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Fossilized Nuclei and Chromosomes Reveal 180 Million Years of Genomic Stasis in Royal Ferns

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Science  21 Mar 2014:
Vol. 343, Issue 6177, pp. 1376-1377
DOI: 10.1126/science.1249884

Cytologically Informative Fossils

Fossilization processes tend to destroy fine-cell structure but, exceptionally, Bomfleur et al. (p. 1376) have found examples of fossil ferns from the Jurassic in which subcellular structures, including organelles such as nuclei and chromosomes, are well-preserved. Comparative and quantative analyses show that these cells closely resemble the fossil nuclei of extant cinnamon ferns, Osmundastrum cinnamomea, which indicates that this group of ferns has remained virtually unchanged for 180 million years.

Abstract

Rapidly permineralized fossils can provide exceptional insights into the evolution of life over geological time. Here, we present an exquisitely preserved, calcified stem of a royal fern (Osmundaceae) from Early Jurassic lahar deposits of Sweden in which authigenic mineral precipitation from hydrothermal brines occurred so rapidly that it preserved cytoplasm, cytosol granules, nuclei, and even chromosomes in various stages of cell division. Morphometric parameters of interphase nuclei match those of extant Osmundaceae, indicating that the genome size of these reputed “living fossils” has remained unchanged over at least 180 million years—a paramount example of evolutionary stasis.

Royal ferns (Osmundaceae) are a basal group of leptosporangiate ferns that have undergone little morphological and anatomical change since Mesozoic times (16). Well-preserved fossil plants from 220-million-year-old rocks already exhibit the distinctive architecture of the extant interrupted fern (Osmunda claytoniana) (2), and many permineralized osmundaceous rhizomes from the Mesozoic are practically indistinguishable from those of modern genera (35) or species (6). Furthermore, with the exception of one natural polyploid hybrid (7), all extant Osmundaceae have an invariant and unusually low chromosome count (7, 8), suggesting that the genome structure of these ferns may have remained unchanged over long periods of geologic time (8). To date, evidence for evolutionary conservatism in fern genomes has been exclusively based on studies of extant plants (9, 10). Here, we present direct paleontological evidence for long-term genomic stasis in this family in the form of a calcified osmundaceous rhizome from the Lower Jurassic of Sweden with pristinely preserved cellular contents, including nuclei and chromosomes.

The specimen was collected from mafic volcaniclastic rocks [informally named the “Djupadal formation” (11)] at Korsaröd near Höör, Scania, Sweden [fig. S1 of (12)]. Palynological analysis indicates an Early Jurassic (Pliensbachian) age for these deposits (11) (fig. S2), which agrees with radiometric dates obtained from nearby volcanic necks (13) from which the basaltic debris originated. The fern rhizome was permineralized in vivo by calcite from hydrothermal brines (11, 14) that percolated through the coarse-grained sediments shortly after deposition (table S1). The fossil is 6 cm long and 4 cm wide and consists of a small (~7 mm diameter) central stem surrounded by a dense mantle of persistent frond bases with interspersed rootlets (Fig. 1). Its complex reticulate vascular cylinder (ectophloic dictyoxylic siphonostele), parenchymatous pith and inner cortex, and thick fibrous outer cortex are characteristic features of Osmundaceae (1, 35, 12) (fig. S3). Moreover, the frond bases mantling the rhizome contain a heterogeneous sclerenchyma ring that is typical of extant Osmunda sensu lato (1, 3, 4, 12) (fig. S4). The presence of a single root per leaf trace favors affinities with (sub)genus Osmundastrum (1, 3, 6, 12).

Fig. 1 Cytological features preserved in the apical region of the Korsaröd fern fossil.

(A) transverse section through the rhizome; (B) detail of radial longitudinal section showing typical pith-parenchyma cells with preserved cell membranes (arrow), cytoplasm and cytosol particles, and interphase nuclei with prominent nucleoli; (C) interphase nucleus with nucleolus and intact nuclear membrane; (D) early prophase nucleus with condensing chromatin and disintegrating nucleolus and nuclear membrane; (E and F) late prophase cells with coiled chromosomes and with nucleolus and nuclear membrane completely disintegrated; (G and H) prometaphase cells showing chromosomes aligning at the equator of the nucleus; (I and J) possible anaphase cells showing chromosomes attenuated toward opposite poles. (A), (C to E), (G), and (I) are from NRM S069656. (B), (F), (H), and (J) are from NRM S069658. Scale bars: (A) 500 μm; (B) 20 μm; (C to J) 5 μm.

The specimen is preserved in exquisite subcellular detail (Fig. 1 and figs. S4 and S5). Parenchyma cells in the pith and cortex show preserved cell contents, including membrane-bound cytoplasm, cytosol granules, and possible amyloplasts (Fig. 1 and fig. S5). Most cells contain interphase nuclei with conspicuous nucleoli (Fig. 1, figs. S4 and S5, and movies S1 and S2). Transverse and longitudinal sections through the apical part of the stem also reveal sporadic dividing parenchyma cells, mainly in the pith periphery (Fig. 1). These are typically preserved in prophase or telophase stages, in which the nucleolus and nuclear envelope are more or less unresolved and the chromatin occurs in the form of diffuse, granular material or as distinct chromatid strands. A few cells contain chromosomes that are aligned at the equator of the nucleus, indicative of early metaphase, and two cells were found to contain chromosomes that appear to be attenuated toward opposite poles, representing possible anaphase stages. Some tissue portions in the stem cortex and the outer leaf bases show signs of necrosis and programmed cell death (fig. S6).

Such fine subcellular detail has rarely been documented in fossils (1517) because the chances for fossilization of delicate organelles are small (16) and their features are commonly ambiguous (17). The consistent distribution and architecture of the cellular contents in the Korsaröd fern fossil resolved via light microscopy (Fig. 1 and fig. S4), scanning electron microscopy (fig. S5), and synchrotron radiation x-ray tomographic microscopy (SRXTM) (fig. S5 and movies S1 and S2) provide unequivocal evidence for three-dimensionally preserved organelles.

Positive scaling relationships rooted in DNA content can be used to extrapolate relative genome sizes and ploidy levels of plants (1821). We measured minimum and maximum diameters, perimeters, and maximum cross-sectional areas of interphase nuclei in pith and cortical parenchyma cells of the fossil and of its extant relative Osmundastrum cinnamomeum. The measurements match very closely (Fig. 2), with mean nuclear perimeters of 32.2 versus 32.6 μm and mean areas of 82.2 versus 84.9 μm2 in the fossil and in extant Osmundastrum, respectively. The equivalent nuclear sizes demonstrate that the Korsaröd fern fossil and extant Osmundaceae likely share the same chromosome count and DNA content, and thus suggest that neither ploidization events nor notable amounts of gene loss have occurred in the genome of the royal ferns since the Early Jurassic ~180 million years ago [(8), see also discussion in (9, 10)]. These results, in concert with morphological and anatomical evidence (16), indicate that the Osmundaceae represents a notable example of evolutionary stasis among plants.

Fig. 2 Morphometric parameters of interphase nuclei of extant O. cinnamomeum compared to those of the Korsaröd fern fossil.

Colored box-and-whiskers plots in upper graph indicate interquartile ranges (box) with mean (square), median (solid transverse bar), and extrema (whiskers); dashed colored lines in lower graph indicate linear fits (n = 76 versus n = 37 measured nuclei for extant O. cinnamomeum versus the fossil).

Supplementary Materials

www.sciencemag.org/content/343/6177/1376/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S6

Table S1

References (2236)

Movies S1 and S2

References and Notes

  1. See supplementary materials available on Science Online.
  2. Acknowledgments: We thank E. M. Friis and S. Bengtson (Stockholm) and F. Marone and M. Stampanoni (Villigen) for assistance with SRXTM analyses at the Swiss Light Source, Paul Scherrer Institute (Villigen); G. Grimm (Stockholm) for assistance with statistical analyses; B. Bremer and G. Larsson (Stockholm) for providing live material of Osmunda; M. A. Gandolfo Nixon and J. L. Svitko (Ithaca, New York) for permission to use images from the Cornell University Plant Anatomy Collection (CUPAC; http://cupac.bh.cornell.edu/); the members of Tjörnarps Sockengille (Tjörnarp) for access to the fossil locality; A.-L. Decombeix (Montpellier), I. Bergelin (Lund), C. H. Haufler (Lawrence, Kansas), N. Tian (Shenyang), Y.-D. Wang (Nanjing), and T. E. Wood (Flagstaff, Arizona) for discussion; and two anonymous referees for constructive comments. This research was jointly supported by the Swedish Research Council (VR), Lund University Carbon Cycle Centre (LUCCI), and the Royal Swedish Academy of Sciences. The material is curated at the Swedish Museum of Natural History (Stockholm, Sweden) under accession nos. S069649 to S069658 and S089687 to S089693.

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