Cytologia, 37: 175-196, 1972

Evolutionary Status of the Asplenioid and Athyrioid Ferns with Particular Reference to the Himalayan Forms

S. S. Bir

Department of Botany, Panjabi University, Patiala, India

Received June 8, 1970

Introduction

The shift from morphological and taxonomical studies in Pteridophyta to cytological aspect was the direct result of the publication of excellent work, 'Pro blems of cytology and evolution in the Pteridophyta' in 1950 by Irene Manton. It mainly concerned the European flora. With the counting of chromosomes made easier by acetocarmine squash technique, the work on the cytology of ferns was started almost simultaneously in several parts of the world. Chiarugi (1960), Fabbri (1963, 1965) and Ornduff (1967-69) have compiled complete information of all the contributions made till the end of 1967. Since then a few more papers have appeared and those dealing with the group of ferns under review pertain to Bir (1965a), Chaudhuri (1966), Gupta (1966), Jarret et al. (1968), Kuriachan (1967), Lovis and Reichstein (1968a, b, 1969), Manton and Vida (1968), Mickel et al. (1966), Sleep (1967) and Shivas (1969). Therefore, sufficient cytological data have now accumulated about the Asplenioid and Athyrioid ferns so as to enable us to assess their evolutionary status.

Review of the work done on Asplenioid and Athyrioid ferns of Himalayas

Out of about 1151 species recorded from the Himalayas, as a consequence of writer's cytomorphological work during the last 17 years, chromosomal information for 79 species (67.8%) is available (cf. Bir 1958, 1959, 1960, 1961a, b, 1962, 1963, 1964b, 1965a, b, 1969, Bir and Shukla 1967, Mehra and Bir 1957, 1960a, b). Re ference to the chromosome numbers in most of the Himalayan forms can be found in Mehra's (1961) list. This information is enough to warrant some comments which pertain mainly the four principal genera worked out, namely, Asplenium, Cystopteris, Athyrium and Diplazium. Analysis given in Table 1 shows that poly ploidy is wide spread in Asplenium (76.3%) and Cystopteris (83%) as compared to its low incidence in Athyrium (29%) and Diplazium (18.2%). Tetraploids are abundant and the highest grade of ploidy attained in the region is at the octoploid level. Diploids are rather poorly represented in Asplenium as compared to Athyrium

1 This estimate is based on writer's taxonomic revision of these ferns from the Himalayas (unpublished). Table 1. Summary of cytological work done on Asplenioid and Athyrioid ferns of the Himalayas*

* Information is based on writer's work (Bir 1958, 1959, 1960, 1961a, b, 1965a, 1969; Bir and Shukla 1967; Mehra and Bir 1957, 1960a, b). ** So far only one species , Asplenium unilaterale Lamk. (var. udum Atk. and var. delicatulum Par. also) is known to be based on x=40 (cf. Bir, 1960). Chaudhuri's (1966) report of n=40 for Asplenium phyllitidis is wrong because this fern has been recorded to a tetraploid (n=72) from South India by Abraham et al. and from the Eastern Himalayas by the writer (Bir 1960, 1965a) under A. nidus Linn. var. phyllitidis (Don) Bir. Similarly, Gupta's (1966) record of 2n=80 (also 2n=52, 82) for A. bulbiferum Forst. (=A. bullatum Wall.) may also be wrong because several populations of it from North Sikkim unequivocally showed the presence of n=72 (Bir 1960). Therefore, both these species are based on x=36. *** Abraham et al . (1962) wrongly attributed 2 species of Athyrium to be triploid apogamous. This is because the authors included some species of Diplazium under Athyrium (cf. Table 9 of authors) and thus erroneously recorded two basic numbers x=40, 41 for the genus Athyrium. 1972 Evolutionary Status of the Asplenioid and Athyrioid Ferns 177

Table 2. List of Asplenioid and Athyrioid ferns with more than one cytological race in the Himalayas and South India (excluding Ceylon)

and Diplazium. In the Himalayas Ceterach, Dryoathyrium and Diplaziopsis are represented by one species each whereas Cornopteris has two species of which only one is worked out. Occurrence of sterile taxa (hybrids)1 is quite common-about 16% in Asplenium, 17% in Cystopteris and as much as 27.2% in Diplazium. It is only in Athyrium that the percentage of hybridity1 (relating to naturally occurring species or other taxonomic entities) is relatively meagre, only nearly 9.7%. Apoga my is poorly established in the Himalayan species of Asplenium (only one apomict), 1 The term hybrid or hybridity throughout the paper (especially Tables 5 and 7 pertains only to those taxa in which as a consequence of irregular meiosis in the spore mother cells, abortive spores are produced. It certainly excludes the allopolyploids with normal meiosis and spores. 178 S. S. Bir Cytologia 37

altogether absent in Athyrium and quite abundant in Diplazium which shows the presence of about one fifth of the investigated species to be apogamous with abun dance of triploids. Taken as a whole the Himalayan members of Asplenioid and Athyrioid ferns show polyploidy to the extent of 44%. The incidence of hybrid taxa is also high, as much as 14.7%1. Cytotypes are known to exist in 5 species of Asplenium, 6 species of Athyrium, 2 species of Diplazium and one of Cystopteris (for details see Table 2). Before passing on to the consideration of evolutionary mechanisms at work in these important genera of ferns and also to a consideration of results on world-wide basis, it is imperative to evaluate the findings of the writer which primarily concern the Himalayas-a region of utmost phytogeographic importance. The salient fea tures brought out to light are the occurrence of: A. Species complexes in the Himalayas: Several species complexes have been found which may be classified into the following three categories: I. More than one stable species lumped together under a common name. These usually possess the same cytological status but differ morphologically. i) Asplenium laciniatum complex: Segregated into A. indicum Sledge var. indicum and var. obtusa Bir2, and A. laciniatum Don var. laciniatum, var. subinte grifolium Hook. and var. acutipinna Bir. All of these five distinct taxonomic entities are tetraploid sexual (n=72). ii) A. unilaterale complex: Consisting of A. unilaterale Lamk. var. unilaterale,3 var. delicatulum Par., and var. udum Atk. Because of large scale differences, it would be more appropriate to raise the last two varieties to specific rank. All of these are diploid sexual with n=40. iii) Athyrium spinulosum complex: Comprised of two distinct tetraploid (n=80) species, A. spinulosum (Maxim.) Milde and A. subtriangulare (Hook.) Bedd. iv) A. nigripes complex: Now separable into three distinct diploid species with n=40, namely, A. nigripes (Bl.) Moore, A. setiferum C. Chr. and A. clarkei Bedd. v) A. filix-foemina complex: Consisting of A. filix-foemina (L.) Roth, A. mehrae Bir, A. attenuatum (Clarke) Tagawa, A. rupicola (Hope) C.Chr. A. dentigerum (Clarke) Mehra and Bir and A. rubicaule (Edgw.) Bir. All these are diploid sexuals with n=40. vi) A. puncticaule (macrocarpum) complex: Now consisting of two different species, A. puncticaule (Bl.) Moore, diploid with n=40 and A. anisopterum Christ with diploid (n=40) and tetraploid (n=80) races. vii) Diplazium latifolium complex: The array of forms met within this 'spe cies' complex are separated into four distinct taxonomic units. All of these are

1 Out of a total of 109 investigated taxa , 16 taxa turned out to be hybrids. 2 The variety obtusa was originally published by the writer (Bir 1964a) under Asplenium planicaule Wall. but the specific name was shown by Sledge (1965) to be illegitimate and he gave the species a new name as A. indicum Sledge. 3 As pointed out elsewhere (Bir and Shukla 1968) the Darjeeling specimens on which the cytolo gical observations are based, conform to A. unilaterale var. majus (C. Chr.) Sledge (=A. excisum Presl). 1972 Evolutionary Status of the Asplenioid and Athyrioid Ferns 179 tentatively called 'forma' and are diploid sexuals (n=40) . viii) D. polypodioides complex: Now recognizable by the presence of unmistakably four distinct diploid species with n=41 , namely D. polypodioides BI., D. asperum Bl., D. giganteum (Bak,) Ching, and D. torrentium (Clarke) Tard-Blot. ix) D. maximum complex: The triploid and tetraploid apomictic forms with 2n='n'=123 and 2n='n'=164 are morphologically quite inseparable. Another collection from the area with simple pinnate fronds turned out to be triploid apomict with 2n='n'=123. Though morphologically distinct this may represent pre cociously fertile individuals of D. maximum because the fronds resemble in size and form one pinna of D. maximum. x) D. umbrosum complex of Beddome (1892): It now consists of three diploid sexual species, namely, D. australe (R. Br.) Bir., D. spectabile (Wall. ex Mett.) Bir and D. muricatum (Mett.) v. A. v. R. in the Himalayas. The true D. umbrosum is not found in this region. II. Complexes with different cytotypes within a long established species. These polyploid series are usually morphologically non-recognizable and often intergrade into each other. As far as the Himalayas are concerned, the different cytological races do not have any specific ecological preference. These can be distinguished only on the basis of micro-characters such as the spore-size, the size of the stomata and their frequency, number of annulus cells etc. These do not merit any taxonomic recognition (see however page 183). Examples1: i) Asplenium trichomanes 2x, 4x (for details see Bir and Shukla) 1967. ii) A. varians 2x, 4x iii) A. unilaterale (2x sexual and hybrid), 3x iv) A. unilaterale var. udum 2x, 3x v) Cystopteris tenuisecta 3x, 4x vi) Athyrium attenuatum 2x, 4x vii) A. japonicum 2x, 4x, 5x III. Complexes with appreciable morphological differences between different cy tological races. These do merit taxonomic status as new species, subspecies or varieties. i) Asplenium paucivenosum 4x, 8x ii) Athyrium anisopterum 2x, 4x iii) A. setiferum 2x, 4x (for details see Bir and Shukla, 1967)

B. Hybrid taxa: I. Well established taxonomic species which are hybrids: Asplenium griffithianum Hook. (diploid) with 2n=72 and commonly met with in the Eastern Himalayas be longs to this category.

II. Interspecific or intervariental crosses have been noted in: i) Diplazium polypodioides•~D. asperum Diploid (2n=82) ii) Asplenium laciniatum var. subintegrifolium•~var. actuipinna Tetraploid (2n=144)

1 In ferns 2 X-diploid, 3 X-triploid, 4 X-tetraploid etc. Table 3. Comparison of cytological results of Asplenioid and Athyrioid ferns from Himalayas with South India (excl. Ceylon)

* Compiled on the basis of the basis of Mehra and Bir (1957, 1960a, b), Bir (1959, 1960; 1961a, b, 1963, 1965a, b), Bir and Shukla (1967), Mehra and Verma (1957). ** Compiled on the basis of Abraham et al. (1962), Bir (1965b), Kuriachan (1964, 1965, 1967). 1972 Evolutionary Status of the Asplenioid and Athyrioid Ferns 181

iii) Asplenium•~unilaterale Diploid (2n=76) iv) Asplenium•~unilaterale Triploid (2n=112) v) A thyrium•~pectinatum Diploid (2n=80)

III. Crosses between different cytological races of the species' complex:

i) Athyrium japonicum 5x (Tetraploid•~Hexaploid) with 2n=200 . ii) Cystopteris tenuisecta 3x (Diploid•~Tetraploid) with 2n=126 . C. Autoploidy:

It is noticed only in Asplenium unilaterale var . udum with 2n=120.

D. Apogamous taxa:

Several instances of obligate apogamous species are encountered in the region: i) Asplenium chielosorum Triploid apogamous (2n='n'=108) ii) Diplazium stenochlamys Diploid apogamous (2n='n'=82)

iii) D. simplicivenium Triploid apogamous (2n='n'=123) iv) D. maximum (Coll. No. 1116) Triploid apogamous (2n=`n'=123) v) D. maximum (Coll. No. 1118) Tetraploid apogamous (2n=n=164)

vi) D. maximum (Coll. No. 1114) Triploid apogamous (2n='n'=123) Evidently triploid apomicts are predominant in the Himalayas.

Comparison of the Himalayas with other areas

The Himalayas (above 1,200m.) have predominantly warm to cold temperate climate with high annual range of mean temperature (about 11•Ž for Darjeeling)

and marked diurnal differences. The vegetation experiences seasonal variations as compared to the equitable climate throughout the year of the tropical areas.

The cytological results from the Himalayas for Asplenium, Athyrium and Diplazium are compared with two areas with temperate climate (Europe and North America)

and with those from Ceylon, Malaya and West Africa having tropical climate

(cf. Tables 5-6). It has been found that in the Himalayas as well as temperate areas the diploids and tetraploids are common (Table 5). The maximum grade of poly

ploidy in the Himalayas is upto octoploid level (Asplenium paucivenosum, n=144 and Cystopteris sikkimensis, n=168). But in the tropical areas the diploids are few while the tetraploid, hexaploid and octoploids are quite frequent (cf. Table 5) and further three cases of even dodecaploids are also known for Asplenium. On

comparing the results from Himalayas with those from South India (cf. Table 3) it is clear that the incidence of polyploidy is definitely lower in the region thus

differeing from rest of the Indian subcontinent. Even for the same species which have been investigated from different areas the grade of polyploidy is higher from

South India and Ceylon in contrast to the Himalayas. Datta for such species are

presented in Table 4. As far as the frequency of the polyploid taxa is concerned the data for all the areas taken together reveal that in tropics the percentage of polyploidy is definitely much higher than in the Himalays or any other temperate region (cf. Tables 5 and 6). In Athyrium the polyploids are about 22% in the Himalayas and 14% in North America in contrast to about 92% in the tropics whereas in Diplazium only 18% 182 S. S. Bir Cytologia 37

of the Himalayan taxa are polyploids as compared to 85% of the tropics. Similarly, in Asplenium the polyploidy is 76% in the Himalayas and North America (only about 51% in Europe) as compared to about 92% under tropical climate. The statistical datta are presented in Tables 5 and 6. Therefore, it may be concluded that the flora of the Himalayas with very high percentage of diploids resembles floras of Europe and North America and differs from those of South India, Ceylon,

Table 4. Comparison of cytological results* for the same species of Asplenioid and Athyrioid ferns from the Himalayas and other regions

* Information about investigators is available from Tables 2 and 8 . ** Mehra and Bir (1960a)

•õ Manton and Sledge (1954) •õ•õ Bir (1965b)

•õ•õ•õ Bir (1961b)

Malaya or West Africa. Even in case of Asplenioid and Athyrioid ferns from the comparisons given in Tables 5 and 6, it is amply clear that the frequency and grade of polyploidy is markedly higher in tropics as compared to the temperate climate as well as the Himalayas thus supporting the earlier conclusion of Manton (1953) for ferns as a whole that evolution is proceeding at a faster rate in the tropics than in temperate latitudes.

Evolutionary machanisms

An analysis on world wide basis of sexual, sterile, and apogamous taxa and percentage of polyploidy for principal genera investigated is given in Table 7. Though on the basis of accumulated data the three genera, namely, Asplenium, Athyrium and Diplazium are considered among the cytologically best known fern genera yet the percentage of investigated species on the whole in these is rather low; 21.1% for Asplenium, 12.6% for Diplazium and 37.7% for Athyrium. Nevertheless, 1972 Evolutionary Status of the Asplenioid and Athyrioid Ferns 183

one is able to analyse the various evolutionary mechanisms at work . 1. Polyploidy: The genus Asplenium shows a marked predisposition for evolution through polyploidy which is detected in 72.2% of total taxa investigated throughout the world. The diploids are rather poorly represented. Higher levels of polyploidy i.e. at octoploid and dodecaploid levels are not scarce. Asplenium flabellifolium Sw. from New Zealand (cf. Brownlie 1958)and A. erosum from Jamaica (cf. Walker 1964 65) are 16-ploid. Cystopteris also shows equally high (77.4) frequency of polyploidy as Asplenium but the maximum grade is only upto octoploid level. Next comes the genus Dipla zium which shows 51.7% polyploid taxa, largely composed of tetraploids, hexaploids and octoploids and D. hians from Jamaica (Walker 1964-65) is 16-ploid. A notable feature about Asplenium and Diplazium is that amongst the higher leptosporangiate ferns these show the highest grade of polyploidy. In the genus Athyrium the great majority of species are at the diploid level. 77.5% of the species in the Himalayas, 85.7% in North America and 100% in Europe are diploid but from Ceylon no diploid is reported. Of the 11 taxa investigated from this region 6 are tetraploid and 5 hexaploid. On the world-wide basis only 44.3% taxa of Athyrium are polyploid and usually at no higher level than tetraploidy or hexaploidy. For Cornopteris also 60% taxa are polyploides with abundance of tetraploides. i) Intraspecific polyploidy: Investigation of the same species from wide areas has revealed the existence of populations in various grades of polyploidy (see Tables 2 and 6). Multiple sexual cytotypes are now known in as many as 41 species of Asplenioid and Athyrioid ferns (cf. Table 8). The largest number (22) exists in Asplenium alone. Under tropical climate the grade of polyploidy of such cytotypes is often very high (A. aethiopicum-4x, 8x, 12x; A. affine-8x, 12x; A. zenkerianum-8x, 12x etc.) whereas in temperate climate these are generally diploid and tetraploid or at the most hexaploid. Further, the number of recorded cases of 'species' complexes from Ceylon is the highest in any region as revealed by the studies of Manton (1953) and Manton and Sledge (1954). Such a situation of several sexual forms being grouped under one name raises some pertinent questions re garding the concept of species. Whether to accord some taxonomic status to such races or not is the real problem. Morphological comparisons of voucher specimens from different regions is very essential before this could be decided. To the writer, the best way appears to be to follow the middle path. If the differences within the cytotypes are easily perceptible and if they are of distinct ecological or geogra phical importance, then these may be separated as was done by Meyer (1961) and Lovis (1964b) and Lovis et al. (1966) in the case of Asplenium trichomanes, Lovis and Rechstein (1964) in A. rutamuraria and Vida (1963) in Ceterach officinarum. However, such differences are not noticeable in the Himalayan populations of A. trichomanes (see Bir and Shukla 1967). In the event of absence of easily perceptible morphological differences different cytological races may be retained under the present acceptable names. ii) Allopolyploidy: The role of allopolyploidy in speciation is well recognised. Quite a large number of British polyploid ferns are allopolyploids (Manton 1950). Table 5. Regional analysis* of three cytologically best known Asplenioid and Athyrioid genera Table 6. Frequency of polyploidy in the Himalayas and other temperate and tropical regions

* Data presented in this table and subsequent ones are compiled from list of chromosomes provided by Chiarugi (1960) , Fabbri (1963, 1965), Ornduff (1967-69) and subsequent publications of Bir (1965a), Chaudhuri (1966), Cupta (1966), Jarret et al. (1968), Kuriachan (1967), Lovis and Reichstein (1968a, b) and Manton and Vida (1968). ** At the foot of the Himalayas particularly in the East , upto 1,200m altitude, the climate is tropical to sub-tropical but only six species

(Asplenium nidus, A. finlaysonianum, A. falcatum, A. nitidum, Diplazium esculentum and D. australe) grow in this zone. Majority of the species worked out from the Himalayas grow under warm to cold temperate climate between 1,200-4,200m altitude. •õ No member of genus Athyrium has been worked out from Malaya.

•õ•õ Genus Diplazium is not represented in North America. •õ•õ•õ The European fern, Athyrium crenatum (Sommerf.) Rupr. has been shown by Brogger (1960) to possess n=41. This is a Diplazium with short, linear-oblong or globose and rarely 'double sori. Hope (1902) expresses the possibility that in spreading westwards to Norway, the

Japanese plant of A. squamigerum (Mett.) Ohwi [=D. squamigerum (Mett.) Christ] with n=80 (cf. Mitui 1965) has lost in length of sori to become this fern. 186 S. S. Bir Cytologia, 37

Wagner (1954), in case of Appalachian Aspleniums has brought out clearly the part

played by alloploidy in evolution of new species. Here three basic diploid Asple nium species, namely, A. montanum, A. platyneuron and A. rhizophyllum with dis tinct morphology have given rise by hybridization, doubling of chromosomes and

back-crossing to a total of 8 additional taxa which include the familiar, fully fertile, allotetraploid species A. pinnatildum, A. ebenoides, and A. bradlevi with morphology intermediate between the parents. Further, Wagner and Whitmire (1957) have

reported a distinct allotetraploid form of A. ebenoides (A. playtneuron•~A. rhizo

phyllum) having 2n=144 and 72 bivalents at meiosis, this having arisen spontaneous ly through polyploid spores. Also, an allotetraploid form of A. unilaterale with n=76 and 2n=152 has

recently been reported from South India by Kuriachan (1964, 1967). A diploid hybrid of the species with 76 bivalents at meiosis grows in the Himalayas (cf. Bir

1960, 1963). So the form with 76 bivalents may have arisen by the doubling of chromosomes of such a hybrid which, though yet undetected in S. India, may

even be growing in the vicinity of the sexual tetraploid. Several other European species of Asplenium, namely, A. majoricum (cf. Jermy and Lovis 1964, Sleep 1967),

A. lepidum (cf. Lovis et al. 1965, 1966), A. adulterinum (cf. Lovis 1968), A. adian tumnigrum and A. balericum (cf. Shivas 1969) have been shown to have originated

through alloploidy. Several of these are allotetraploids. Lovis and Reichstein

(1968b) have also described the abundant production of tetraploid progeny from A. protoadulterinum and A. adulteriniforme hybrids by means of diplospores. Formation of diploid spores is of great significance for the production of allopoly

ploids in nature. These above instances, few among the many unnoticed, indicate that in Asplenium considerable part has been played by allopolyploidy in evolu

tion of new taxa. According to Blasdell (1963) some species of Cystopteris (C.•~ tennesseensis, n=84; C,•~laurentiana, n=126 and C,•~alpina, n=126) have

evolved through allopolyploidy. Such processes are surely at work also in Di

plazium and Athyrium though not yet brought to light. iii) Autopolyploidy: There are apparently little indications of autoploidy having played a significant role in evolution of new taxa in the genera studied from the Himalayas. The only positive case is that of an autotriploid of A. unilaterale var. udum which exhibits both the important criteria for diagnosis of autoploidy exact phenotypic resemblance with the parent and formation of multivalents. The recently investigated other two cases of triploid Cystopteris tenuisecta (cf. Bir 1965a) and pentaploid Athyrium japonicum (cf. Bir. 1961b) where multivalent formation is noticeable, are not true autopolyploids because these races do not show exact resemblance with the sexual forms. The multivalents are probably due to close homology of the two genomes, they may be segmental allopolyploids. A. ruta muraria has been shown by Lovis (1963) to be either a segmental allopoloyploid or an ancient autopolyploid. Here, in this connection it is pertinent to draw atten tion to a report by Verma and Loyal (1960) and Loyal and Verma (1961) of the presence on an average of 12Iv+36II (2n=120), their regular disjunction and production of normal viable spores in colchicine-induced fertile autotetraploid of Adiantum capillusveneris. It is, therefore, not improbable that some present day 1972 Evolutionary Status of the Asplenioid and Athyrioid Ferns 187

polyploid species may be ancient autopolyploids that have lost power of multivalent formation during their long period of evolution. Support for such an idea comes from Lovis (1964a) who considered the two European tetraploid species, Asplenium ruta-muraria and A. septentrionale to be of autopolyploid origin. Similarly A. trichomanes ssp. quadrivalens (Lovis et al. 1966, Lovis and Reichstein 1969) and A. petrarachae (Sleep 1966) are of autoploid origin. iv) Aneuploidy: There is a uniformity of basic chromosome number in so far as the species belonging to different genera of Asplenioid and Athyrioid ferns are concerned. The only glaring exception is of Asplenium unilaterale with x=40 and this number may have been evolved from 36 through aneuploidy. Thus, aneuploidy is practically of no importance in the species evolution in these groups of ferns. It is only at generic level in Athyrioid ferns that aneuploidy has had to play some role in the evolution of different based numbers 40, 41, 42 in Athyrium, Diplazium and Cystopteris respectively. 2. Hybridity: Hybrids (taxa producing abortive spores as a sequence of irregular meiosis) in Asplenium and Cystopteris have been recorded since long on the basis of certain established criteria i.e. intermediate morphology between the supposed parents, their occurrence with or in the near vicinity of the parents usually as single plants unless reproducing vegetatively and complete or partial sterility. It is not intended to refer to all the published literature but mention must be made of the work of Alston (1940) who recorded the occurrence of hybrids in the genus Asplenium in Britain and also of Meyer (1960) who listed as many as 23 hybrids (wild and syn thetic) in Asplenium from North-Eastern and Central Europe.1 Majority of the hybrids are sterile and reproduce vegetatively but few may be

fertile. One such example is of a tetraploid hybrid, A. septentrionale•~A. rutamuraria from Switzerland which showed complete pairing in a high proportion of cells, the bivalents observed being produced by autosyndetic pairing (cf. Lovis 1964a). Large cases of speciation through hybridity followed by polyploidy or apogamy are known in Asplenium and Diplazium. On world-wide basis the percentage of hybridity is well marked in Asplenium 13.9% and Diplazium 19.4% as compared to 8% in Athyrium and 4.5% in Cystopteris (cf. Table 7). Of all the categories, triploid hybrids are in great profusion. Asplenium is also known to form intergeneric hybrids with Phyllitis (Alston

1940), Camptosorus (Slosson 1902, Wagner 1954, Wagner and Whitmire 1957, Morton 1956) and Ceterach (Mayer 1957, 1960). Girard and Lovis (1968) and

Lovis and Vida (1969) have respectively discovered in nature and synthesised two intergeneric hybrids, namely, •~Asplenophyllitis microdon (A. billotii•~Phyllitis scolopendrium) and •~A. jacksonii (A. adiantum-nigrum•~P. scolopendrium). To

the writer's knowledge, there is no report of Athyrium, Diplazium and Cystopteris forming hybrids amongst themsleves or with other allied genera. The higher

frequency of hybrids and the ease with which Asplenium forms intergeneric crosses

1 Further information about wild as well as synthetic hybrids of Asplenium species is available from works of Emmot (1964), Lovis and Reichstein (1968a, b, 1969) and Lovis et al. (1965, 1966). Table 7. World wide analysis of principal genera of Asplenioid and Athyrioid frens

* This excludes synthetic hybrids . Only wild hybrid taxa with irregular meiosis resulting in abortive spores (2x-10x) and apomictic taxa considered. 1972 Evolutionary Status of the Asplenioid and Athytioid Ferns 189

indicate that the genus has great potentialities for evolving new types . 3. Apogamy It is the most prevalent form of apomixis in the higher leptosporangiate ferns. Only five cases of obligate apogamy are known in the genus Asplenium. Three of these namely, A. cheilosorum (Manton and Sledge 1954; Mehra and Bir 1960a, Bir 1965b), A. monanthes (Manton 1950, 1959, Manton and Vida, 1968) and A. resiliens (Wagner 1963, 1966, Wagner and Wagner 1966, Morzenti 1966, Walker T. 1964-65) are triploid apogamous with 2n='n'=108 . A. aethiopicum is octoploid apogamous 2n='n'=288 (cf. Braithwaite 1964). The fifth species, A. heterore siliens is morphologically intermediate between A. heterochorum and A. resiliens and is a pentaploid apogamous fern with 2n='n'=180 (Morzenti 1966, Wagner 1966). It is a new species heretofore unrecognised in North America (cf. Morzenti 1966, Morzenti and Wagner 1962, Wagner 1963, 1966). Among the athyrioid ferns apogamy has been reported in nine taxa (8 species) of Diplazium i.e. D. syl vaticum-2n='n'=200 (pentaploid apogamous) , D. dilatatum, D. doederleinii, D. procumbens, D. simplicivinium, and Diplazium sp. (Coll. No. 1114 provisionally included under D. maximum)-2n='n'=123 (trip. apogam), D. maximum-2n=' n'=123 (trip. apogm.) and 2n='n'=164 (tetraploid apogam.) and D. stenochla mys-2n='n'=82 (diploid apogm.) (cf. Manton 1954, Manton and Sledge, 1954, Bir 1965a, 1969, Kurita 1965, 1966). In all these apomictic species 32 spores with zygotic number from 8-spore mother cells are the result as a consequence of failure of premeiotic mitotic division in the sporangium. A new type of apogamy in A. aethiopicum from Africa is recorded by Braithwaite (1964) where the first meiotic division in each of 16-spore mother cells ends in restitution nucleus which then di vides regularly to produce 32 diplospores. The cytological status of both game tophytic and sporophytic generations is the same. Apart from classical examples of Athyrium filix-foemina var. 'clarissima Jones', var. 'clarissima Bolton' and var. 'unconglomeratum Stansfield' (Farmer and Digby 1907) where apospory is followed by apogamy, the phenomenon of apogamy has been noticed so far in only A. unifurcatum, tripoid apogam. with 2n='n'=120 (Kurita 1966), out of 64 species (80 taxa) of Athyrium worked out and this disproves the statement of Mahabale and Vanketeswaran (1953) that most of the species of Athy rium are apogamous. A. filix-foemina, it may be mentioned is a diploid sexual species (n=40) as investigated from Europe, North America and the Himalayas. Both the cases of tropical apogamous species of Athyrium recorded by Manton (1953) and quoted by Mehra and Verma (1957) belong to Diplazium as that genus is now understood with the present day information of their chromosome numbers. In Cystopteris also apogamy is absent except for Heilbronn's (1910) report of a race of C. fragilis whose protahalli formed normal embryos during winter while in spring and summer apogamous sporophytes were produced, there being no cytolo gical evidence to support it. The incidence of apogamy in Asplenioid and Athyrioid ferns as a whole, is very low. It has been recorded in only 14 species (15 taxa) out of a total of about 300 species cytologically investigated so far. In Asplenium 2.4%, Athyrium 1.2% 190 S. S. Bir Cytologia 37

Table 8. World-wide distribution of sexual 'species' of Asplenioid and Athyrioid ferns with several cytotypes 1972 Evolutionary Status of the Asplenioid and Athyrioid Ferns 191

Table 8. (Continued) 192 S. S. Bir Cytologia 37

Table 8. (Continued)

* According to the data available at present , this species is dibasic with x=36,40. Evidently there are morphological differences in the specimens worked out from various regions and the taxonomy of the species is still confusing which is being worked out. ** This has arisen through a cross between 36 and 40-chromosome forms and subsequent doubling of chromosomes. *** Polyploidy level determined by Blasdell (1963) on the basis of spore size. Actual chro mosome determinations were not done. and in Diplazium 13.4% of investigated species are apogamous (cf. Table 7). From the foregoing discussion, it is clear that in Asplenium, Cytopteris, and Diplazium hybridization and polyploidy (auto-allo and allo-) have played signi ficant role in the evolution of species but not to that extent in Athyrium. The few hybrids recorded, the preponderance of diploids, and the almost absence of apo gamy reveal that these factors have played relatively insignificant part in the evolu tion within the genus Athyrium which has been principally through genic muta tions (Mehra and Bir 1960b). On the other hand Diplazium appears to be actively evolving along these lines because of low percentage of diploids (cf. Table 7), greater number of hybrids, and the consequent establishment of apogamy.

Summary

A review of the results of cytomorphological observations so far carried out on members of Asplenioid and Athyrioid ferns from the Himalayas is given. The results from this phytogeographically important region are compared with other areas having temperate and tropical climates. It is concluded that the percentage and rate of polyploidy is definitely lower in the Himalayas as compared to the tropi cal areas. This region contains a very high proportion of diploids and is different from rest of the Indian sub-continent (South India and Ceylon) where there is greater incidence of polyploidy. Also the flora of the Himalayas resemble the flora of Europe and North America in this respect. Evolutionary mechanisms at work in members of the four principal investi gated genera, namely, Asplenium, Cystopteris, Athyrium and Diplazium are also 1972 Evolutionary Status of the Asplenioid and Athyrioid Ferns 193 considered. Asplenium shows a marked predisposition for evolution through polyploidy which is detected in 72.2% of total taxa investigated throughout the world. As compared to this, in Athyrium the great majority of the species are at diploid level (about 56% taxa). Diplazium shows 56.7% polyploid taxa, largely composed of tetraploids. Apogamy is frequent in Asplenium and Diplazium whereas it is almost absent in Athyrium and Cystopteris. It is now evident that in Asplenium, Cystopteris and Diplazium hybridization and polyploidy (both allo- and auto-allo) have played a significant role in the evolution of species, but not to that extent in Athyrium where the evolution has been principally through genic mutations.

Acknowledgement

This paper is gratefully dedicated to my teacher Prof. P. N. Mehra (Chandi garh) in honour of his 65th birthday (28th October, 1972). He took keen interest in the progress of the work and from time to time offered very valuable suggestions.

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