Alejandra Vasco

The evolution, morphology, and development of fern leaves

Leaves are lateral determinate structures formed in a predictable sequence (phyllotaxy) on the flanks of an indeterminate shoot apical meristem. The origin and evolution of leaves in vascular plants has been widely debated. Being the main conspicuous organ of nearly all vascular plants and often easy to recognize as such, it seems surprising that leaves have had multiple origins. For decades, morphologists, anatomists, paleobotanists, and systematists have contributed data to this debate. More recently, molecular genetic studies have provided insight into leaf evolution and development mainly within angiosperms and, to a lesser extent, lycophytes. There has been recent interest in extending leaf evolutionary developmental studies to other species and lineages, particularly in lycophytes and ferns. Therefore, a review of fern leaf morphology, evolution and development is timely. Here we discuss the theories of leaf evolution in ferns, morphology, and diversity of fern leaves, and experimental results of fern leaf development. We summarize what is known about the molecular genetics of fern leaf development and what future studies might tell us about the evolution of fern leaf development.

Challenging the paradigms of leaf evolution: Class III HD-Zips in ferns and lycophytes.

Despite the extraordinary significance leaves have for life on Earth, their origin and development remain vigorously debated. More than a century of paleobotanical, morphological, and phylogenetic research has still not resolved fundamental questions about leaves. Developmental genetic data are sparse in ferns, and comparative studies of lycophytes and seed plants have reached opposing conclusions on the conservation of a leaf developmental program. We performed phylogenetic and expression analyses of a leaf developmental regulator (Class III HD-Zip genes; C3HDZs) spanning lycophytes and ferns. We show that a duplication and neofunctionalization of C3HDZs probably occurred in the ancestor of euphyllophytes, and that there is a common leaf developmental mechanism conserved between ferns and seed plants. We show C3HDZ expression in lycophyte and fern sporangia and show that C3HDZs have conserved expression patterns during initiation of lateral primordia (leaves or sporangia). This expression is maintained throughout sporangium development in lycophytes and ferns and indicates an ancestral role of C3HDZs in sporangium development. We hypothesize that there is a deep homology of all leaves and that a sporangium-specific developmental program was coopted independently for the development of lycophyte and euphyllophyte leaves. This provides molecular genetic support for a paradigm shift in theories of lycophyte leaf evolution.

Phylogeny and classification of the Cuban species of Elaphoglossum (Dryopteridaceae), with description of Elaphoglossum sect. Wrightiana sect. nov.

Although a worldwide phylogeny of the bolbitidoid fern genus Elaphoglossum is now available, little is known about the phylogenetic position of the 34 Cuban species. We performed a phylogenetic analysis of a chloroplast DNA dataset for atpß-rbcL (including a fragment of the gene atpß), rps4-trnS, and trnL-trnF. The dataset included 79 new sequences of Elaphoglossum (67 from Cuba) and 299 GenBank sequences of Elaphoglossum and its most closely related outgroups, the bolbitidoid genera Arthrobotrya, Bolbitis, Lomagramma, Mickelia, and Teratophyllum. We obtained a well-resolved phylogeny including the seven main lineages recovered in previous phylogenetic studies of Elaphoglossum. The Cuban endemic E. wrightii was found to be an early diverging lineage of Elaphoglossum, not a member of E. sect. Squamipedia where it was previously classified. We propose a new section for this species: E. sect. Wrightiana. The early diverging position of E. wrightii is of particular interest because the species is a root climber (i.e., climbing from the soil on the lower portions of tree trunks and not losing its connection with the soil), a growth habit it shares with its closest bolbitidoid outgroup genera. This suggests that holoepiphytism evolved later in Elaphoglossum, and the primary hemiepiphytism of E. amygdalifolium, which is sister to the rest of the genus, was derived independently from ancestors that were root climbers. Based on our phylogenetic analysis and morphological investigations, the species of Cuban Elaphoglossum were found to occur in E. sects. Elaphoglossum, Lepidoglossa, Polytrichia, Setosa, and Squamipedia.

CRYPTOCHLOROPHYLLOUS SPORES IN FERNS: NONGREEN SPORES THAT CONTAIN CHLOROPHYLL

In terms of color, fern spores have traditionally been classified as either green or nongreen. This study reports a new kind of spore, here termed “cryptochlorophyllous.” Such spores contain chlorophyll but do not appear green when examined with either the naked eye or a light microscope. Epifluorescence microscopy was used to detect chlorophyll in cryptochlorophyllous spores. To test one aspect of the reliability of this method, we examined whether chlorophyll degradation over time would give false negative results. We found that in two species with green spores (Osmundastrum cinnamomeum and Terpsichore asplenifolia), fluorescence weakened with but could still be detected in specimens that were 110 yr old. We surveyed the spores of species in three fern genera under white light and with epifluorescence microscopy. Under white light, green spores were seen in 10 (26%) of 39 species of Elaphoglossum, two (17%) of 12 species of Lomariopsis, and seven (25%) of 28 species of Pleopeltis. When spores from the same species were examined with epifluorescence microscopy, fluorescence was detected in 22 (56%) of 39 species of Elaphoglossum, 10 (83%) of 12 species of Lomariopsis, and 12 (42%) of 28 species of Pleopeltis. We also found conflicting results in nine of the 33 species represented in our study by more than one specimen. In these species with conflicting results, at least one specimen fluoresced whereas the other(s) did not. The specimens that failed to fluoresce might have been collected and dried before the chloroplasts in its spores had developed, or they may represent infraspecific variation in this characteristic. Chlorophyll-containing spores have probably been overlooked in Elaphoglossum and Lomariopsis because the dark perispore obscures the green contents of the cell. In some species of Pleopeltis, such spores have probably been overlooked because of their paleness and the short duration of their greenness. Chlorophyll-containing spores are probably more widespread than is currently suspected, especially in clades known to contain both green and nongreen spores. Overall, the dichotomy of fern spores into green and nongreen is too simplistic. It must be investigated experimentally whether cryptochlorophyllous spores behave like visibly green spores in terms of germination and viability times.

Monograph of the Elaphoglossum ciliatum group (Dryopteridaceae)

The Elaphoglossum ciliatum group was supported by a previous study as monophyletic on the basis of cpDNA sequence data. The present paper provides a monograph of this group. It includes keys, full synonymy, descriptions, specimens examined, index to collectors’ names and numbers, photographs, and distribution maps for all species. Morphologically, these species are characterized by abaxial laminar scales reduced to minute dots, resinous rhizomes, and (except for E. dussii) spiny perispores. These characteristics are not unique to the E. ciliatum group because they are found in other species of Elapholgossum sect. Lepidoglossa. Nine species are here recognized in the E. ciliatum group, with no subspecies or varieties. Six species are neotropical and primarily montane, being largely absent from Amazonia. The remaining three species are endemic to the island of St. Helena in the South Atlantic Ocean. A lectoype is designated for E. calaguala.ResumenSe presenta la monografía del grupo de Elaphoglossum ciliatum, un grupo definido como monofilético basado en análisis de secuencias de ADN del cloroplasto. La monografía incluye claves, sinonimia completa, descripciones, especimenes examinados, un índice de colectores y sus números de colección, fotografías y mapas de distribución de todas las especies del grupo. Las especies de este grupo se caracterizan por tener las escamas de la superficie abaxial de la lámina reducidas a puntos muy pequeños, rizomas resinosos y (exceptuando E. dussii) perisporas equinadas. Sin embargo estas características no son únicas al grupo de E. ciliatum, y pueden encontrarse en otras especies de Elapholgossum sect. Lepidoglossa. Se reconocen nueve especies en el grupo de E. ciliatum, sin subespecies ni variedades. Seis especies son neotropicales y principalmente de regiones de montaña. El resto de las especies son endémicas de la isla de St. Helena en el sur del océano Atlántico. Se designa un lectotipo para E. calaguala.

Divided Leaves in the Genus Elaphoglossum (Dryopteridaceae): A Phylogeny of Elaphoglossum Section Squamipedia

Abstract Elaphoglossum is comprised almost entirely of epiphytic species with simple and entire leaves. Elaphoglossum section Squamipedia is intriguing because four of its species have dissected leaves. To generate a phylogenetic hypothesis of the relationships among all taxa belonging to section Squamipedia, we assembled a three-locus plastid dataset that included all recognized species in the section plus taxa representing all other sections of Elaphoglossum and three bolbitidoid genera. Our results support section Squamipedia as monophyletic. The species belonging to section Squamipedia are recovered in two well supported clades. The first clade includes two species endemic to Madagascar, whereas the second includes 16 species endemic to the Neotropics. Echinulate spores are synapomophic for the Neotropical species of section Squamipedia. Other characters that characterize most species of section Squamipedia are long-creeping rhizomes and absence of phyllopodia; however, two species of section Squamipedia (E. nidusoides and E. nidiforme) have phyllopodia and short-creeping rhizomes. The four species with dissected leaves belong to different clades and had independent origins from ancestors with simple, entire leaves. Dissected leaves have evolved at least six times during the history of Elaphoglossum. Elaphoglossum lloense and E. squamipes, two species defined on the basis of morphology, are not supported by our molecular analyses, being recovered in three and two different clades, respectively.

Elaphoglossum doanense and Elaphoglossum tonduzii: New Members of Elaphoglossum Sect. Squamipedia (Dryopteridaceae) and Their Significance for Inferring the Evolution of Rhizome Habit and Nest-Forming Leaves within the Genus

Premise of research. Elaphoglossum is a large fern genus with about 600 species. Our continuing fieldwork and molecular investigations have revealed a surprising placement of two Elaphoglossum species in sect. Squamipedia. The two species differ from most others in the section by short-creeping rhizomes and large leaves. Their placement in sect. Squamipedia has implications for the evolution of rhizome form and nest-forming habit within the genus. Methodology. Phylogenetic relationships within Elaphoglossum were investigated using a three-locus plastid data set. Sampling encompassed representatives of all major clades in the genus, including all recognized species of sect. Squamipedia. We included all species of Elaphoglossum with nest-forming habit from which material was available. To better understand the evolution of rhizome form and nest-forming habit, those characters were scored and optimized onto the resulting phylogenetic tree. Pivotal results. Elaphoglossum doanense and Elaphoglossum tonduzii are resolved in sect. Squamipedia. They are sister to each other, and they in turn are sister to the rest of the Neotropical species in the section. Long-creeping rhizomes and non-nest-forming habit are plesiomorphic in Elaphoglossum. There have been several transitions in rhizome form within Elaphoglossum, either from short-creeping rhizomes to long-creeping ones or vice versa. For sect. Squamipedia, it is equivocal what the plesiomorphic rhizome form was. The nest-forming habit has evolved independently in each of the major clades of Elaphoglossum, and in sect. Squamipedia it has evolved at least three times. Keys, illustrations, nomenclatural data, specimens examined, dot-distribution maps, and spore images are given for all species of sect. Squamipedia with short-creeping rhizomes. Conclusions. Three out of four members of sect. Squamipedia with short-creeping rhizomes have short-petiolate leaves forming a basket that traps organic matter falling from above. The only exception is E. tonduzii, which has longer petioles and leaves not forming a basket. The nest-forming habit is likely an adaptation to the epiphytic environment, being also known from other plant families.

Elaphoglossum gemmatum (Elaphoglossaceae), a new species from Colombia

Elaphoglossum gemmatum, a species of proliferous fern from Colombia, is described, illustrated, and compared to the most similar species,E. exsertipes, E. phyllitidis, andE. proliferans.