Reticulate Evolution in the Appalachian Aspleniums Warren H
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Reticulate Evolution in the Appalachian Aspleniums Warren H. Wagner, Jr. Evolution, Vol. 8, No. 2. (Jun., 1954), pp. 103-118. Stable URL: http://links.jstor.org/sici?sici=0014-3820%28195406%298%3A2%3C103%3AREITAA%3E2.0.CO%3B2-Q Evolution is currently published by Society for the Study of Evolution. Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/journals/ssevol.html. 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For more information regarding JSTOR, please contact [email protected]. http://www.jstor.org Fri Feb 8 10:53:51 2008 RETICULATE EVOLUTION IN THE APPALACHIAN ASPLENIUMS Received, October 20, 1953 point species and comparisolis of them with their various intermediates suggest To he able to postulate the course of that within the complex totalling 11 de- evolution in a group of organisnls on the scribed entities, the real extremes are only basis of indirect evidence is one of the three in number. These three species are goals of phylogenetic research, since in A. z?contc~nunt,A, plntyneuron, and A. Inany groups direct evidence is difficult l-lti:ophyllt~71~.A11 the remaining taxa. or impossible to obtain. Occasionally a comn~onlytreated as species or hybrids- worker may be fortunate enough to con- including the familiar A. pilznntifidzwl and firm his indirect and comparative tech- A. b~ndleyi-lie someri~herebetween these niques by direct evidence, such as the dis- three extremes in their morphology. covery of a postulated ancestral type, or ' Anlong students of the Filici~leaeit has the production of a postulated form by been rather conventio~lal to consider as hybridization experiments. In the pres- hybrids only those intertnediate forms ent study of the evolution of the Appala- ~vl~ichoccur as single, sterile plants with chian Aspleniums, a complex group of obvious parents growing nearby. But small ferns of the eastern United States, with the increasing recognition of allopoly- the methods used have been indirect ones, ploidy as a factor in species forination in morphological, anatomical, and cytological. plants, sterility per se is 110 longer the sole But in future years it is to he expected signpost of hybrid origin Indeed, the that experimental proof of the conclusions best-known fern hybrid. Asjlenitl~webe- of this study will be forthcoming. The noides (A. plntynez~ronx A rhizophyl- indirect steps used as evidence of reticu- lt~i~z),though usually sterile, occurs as a late evolution in the Appalachian Asple- fertile for111 in one large population in niums are thus subject to valiclatlo~l by Alabama. It has not, however, been ex- direct tests. anlined cytologically until the present 1tTherry(1925, 1936) pointed out that study (IVagner, 1953), nor have any the Appalachian Aspleniutns "form a ser- others of the Appalachian Aspleniu~ms ies showing intermediates between cer- been so investigated. The aim of the pres- tain long-recognized species." His basic end-point species were five in number: ent work has been not only to clarify the Asp2eniz~nz pinnatifidztnz (lobed spleen- cytological picture of the group, but also wort), A. nzo~ttnnzt~lz(mountain spleen- to use any other available illdirect ap- proaches to its evolutionary history. Thus IT ort), A. bl-ndleyi (cliff spleen~vort). A. fllnt~we?~l-on(ebony or brownsteln spleen- certain lnorphological and anatomical fea- wort), and A. hi,-oplzyllzt~~z(the ~valk- tures were found valuable in interpreting ing-fern, usually treated as Cn~~tptosortts the intermediate for~ns. The evidence rlzi,-oplzyllz~sbut for sake of sinlplicity re- now itldicates that n complex of eight cle- tained in Asplenit~tnhere). Detailed stud- scribed entities, A. ebenoides, A. pinna- ies of the morphology of these five end- tifidtiiqc, A. t~*?~dellii,'4. ke?ztz~clziense,A. gvnvesii, A. brndleyi, A. brndleyi x A. 1 This study \\as aided by a Faculty Research Grant from the Horace H. Raclihatll School of platyneui,on, and A. brc~dleyix A. ?%on- Graduate Studies. fnnlcnc, has arisen as a result of hybridiza- ~VOI~~TIOX8: 103-118. June, 1954. 103 104 WARREN H. WAGNER, JR. tion between three ancient and original blades, (2) dark-brown color of the leaf species. axis present only at the base of the flat- The three basic species are entirely tened petiole, (3) "glossy" upper leaf distinctive, whereas the remaining taxa surface, resulting from the markedly elon- constitute a difficult group, as shown by gate, practically straight-walled form of diverse herbarium identifications and the epidermal cells, and (4) occurrence taxonomic interpretations. Aspleniuvn strictly in acid-rock crevices. The closest montanum (fig. 1, RI) shows notably the relative of A. nzontanuvn appears to be the foIlowing peculiarities : (1) triangular, wide-ranging A. adiantuwz-nigruvn of the long-petiolate, 2-3-times dissected leaf western United States, Africa, Eurasia, FIG.1. Concept of relationships in the Appalachian Aspleniums. R. Asplenizhm vhizophyllum; P. A. platyneuron; M. A. monta~tum;PR. A. ebenoides; RM. A. pilz~tatifidzrm;PM. A. bradleyi (frond on left, Frederick Co., Va., Gilbert 250; frond on right, Madison Co., Mo., Russell) ; RMM. A. trtfdellii; RMPM. A. gravesii; PRM. A. ke,ltuckiet~se. RETICULATE EVOLUTIOIV 10.5 TABLEI Species 2?2 11 Asplerziuin nzontanuriz Harford Co., Md. Pilre Co., Ohio -4splenium platynez~ron Montgomery Co., Md. Harford Co., Md. Hoclring Co., Ohio Licking Co., Ohio Hardy Co., U'. Va. .lspleniu~~rrhizophyllz~,iz Ross Co., Ohio - 36 Fairfield Co., Ohio 72 - Shellandoah Co., Virginia 72 36 Alonroe Co., Illdialla - 36 .4spleniuin ebc?zoides Alontgomery Co., Md. 72 (72 univalents) Hale Co., Alabama (Groff) 144 72 Hale Co., Alabama (Logue) 144 72 Aspleniunz pinnati$dz~nz Shenandoah Co., \la. Hardy Co. W. Va. Yorlr Co., Pa. Licking Co., Ohio hlonroe Co., Indiana Aspleniunz brndleyi Harford Co., Md. Pilce Co.. Ohio Asplcnz~tnztrudellzi Yorli Co., Pa. 108 (ca. 36 univalents, ca. 36 bivalents) Hardy Co., \V. Va. 108 (ca. 36 univalents, ca. 36 bivalents) Pilre Co., Ohio 108 (ca. 36 univalents, ca. 36 bivalents) (Voucher specimens deposited in the Department of Botany University of Michigan) and Hawaii, a species which also possesses derillal cells oblong, undulate-walled, and the "glossy" upper epidermal cells. A. (4) occurreilce on roclis and soils of a 1l~onfnn1c1lzis confined to the eastern variety of pH reactions. It appears to be United States, occurring from northern illost closely related to A. t~iclzoutanesand Georgia and Tennessee ill the Appalachian A. rpsiliens, and its hybrids with the Region to western R/Iassachusetts. A. foriner have been twice recorded (A.vir- platyneztron (fig. 1, P), the second of the giniculn Mason). A. plntynezr~onhas a basic species, has the follo\ving distinc- very wide range, extending from Texas tions : (1) linear- to oblanceolate-elliptic, and Florida in the south to IYisconsin once-pinnate, short-petiolate leaf blades, and I<ansas to southern Quebec in the (2) leaf axis entirely dark-brown, includ- north. A. rlzizoplzyllu~~z(Cnttzptosovtts ing the rouilded midrib, (3) upper epi- ~lzizoplzyllt~s)possesses these character- 106 WARREN H. WAGNER, JR. istics : (1) triailgular-atteiluate, simple and young leaflets placed in saturated leaf-blades, with extremely long, rooting aqueous solution of paradichlorobenzene tips, (2) leaf axis green except for the for 3 hours to shrink the chromosomes, base of the petiole, (3) upper epidermal then placed ill fixative. Meiotic studies cells as in A. platyneuron, but the venation were made from young sori, fixed directly. pattern anastomosing, and (4) occurrence The fixative used was 4 parts chloroforn~; confined almost esclusively to well-shaded, 3 parts ethyl alcohol; and 1 part glacial moss-covered tops and sides of rocks and acetic acid. The specimens were squashed boulders, the pH reaction of the rock sub- after 24 hours in the solution, and stained stratum predominantly circumneutral. ill aceto-orcein. Its closest relative is A. sibi~icuwzof north- eastern Asia. A. rlzizoplz)lllum, like A. platyfzeztron, is a rather commoil fern and has a broad range, extending from Ala- The first of the intermediates to be bama and Georgia north to Minnesota discussed is Asplenium ebenoides. Prob- and Quebec. The i~lorphological char- ably no other fern of the New World acteristics of these three basic species are has attracted so much attentioil as this intricately blended aillong the members hybrid. Wherever its parents, A. platy- of the Appalachian Asplenium complex. neuron and A. ~hizophyllu~~z,grow in close The chroinosome numbers (table 1) of proximity, this intermediate appears with the basic species cannot be used to distin- gratifying regularity, but usually as a guish them since they all have the same solitary plant, and with sufficient rarity to make its discovery a challenge to the -2n = 72, with 36 pairs at meiosis.