Amer. J. Bot. 54(9): 1061-1068. 1967. ASPLENIUM PLENUM: A FERN WHICH SUGGESTS AN UNUSUAL l\1ETHOD OF SPECIES FORMATIONl VIRGINIA M. MORZEN'rI Department of Botany and Botanical Gardens, University of Michigan, Ann Arbor ABST RAe T Morphological data, chromosome behavior, and occurrence strongly suggest that the Florida fern known as Asplenium plenum arose as a cross (AAB1B2) between a sexual 2x species, A. abscissum (AA), and a "sterile" 3x hybrid, A. curtissii (AB1B2). A mechanism is described where­ by such an origin is possible: A. curtissii, among other so-called "sterile" hybrids, has the ability to produce unreduced spore mother cells capable of becoming spores directly. Such spores are able to form garnetophytes with viable sperms and have the potential of producing sporophytes apogamously. It is highly probable that this heretofore overlooked process has operated in the origin of the hybrid fern, A. plenum, and is the first to be reported in the literature. THE EXCEEDINGLY RARE Florida fern known as Fig. 1: A. cristatum Lam., A. abscissum Willd., Asplenium plenum E. P. St. John was first dis­ A. plenum E. P. St. John, A. curtissii Underw., covered in 1936 in a limestone cavern near and A. oerecundum Chapm. The habitats of all Lecanto, Fla. In 1938 St. John (Small, 1938) are similar. As summarized by Wherry (1964), described this plant as a normal species, known they grow on shady, wooded limestone ledges then from that single locality. Small (1938) says and walls of sinks. All five have been reported that A. plenum "was at first considered a form from at least one locality, Indian Field Ledges. of Asplenium curtissii, from which it differs Darling (1962) reported that "Asplenium ab­ in similar ways and to about the same extent as scissum and A. oerecundum were common every­ that fern diff~rs from A. verecundum." In view of where, and A. plenum was luxuriant on a long, the studies to be described below the latter state­ low-lying boulder, with A. curtissii less plentiful ment has proved to be significant. In the mid­ on rocks to the southwest." Asplenium cristatum, 1950's specimens of A. plenum were discovered at however, was not recorded from this locality a new locality (Indian Field Ledges in Sumter Co., until Wagner (1963a) found a single plant. Fla.) by E. S. Ford and Thomas Darling, Jr. In Recent studies on cultivated plants have describing his observations at this locality, resulted in the following data: Three are normal Darling (1961) wrote: "At the time of my 1957 species, two being diploids and one a tetraploid; visit I ... supposed the entire lot to be A. cur­ the other two are hybrids, a triploid and a tetra­ tissii. Not until I revisited .Indian Field Ledges ploid (Fig. 2-6). Both A. crisiaium and A. in 1958 and collected more specimens of these abscissum are 2x with n = 36 and have 64 ferns did I suspect that two different species of normal, monolete, meiotic spores per sporangium; Asplenium (A. curtissii and A. plenum) were A. verecundum is 4x with n = 72 and it also has represented." More recently Wagner (1963a, b) 64 meiospores per sporangium. Asplenium cur­ suggested that A. plenum is of hybrid origin and tissii is a 3x hybrid with 108 chromosomes, that its most probable parents are A. abscissum forming mostly univalents at meiosis with an and A. crisiaium, The purpose of the current paper irregular number of bivalents, varying from 1 to is to present evidence in support of a new hy­ 17 pairs in the observations made. In sporo­ pothesis for the origin of Asplenium plenum, genesis A. plenum also behaves as a hybrid but is which may involve an undescribed process in the a tetraploid, 2n = 144, and exhibits many more fprmation of certain fern species. pairs, usually some 60-odd bivalents. Up to the present, where the spores produced by these two THE TAXA INVOLVED-The five taxa included hybrids have been described, they have been in the complex under discussion are shown in called "abortive." Asplenium curtissii is obviously a simple hy­ 1 Received for publication 5 November 1966. This study was supported in part by National Science brid between two normal species. It is analogous Foundation Grants G-10846 and GB-3366. The author to dozens of such taxa now known in the literature thanks Professor Warren H. Wagner, Jr., for helpful of pteridology. This taxon was described by suggestions; Dr. E. S. Ford, Mr. Thomas Darling, Jr., and Mr. C. E. Delehamps for sending living plants upon Underwood from the vicinity of Ocala, Fla., in which this research is based; and the staff of the University 1906, although he did not regard it as a cross. of Michigan Botanical Gardens for cultivating the plants. As discussed by Wagner (l963a), the plant is, [The JOURNAL for September (1)4:931-1059) was issued 17 October 1967J [AMERICAN JOURNAL OF BOTANY, Vol. 54: No.9, 1967 1061 1062 A~IERICAN JOURNAL OF BOTANY [Vol. 54 Fig. 1. (left to right) A. cristaium (Jamaica); A. absciseuni (Marion Co., Fla.); A. plenum (Sumter Co., Fla.); A. curtissii (Sumter ce., Fla.); A. verecunduni (Sumter Co., Fla.). however, a morphological intermediate between of bivalent formation at meiosis but usually with A. verecundum and A. abscissum, though in some 10 to 20 unpaired chromosomes. In his overall appearance it tends to look somewhat discussion of the nature of this plant, Wagner more like the former parent. Since A. oerecunduni (1963a) stated that such a cytological condition contributes a genome of 72 chromosomes while must mean that either two 2x or two 4x species A. abscissum contributes a genome of 36 chromo­ participated in the formation of A. nlenum if it somes, it is not surprising that their cross inclines is, in fact, a hybrid. At the time of his writing the toward the former. As shown in Table 1, however, most logical parental candidates seemed to be a more detailed study of the morphology of these A. abscissum, n = 36, and A. cristatum, also plants has demonstrated that in some characters with n = 36. At first sight A. plenum appears A. curtissii is almost exactly intermediate be­ to be somewhat intermediate between these two tween its parents. In the degree of cutting of the sexual species. However, if it is really the al­ pinnae it tends to resemble A. verecundum more. lotetraploid cross of these two taxa (excluding But in the average length of the longest pinnae, morphology for the present), there are several the hybrid is more nearly intermediate, as shown major difficulties: (1) There is usually incomplete by the proximity of the actual and predicted pairing at meiosis. Even though there are numer­ averages (predicted average based on parental ous pairs, the number of univalents is more measurements) given in Table 1. Another charac­ than one would expect. A normal allotetraploid ter of the hybrid showing intermediacy is the fern usually has complete pairing. (2) Normal pinna angle from the rachis (using dried speci­ monolete meiotic spores are not formed in A. mens). As shown in the table the predicted pinna plenum. Its spores have been described as "abor­ angle average for A. curtissii is 59° while the tive" and unable to germinate, which is unlike actual average measured 60°. typical fern allotetraploids. (3) The only time A. The pairing behavior of A. plenum is rather cristatum has ever been reported as growing to­ peculiar in view of the seemingly abortive spores. gether with A. abscissum in Florida was by Wag­ There are 144 chromosomes with a high degree ner (l963a) and there was but a single plant of o t:3 C'> A. cristatum (2x) A. abscissum (2x) A. verecundum (4x) A. curtissii (3K) A. plenum (4~) tl:l M­ to' o 1':1 0­ 1. Chromosome number. 2.<i = 12 2.<i = 12 4~ = 144 3~ = lOS 4~ = 144 ro ~ ~ 2. pairing. behavior. 36 II 36 II nIl 1-17 II's + I's 60-70 II's + I's co..... 0;, 3. Spores. Normal monolete, Same Same Mostly abortive Mostly abortive f ~ meiotic: 64/ but some unreduced but some unreduced £; sporangium. spores present. spores present. ~. 4. Life cycle. Sexual Sexual Sexual Sporophytes apog­ Sporophytes apog­ ~ amous(?) and from amous(?) and from root prolifera­ root prolifera­ ';:0, tions. tions. '"co ~ 5. Longest pinna lengths, Ave. 5.6 cru, 5.1 2.1 3.7 (predicted 3.0* (predicted ave. '" attachment to tip ave. 3.6) absc. x crist. 5.4 g~ (20-22 fronds). absc. x curt. 4.4) -e Range 3.S - 7.0 cm. 3.9 - 6.S 1.6 - 2.S 2.0 - 5.1 2.0 - 4.4 ~ 0 0 !;.oJ ~ngles o 6. Pinna from rachis, Ave. S5° 44 74° 60 4S * <3 measured 1 cm. from pt. (predicted ave. (predicted ave,, ~ ;l;I N of attachment (20 pinnae, 590 ) absc. x crist. 64.5°, t."l 10 plants). absc. x curt. 52 0 ) ",' S· Z 7. Pinna outline. Pinna width most­ Pinnae broadest Same Same Same* "" ly equal to width at base or below § R. of base, tapering middle, gradually !tn ;>- "C near tip of pinna. tapering. r­ t."l S. Cutting of pinnae. 3-pinnate l-pinnate 3-pinnate 3-pinnate 2-pinnate** ~ Z ~. a 9. Stalking of pinnae. Completely ses­ c. 1 mm , long Completely ses- Less than 1/2 mm. More than 1/2 mm. co Ii:: sile sile (but less than 1 mm.)* 'l:l -e ~ t< 10. Number of anterior sec- Ave. 12.3 12.3 4.6 7.2 6.1* t."l ondary vein departures, Z median pinnae (10 Range 10-16 11-14 4-6 5-S 5-S c1 plants).