Keeley 1984). The taxonomic outsociated with germinatingmegaspores or true groups of the Isoetaceae,the Lycopodiaceae and embryos.In addition to their unusual origin, the Selaginellaceae, are terrestrialand have the gemmae of I. eshbaughii are unique in that laminate leaves lacking lacunae. The nominalthey are initially or ultimately enclosed in ly distinctfossilIsoetites(the sistergroup of Isosclerifiedleaf primordia(scales). The outermost etes) has a variously,albeit well developed, ala leaves of gemmae on peripheralsporophyllsare and distinctlacunae (Bock 1962; Bose and Roy

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1964; Brown 1939, 1958). Three extant,aquatic Neotropical species of Isoetes(I. gigantea,I. bradei,and I. baculata)also possess laminate, lacunate leaves. These species do not share any uniquely derived characterconditionsand thus, togetherwith Isoetites, they formfour unrelated subsets within the Isoetaceae (fig. 13). The unique condition of a distal, non-laminatesubula and the concomitantrestrictionof the alae to the basal portions of the leaf can thus be regarded as charactersdefiningthe rest of the Isoetaceae (fig.13). The reductionof the lamina appears to be initiallyassociated with a group of taxa whose primaryhabitat is terrestrialor seasonal and amphibious,and whose leaves are supplied with peripheral fibrousbundles. It is temptingto consider the essentially teretesubula then as adaptive by providing an upright, radiallysymmetricalphotosyntheticorgan. It is equally likely, however, that the reduced ala originated as a response to other photosynthetic requirements. Keeley (1982, 1983), for example, has shown that submerged leaves of Isoites undergo CAM photosynthesisand recycle CO2 while submerged. The reduction of the lamina to a small band of photosynthetic tissue surrounding these lacunae may be significantin that it would drasticallyreduce the distances involved in intercellularmovement

of CO2.

Peripheralfibrousbundles. Nearly all of the terrestrialand amphibious species, and many of the aquatic species, produce specialized, subepidermal packets of longitudinally aligned, elongate cells in theirleaves. These structures, peripheralfibrousbundles, appear to be collenchymatousin nature and are unique to Isoetes.

They are completely absent in the Lycopodiaceae, Selaginellaceae, and Isoetites. They function in supportingthe leaf particularlywhere the whole plant or portions of the leaves are emergent.Fibrous bundles are absent in obligately submerged aquatics, in certain high altitude,secondarilyterrestrialspecies fromNew Guinea and South America (e.g., I. hopeiCroft, I. novo-granadensis, I. andina Spruce ex Hook.), and in the completelyalate species fromSouth America. All of the Indian species have well developed fibrousbundles. The taxonomicdistribution and function of peripheral fibrous bundles suggest that their absence in most aquatic species and in the high altitudeparamo species is the result of a secondary loss. The

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presence of peripheral fibrousbundles thereforedefines the same subset within the Isoeaceae as thatcircumscribedby the presence of a subula (fig. 13).

Scales and phyllopodia. The coloration and textureof the scales in I. eshbaughii(figs. 6-7) are reminiscentof the phyllopodia of I. histrix Dur. ex Bory and I. durieuiHook. (figs. 8-9; Braun 1864; Motelay and Vendryes 1882; Maire 1952). Phyllopodia are the sclerifiedremnants of the bases of fully developed leaves and are only noticeable afterthe disintegrationof the surroundingsoftertissues.The presenceof such phyllopodia was in part the basis forthe establishmentof the Section Terrestres A. Br. as well as the segregate genera Cephaloceraton Gennari and Isoetella Gennari.Scales, however,are complete leaf primordia which become arrested early in their development and are entirely sclerified. The discrete morphology of the phyllopodia and scales is the result of differences in ontogeneticdevelopment. That scales representwhole leaf primordiais evidenced by the presence of ligules, abortedsporangia,alae, and subulae (figs.6-7; see also Wanntorp1970).

Personal examination of Isoetes collections on a world-wide basis as well as a survey of the literatureindicate that phyllopodia and scales are produced by a surprisinglylarge number of species (table 3). Even thislistshould be considered preliminary because the absence of scales and phyllopodia on a specimen is not conclusive; the retentionof such structuresis largely dependent upon the time of year in which the collectionswere made as well as their quality.

Virtually all of the species which produce phyllopodia or scales in any abundance (table 3) are seasonally inundated and subject to periods of drought-associateddormancy.The first descriptionof scales in Isoetes was given by Duthie (1929) who intimated that there was a strong correlation between the presence of these structuresand a terrestrialhabit. The increasinglylarge number of terrestrialand seasonal species now known to have scales (table 3) stronglysupports this relationshipand suggests that the correlationis evidence of adaptive significance.That is, scales and phyllopodia enclose the meristematictissues of dormant corms and gemmae providing protection for these tissuesfromboth mechanical damage and desiccation.

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13

[soeiteI]

A L R

-restrictedala subulae

1

m

-fibrousbundles

14

liii

-~~~scales, -scales, phyllopodia -sporangial pigmentation

FIGS. 13-14. Characterset and subset relationshipswithin the Isoetaceae. 13. The five primarysets definedby ala developmentand the possession of fibrousbundles. 14. Subset relationshipswithinthe primary, subulate set. The outer box in each diagram representsthe familyIsoetaceae; ba = 1. baculata,Br = I. bradei, gi = 1. gigantea.See textfor furtherdiscussion.

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TABLE3. Distributionof scales and phyllopodia in Isoetes.

Species

Distribution

Form

I. durieuiBory I. histrixBory & Dur. I. drummondii A. Br. I. mexicanaUnderwood I. nuttalliiA. Br.

Mediterranean Mediterranean Australia north-centralMexico western North America

phyllopodia phyllopodia scales scales phyllopodia scales intermediate

I. melanopodaGay & Dur. I. stephansenii Duthie I. capensisDuthie I. stellenbosiensis Duthie

central United States South Africa South Africa South Africa

I. kersiiWanntorp

South West Africa

I. montezumae Eaton I. cubanaBaker I. panamensisMaxon & Morton I. pringleiUnderwood I. hieronymii Weber I. triangulaWeber I. weberiHerter I. eshbaughii Hickey I. butleriEngelm.

I. piedmontana (Pfeiffer)Reed

central Mexico Caribbean Neotropical America

"phyllopodia" scales scales scales phyllopodia intermediate forms scales phyllopodia scales scales scales

west central Mexico Argentina Venezuela, Mexico Brazil, Uruguay Bolivia central United States southeasternUnited States

scales scales scales scales scales scales "phyllopodia"

A numberof species produce both scales and phyllopodia (I. nuttalliiA. Br., I. stellenbosiensis Duthie, I. kersiiWanntorp) as well as an occasional intermediateform(I. nuttallii,I. stellenbosiensis).The intermediatecondition is represented by structureswhich have undergone more extensive ontogeneticdevelopment than typical scales and by incomplete sclerification (figs. 10-11) of the resulting primordia. The sclerificationin I. melanopodaGay and Durieu (Pfeiffer)Reed dif(fig. 12) and I. piedmontana fersfromthat in either I. histrixor I. durieuiin thatit occurs over greaterleaf area but is often confinedto superficialtissues. Because of these differencesthe sclerifiedstructuresare listed as "phyllopodia" in table 3. The similar development and intergradationseen in all of these structuressuggest that all are essentially homologous; i.e., scales, intermediates,and phyllopodia representa continuum in the expression of a single character.

Because scales and phyllopodia are structures

Data source

Braun 1864; and others Braun 1864; and others Braun 1868; Osborne 1922 Pfeiffer1922 Pfeiffer1922; Howell s.n. (MU); Suksdorf 836 (GH); 917 (GH) Suksdorf Pfeiffer1922 Duthie 1929 Duthie 1929 Duthie 1929 Tryonand Tryon6361 (GH) Wanntorp 1970 Stolze and Hickey 1983 Hickey 1985 Hickey 1985 Hickey 1985 Hickey 1985 Hickey 1985 Hickey 1985 Hickey 1985 Bush231 (GH) et al., 1740 GH Spongberg

unique to the Isoetaceae, outgroup comparison with the Selaginellaceae and Lycopodiaceae does not allow for an assessment of polarity along this morphocline. While alternative methods of determiningcharacterstate polarityhave been criticized(Stevens 1980), thereis evidence to indicate thatscales are ancestralto phyllopodia. For example, it has been argued thatcorrelationamong several polarized transformationseries (morphoclines) can provide a basis for assessing the polarityof an undirected, parallel characterseries (Wiley 1976; Crisci and Stuessy 1980). In the case of Isoetes, those species which produce only scales show plesiomorphic states for a large number of other characterssuch as mega- and microsporemorphology, velum coverage, and labium development. The alternative is also true: species which produce phyllopodia invariablypossess derived states for these same charactersindicating that scales are ancestral to phyllopodia.

For example, the clade of Neotropical Isoetes,

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which includes I. panamensis,is closely related majorityof species in the genus (fig. 14). This but ancestral to the I. melanopodacomplex (in patternis somewhatobscured by the secondary partsensu Boom 1982) of NorthAmerica(Hick- loss of the synapomorphyin obligate aquatic ey 1985). Within this clade, phyllopodia are and nonseasonal amphibious taxa. The only found only in the I. melanopodacomplex while austral, terrestrialspecies group which lacks scales are found in all of the tropical species. scales and/or phyllopodia are those species Because many of the African species are sea- originatingin the Indian subcontinent.All of sonal savanna plants it would be particularly these species undergo periods of drought-asrewarding to observe their occurrence on that sociated dormancyyet none produce sclerified tissue associated with the leaf base. The dorcontinent.

The absence of scales in several species of the mantcormapex in these species is protectedby and unique, light brown, papyraceous leaf bases I. melanopodacomplex, e.g., I. piedmontana I. tegetiformans, also indicates that a reversion rather than by sclerified tissue. The Indian to a more aquatic habitat may be accompanied species formthe second set or clade within the by a subsequent loss of sclerified tissue. The lineage characterized by subulate leaves (fig. loss of sclerifiedtissue in aquatic and strongly 14).

Sporangialpigmentation.The distributionof amphibious taxa supports the premise that scales are an adaptation to droughtconditions. sporangialpigmentationis widespread in terms In this regard,the occasional production of in- of both geography and systematic relationWeber is of ships. The absence of sporangial pigmentation termediatescales in I. hieronymii particular interest because this species is an in the outgroupsof the Isoetaceae suggeststhat aquatic. Its scales typicallynumberonly one or pigmentationis an apomorphous state relative two per plant and, as evidenced by the greater to them. The systematicdistributionindicates development of the subula, are arrested and that either the characterhas evolved indepensclerifiedlater in their development (fig. 10). dently numerous times or has been lost in a The reduced scale number and more complete number of lineages. In light of the similarity ontogeneticdevelopmentsuggestthatthe scales in the appearance of these pigmented cells are vestigial. Isoetes hieronymii throughoutthe genus and the unlikelihood of of I. hieronymii belongs to a clade whose sistergroup is I. weberi derivingsuch a characterseveral times,the latHerter, a distinctlyterrestrialspecies which ter hypothesisseems most probable. With this produces numerous scales (Hickey 1985). It is assumption in mind, however, there still rehypothesizedthatthe clade including I. hieron- mains the problem as to how much of the geymii has become secondarily adapted to an nus this charactercircumscribes.Pigmentation aquatic habitat with the subsequent loss of is completely absent among the Indian (Pant the loss is only and Srivastava 1962) and primitive, alate scales. In the case of I. hieronymii a functionalone whereas in the more derived species. Within the set of taxa defined by the etc.) presence of scales, however, all of the more membersof the clade (I. glacialis,I. lechleri, primitivespecies (as defined by labium morthe loss is complete (Hickey 1985).

All of the species which produce scales or phology, velum development,etc.) show some phyllopodia (table 3) are austral in distribution degree of sporangial pigmentation. This coror have strongaustral affinities(e.g., the rela- relationindicates thatsporangial pigmentation tionshipsof the North American I. melanopoda and the presence of scales or scale-like struccomplex are with lowland Neotropical species tures define the same monophyletic assemand the relationshipsof the basically Mediter- blage (fig.14). The correlationalso suggeststhe appear to be with possibilitythat these charactersare intimately ranean I. durieuiand I. histrix I. capensisDuthie and the other South African related (metabolicallyhomologous). species listed in table 3). The absence of scales EVOLUTIONARY RELATIONSHIPS and phyllopodia in most north temperate When these characters (scales and phyllospecies can be explained by theiraquatic or amphibious, nonseasonal habit. Scales and related podia, subulae and reduced alae, peripheral fistructuresare the basis forestablishingthe ma- brous bundles, and sporangial pigmentation) jor subsetwithinthe subulate-leavedIsoetes and are considered together,certain relationships hence they represent a synapomorphy for a among species of the family become evident

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1986]

*

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~acunate

(U

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,

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? B\spapyaceost

~ ~ ~ ~~~e t

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319

HICKEY: ISOETES

/

|

rite aalae,

~

_

,

phyllopodia ~~~~~~~~~~~azles,

porangialpigmentation

subulae

~~~~~fibrous bundles

leaves

FIG. 15. Cladistic interpretationof the evolutionaryand morphological divergence within the Isoetaceae as evidenced by a number of novel leaf characters.Only those synapomorphiesdiscussed in this paper are shown.

(fig. 15). The three lineages thus defined are supported by morphology,outgroup comparisons, and geography.

The ancestralIsoetes is hypothesized to have been an aquatic with completely alate leaves and lacking sporangial pigmentation,peripheral fibrous bundles, and scales. Initial morphological divergence within the genus is indicated by the clade definedby distinctsubulae and peripheral fibrousbundles. These changes appear to be correlatedin part with a seasonal, amphibious habit and possibly with a change to CAM metabolism.The group of species lacking these derived features(the alate species and Isoetites) representsa primitive,basal, and perhaps relictualgrade. The subulate clade underwent additional divergence to form two distinct lineages. The first of these two is characterized by papyraceous leaf bases surrounding the dormantapex. This group is centered in the Indian subcontinent and, except forthe presence of I. coromandelina L. f. in Australia(Marsden 1976), thereis littleevidence to suggest that it has spread beyond this region.

Their distinct mode of seasonal corm protection, inability to produce sporangial and leaf

base pigmentation,and their restricteddistribution indicate that the Indian species have been isolated fromthe rest of the genus for a considerable period of time. The second clade, which is characterized by the production of scales, phyllopodia, and sporangial wall pigmentation,is completely absent from the Indian subcontinent.Elements derived fromradiations within this clade represent the vast majority of extant Isoetes and are found throughoutthe rest of the world.

Isoetesspecies in general are characterizedby geographicallyrestrictedranges and by a great many locally derived endemics. These facts coupled with an absence of any substantiated means of long range dispersal (Cox and Hickey 1984) suggest that the patternsof distribution seen in Isoetes,at least at the species group level, have theirbasis in vicariantbiogeography.The major lineages of Isoetesoutlined in figure 15 are thusexplainablewithinthe confinesof plate tectonic theory. Isoetiteswas apparently cosmopolitan in distribution(Bock 1962; Bose and Roy 1964; Brown 1939, 1958) and the extant alate species represent relictual morphotypes of this once widespread form. The distinctly

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320

SYSTEMATIC BOTANY

southern distribution of morphologically primitivespecies within the subulate clade indicatesthatthisconditionevolved in and spread throughout Gondwana. Initially at least this clade appears to have been isolated fromLaurasia until northward raftingof the southern continentsbroughtthe two major land masses back together.This hypothesis thereforesuggests that the firstmajor diversificationoccurred during or near the Cretaceous Period when the Tethys Sea divided Laurasia and Gondwana.

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posed phylogeny provides a functionalsource of comparison by which additional relationships within the genus can be elucidated: that is, the alate species (plus Isoetites) and the Indian species can serve as functionaloutgroups (Watrous and Wheeler 1981) for the clade circumscribingthe vast majorityof Isoetes.

I would like to thankGregACKNOWLEDGMENTS. oryJ.Anderson,David Barrington, Anne Bruneau, EricChristenson, Christopher Haufler,MichaelLefor,and Aliceand RollaTryonfortheirsuggestions and criticisms of thismanuduringthepreparation script.I especiallythankW. HardyEshbaughforthe opportunity to studyIsoetesin Boliviaand Peruand DISCUSSION bothhe and G. J.AndersonfortheirconstantenA great deal of the taxonomic confusion in couragement.

Thisstudywas supportedby twoSigIsoetes has been due to morphological conver- ma Xi grants(R. J.Hickey),a University ofConnectgence resulting from secondary reversions to icutResearchGrant(G. J.Anderson), and NSF grants an aquatic habitat,with the associated loss of BSR 78-23389(W. H. Eshbaugh)and BSR 82-07125 sclerifiedleaf tissue and fibrousbundles. Ad- (G. J.Anderson). ditional convergence,throughthe loss of sporangial wall pigmentation,and the tremendous LITERATURE CITED phenotypic plasticityof most vegetative charactershave obscured this evolutionarypattern BERTHET, P. and M. LECOCQ. 1977. Morphologie sporale des especes franqaisesdu genre IsoetesL. and greatly hampered our ability to resolve Pollen & Spores 19:329-349. phylogenetic lineages within this ancient genus. Previous classifications have circum- BOCK,W. 1962. A studyon fossilIsoetes.J.Paleontol.

36:53-59. scribed lineages either by "pigeon-holing" BOOM,B. M. 1982. Synopsis of Isoetesin the southspecies or by excluding charactersshowing hoeastern United States. Castanea 47:38-59. moplasy or phenotypic variation, rather than BOSE,M. N. and S. K. Roy. 1964. Studies on the by tryingto understand the patternsand unupper Gondwana of Kutch-2. Isoetaceae. Paleoderlyingcauses forsuch variation. As a result, botanist 12:226-228. species are currentlyclassified on the basis of BRAUN,A. 1862. Anhang iiber einige auslandische Arten der Gattung Isoetes. Verh. Bot. Vereins a single charactersource-a megaspore surface Prov. Brandenburg3:326-333. morphology.This systemhas persistedin spite . 1864. Les especes d'Isoites de l'Ile Sarof the lack of correlationswith othercharacters Ann. Sci. Nat. Bot. V, 2:306-377. daigne. in of and the face a growing body of evidence 1868. Uber die AustralischenArtender Gatwhich suggests that megaspore morphologyis tung Isoites. Monatsber. Konigl. Preuss. Akad. equally susceptible to convergence and pheWiss. Berlin 1868:523-545. notypicvariation.

BROWN,R. W. 1939. Some American fossil plants The phylogenetic relationships depicted belonging to the Isoetales. J.Wash. Acad. Sci. 29: herein (fig. 15) representnovel hypotheses re261-269. . 1958. New occurrences of the fossil quillgarding the early morphological and evoluworts called Isoetites.J.Wash. Acad. Sci. 48:358tionarydiversitywithinthe Isoetaceae. In great 361. part, they are in conflictwith previous classificationsystemsand theirimplied evolutionary Cox, P. A. and R. J. HICKEY. 1984. Convergent megaspore evolution and Isoetes.Amer. Naturalrelationships.Unlike other systems,however, ist 124:437-441. the relationships proposed here are open to CRISCI,J. V. and T. F. STUESSY. 1980. Determining testingby homology assessment of scales (and primitive character states for phylogenetic reallied structures),fibrousbundles, and sporanconstruction.Syst. Bot. 5:112-135. gial pigmentation,and by characterizationof CROFT,J.R. 1980. A taxonomicrevision of IsoetesL. photosyntheticmodes in the primitivelyalate (Isoetaceae) in Papuasia. Blumea 26:177-190. species of South America. In addition, the pro- DUTHIE, A. V. 1929. The species of Isoetesfound in

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HICKEY: ISOETES

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