Browsing by Subject "heterospory"

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    Assessing phylogenetic relationships in extant heterosporous ferns (Salviniales), with a focus on Pilularia and Salvinia
    (Botanical Journal of the Linnean Society, 2008-08-01) Nagalingum, NS; Nowak, MD; Pryer, KM
    Heterosporous ferns (Salviniales) are a group of approximately 70 species that produce two types of spores (megaspores and microspores). Earlier broad-scale phylogenetic studies on the order typically focused on one or, at most, two species per genus. In contrast, our study samples numerous species for each genus, wherever possible, accounting for almost half of the species diversity of the order. Our analyses resolve Marsileaceae, Salviniaceae and all of the component genera as monophyletic. Salviniaceae incorporate Salvinia and Azolla; in Marsileaceae, Marsilea is sister to the clade of Regnellidium and Pilularia - this latter clade is consistently resolved, but not always strongly supported. Our individual species-level investigations for Pilularia and Salvinia, together with previously published studies on Marsilea and Azolla (Regnellidium is monotypic), provide phylogenies within all genera of heterosporous ferns. The Pilularia phylogeny reveals two groups: Group I includes the European taxa P. globulifera and P. minuta; Group II consists of P. americana, P. novae-hollandiae and P. novae-zelandiae from North America, Australia and New Zealand, respectively, and are morphologically difficult to distinguish. Based on their identical molecular sequences and morphology, we regard P. novae-hollandiae and P. novae-zelandiae to be conspecific; the name P. novae-hollandiae has nomenclatural priority. The status of P. americana requires further investigation as it consists of two geographically and genetically distinct North American groups and also shows a high degree of sequence similarity to P. novae-hollandiae. Salvinia also comprises biogeographically distinct units - a Eurasian group (S. natans and S. cucullata) and an American clade that includes the noxious weed S. molesta, as well as S. oblongifolia and S. minima. © 2008 The Linnean Society of London.
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    Marsileaceae sporocarps and spores from the late cretaceous of Georgia, U.S.A.
    (International Journal of Plant Sciences, 2000-01-01) Lupia, R; Schneider, H; Moeser, GM; Pryer, KM; Crane, PR
    A new species provisionally assigned to the extant genus Regnellidium Lindm. (Regnellidium upatoiensis sp. nov.) is established for isolated sporocarps assignable to the heterosporous water fern family Marsileaceae. Three sporocarps and hundreds of dispersed megaspores were recovered from unconsolidated clays and silts of the Eutaw Formation (Santonian, Late Cretaceous) along Upatoi Creek, Georgia, U.S.A. The sporocarps are ellipsoidal and flattened, contain both megasporangia and microsporangia, and possess a two-layered wall - an outer sclerenchymatous layer and an inner parenchymatous layer. In situ megaspores are spheroidal, with two distinct wall layers - an exine, differentiated into two layers, and an outer ornamented perine also differentiated into two layers. The megaspores also possess an acrolamella consisting of six (five to seven) triangular lobes that are twisted. In situ microspores are trilete and spheroidal, with a strongly rugulate perine, and show modification of the perine over the laesura to form an acrolamella. Comparison of the fossil sporocarps with those of four extant species of Marsileaceae reveal marked similarity with Regnellidium diphyllum Lindm., particularly in megaspore and microspore morphology. If found dispersed, the in situ megaspores would be assigned to Molaspora lobata (Dijkstra) Hall and the microspores to Crybelosporites Dettmann based on their size, shape, and ornamentation. Regnellidium upatoiensis sp. nov. extends the stratigraphic range of the genus back to the Santonian, nearly contemporaneous with the first evidence of Marsilea, and implies that the diversification of the Marsileaceae into its extant lineages occurred in the mid-Cretaceous.
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    Phylogeny and divergence time estimates for the fern genus Azolla (Salviniaceae)
    (International Journal of Plant Sciences, 2007-10-22) Metzgar, JS; Schneider, H; Pryer, KM
    A phylogeny for all extant species of the heterosporous fern genus Azolla is presented here based on more than 5000 base pairs of DNA sequence data from six plastid loci (rbcL, atpB, rps4, trnL-trnF, trnG-trnR, and rps4-trnS). Our results are in agreement with other recent molecular phylogenetic hypotheses that support the monophyly of sections Azolla and Rhizosperma and the proposed relationships within section Azolla. Divergence times are estimated within Azolla using a penalized likelihood approach, integrating data from fossils and DNA sequences. Penalized likelihood analyses estimate a divergence time of 50.7 Ma (Eocene) for the split between sections Azolla and Rhizosperma, 32.5 Ma (Oligocene) for the divergence of Azolla nilotica from A. pinnata within section Rhizosperma, and 16.3 Ma (Miocene) for the divergence of the two lineages within section Azolla (the A. filiculoides + A. rubra lineage from the A. caroliniana + A. microphylla + A. mexicana complex). © 2007 by The University of Chicago. All rights reserved.
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    Structure and function of spores in the aquatic heterosporous fern family Marsileaceae
    (International Journal of Plant Sciences, 2002-01-01) Schneider, H; Pryer, KM
    Spores of the aquatic heterosporous fern family Marsileaceae differ markedly from spores of Salviniaceae, the only other family of heterosporous ferns and sister group to Marsileaceae, and from spores of all homosporous ferns. The marsileaceous outer spore wall (perine) is modified above the aperture into a structure, the acrolamella, and the perine and acrolamella are further modified into a remarkable gelatinous layer that envelops the spore. Observations with light and scanning electron microscopy indicate that the three living marsileaceous fern genera (Marsilea, Pilularia, and Regnellidium) each have distinctive spores, particularly with regard to the perine and acrolamella. Several spore characters support a division of Marsilea into two groups. Spore character evolution is discussed in the context of developmental and possible functional aspects. The gelatinous perine layer acts as a flexible, floating organ that envelops the spores only for a short time and appears to be an adaptation of marsileaceous ferns to amphibious habitats. The gelatinous nature of the perine layer is likely the result of acidic polysaccharide components in the spore wall that have hydrogel (swelling and shrinking) properties. Megaspores floating at the water/air interface form a concave meniscus, at the center of which is the gelatinous acrolamella that encloses a "sperm lake". This meniscus creates a vortex-like effect that serves as a trap for free-swimming sperm cells, propelling them into the sperm lake.