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Megagametophyte Development in the Chaloneriaceae Fam. Nov., Permineralized Paleozoic Isoetales (Lycopsida) Kathleen B. Pigg; Gar W. Rothwell Botanical Gazette, Vol. 144, No. 2. (Jun., 1983), pp. 295-302. Stable URL: http://links.jstor.org/sici?sici=0006-8071%28198306%29144%3A2%3C295%3AMDITCF%3E2.0.CO%3B2-J Botanical Gazette is currently published by The University of Chicago Press.

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BOT. GAZ. 144(2):295-302. 1983.

0 1983 by The University of Chicago. All rights reserved 0006-807 118314402-0010502.00

MEGAGAMETOPHYTE DEVELOPMENT IK T H E CHALONERIACEAE

FAM. KOV., P E W I N E R A L I Z E D PALEOZOIC ISOETALES (LYCOPSIDA)

KATHLEEK B. PIGG AxD GAR W. ROTHWELL

Department of Botany, Ohlo Un~versity,Athens, Ohlo 45701

Megaspores and megagametophytes that exhibit a wide range of variability were discovered within sporangia of the Pennsylvanian fossil lycophyte, Chaloneria cormosa. The smallest megaspores occur in tetrahedral tetrads, with larger specimens preserved individually. Full-sized megaspores are often devoid of contents, but some show cellular megagametophytes entirely within the spore, or megagametophytes that protrude out of the trilete and display several archegonia. The range of variability is compared with developmental variations in extant and other fossil lycophytes to interpret megaspore and megagametophyte ontogeny and several aspects of reproductive biology in Chaloneria. Based on numerous similarities to Isoetes and to the hlesozoic Pleurotneia complex, Chaloneria and several other Paleozoic lycophytes are grouped into the Chaloneriaceae fam. nov. and placed within the Isoetales.

Introduction Valvisisporites and Endosporites formed a significant component of the sporae dispersae flora in Upper Carboniferous and Pennsylvanian coals of the Northern Hemisphere ( P O T O K Iand ~ KREMP 1954) and, until recently, were thought to represent a single species, Polysporia mirabilis (CHALOKER 1958). However. we now recognize that spores of these types were produced by several plants with features that do not conform to Lepidodendrales (PIGGand ROTHWELL1983). Among these. Clzaloneria Pigg and Rothwell (1983) was a heterosporous lycophy-te that inhabited Pennsylvanian coal swamps of Korth America. The plant was unbranched, with a rounded or lobed rooting base. and had fertile regions along the stem. Taxonomically diagnostic leaf cushions and compact cones were not produced. In C . cormosa the apical region was all fertile, but in C . periodica there were alternating fertile and vegetative regions along the length of the stem (PIGGand ROTHWELL1983). In this paper we describe several aspects of reproductive biology for Chaloneria cormosa and interpret the systematic affinities of plants t h a t produced Valvisisporites and Endosporites. Included are descriptions of megaspore and megagametophyte development in C . c o r m o s a a n d interpretations of reproductive biology in Clzaloneria. Also included are comparisons of Chaloneria with other heterosporous lycophytes and the proposal of an additional family of Isoetales, the Chaloneriaceae fam. nov. This represents the first recognition of Isoetales in Paleozoic deposits. Chaloneria cormosa megaspores and megagametophytes were obtained primarily from within Manuscript received M a y 1982; revised manuscript received Septenzber 1982. Address for correspondence and reprlnts GARW ROTHDepartment of Botanq, Ohlo Univers~tq.Athens, Ohlo 45701

WELL,

intact sporangia. The specimens were collected and prepared as described by PIGG and ROTHWELL (1983) and are housed in the Paleobotanical Herbarium, Department of Botany, Ohio University, acquisition nos. 3284-3293, 3443-3459, 3487-3705, 3767-3791.

Description Megaspores and megagametophytes of Chaloneria cormosa display a wide range of variability with many of the stages often preserved in a single sporangium. The smallest megaspores occur as small tetrads (fig. 1). Tetrads are ca. 90 k m across, about one-sixth the size of a single mature megaspore. Three of the spores of a tetrahedral tetrad are often visible in a single section, while adjacent sections reveal the attachment of the fourth sDore. Somewhat larger individual spores also occur throughout the fructification (fig. 2). Most megaspores are empty, but some exhibit poorly preserved contents, which may represent either torn fragments of nexine or remnants of prothallial tissue. Other megaspores contain cellular megagametophytes (figs. 4-13); some of these have a closed or only slightly open suture and contain gametophyte tissue enclosed entirely within the megaspore wall (figs. 3-5). Prothallial cells occupying the distal two-thirds of the spore are large, while those of the proximal third are much smaller and more compact (fig. 5). A similar configuration also characterizes Isoetes (CAMPBELL1891, LA MOTTE 1933), Selaginella (LYON1901), and the Carboniferous lycophytes described by GALTIER (1970) and BRACK-HANES (1978). Ko archegonia have been found in gametophytes of this type. Other C . cormosa megagametophytes protrude through the open trilete mark (figs. 6-9), as reported among both extant lycophytes (Selaginella, BRUCHMAKN 1912; Isoetes, LA MOTTE1933; Stylites, RAUHand FALK 1959) and some Carboniferous taxa (GAL-

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PIGG & ROTHWELLhIEGAGAR.IE'TOPHYTES OF CHALONERIACEXE

1970; BRACK-HANES 1978). The protruding gametophytes of C. cornzosa are lobed (fig. 6), resembling closely some species of Selagiizella (BRUCHMANK 1912) a n d Lepidodeizdvoiz estouense (GALTIER1970). Protruding megagametophytes of C. cormosa only rarely exhibit unicellular rhizoids, which arise from superficial cells on the prothallial surface (figs. 9, 11). This is in contrast to the megagametophytes of Lepidostrobus schopjii (BRACK1970), which, like many extant lycophytes (Stylites, RAUH a n d FALK 1959; Selaginella, BRUCHMAKN 1897, 1908), have prominent rhizoidal tufts. Rhizoids of C. cormosa are ca. 1 6 1 9 p m in diameter, up to 104 p m long, and have a swollen base that may reach 28 p m in diameter (fig. 11). Archegonia are embedded in the densely packed cells along the proximal surface of protruding gametophytes (figs. 6, 8). U p to seven archegonia have been found within a single megagametophyte Archegonia have two to four tiers of neck cells (fig lo), with each tier composed of four cells Fourtiered archegonia protrude slightly from the prothallial mass and have an asymmetrical, beaklike neck (fig. 10). Other archegonia retain only the basal two tiers of neck cells as in mature gametophytes of extant lycophytes. I n these specimens the archegonium is recessed between two crests (fig. 13). Individual neck cells are 16-29 p m across. Below the neck region is the ventor (figs 12-13) Ventors are ca. 60 p m high in longitudinal section (fig. 13) a n d ca. 40 p m in diameter in cross section (fig. 12). Many ventors contain a dark, globular mass up to 24 p m in diameter (figs. 8, 13). A similar mass, which may represent an egg or zygote, is also present in ventors of L schopjii (BRACK1970) and is particularly well preserved in L . estotzense (GALTIER 1970).

TIER

Megaspore and megagametophyte development The full range of megaspore size and megagametophyte structure is found within a single fructification of Chalonevia cornzosa, with several configurations typically occurring in the same sporangium. This is in contrast to Lepidocarpon cone fragments described by BALBACH(1962) and F h MANUJAM and STEWART(1969). I n the latter studies, ontogeny of the sporangium and distal lamina are interpreted by comparing sporophylls from proximal levels with those from progressively more distal levels; the most distal is interpreted as the least mature. Among C. cormosa megagametophytes, which probably did not undergo synchronous development, the full-sized spores containing cellular gametophytes probably represent the normal range of ontogenetic variation present a t a single time in a. maturing sporangium. Smaller spores and those without cellular contents may be abortive. Assuming that the abortive spores retain the

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structure of earlier developmental stages, we may interpret the ontogenetic sequence of spore development. Variations in megagametophyte structure may also represent developmental stages in light of the similar structural variations that occur in ontogenetic sequences of extant heterosporous lycophytes (CAMPBELL 1891; LYON1901; LA ~ I O T T E 1933). The least mature megaspores in our material are represented by the small tetrads (fig. I). T h e small size of these tetrads suggests that megaspores disassociated early in development and continued to grow after separation. Early disassociation of both extant (Selagitzella, LYON 1901; TRYOK and L r GARDON 1978) and fossil megaspore tetrads is common. In the earliest stage of megagametophyte development, lycophyte megaspores undergo numerous mitotic divisions without forming cell walls (CAMPBELL 1891; LYON1901). This results in an acellular configuration referred to as the free nuclear stage of development. Small C. contzosa megaspores, slightly larger than the tetrads (fig. 2 ) , may represent either megaspores prior to mitotic divisions or the megaspore wall, which contained megagametophytes in the free nuclear stage of development. LYON(1901) commented that, in Selaginella apus, gametophyte development began while megaspores were much smaller than their mature size and that there was a rapid expansion of the megaspore wall along with the onset of mitotic divisions. The presence in the same fructification of both earlier and later ontogenetic stages supports the conclusion that the free nuclear stage of development would also be represented in C. covmosa. The preservation of nuclei and cytoplasm, which could demonstrate premitotic or free nuclear stages, has rarely been observed among Paleozoic plant material (MILLAYand EGGERT1974; BRACKHAKESa n d V A ~ G H K 1978). If a rapid expansion of the megaspore wall occurred during the free nuclear stage in C. cormosa, this stage m i g h ~be represented by both small individual spores and some full-sized specimens lacking internal contents (fig. 2 ) . Other full-size megaspores probably lack contents because of incomplete preservation. Lycophyte megagametophytes are typified by centripetal and basipetal cellularization (CAMPBELL 1891; LYON 1901). As in Isoetes (LA MOTTE 1933), Stylites (RAuH and FALK 1959), and some Carboniferous lycophyte gametophytes (BRACK 1970; GALTIER1970; BRACK-HANES 1978), mature C. cormosa specimens are completely cellular, lacking the central hollow areas and diaphragm that are typical of some Selaginella gametophytes (CAMPBELL 1902; BRUCHMANN1908, 1912; ROBERT 1971). The occurrence of small cells in the more proximal regions of C. cormosa (fig. 5) may be due both to the direction of cellularization and to the

FIGS. 8-13.-Megagametophytes. Fig. 8, Oblique longitudinal section through mature megagametophyte with several archegonia in protruding tissue; 1408, K(l)b(l) side, no. 85; X 72. Fig. 9, Longitudinal section through mature megagametophyte showing rhizoids; 1408, K(l)b(l) side, no. 28; x 143. Fig. 10, Cross section showing rhizoid; 1408, K(l)b(l) side, no. 85; x 77. Fig. 11, Cross section of several archegonia; note jacket and egg cell of archegonium a t center; 1408, K(l)b(l) side, no. 52, x 360. Fig. 12, Archegonium with four tiers of neck cells; 1408, K(l)b(l) side, no. 51; x 340. Fig. 13, Archegonium with two tiers of neck cells; 1408, K(l)b(l) side, no. 92; x 370. ( e = egg, nc = neck cells, r = rhizoid.)

PIGG & R O T H W E L G M E G X G A M E TOPHYTES OF CHALONERIACEAE

299

the stem, wood, secondary cortex, and ligulate lea\,es with parichnos. Both are heterosporous, but Chaloneria is not an arborescent form with a large, well-de~relopedcrown and taxonomically diagnostic leaf cushions. Chaloneria also shows no evidence of branching or small twigs, and when the leaves are lost, the stem surface shows irregular scars. Furthermore, the bisporangiate fertile region of Chaloneria is unlike the highly specialized fructifications linked with some Lepidodendrales (lepidocarps: LEISMANand RIVERS 1974; PHILLIPS 1979; THOMAS1981). Perhaps most significantly, Chaloneria lacks the stigmarian rooting system that characterizes Lepidodendrales and may also ha\,e a distinctively different embryogeny (STUBBLEFIELD and ROTHWELL1981). A second group of lycophytes to which Chaloneria bears a resemblance is Isoetales. Both are heterosporous, h a v e r o u n d e d or lobed rooting structures, and exhibit bipolar growth. Histological details that both share with some Lepidodendrales include air channels (Isoetales) or parichnos (Lepidodendrales and Chaloneria) in the lea\,es and trabeculae in the sporangia (FELIX1954; BRACK1970; PIGGa n d ROTHWELL1983). A less common feature for Lepidodendrales is the highly contorted tracheids found in Isoetes a n d Chaloneria (ANDREWS and MURDY1958). Lack of intact plant-base cortex on available specimens of C . cormosa precludes interpretations regarding the presence of cortical lobes and furrows similar to those that characterize Isoetes corms, but neither Chaloneria nor Isoetes produces specialized compact cones. A third group with which Chaloneria may be allied is the Mesozoic Pleuromeiaceae. DELEVORYAS (1962), FOSTERa n d GIFFORD(1974), and TAYLOR(1981) recognized this group as distinct a t the ordinal level, while K~~~~~~ (1961), CHALONER (1967), RETALLACK(1975), and WHITE (1981) retained it as a family of Lepidodendrales. SMITH (1938) a n d BIERHORST(197 1) recognized it as a family of Isoetales. None of the authors who conSystematics of lycophytes sidered the group as distinct a t the ordinal level A survey of lycophyte classification schemes formally circumscribed it as such. I t is, therefore, difficult to determine the exact features upon which adopted by SOLMS-LAUBACH (189 l ) , SCOTT(1920), (1967), and TAYLOR they distinguish plants assignable to Pleuromeiales AXDREWS(1961), CHALOA-ER from Isoetales or Lepidodendrales. (1981) reveals both changing ideas of the structure The most well-characterized representatives of of these plants a n d a n increasing understanding of the Plez~romeiacomplex include P. sternbergii from their relationships. Among fossil taxa, the descripGermany, Russia, and China (MAGDEFRAU193 1; tion of newly recognized plants and the emergence HIRMER1933; NEYBURG1961; WANG, X I E , and of whole plant biology concepts for many forms furWANG 1978), the Russian a n d Chinese P. rossica ther refine our ideas concerning evolution a n d phy(NEYBURG1961; DOBRUSKIXA 1974; WAA-Get al. logeny of the group (PHILLIPS1979; THOMAS1981). 1978), P. hataii from Japan (KOX'XO 1973), P. Chaloneria was originally interpreted as a memjiaochengensis from China (WAXG and WANG ber of the Lepidodendrales, based on anatomical 1982), a n d the Australian P. longicaz~lis(RETALand morphological similarities with the arborescent LACK 1975). T h e last has been placed in Cylomeia lepidodendrids a n d the similar geologic age of all (WHITE 1981). I n general, plants of this type are (PIGGand ROTHWELL1979). Both Chaloneria a n d very much like both Chaloneria a n d Isoetes. They Lepidodendrales have a medullated protostele in

distribution of nuclei prior to cellularization (LYON 1901; LA MOTTE 1933). Cellular megagametophytes, contained within the megaspores displaying a closed trilete, represent young specimens in which archegonia have not yet differentiated. Further increase in size of existing gametophyte cells, and possibly proliferation of additional cells near the trilete (LA MOTTE 1933), leads to the pressing open of the trilete and the protrusion of prothallial tissue, with a concomitant differentiation of archegonia (ROBERT197 1). There is a gradation among both li\,ing a n d fossil lycopsids in the extent to which megagametophyte tissue protrudes through the trilete. Mature C . cormosa specimens resemble gametophytes of Lepidodendron estonense (GALTIER1970) as well as Stylites (RAUHand FALK193'91, some species of Isoetes (LA 1912) MOTTE 19331, and Selaginella (BRUCHMANN in protruding extensively through the open trilete (figs. 6, 8). Such protrusion may aid in fertilization by placing the archegonia in closer proximity to the microgametophytes. This is in marked contrast to the spores of Bothrodendrostrobus m u n d u s , in which even the young embryo is retained entirely with the megaspore wall, which may be a n adaptation to a specialized a q u a t i c environment (STUBBLEFIELD a n d ROTHWELL1981). Although numerous well-developed megagametophytes, some apparently with egg cells or newly fertilized zygotes, occur with C . cormosa megaspores (fig. 13), no evidence of multicellular embryos has been found. T h e lack of preservation of embryos may be due to the extruded nature of gametophytes. Most previously reported anatomically preserved lycophyte embryos are retained primarily within the megaspore wall (Lepidocarpon: PHILLIPS, AVCIN, and SCHOPF 1975; PHILLIPS 1979; Bothrodendrostrobusmundus: STUBBLEFIELD and ROTHWELL 198 I), therefore allowing for the protection, a n d the more likely preservation, of the delicate tissue.

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BOTASICAL GAZETTE

[JUNE

also assignable to Chaloneriaceae. These include both compressed fructifications a n d permineralized cones with spores similar to Chaloneria. Among the compressed fructifications, Polysporia mirabilis (CHALONER1958) a n d P. robusta (DRABEK1976) are bisporangiate, while Lepidostrobopsis missouriensis a n d L . mansjieldi are compact monosporangiate cones with Tralvisisporites megaspores (ABBOTT1963). Permineralized cones include the two species from Pennsylvanian deposits assigned to Polysporia by HAXES(19i5). Because the compact cones have narrow peduncles, it is possible that they were borne on branched plants with small twigs. If so, then the family includes taxa with a variety of plant habits. Additional specimens that we interpret as members of Chaloneriaceae have been described as species of Sporangiostrobus, which was originally described from compressions with distinctive spores (BODE 1928; CHALONER1956; REMY a n d REMY 1975); permineralized specimens were later assigned to the genus (LEISMAX19i0; LEISMAXand STIDD 1977). Like Chaloneria, the microspores (Densosporites) are trilete a n d saccate, and the megaspores (Zonalesporites) are trilete with a n equatorial elaboration of the exine. I n Chaloneria the elaboration is in the form of auriculae, while in Sporangiostrobz~s it forms a continuous cingulum. Sporangiostrobus has been reconstructed as a n unbranched or once-branched plant with fertile zones rather than compact cones (REMYa n d REMY 19i5) and in these features is similar to Chaloneria. There are apparently no taxonomically diagnostic leaf cushions on either the vegetative or fertile portions of the stems (REMYa n d REMY 1975), and if associated permineralized vegetative organs belong to Sporangiostrobz~s(LEISMAN19i0), then the vascular architecture a n d anatomical features of the leaf bases are also similar to those of Chaloneria (PIGGand ROTHWELL1983). Other Paleozoic lycophytes that do not conform comfortably to Lepidodendrales may also represent members of Chaloneriaceae. However, our growing awareness of the diversity among Paleozoic Lycopsida emphasizes the probability that isolated organs do not always represent close relatives of plants for which attachments of similar organs have been demonstrated. Common growth habits and organs of similar structure may have evolved inwithin only distantly related groups ~ O ~ ~ ~ ~ dependently , FAMILIALD I A G X O S I S . - F ~ ~ ~ - S heterosporous lycophytes with rounded andlor lobed rootin response to similar selective pressures and ecological conditions. ing base. Leaves ligulate, consistently produced leaf In formulating the concept of Chaloneriaceae and cushions absent. Trilete megaspores with equatoplacing the family in Isoetales rather than in Lepidorial extension of the exine in the form of separate dendrales, we have attempted to employ suites of auriculae or continuous cingulum; trilete microcontrasting characters to interpret the systematic spores with saccus. relationships among heterosporous Paleozoic lycoTYPE ~ ~ ~ u s . - c h a l o n e r i Pigg a a n d Rothwell (1983). phytes. Features that we consider to be of greatest Several other Carboniferous lycophytes are importance include similar spore types, nature of

have a n upright unbranched stem ca. 0.2-2.0 m tall, bear helically arranged leaves, a n d have a terminal fructification. Taxonomically diagnostic leaf cushions are not produced. At the base, the plants are lobed with curved rows of rootlet scars that indicate structural a n d developmental homologies to Isoetes (JENXINGS, KARRFALT,and ROTHWELL1983). Pleuromeia-type plants are either clearly or apparently heterosporous, and many show evidence of parichnos a t the leaf bases. Currently known plants assignable to the Pleuromeiaceae are from Mesozoic deposits and occur as compressions or mold casts; therefore, little is known of their internal anatomy. Kevertheless, the known features of the Pleuromeia-type plants are strikingly similar to both Chaloneria a n d Isoetes and allow for reconstructions that resemble closely that of Chaloneria (HIRMER 1933; MAGDEFRAU 1968; RETALLACK19i5; WHITE 1981; PIGG and ROTHWELL1983). For many years, Isoetales has been interpreted as having arisen by a reduction series from the Carboniferous Lepidodendrales through the Mesozoic Pleuromeiales to the extant Isoetes ( P O T O N I ~ 1894; MAGDEFRAU193 1; STEWART1947). However, it is now apparent that lycophytes with rounded or lobed bases, among them Chaloneria, grew in contemporaneous strata with arborescent lepidodendrids. Evidence from structure, development, a n d embryogeny indicates that there are a t least two parallel lineages (JEXNIXGS1975; STUBBLEFIELD a n d ROTHWELL198 1; ROTHWELL, KARRFALT,a n d JENNIXGS 1982; JEXNIXGSet al. 1983), plants with lobed rooting bases and Lepidodendrales with stigmarian rooting systems. Because of t h e n u m e r o u s similarities a m o n g heterosporous lycophytes with lobed or rounded rooting bases, we interpret them to be more closely related to one another than they are to other major groups of lycophytes (Lycopodiales, Selaginellales, Lepidodendrales). This allows us to recognize three groups of Isoetales: Isoetaceae for the Mesozoic and Cenozoic Isoetes a n d Isoetes-like plants, Pleuromeiaceae for the Mesozoic forms for which internal anatomy is little known, and Chaloneriaceae fam. nov. for Chaloneria a n d similar Paleozoic forms.

19831

PIGG & R O T H W E L G M E G A G A M E T O P H Y T E S OF CHALOSERIACEAE

the rooting system, internal anatomy and vascular architecture, presence or absence of taxonomically diagnostic leaf cushions, features of reproductive biology, and possibly embryogeny. If correctly interpreted, Lepidodendrales are characterized by stigmarian rooting systems, taxonomically diagnostic leaf cushions, often prolific branching, compact cones, a n d several forms of extreme heterospory. By contrast, Chaloneriaceae (of the Isoetales) have lobed andlor rounded rooting bases, lack taxonomically diagnostic leaf cushions, are typically un- or little-branched, and exhibit freesporing heterospory. While the contrasting nature of incomplete suites of characters exhibited by many only partly characterized plants is currently consistent with our interpretation of Paleozoic lyco-

301

phyte systematics, only whole-plant reconstructions for a large number of plants will determine if two divergent sets of characters typify Lepidodendrales and Isoetales, or if many of the characters evolved independently within a more complex array of lycophytes.

Acknowledgments We thank Dr. ERIC E . KARRFALT for the loan of a sectioned Stylites megagametophyte containing archegonia and a young e m b v o , and Dr. SHEILA BRACK-HANES f or providing sections of Lepidostrobz~sschopfii megagametophytes. This study was supported in part by National Science Foundation grants DEB80-10793 and DEB81-09682.

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