Received Date: May 26, 2017; Accepted Date: August 08, 2017; Published Date: August 11, 2017
Citation: Miao Y, Liu ZJ, Wang M, Wang X (2017) Fossil and Living Cycads Say No More Megasporophylls. J Morphol Anat 1: 107.
Copyright: © 2017 Miao Y, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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The origins of angiosperms and cycads are still mysterious. To understand the evolution of these groups as well as other gymnosperms it was impossible without mentioning a frequently used term “megasporophyll”. “Megasporophyll” is a concept that has been used widely in botany. This term is more or less related with the famous saying “Alles ist Blatt” by Goethe. This term became popular since Arber and Parkin hypothesized that the carpels in the Magnoliales were equivalent to and derived from former foliar parts bearing ovules along their margins (“megasporophyll”). Many botanists uncritically called the parts in all the reproductive organs of seed plants as “sporophylls”, no matter what they actually saw in the plants. However, the fact is that none of the reproductive parts (fossil or living), except those in the Cycadales, are foliar or leaf-like. The female parts in (fossil and extant) Ginkgoales, Coniferales, and Gnetales apparently have nothing to do with any foliar parts, as proven by previous studies. Among the living gymnosperms, Cycadales usually are taken as the most primitive and ancestral, therefore understanding the reproductive organs and their evolution in Cycadales will not only enhance our understanding of Cycadales, but also is hinged with our understanding of all seed plants in general. Interestingly, the female parts of Cycadales are the ones that demonstrate by far the greatest resemblance to leaves, at least in appearance. Thus whether the female parts of Cycadales are truly foliar is a key but rarely asked question hinged with the validity of the term “megasporophyll” and the systematics of seed plants. To verify the validity of this term and its implications, we examined the morphology of both fossil and extant cycads. Our fossil evidence includes the earliest unequivocal fossil of cycad reproductive organ, Crossozamia chinensis (Zhu and Du) Gao and Thomas, recovered from the Permian of China. Unlike seen in living cycads, the ovules in this fossil reproductive organ are apparently inserted on the adaxial rather than strictly the laterals of the female parts. Such an arrangement is not expected for a typical leaf, but rather demonstrates certain resemblance to the sporangia arrangement in Archaeopteris. Parallel to and agreeing with this, the ovules in living cycads, Cycas taitungensis Shen, Hill, Tsou and Chen, have their micropyles oriented to the adaxial rather than the laterals of the parts. Taking into consideration of previous experiment proving that the leaf-like appearance of Cycas “megasporophylls” is due to mechanical pressure and unexpected occurrence of amphicribral vascular bundles with secondary growth in the so-called “megasporophylls” of cycads, we conclude that the female parts of these Cycadales are actually branches bearing ovules. This conclusion rejects foliar nature of female parts in cycads and undermines the validity of the term “megasporophylls”, although the latter has been the most-widely accepted misnomer in botany. The by-product of eliminating this term is that the origins of Cycadales and angiosperms as well as homology of carpels become much easier than assumed before. Considering the long time the term “megasporophyll” inflicting botany and misleading botanists, the influence of eliminating this misnomer in botany cannot be exaggerated. It is not restricted to a single term and its usage, but permeates into all branches of botany, especially the systematics of Cycadales, Angiosperms, and other seed plants.
Morphology; Megasporophyll; Cycas sexseminifera; Mechanical pressure
Cycads are the oldest group among the extant seed plants . Their long history dated back to the Permian and basalmost position among extant seed plants make cycads important and more insightful for seed plant evolution. Therefore cycads have been the focus of many botanical studies. “Megasporophyll” is a term frequently used to describe the female parts of gymnosperms, and carpels of angiosperms are frequently interpreted as metamorphosed Megasporophylls [2,3]. The term “Megasporophylls” per se implies that it is essentially a leaf in nature. However, this implication appears spurious in non-cycad gymnosperms (at least including Ginkgoales, Coniferales, Gnetales, Bennettitales, Caytoniales, etc.), the female parts of which are far from leaf-like in morphology [4-6]. “Megasporophylls” seems to find a foothold for its rationality in Cycas, the female parts of which appear leaf-like. Previously, doubt has been cast on the foliar nature of female parts in Cycas [7,8]. In their developmental experiment, demonstrated that the female part of Cycas sexseminifera, when growing free of pressure from its adjacent peers, has its ovule shift to the adaxial rather than remain lateral in other naturally growing female parts in the same plant, implying that the leaf-like appearance of Cycas female parts is a consequence of mechanical pressure exerted by their neighbouring peers . However, this conclusion is largely ignored probably due to inertia of thinking. To further enquire the nature of female parts in Cycas, we re-investigate the female parts of the oldest fossil cycad from the Permian of China and extant Cycas growing both in natural and man-made conditions. Both fossil and extant cycads reject the foliar nature formerly assumed for cycad female parts.
The fossil specimens investigated here have been reported before [9,10]. These coalified compression specimens were collected from the Xiashihezi Formation (late Early Permian, >272 Ma) of Dongshan, Taiyuan, China in 1980s. Our re-investigation confirms the conclusion that these fossils currently represent the oldest unequivocal cycads and they are closely comparable to living Cycas. Furthermore formerly ignored important features of the fossils are revealed this time.
Cycas taitungensis Shen, Hill, Tsou and Chen is a cycad endemic to eastern region of Taiwan, China. Our materials were collected from an individual grown in National Orchid Conservation Center of China.
The general morphology of the specimens was recorded with digital camera, and the details are photographed with stereomicroscopes equipped with digital cameras. All pictures were organized using a Photoshop 7.0 for publication.
Just like in extant Cycas, a fossil female part of Crossozamia chinensis [9,10] includes a terminal lamina and a pedicel that bears ovules/seeds on its sides (Figure 1a-1c). Different from the idealized “megasporophyll”, ovules/seeds on the same side are not arranged in two strict ranks (Figure 1d and 1e). Instead one of the funiculi of two adjacent ovules in the female part in overlaps the other, suggestive of different ovule orientations and that one is more oriented to the adaxial than the other. This implication from this specimen is confirmed by observation on more specimens. Several ovules/seeds of Crossozamia chinensis [9,10] are directly inserted on the adaxial surface of the female part. The ovule in is obviously located on the surface, not margins, of the pedicel, as indicated by the presence of sediment between the ovule funiculus and the pedicel. Furthermore, a more mature seed is apparently attached to the surface of a female part (Figure 1f and 1i). Parallel to these, an ovule in (Figures 1g and 1h) is apparently located on the surface rather than along the margin of the female part. These observations suggest that the formerly assumed two dimensional foliar morphology assumed for fossil cycad female parts is an illusion resulted from over-simplification and careless observation. Instead the female parts of these fossil cycads are axial structure with three dimensional configurations.
The aforesaid conclusion on the nature of fossil cycads is further strengthened by observations on extant cycads, either grown in natural or man-made condition. All ovules and seeds in the strobilus of C. taitungensis growing naturally are apparently consistently oriented to the adaxial of female parts (Figure 2a). This orientation is similar to that seen in the above fossil cycads. Cycas rumphii is another cycad frequently seen in south-eastern Asia, northern Australia, and eastern Africa. Although there is certain variation in ovule orientation in the female parts of naturally grown C. rumphii, at least some of the ovules are on the lateral of the female parts  as expected for foliar Megasporophylls. To test whether such arrangement is due to mechanic pressure (as suggested by Wang and Luo, we perform similar manipulation on C. rumphii, namely, removing adjacent female parts to let the remaining female parts grow freely. As seen in Figure 1f and 1i of all ovules of Cycas rumphii unexceptionally turn to the adaxial of the female parts (Figure 2b and 2c). Considering morphology of plant part is a function of its genetics and available space , we assume that plant parts growing in pressure-free environment, where genetics overwhelms, tend to display their “real” morphology. Observations on both living Cycas taxa concurrently suggest that the ovules in the precursor of cycads are most likely attached to the adaxial of female parts, just as seen in the above fossil cycads.
Our observations on both extant and living cycads reject the foliar nature previously assumed for cycad female parts, which are frequently termed as “megasporophyll” [1,3,9]. The term “megasporophyll” implies that female parts in seed plants are foliar in nature. Theoretically, a foliar organ (phyllome) should have bilateral symmetry (ad/abaxial polarity) . Previous works have indicated that female parts in Ginkgoales, Coniferales, Gnetales, Bennettitales, and Caytoniales are not foliar or leaf-like [4,5,6,14]. Among Cycadales, female parts in Zamiaceae are not leaf-like, either . All these make the commonly-used term “megasporophyll” spurious. The only female parts that demonstrate by far the greatest resemblance to leaves are those of Cycas, which is the last foothold for the rationality of “Megasporophylls”. Now both fossil and extant cycads consistently point to the non-foliar nature of cycad female parts. Actually, incipient sign of invalidity of this term occurred long time ago. For example, concentric bundles (some even with secondary growth), abnormal ovule arrangement in cycad  and three dimensional vascular bundles branching in cycad Megasporophylls  have cast doubt over the foliar nature of cycad female parts. Back to 1963, Meeuse cast doubt over the foliar nature of female parts in gymnosperms and suggested to replace “Megasporophylls” with “megasporocladode” . In short, all evidence converges to the same conclusion that the term “Megasporophylls” is apparently a misnomer that should be discarded in botany.
The removal of the term “Megasporophylls” paves the road for better understanding on origin and evolution of cycads and other seed plants. The assumed foliar nature of cycad female parts makes cycad ancestor mysterious, especially when the Phasmatocycas-based hypothesis is ruled out . Searching for a precursor of the assumed foliar female parts in early land plants is now proven in vain, at least partially because all parts in early land plants are axes or axis-derived structures. However, this conundrum vaporizes when a female part of Cycas is interpreted as an axis or shoot. The fertile parts as that of Archaeopteris with sporangia borne on their adaxial [18,19] may give rise to Cycas female parts simply by shifting its adaxial sporangia to more lateral positions, owing to mechanical pressure or whatever other reasons. In this way, the origin of cycads seems to be in the reach of botanists. At the same time, female parts of all seed plants become homologous and comparable, making the evolutionary relationship among them easy to decipher. For example, the homology of angiosperm carpels has been a headache question for many, but this problem disappear when a “carpel” is separated into a placenta (ovulebearing branch) and enclosing foliar part (carpel wall or ovarian wall). This interpretation has been proposed before [20,21] and is favored by recent studies on magnoliaceous carpels [22,23]. Apparently, the eradication of “megasporophyll” paves the road leading to more reasonable understanding of plant evolution and systematics.
A female part of Crossozamia chinensis (Zhu and Du) GAO and Thomas (Gao and Thomas 1989; Zhu and Du 1981) with a fan-shaped distal lamina and several ovules (arrows) attached to its pedicel (P). BMNH115131. Bar=10 mm (Figure 1a).
Two partially preserved female parts (separated by white line) with an attached ovule/seed of Crossozamia chinensis (Zhu and Du) GAO and Thomas (Gao and Thomas 1989; Zhu and Du 1981). Note a seed (blue arrow) attached to a lamina segment and an ovule (red arrow) attached to the pedicel (P) of another female part. GP0001. Bar=10 mm (Figure 1b).
Detailed view of the ovule arrowed. Note the funiculus (between the blue arrows) of the ovule (O) is directly inserted on the surface at a position far from the lamina/pedicel margin (black arrow). Bar=0.5 mm (Figure 1c).
Two ovules physically connected to the same pedicel (P). Bar=1 mm (Figure 1d).
Detailed view of two funiculi of ovules in Figure 1b. Note one of them (blue arrow) overlaps the other (black arrow). Bar=0.5 mm (Figure 1e).
Detailed view of red-arrowed ovule Note the ovule (O) is above the surface of the pedicel, a piece of sediment (white arrow) is between the funiculus (F) and pedicel (P) margin (black arrow), suggestive of surface attachment of the ovule. Bar=1 mm (Figure 1f).
Detailed view of the seed blue-arrowed connected to the lamina segment. Bar=2 mm (Figure 1g).
A female part with a typical fan-shaped distal lamina, a pedicel (P), and an ovule (blue arrow) of Crossozamia chinensis (Zhu and Du) GAO and Thomas (Gao and Thomas 1989; Zhu and Du 1981). BMNH114993. Bar=10 mm (Figure 1h).
The same seed (S) as is showing its physical connection (arrow) to the lamina segment. Viewed following the direction of the arrow in Fig. 1f, after warped certain degrees. Bar=2 mm (Figure 1i).
Several female parts of Cycas taitungensis Shen, Hill, Tsou and Chen, with their ovule/seed tips (arrows) pointing to the adaxial (Figure 2a).
Adaxial surface view of a female part of Cycas rumphii, which grows free of peer pressure, with four ovules pointing to the adaxiallateral (Figure 2b).
Side view of the part showing the ovule tips (arrows) pointing to the adaxial (to the right) (Figure 2c).
Although widely-used in botany, “megasporophyll” is a misnomer in botany. This is suggested by both fossil and extant cycads. Eliminating this misleading term will help to decipher the mysteries about the origins of cycads and angiosperms.
This research is supported by the National Natural Science Foundation of China (41688103, 91514302, 91114201), the Strategic Priority Research Program (B) of Chinese Academy of Sciences (Grant No. XDB18000000) awarded to X.W.; and State Forestry Administration of China (No. 2005–122), Science and Technology Project of Guangdong (No. 2011B060400011), and Special Funds for Environmental Projects of Shenzhen (No. 2013-02) awarded to Z. J. L.