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Megaphylls, Microphylls and the Evolution of Leaf Development

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Megaphylls, Microphylls and the Evolution of Leaf Development Opinion Megaphylls, microphylls and the evolution of leaf development Alexandru M.F. Tomescu Department of Biological Sciences, Humboldt State University, Arcata, CA 95521, USA Originally coined to emphasize morphological differ- However, the microphyll–megaphyll divide is not as ences, ‘microphyll’ and ‘megaphyll’ became synon- clear cut, and morphological definitions that contrast ymous with the idea that vascular plant leaves are not microphylls and megaphylls as mutually exclusive con- homologous. Although it is now accepted that leaves cepts of leaves are inconsistent. Moreover, current under- evolved independently in several euphyllophyte standing of plant phylogeny and leaf development fails to lineages, ‘megaphyll’ has grown to reflect another type shed light on the origin of microphylls, and supports of homology, that of euphyllophyte leaf precursor struc- several independent origins of megaphylls. Here, I review tures. However, evidence from the fossil record and plant phylogeny and developmental data from fossil and developmental pathways fails to indicate homology extant trachephytes, as well as the current understanding and suggests homoplasy of precursor structures. Thus, of genetic pathways controlling leaf development, to argue as I discuss here, ‘megaphyll’ should be abandoned that the megaphyll concept should be abandoned because because it perpetuates an unsupported idea of it perpetuates misconceptions and confusion based on homology, leading to misconceptions that pervade plant unsupported homology. biology thinking and can bias hypothesis and inference in developmental and phylogenetic studies. Alternative Morphological inconsistencies and overlap in the definitions are needed that are based on development microphyll–megaphyll dichotomy and phylogeny for different independently evolved leaf Microphylls are defined as leaves of small size, with simple types. venation (one vein) and associated with steles that lack leaf gaps (protosteles). By contrast, megaphylls are defined as The microphyll–megaphyll dichotomy leaves of generally larger size, with complex venation and In vascular plant sporophytes, leaves are lateral appen- associated with leaf gaps in the stele [3]. However, each of dages that share four defining features: vascularization, these criteria has inconsistencies [4] that highlight the determinate growth, bilateral symmetry (adaxial–abaxial disconnection between phylogeny and current morphologi- polarity; hereafter referred to as ad/abaxial polarity) and cal definitions at the level of the microphyll–megaphyll definite arrangement (phyllotaxis). Despite these shared divide. characteristics, leaves are not homologous across all vas- First, the presence or absence of leaf gaps does not cular plants. An early step toward the recognition of this provide a basic distinction between the two types. In many situation [1] was the defining of a major dichotomy be- plants nested among the megaphyll-bearing euphyllo- tween microphylls, small and simple leaves, and mega- phytes, leaf trace divergence is not associated with a leaf phylls, larger and more complex leaves. The dichotomy, gap (e.g. the non-protostelic Equisetum, cladoxylalean based on morphological criteria, was reinforced as sub- pteridophytes and all extant seed plants [5]). Moreover, sequent progress in paleobotany led to the realization that several groups of euphyllophytes are similar to microphyll- vascular plant phylogeny was itself divided by a major bearing lycophytes in that they have protosteles (e.g. the dichotomy [2], dating back 415 million years to the Late extant filicalean ferns Lygodium and Gleichenia, fossil Silurian–Early Devonian, that paralleled, to some extent, Kaplanopteris and Botryopteris, some extinct coenopteri- the taxonomic distribution of the two leaf types. The two dalean ferns and sphenophyllales, the aneurophytalean major lineages of that phylogenetic divide originated from progymnosperms, and even early seed plants such as among two distinct grades of early vascular plants, the Elkinsia). zosterophylls and the trimerophytes. The descendants of Second, leaf size and venation complexity are also those two lineages form the two clades that comprise most inconsistent as distinguishing criteria between microphylls vascular plant phylogeny, the lycophytes and the euphyl- and megaphylls. Equisetum and some fossil sphenophyl- lophytes, and which were regarded as having evolved lales, as well as several extant and fossil gymnosperms, all of microphylls and megaphylls, respectively. These ideas which are megaphyllous euphyllophytes, have highly provided the impetus and framework for an early wave reduced leaves supplied by one vein. Conversely, some of thinking that would later become known as evo-devo, lycophyte microphylls are large (up to 1 m in some of the which led to the formulation of seminal hypotheses of leaf extinct lepidodendrales or up to 0.5 m in extant Isoetes evolution. species), whereas others have complex venation patterns (some Selaginella species [6]) or morphologies (e.g. the dis- Corresponding author: Tomescu, A.M.F. ([email protected]). sected leaves of extinct protolepidodendraleans such as 1360-1385/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2008.10.008 Available online 11 December 2008 5 Opinion Trends in Plant Science Vol.14 No.1 Colpodexylon, Protolepidodendron, Leclerqia and Estinno- precursor structure at the origin of each, as reflected by phyton [7]). their respective leafless sister groups. That requires inclusion of as many fossil taxa as possible in Phylogenetic and developmental perspectives morphology-based phylogenies. In lycophytes, three com- Definitions based on morphology thus seem to create more peting hypotheses propose evolution of the leaf (‘micro- confusion and overlap between the microphyll and mega- phyll’): (i) by vascularization of enations; (ii) from phyll concepts instead of clarifying them. The validity of sporangia by sterilization; or (iii) from telome trusses by the two concepts needs to be verified against comparative reduction [7,8]. However, although monophylly of the developmental data, genetic developmental pathways con- group is generally accepted, neither the scarce data on trolling leaf development and plant phylogenies that illu- genetic pathways controlling leaf development [15,16] nor minate the relationships between extant taxa and their phylogenetic studies [8] lend unequivocal support to any of closest leafless relatives in the fossil record. Leaf evolution the three hypotheses. was the earliest major plant biology question to be Our understanding of euphyllophyte phylogeny is ham- addressed using a comprehensive evo-devo approach. pered by major disparities between the results of studies Work on leaf evolution integrated all the data types avail- based exclusively on extant taxa and those of studies that able (morphology, anatomy, development, phylogenetic include fossils [8–12]. ‘Extant-only’ studies support mono- hypotheses and the fossil record) to produce consistent phylly of extant seed-free euphyllophytes (the ‘monilo- theories. Such pre-molecular era evo-devo approaches phyte’ clade), whereas studies including fossils support a led to several hypotheses for the evolution of leaves. Of sequence of paraphyletic grades comprised of extant and those, two prevailing hypotheses explained the origin of fossil seed-free euphyllophytes that led to the seed plants. microphylls from enations (small unvascularized flaps of Although they have been gaining wide acceptance, the tissue that characterized early vascular plants), and that of results of extant-only studies are equivocal to the question megaphylls from three-dimensional (3D) branching sys- of euphyllophyte leaf origin because they lack fossil taxa tems of undifferentiated photosynthetic axes [7]. that: (i) could represent transitional stages bearing pre- Most modern phylogenies support the lycophyte– cursor structures in transformational series; and (ii) gen- euphyllophyte divide [8–12], but phylogenetic studies have erate the amount of phylogenetic resolution needed to so far failed to provide unequivocal answers on the nature answer such a question. Nevertheless, the fossil record of the ancestral structures and evolutionary processes that shows unequivocally that the common ancestor of ‘mega- generated the lycophyte leaf and have produced conflicting phyllous’ euphyllophytes was leafless. This indicates that, hypotheses of euphyllophyte phylogeny [9,10,12]. A devel- among euphyllophytes, leaves are not homologous because opmental feature potentially congruent with the lyco- they evolved independently in at least two lineages or, as phyte–euphyllophyte dichotomy could be the mode of indicated by phylogenies that include fossils, in as many as origination of leaf primordia on the flanks of the shoot nine different lineages. A close look at leaf development apical meristem (SAM). A study of Selaginella [13] led to and the genetic pathways that control it across euphyllo- the idea that lycophyte leaves originate from a few initials phytes provides strong support for homoplasy of the leaf specified exclusively in the outermost cell layer of the SAM; across euphyllophytes. this is in contrast with euphyllophyte leaves, which originate from larger populations of initials to which both How many different ‘megaphylls’ are there? the outermost cell layer, as well as subjacent layers, con- Multiple lines of evidence indicate that leaves that have tribute. The mode of origination
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