SPORE MORPHOLOGY IN THE CYATHEACEAE. I. THE PERINE AND SPORANGIAL CAPACITY: GENERAL CONSIDERATIONS

The literature on cyatheaceous spore morphology relative to the presence of a perine layer is reviewed, and evidence based on a sodium-hydroxide assay is presented indicating that the outer scultpine layer in certain cyatheaceous spores is perine. Perine so defined characterizes Metaxya, paleotropical and certain neotropical species of Sphaeropteris, nearly all species of Alsophila, all species of Nephelea, and certain species of Trichipteris and Cyathea. It is lacking in Lophosoria, many species of Trichipteris and Cyathea, and all species of Cnemidaria. Two major patterns of spore number per sporangium in the family are reported. Lophosoria, Sphaeropteris, Trichipteris, Cyathea, Cnemidaria, and probably Metaxya are characterized by 64-spored sporangia, whereas most species of Alsophila and all species of Nephelea are characterized by 16-spored sporangia. The congruence of this generic distribution of sporangial-capacity types with Tryon's phyletic arrangement of cyatheaceous genera supports the naturalness of his system. The intrasporangial germination of spores retained in dehisced and dispersed sporangia supports the suggestion that decreased spore number per sporangium in Alsophila and Nephelea may relate to the role of the sporangia as dispersal units. The decreased number of spores per sporangium is associated with a trend toward increase in the number of sporangia per sores, with the highest known count approaching 1000 sporangia per sorus. The Alsophila-Nephelea evolutionary line has probably not been ancestral in the phylogeny of the more advanced groups of ferns.     

中华水韭孢子囊发育研究

采用半薄切片法,连续观察了极度濒危级(CR)植物中华水韭大小孢子囊的发育过程,以期从无性生殖的角度,为探讨其濒危原因提供直观可靠的理论根据。结果显示:(1)中华水韭的大小孢子叶相间排列,无混生孢子囊。(2)隔丝为孢子供给营养,其体积直接影响孢子的大小、产量和育性。(3)大小孢子囊都近半数败育,小孢子囊为整齐发育,大孢子囊为不整齐发育。(4)大小孢子囊均无柄,且都不存在开裂结构,只有孢子囊壁腐烂后才能散播孢子。研究认为,中华水韭的濒危与孢子囊的发育特征密切相关,孢子囊的高频率败育、没有开裂结构以及对环境的依赖,是造成中华水韭濒危的重要因素之一;通过与近缘类群孢子囊的比较,发现仅水韭孢子的散播借助外力,对生境要求较高,即验证了水韭古老的系统学地位,同时说明水韭更具有监测生境地区环境指标的能力。     

ULTRASTRUCTURE OF DISPERSED AND IN SITU SPECIMENS OF THE DEVONIAN SPORE RHABDOSPORITES LANGII: EVIDENCE FOR THE EVOLUTIONARY RELATIONSHIPS OF PROGYMNOSPERMS

Abstract: The spore Rhabdosporites (Triletes) langii (Eisenack) Richardson, 1960 is abundant and well preserved in Middle Devonian (Eifelian) ‘Middle Old Red Sandstone’ deposits from the Orcadian Basin, Scotland. Here it occurs as dispersed individual spores and in situ in isolated sporangia. This paper reports on a detailed light microscope (LM), scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis of both dispersed and in situ spores. The dispersed spores are pseudosaccate with a thick walled inner body enclosed within an outer layer that was originally attached only over the proximal face. The inner body has lamellate/laminate ultrastructure consisting of fine lamellae that are continuous around the spore and parallel stacked. Towards the outer part of the inner body these group to form thicker laminate structures that are also continuous and parallel stacked. The outer layer has spongy ultrastructure. In situ spores preserved in the isolated sporangia are identical to the dispersed forms in terms of morphology, gross structure and wall ultrastructure. The sporangium wall is two‐layered. A thick coalified outer layer is cellular and represents the main sporangium wall. This layer is readily lost if oxidation is applied during processing. A thin inner layer is interpreted as a peritapetal membrane. This layer survives oxidation as a tightly adherent membranous covering of the spore mass. Ultrastructurally it consists of three layers, with the innermost layer composed of material similar to that comprising the outer layer of the spores. Based on the new LM, SEM and TEM information, consideration is given to spore wall formation. The inner body of the spores is interpreted as developing by centripetal accumulation of lamellae at the plasma membrane. The outer layer is interpreted as forming by accretion of sporopollenin units derived from a tapetum. The inner layer of the sporangium wall is considered to represent a peritapetal membrane formed from the remnants of this tapetum. The spore R. langii derives from aneurophytalean progymnosperms. In light of the new evidence on spore/sporangium characters, and hypotheses of spore wall development based on interpretation of these, the evolutionary relationships of the progymnosperms are considered in terms of their origins and relationship to the seed plants. It is concluded that there is a smooth evolutionary transition between Apiculiretusispora‐type spores of certain basal euphyllophytes, Rhabdosporites‐type spores of aneurophytalean progymnosperms and Geminospora‐/Contagisporites‐type spores of heterosporous archaeopteridalean progymnosperms. Prepollen of basal seed plants (hydrasperman, medullosan and callistophytalean pteridosperms) are easily derived from the spores of either homosporous or heterosporous progymnosperms. The proposed evolutionary transition was sequential with increasing complexity of the spore/pollen wall probably reflecting increasing sophistication of reproductive strategy. The pollen wall of crown group seed plants appears to incorporate a completely new developmental mechanism: tectum and infratectum initiation within a glycocalyx‐like Microspore Surface Coat. It is unclear when this feature evolved, but it appears likely that it was not present in the most basal stem group seed plants.     

Evolution of a multi‐step phosphorelay signal transduction system in Ensifer: recruitment of the sigma factor RpoN and a novel enhancer‐binding protein triggers acid‐activated gene expression

Most Ensifer strains are comparatively acid sensitive, compromising their persistence in low pH soils. In the acid‐tolerant strain Ensifer medicae WSM419, the acid‐activated expression of lpiA is essential for enhancing survival in lethal acidic conditions. Here we characterise a multi‐step phosphorelay signal transduction pathway consisting of TcsA, TcrA, FsrR, RpoN and its cognate enhancer‐binding protein EbpA, which is required for the induction of lpiA and the downstream acvB gene. The fsrR, tcrA, tcsA and rpoN genes were constitutively expressed, whereas lpiA and acvB were strongly acid‐induced. RACE mapping revealed that lpiA/acvB were co‐transcribed as an operon from an RpoN promoter. In most Ensifer species, lpiA/acvB is located on the chromosome and the sequence upstream of lpiA lacks an RpoN‐binding site. Nearly all Ensifer meliloti strains completely lack ebpA, tcrA, tcsA and fsrR regulatory loci. In contrast, E. medicae strains have lpiA/acvB and ebpA/tcrA/tcsA/fsrR co‐located on the pSymA megaplasmid, with lpiA/acvB expression coupled to an RpoN promoter. Here we provide a model for the expression of lpiA/acvB in E. medicae. This unique acid‐activated regulatory system provides insights into an evolutionary process which may assist the adaptation of E. medicae to acidic environmental niches.     

Exceptional preservation in Lower Devonian coalified fossils from the Welsh Borderland: a new genus based on reniform sporangia lacking thickened borders

Reniform sporangia, comprising two equal valves and containing retusoid spores, recovered from Lower Old Red Sandstone strata of Devonian age (micrornatus-newportensis Spore Biozone: lower Gedinnian lower Lochkovian) on North Brown Clee Hill in the Welsh Borderland are placed in Resilitheca salopensis gen. et sp. nov. Conventional compression fossils from Targrove, Ludlow of fertile axes showing isotomous branching with limited overtopping are considered conspecific because the terminal reniform sporangia contain the same spores. Spore ultrastructure is described using scanning and transmission electron microscopy. Sections show faint traces of lamellae. Particles associated with spores and sporangium wall are compared with the globules of pteridophytes and Ubisch bodies of angiosperms, and related to the development of the sporangium. The new plants are compared with Cooksonia caledonica Edwards known only from impressions, and with Renalia Gensel showing far more pronounced pseudomonopodial branching.     

Phylogeny,Evidence for a Cryptic Plastid,and Distribution of Chytriodinium Parasites (Dinophyceae) Infecting Copepods

Spores of the dinoflagellate Chytriodinium are known to infest copepod eggs causing their lethality. Despite the potential to control the population of such an ecologically important host, knowledge about Chytriodinium parasites is limited: we know little about phylogeny, parasitism, abundance, or geographical distribution. We carried out genome sequence surveys on four manually isolated sporocytes from the same sporangium, which seemed to be attached to a copepod nauplius, to analyze the phylogenetic position of Chytriodinium based on SSU and concatenated SSU/LSU rRNA gene sequences, and also characterize two genes related to the plastidial heme pathway, hemL and hemY. The results suggest the presence of a cryptic plastid in Chytriodinium and a photosynthetic ancestral state of the parasitic Chytriodinium/Dissodinium clade. Finally, by mapping Tara Oceans V9 SSU amplicon data to the recovered SSU rRNA gene sequences from the sporocytes, we show that globally, Chytriodinium parasites are most abundant within the pico/nano‐ and mesoplankton of the surface ocean and almost absent within microplankton, a distribution indicating that they generally exist either as free‐living spores or host‐associated sporangia.     

SULLITHECA DACTYLIFERA GEN. ET SP. N.: A NEW MEDULLOSAN POLLEN ORGAN AND ITS EVOLUTIONARY SIGNIFICANCE

The pteridosperm (Medullosaceae) pollen organ Sullitheca dactylifera gen. et sp. n. is described from middle Pennsylvanian coal balls. The proximally fused units of the obpyriform compound synangium separate and extend distally as finger-like projections. Each projection contains 4–6 vertically oriented cylindrical sporangia arranged in pairs along the radius of the unit; each unit extends from the outer cover wall toward the center. The distal portion of the compound synangium is hollow as a result of the lateral separation of the centripetally and distally directed synangial units. About 40 tubular sporangia are present in all and dehiscence occurs along a lateral slit in each sporangium. Vascular strands are disposed around the periphery of the organ in addition to a single strand paralleling each sporangium. Two- or three-cell trichomes and stomata are present on the organ surface. Pollen of the Monoletes type is present. A paired row of sporangia in Sullitheca composing a synangial unit is considered the homologue of a paired row of sporangia in the more compact and highly evolved genus, Dolerotheca.     

THE SPORE PATTERN IN SOME SPECIES OF CHEILANTHES

Of the 32 taxa examined, 13 contained 32 spores in each sporangium and are considered apo-gamous, 14 were sexual species with 64 spores per sporangium, and 5 had 32 spores in some sporangia and 64 in others. When considered as a whole, the spores ranged in size from 29.9 to 74.88μ. Most species had oval or globose spores but several had tetrahedral spores. The spores of all were radially symmetrical. Almost all of the species possessed a crassimarginate type of laesura and all except C. cooperae and C. viscida had a perispore. The ornamentation of the perispore showed the following patterns: napate, granulate, psilate, lobate, foveate, and echinate. The exine pattern was predominantly psilate but foveate, rugulate, napate, and granulate conditions were observed. Seventeen taxa were found to have some degree of spore abortion.     

FURTHER OBSERVATIONS ON ZOSTEROPHYLLUM LLANOVERANUM FROM THE LOWER DEVONIAN OF SOUTH WALES

Zosterophyllum llanoveranum was first described by Croft and Lang in 1942. This account presents the anatomy of the axes. Details of the sporangium wall in the vicinity of the dehiscence line are reported and the spores are described. The plant is compared with other members of the genus Zosterophyllum and also with those Devonian plants possessing exarch protosteles and lateral sporangia, which Banks has placed in the subdivision Zosterophyllophytina.