Edward L. Schneider

Phylogeny, Classification and Floral Evolution of Water Lilies (Nymphaeaceae; Nymphaeales): A Synthesis of Non-molecular, rbcL, matK, and 18S rDNA Data

The water lilies (Nymphaeaceae) have been investigated systematically for decades because they are believed to represent an early group of angiosperms with relatively unspecialized floral organization. Although this group is small taxonomically, the relationships among genera of water lilies have eluded clarification and no single classification has become widely accepted. We present a well-corroborated phy- logeny of water lily genera that is based on agreement between non-molecular data and DNA sequences obtained from both organellar and nuclear genomes. For specific portions of the resulting phylogeny, we evaluate the support conferred by each separate data set in comparison to various combinations. This ap- proach enabled us to assess the potential benefits of further data acquisition, and also allowed us to evaluate the fundamental advantages and disadvantages of each data partition. Every data set contributed differently to the overall phylogenetic analysis and resolution of the cladogram. The 18S rDNA performed the most poorly, with homoplasious sites confounding some topological assessments in comparisons of closely related genera. However, as taxonomic distance increased, phylogenetic signal in the 18S rDNA data increased due to the expression of sequence variation in highly conserved sites. Even the 18S rDNA data were relatively congruent with the other data evaluated, and the resulting combined data analysis rendered a single maxi- mum parsimony tree with strong nodal support throughout. When floral features were evaluated using this well-corroborated phylogeny, the pleiomerous condition of water lily flowers showed several instances of secondary derivations. Although the actual morphological details of the first water lily flowers remain un- certain, it is clear that the flowers of extant water lilies do not necessarily depict the ancestral organization. Results of the phylogenetic analysis are used to encourage the adoption of an evolutionarily based classifi- cation system for water lilies.

Vessels in ferns: structural, ecological, and evolutionary significance

We have studied macerated xylem of ferns, supplemented by sections, by means of scanning electron microscopy (SEM) in a series of 20 papers, the results of which are summarized and interpreted here. Studies were based mostly on macerations, but also on some sections; these methods should be supplemented by other methods to confirm or modify the findings presented. Guidelines are cited for our interpretations of features of pit membranes. Fern xylem offers many distinctive features: (1) presence of numerous vessels and various numbers of tracheids in most species; (2) presence of vessels in both roots and rhizomes in virtually all species; (3) presence of specialized end walls in vessels of only a few species; (4) multiple end-wall perforation plates in numerous species; (5) lateral-wall perforation plates in numerous species; (6) porose pit membranes associated with perforation plates in all species; and (7) pit dimorphism, yielding wide membrane-free perforations alternating with extremely narrow pits. Multiple end wall perforation plates and lateral wall perforation plates are associated with the packing of tracheary elements in fascicles in ferns: facets of tips of elements contact numerous facets of adjacent elements; all such contacts are potential sites for conduction by means of perforations. This packing differs from that in primary xylem of dicotyledons and monocotyledons. Porosities in pit membranes represent a way of interconnecting vessel elements within a rhizome or root. In addition, these porosities can interconnect rhizome vessel elements with those of roots, a feature of importance because roots are adventitious in ferns as opposed to those of vascular plants with taproots. Fully-formed or incipient (small-to-medium sized porosities in pit membranes) perforation plates are widespread in ferns. These are believed to represent (1) ease of lysis of pit membranes via pectinase and cellulase; (2) numerous potential sites for perforation plate formation because of fasciculate packing of tracheary elements; (3) evolution of ferns over a long period of time, so that lysis pathways have had time to form; (4) lack of disadvantage in perforation plate presence, regardless of whether habitat moisture fluctuates markedly or little, because ferns likely have maintaining integrity of water columns that override the embolism-confining advantage of tracheids. Although all ferns share some common features, the diversity in xylem anatomy discovered thus far in ferns suggests that much remains to be learned.

Cryptic species in an endangered pondweed community (Potamogeton, Potamogetonaceae) revealed by AFLP markers

Cryptic species are morphologically indistinguishable, yet reproductively isolated. Morphological boundaries between species can also be obscured by hybridization and clonality. Determining the roles of reproductive isolation, hybridization, and clonality in morphologically indistinguishable taxa is essential to determining appropriate species-level taxonomic rankings for conservation purposes. The taxonomic status of the endangered Little Aguja pondweed of west Texas, Potamogeton clystocarpus, is uncertain due to a lack of fixed morphological differences between it and two sympatric congeners. Morphology, amplified fragment length polymorphisms (AFLPs), and sequences of the internal transcribed spacer (ITS) region and trnL-F intron and spacer were used to determine the degree of genetic distinctiveness, hybridization and clonality for this rare species. AFLPs indicate that P. clystocarpus is a genetically distinct lineage compared to P. pusillus and P. foliosus. No hybrids involving P. clystocarpus were detected, but two putative hybrids involving P. pusillus and P. foliosus were identified. Clonal growth was only detected in P. pusillus. A combination of morphological and molecular markers was successful in determining the genetic distinctiveness of an endangered cryptic species, Potamogeton clystocarpus. Further sampling in this and adjacent drainages is necessary to assess the degree of endemism of P. clystocarpus and confidently rule out hybridization and clonality in this taxon.

The tracheid–vessel element transition in angiosperms involves multiple independent features: cladistic consequences

Current definitions of tracheids and vessel elements are overly simple. These definitions are based on light microscope studies and have not incorporated information gained with scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Current definitions are based primarily on angiosperms, especially eudicots, and were devised before many basal angiosperms were carefully studied. When all sources of information are taken into account, one can recognize changes in six characters in the evolution of tracheids into vessel elements in angiosperms (or vice versa) as well as in other groups of vascular plants. There is an appreciable number of taxa in which all criteria for vessel origin are not met, and thus incipient vessels are present. At the very least, vessel presence or absence should not be treated as a single binary character state change in construction of cladistic matrices. Increase in conductive area of an end wall by means of lysis of progressively greater areas of pit membrane and increase in pit area on the end wall (as compared to pit area on equivalent portions of lateral walls) are considered the most important usable criteria for recognizing intermediacy between tracheids and vessel elements. Primitive character states in vessel elements are briefly discussed to differentiate them from changes in character states that can be regarded as intermediate between tracheids and vessel elements.

Floral Development of Nelumbo nucifera (Nelumbonaceae)

The floral development of Nelumbo nucifera was compared with that noted in previous studies of Nelumbo, Nymphaeaceae, and other basal angiosperms. Important features include developmental evidence of only two sepals, development of an androecial ring meristem, and an apocarpous gynoecium composed of ascidiate carpels that become embedded in an expanded receptacle. Secretory papillate trichomes cover the stigma and line the stylar canal. The unique apocarpous gynoecium, which lacks a conical residual floral apex, and a greatly expanded receptacle distinguish Nelumbonaceae from the Nymphaeaceae, as does the distinctive androecial ring. Nelumbo is characterized by polysymmetric floral development, with some organs originating spirally (petals) and some in simultaneous whorls (stamens and carpels). This pattern of floral development, as well as the pattern of carpel closure by secretion, is common in several paleoherbs and eudicots and indicates phylogenetic affinity between Nelumbonaceae and basal angiosperms. Because of its unique floral development and anatomy, Nelumbo appears to be an isolated member of the eudicot clade.

Floral development in the Nymphaeales

Numerous phylogenetic analyses and recent paleobotanical findings continue to support the inclusion of Nymphaeales as basal or near‐basal extant angiosperms. The distinctive differences in floral morphology and habit between non‐Nymphaealian taxa of the ANITA grade compared with water lilies, nevertheless, remain perplexing. In this investigation, scanning electron microscope observations on Brasenia (Cabombaceae), Nuphar, Nymphaea, Ondinea, Victoria, and Euryale (Nymphaeaceae) are added to the observations of earlier investigations to enhance our understanding of early floral evolution and shifts in floral ontogeny and to contribute new data for improved cladistic analyses. The enlargement of flowers and amplification of floral parts distinctive in Nymphaeaceae is based on continuing surface area enlargement of the floral apex, allowing numerous organs to be produced acropetally in distinctive parastichies, compared with the ring meristem found in some primitive eudicots (Nelumbonaceae, Papaveraceae) that increase floral organ number but in dense pulses or clusters without clear order. The smaller flowers of the Cabombaceae and their lower number of floral organs are now regarded as more typical of the ancestral angiosperm flower. One may conclude that changes in the size of the floral apex and its geometric conformation alter flower size, number of parts, and phyllotaxy. The ebracteate condition and herbaceous, aquatic habit of Nymphaeales when compared with Archaefructus, a recently discovered herbaceous aquatic angiosperm from the Upper Jurassic/Lower Cretaceous that is proposed as a new basal angiosperm family, continue to question the relationship of early angiosperms to the aquatic habitat. Is the ebracteate state significant, and how did the shift to aquatic habitats impact floral form of early angiosperms?

Phylogenetic interpretations from selected floral vasculature characters in the Nymphaeaceae sensu lato

Abstract The Nymphaeales are assigned a key, basal, pivotal evolutionary position in nearly every old and modern system of classification. Over the past three decades many of the studies on this plant group have focused on floral aspects, especially vasculature and developmental studies. In this investigation, literature on the floral vasculature of the taxon has been reviewed with 12 characters selected for cladistic analysis. Results of phylogenetic analysis of floral vascular features are in agreement with other sets of data (e.g. morphological, biochemical, molecular) in retaining the Nelumbonales as a separate order from the Nymphaeales. Floral vasculature features offer less resolution, however, of relationships within the Nymphaeales.

Vessels in Nymphaeaceae: Nuphar, Nymphaea, and Ondinea

Roots of Nuphar, Nymphaea, and Ondinea were studied with SEM in order to determine presence of perforations in end walls of tracheary elements. Nuphar has occasional small perforations and Nymphaea has a scattering of small perforations, but no perforations were observed in Ondinea. In Nuphar and Nymphaea, metaxylem elements have helical-reticulate wall thickenings, but end walls of tracheary elements have at least some scalariform pitting; in Ondinea, end walls are identical to side walls. In all three genera, striations (thickenings) in primary walls of tracheary elements are common, as in Euryale and Victoria.

SEM studies on vessels in ferns. 2. Pteridium.

Xylem from roots and rhizomes of two infraspecific taxa of Pteridium aquilinum was studied by means of scanning electron microscopy (SEM). All tracheary elements proved to be vessels. End wall perforation plates were all scalariform, lacked pit membrane remnants in at least the central part of the perforation plate, and varied with respect to width of bars, from wide to tenuous, and with respect to presence of pit membrane remnants. In addition, porose pit membranes on walls that are likely all lateral vessel-to-vessel walls must be considered to be perforations also, although different from those on end walls. Lateral wall perforation plates, hypothesized by one worker on the basis of tylosis presence but denied by another on the basis of light microscopy, were confirmed by demonstration of pores with SEM. In addition, lateral walls of Pteridium vessels bear some grooves interconnecting pit apertures; this feature is newly figured by SEM for ferns. Lateral wall pitting that is not porose may either have striate thickenings of the primary wall or be smooth. Vessel presence and degree of specialization in Pteridium vessels may bear a relationship to the wide ecological tolerances of the genus.

Tracheary Elements in Ferns: New Techniques, Observations, and Concepts

ABSTRACT Longisections of xylem were studied with scanning electron microscopy (SEM) for roots of Angiopteris, upright axes of Psilotum, and rhizomes of eight species of leptosporangiate ferns of diverse habits and varied ecological preferences. In contrast to earlier studies using macerations, razor-blade sections of fixed material from living plants were prepared. All materials studied showed porose or reticulate pit membranes present on presumptive end walls of tracheids. Contrasting non-porose pits were observed on lateral walls of some tracheids. Tracheid to parenchyma pit pairs may have porose pit membranes on the tracheid side and nonporose pit membranes on the parenchyma side; thus degree of porosity in a section can represent the degree to which one primary wall or the other is pared away. Reticulate pit membranes on tracheary element end walls are evidently widespread in ferns. Such cells should not be considered vessel elements, although the reticulate pit membranes suggest a degree of transition toward the membrane-free perforations of typical vessels. True vessels (pit membranes absent in perforations) do occur in roots in a limited number of fern genera. The preparation methods of the present study produced results freer from artifacts than did macerations, and interpretations must be altered accordingly. Reports of lateral, multiple, and interrupted perforation plates in ferns are probably the result of loss of pit membranes due to the oxidative action of maceration and should be rejected. Likewise, “pit dimorphism” (alternately wide and narrow pits) and “striate” (corrugated) pit membranes in ferns represent artifacts. True vessel elements in ferns probably always have secondary wall architecture of end walls different from that of lateral walls.