Dynamic ecophenotypy in the Silurian Monograptidae (Graptolithina)

ABSTRACT The monograptids from the Wenlock and Ludlow (mid- to late Silurian) of the palaeotropical Baltic Basin exhibit thickened ring structures (sicular annuli) over their initial phase of growth. Appearing before the lundgreni extinction event, they persisted throughout the remainder of the Silurian, fluctuating in number over that period. To better understand the mechanisms controlling their development and variation, counts of sicular annuli were taken from three well cores in Lithuania, compared between species in each sample and compared with contemporaneous gamma ray data, accompanied by the stable isotope (δ13C), and acritarch diversity data. Mean counts of annuli fluctuated greatly over the studied interval, but showed negligible variation between species, indicating that the trait is ecophenotypic. The fluctuation in annulus presence aligned with variations in fourth- and fifth-order cycles derived from the gamma ray trends, which represent significant sea level fluctuations, δ13C ratios, and primary productivity, suggesting that annuli were more plentiful in high-stand states which are associated with the wetter climate and more productive conditions, whereas dryer, less productive conditions were not conducive to annulus development. In light of this evidence, we hypothesise that the action of upwelling as a result of intensified storm events during wetter periods would have encouraged phytoplankton blooms, increasing construction of annuli. These results show the potential utility of sicular annuli in the study of Silurian climate change and give new insights into graptolite palaeoecology.

Etude ultrastructurale de la différenciation des hétérocystes chez la Cyanobactérie, Anabaena cylindrica Lemm

The differentiation of heterocystes from vegetative cells of Anabaena cylindrica, induced after transfer from a NH4+ medium to a medium without NH4+ under aerobic conditions, was studied by electron microscopy, the material being fixed with osmium tetroxide. The narrow septum separating the heterocyst and the vegetative cell originates from the median region of the normal septum which separates two adjacent vegetative cells. Its peripheral region is transformed into a thickened ring on which rests the complex envelope of the heterocyst. This structure presumably helps to create an anaerobic condition for nitrogenase. At the final stage of heterocyst maturation, the thylakoid membranes are significantly modified. They become flattened while at the previous stages they were expanded and vesiculated. In addition, some of them seem to be closely associated either with the remaining vesicular structures or with the plasmalemma. Polyphosphates bodies, cyanophycin granules, DNA fibres, and polyhedral bodies disappear. The presence of ribosomes, few polyglucosides, and numerous lipid droplets are observed at the end of the differentiation. A possible relationship between these ultrastructural changes and N2 fixation in the heterocysts of Anabaena cylindrica is discussed.

Apodospora, a new genus of the Sordariaceae

Apodospora is a new coprophilous genus of the Sordariaceae with dark, ostiolate perithecia. The eight-spored ascus has a conspicuous thickened ring in the apex. The asci are mixed with abundant filiform paraphyses. The ascospore has one dark cell and no hyaline cells. They are surrounded by a gelatinous sheath and have a single apical germ pore. The three new species described and illustrated are A. simulans (type of genus), on moose dung, Ontario, Alberta, Idaho, and South Dakota; A. thescelina, on rabbit dung, Wyoming; and A. viridis on rabbit dung, Mexico.

ANIXIELLA AND DIPLOGELASINOSPORA, TWO GENERA WITH CLEISTOTHECIA AND PITTED ASCOSPORES

The genus Anixiella Saito and Minoura is validated with a Latin description. Thielavia reticulata Booth and Ebben is transferred to Anixiella and the species redescribed and illustrated. The globose, blackish perithecia are nonostiolate. The broadly clavate asci are arranged in a fascicle, have a thickened ring in the apex, but are evanescent. These characteristics, along with the dark, pitted ascospores, indicate that this species has evolved from the genus Gelasinospora, which differs in having ostiolate perithecia. The new genus Diplogelasinospora with a single new species D. princeps represents another line evolved from Gelasinospora. As in Anixiella, the dark perithecia are nonostiolate and the cylindrical asci are evanescent. However, the ascospores have a hyaline cell formed at the base by means of a transverse septum.

Memoirs: On the Primitive Streak and associated structures in the Marsupial Bettongia cuniculus

1. A series of ten blastocysts of Bettongia cuniculus is described, covering a range of development from the appearance of the primitive streak to the establishment of the medullary plate. 2. In the first blastocyst there has appeared a small area of ectodermal thickening between the centre of the embryonal area and its periphery, and it is suggested that this is the primordium of the primitive streak. In the second blastocyst a very young streak is present which does not reach either to the middle or to the periphery of the area, and whose centre is in nearly the same position as that of the first blastocyst. At its maximum the streak stretches about two-thirds of the way across the embryonal area. A groove develops down its middle and active mesoderm production occurs from its edges. The mesoderm forms an approximately oval area round the streak, and extends into an extra-embryonal position first posteriorly and then laterally. 3. At the anterior end of the streak appears a broader, thicker region of similar but looser tissue, the primitive knot; from here grows forward the head-process in the usual way, and at the same time the endoderm underlying these two structures fuses with them. No lumen is present in the head-process at any stage. The originally circular embryonal area elongates until it becomes somewhat pear-shaped, and later it constricts in the middle. 4. The thickened ring of endoderm at the edge of the embryonal area, the annular zone of proliferation of Hubrecht, is distinguishable at the time of appearance of the primitive streak; but the second thickened area, the prochordal plate, does not appear until the streak is well established, and except in its earliest stages is hard to delimit owing to its relative thinness and to the closely investing mesoderm which soon covers it. The question of mesoderm production from these sources is discussed, and it is concluded that while such does in fact occur in this form it is quite insignificant in amount as compared with that produced from the sides of the primitive streak. 5. In the last stage described the medullary plate and chorda are well established, and the importance of the primitive streak is diminishing. Mesoderm, whose production now appears to be occurring chiefly by division of pre-existing mesoderm cells, is very much more abundant, and can be divided into three parts: thick sheets extending from near the axial line to beyond the edge of the medullary plate, condensations in which the pleuropericardial coelom is appearing anteriorly, and vascular mesoderm in which blood-vessels are forming. Blood-islands are not present within the vessels, and apparently are represented by groups of cells between the vessels. A well-formed prochordal plate is also present.

GELASINOSPORA, A NEW GENUS OF PYRENOMYCETES WITH PITTED SPORES

Gelasinospora is a genus closely related to Sordaria, characterized by the foveolate sculpturing of the spore wall. At present it contains two species, as follows:—I. Gelasinospora tetrasperma, a coprophilous fungus which has been cultivated from spores collected in Manitoba and in Ontario.The species is described, together with illustrations of the chief characters, such as the four-spored condition of the ascus and the thickened ring bordering its apical perforation.Each normal-sized spore gives rise to a homothallic mycelium. The normal spore contains four nuclei. Asci very occasionally produce dwarf spores. Each dwarf spore gives rise to a mycelium which produces archicarps, but no perithecia. The mycelia from dwarf spores fall into two groups, (+) and (−), in regard to their sexual behavior. When (+) and (−) mycelia are paired, perithecia are produced. Sometimes asci contain giant spores. The giant spores usually produce homothallic mycelia and the spores usually contain six nuclei.II. Gelasinospora cerealis, isolated from the crown of wheat and oats in Manitoba.The species is described, together with illustrations of the chief characters, such as the eight-spored condition of the ascus and the two radial thickenings at its apical perforation.Each spore gives rise to a homothallic mycelium. The spores are binucleate.