POLYPLOIDY IN POLYPODIACEAE— ITS EVOLUTIONARY AND ADAPTIVE SIGNIFICANCE By G. Panigrahi and S. N . P a t n a i k Botanical Survey of India, Shillong (Received for publication on December 26, 1962) T h e family Polypodiaceae (Copeland, 1947), comprising of 65 genera in the world flora, are represented by 27 genera and more than 110 species in India. The family occur generally in the tropical and sub­ tropical evergreen forests of Eastern India, mostly as epiphytes on moss- covered tree trunks or on humus-covered rocks and boulders, though a few terrestrial species {viz., Dipteris wallichii) creep on the surfaces of sandy soil on rocky substratum. All the genera are characterized by creeping rhizome (short or extensive) and simple, lobed or pinnate fronds bearing exindusiate sori. Although the family has been subjected to serious studies in orthodox taxonomy, very few genera were cytologically investigated until recently, owing to their inaccessibility to many fern cytologists on account of their occurrence in tropical and subtropical evergreen forests. However, Manton (1950, 1954) and Manton and Sledge (1954) reported the chromosome numbers of 40 species from Ceylon and Malaya including the Polypodium vulgare complex from temperate Europe and America. Nayar (1958), Bir (1960) and Abraham (unpublished) report chromosome numbers of only one species each from India. Realising the inadequacy of the cytological knowledge in this predomi­ nantly epiphytic family, Polypodiaceae and considering the great opportunities for such studies owing to our living in Shillong which is situated in the very heart of Assam’s “ Forest climate”, we have recently studied the cytology of 36 East Indian species (Panigrahi and Patnaik, 1961; Patnaik and Panigrahi, 1963a) collected from the montane ever­ green forests of the Khasi and Jaintia Hills and tropical evergreen and semi-evergreen forests of the lower and upper Assam and foothills of the North-East Frontier Agency. Very recently Malhotra (in Mehra, 1961a) and Pal (1961) have also reported the chromosome numbers in various Indian species. Thus, a total of 85 species belonging to 22 genera from the rain forests of South-East Asia have been cytologically investigated up to date. A perusal of chromosome numbers of the 22 genera referred to above {cf. Table 1) brings out the following broad features of their cytological picture (c/. Panigrahi and Patnaik 1963 6):— (1) Basic chromosome numbers in tiie famil’v range between a : = 11 to X = 47, these two extreme numbers characterizing the species of Pleopehis (= Lepisorus) only. (2) The commonest basic chromosome numbers are x- == 36 charac­ terizing 14 genera and x ~ 37 characterizing 8 genera, whereas 5 genera, viz., Drynaria, Pleopehis (== Lepisorus), Polypodium, Pyrrosia and Goniophlehium each show both these numbers. (3) Only 4 genera possess .v = 35 whereas 2 genera share x — 33 only. (4) Pleopehis (= Lepisorus) is the only genus showing an astonish­ ing range of haploid and diploid chromosome numbers, viz., n — 22, 23, 26, 35, 36, 47, 74 and 2« = 39. (5) All but 14 species investigated are diploids showing regular bivalents at meiosis with the rare exception of Lepisorus pseudonudiis Ching with 2n = 39 and irregular meiosis. The phenomenon of euploidy in Polypodiaceae (sensu Copeland 1947) and its evolutionary and adaptive significance have alieady been discussed in a separate communication (cf., Panigrahi # Patnaik 1963 h). The discovery of tetraploidy in the most primitive terrestrial genus of Polypodiaceae, which are characterized by low percentage and low grades of euploidy in its epiphytic genera and species have led the authors to postulate an hypotehsis ‘Epiphytic habit in closed tropical and subtropical forests works as a ‘ bottle-neck” to the induction of euploidy whereas the terrestrial Iwibit is highly conducivc to it’. E u p l o id y a n d its E f j e c i s Now, let us examine the few examples of euploidy undoubtedly established within the family. Amongst the 85 species studied, only 14 taxa belonging to the following 12 genera turned out to be euj^loids, the respective positions of these 12 genera are shown below in the phylogenetic sequence (Copeland, 1947; Holttum, 1947):— Crypsinus Leptochilus (2n, 4n) Polypodium (2n, 4n) Arthromeris {In, 4 m, 6»i) Colysis {In, 4„) (2n, An) Pleopehis Behisia ( = Lepisorus) (2n, 4n) Loxogramme (2n, 4n) Pyrrosia (2k, 4n) Dipteris (2«, 4 « , 6«) Prosaptim (2«. 4 n ) iln , 4 » ) Xiphopteris ( 2 n , 4 « ) I tt ■fc. CO II H I I I I 'O IU5 CO II S C S I5J I o t <s 8 II a c t I I O f these, we have thoroughly studied the euploidy and its mor­ phological effects in Loxogramme, Pyrrosia and Dipteris, while a few others have been subjected to similar close analysis by Manton (1950) and Manton and Sledge (1954). In the genus Loxogramme, L. lanceolata, L. scolopendrina and L. avenia are diploids, each with 3 5 clear bivalents at meiosis whereas L. involuta is a tetraploid with 70 bivalents at diakinesis. There is no indication of multivalent formation either in the diploid or tetraploid species. Although L. lanceolata is morphologically distinct from the remaining three, one has to look for differences (Table II) to distinguish L. involuta from L. avenia and L. scolopendrina, the shapes and sizes of simple fronds borne on tufted rhizomes being very variable in all the three species. T able II Rhizome scales Midrib on the upper side of the frond L. scolopendrina Large, ovate-lanceolate Not raised, almost flat L. avenia Linear-lanceolate with a Strongly raised and long narrow tip grooved towards base L. involuta Ovate-lanceolate like that Raised like that of avenia of scolopendrina but but not grooved smaller Thus, L. involuta appears intermediate in morphology betwera ijU. aveniaavenuj and L. scolopendrina. But any suggestion of allopolyploid origin of L. involuta by the interspecific crossing between L. scolopendrina and L. avenia must await studies in hybridisation involving the three taxa. Again, Pyrrosia mannii and P. mollis are two very allied species practically indistinguishable from each other except for the colour differences in the scales and so both were graerally known as Niphobolus fissus Bak. or Cyclophorus porosus C. Chr. Ching (1935), however, recog­ nised two species within its limits, and assigned concolourous entire scales to P. mannii and discolourous fimbriate scales to P. mollis. This “ hair-splitting” in taxonomic approach finds, however, coixoboration from studies in their cytology. P. mannii is a diploid with » = 37 whereas P. mollis is a tetraploid with n ~ 74. Both behave as normal species at meiosis, form bivalwits without auy trace of multivalents and form good scores. The mode of origin of tetraploidy in P . mollis Iberefore, an open <^uestion, Manton’s (1950) researches on Polypodium vulgare complex showing 2n, 4n and 6n, which she subjected to a comprehensive hybridization programme, establish undoubtedly the well-known truth, that despite allopolyploid origin of the 6n taxon by the intercrossing of 2n and 4n taxa and subsequent doubling of chromosomes in the Fj hybrid, the morphological differences brought about by allopolyploidy with respect to its putative parents, are of minor nature and are expressed merely by an intermingling of characters already shared by the diploid and tetraploid parents respectively, although polyploidy confers greater plasticity on the organism to sudden changes in the environment. A n e u p l o id B ase N u m b e r s a n d T h e ir E volutionary Significance A perusal of Table 1 and analysis of the cytological pictures provide overwhelming evidences in favour of the great role played by aneuploid base numbers in the evolution of the order Poypodiales (Pichi-Sermolli, 1959), both at generic and family levels. Pichi-Sermolli {he. cit.) and Mehra (1961 b) both recognize 4 families, viz., Dipterid- aceae, Cheiropleuriaceae, Polypodiaceae and Grammitidaceae. Although they do not indicate the genera included in the respective families they consider these families as more or less natural. The discovery of the base numbers x = 11 (viz., n — 33, 66) in Dipteris and x = 11 to jr == 47 with a large array of haploid and diploid chromosome numbers {viz., n — 22, 23, 26, 35, 36, 47, 74 and 2n — 39) in the genus Pleopeltis (— Lepisorus), undoubtedly provides the answer to the successive steps in progressive evolution within the epiphytic family, Polypodiaceae through the mechanism of aneuploid base num­ bers. Thus, the hypothetical phylogenetic relationships of Dipteris with Matoniaceae and of Grammitidaceae with Gelicheniaceae are corroborated by the discovery of very low base numbers, viz., x = 9 {i.e., n = 36), 11 and 13 in Polypodiaceae and x — \'i both in Mato­ niaceae and Gleicheniaceae. With this cytological picture outlining the main lines of evolution of the order Polypodiales (through Dipteridaceae to Pleopeltis-Wkt: ancestors) from the Gleichenioid ancestors {see Pichi-Sermolli, 1959) or from Gleicheniaceous stock {see Mehra, 1961 b), further evolution within the order'Polypodiales follows a stereotyped pattern. Although the basic chromosome numbers in Polypodiales vary between = 11 and x — Al the commonest base numbw, viz., x = 36 characterizes 8 genera, i.e., Microsorium, Phymatodes, Colysis, Leptoohilus, Aglao- morpha and Photinopteris all included within Microsorieae and Lemma- phyllum and Neooheiropteris included in Pleopeltideae (Copeland, 1947). These genera share the common base number, viz., x = 36 with the genus Pleopeltis ( = Lepisorus, 2n = 72 found in one species, viz., longifolius). The similar cytological features and possession of a large array of low base numbers in Pleopeltis ( = Lepisorus) considered together with evidences from comparative morphology postulating divergent lines of evolution, as represented by these eight genera amongst Others, from P lw peltis ( = Lepisorus) {qf. Panigrahi and Patnaik, 1961 b) undoubtedly establish Pleopeltis (= Lepisorus) as an evolutionary plexus (Text-Fig. 1) holding the key to the macro-evolution at generic and family levels within the order Polypodiales Pichi-Sermolli,/oc.aV.).