The biogeography of species,with special reference to ferns

An important element of the biogeography of species is the geographic aspects of speciation. The geography of species has a role in the processes of speciation which have a reciprocal role in species geography. The homosporous ferns provide an especially favorable group for biogeographic studies because nearly all species have an equivalent capacity for dispersal and migration. Species ranges are based on the ecology of the environment, rather than on animal vectors of dispersal or pollination. However, with allowance for these differences, the processes of geographic speciation are basically the same in ferns and other vascular plants, although often on a broader geographic scale in the ferns. Speciation most frequently produces a new species with a small range, which can rapidly expand to occupy the geography of the environment to which the species is adapted. The members of a closely related speciesgroup retain their morphological and geographic relations for a relatively short time. With speciation, changes in distribution, and extinction, the original relations of the species and the biogeographical history of the group will be lost. High regional species diversity occurs in the wet mountainous regions of the tropics, where there is greatest ecological diversity and maximal opportunities for speciation and persistence.     

Composition and chromosomal localization of cetacean highly repetitive DNA with special reference to the blue whale,Balaenoptera musculus

Three highly repetitive DNA components — the common cetacean component, the heavy (GC-rich) satellite and the light (AT-rich) satellite — were studied in the blue whale. Consensus sequences of the common component and the heavy satellite were determined on the basis of three repeats of the common component and eight repeats of the heavy satellite. The tandemly organized common cetacean component, which comprises a large portion of all cetacean — both odontocete (toothed whale) and mysticete (whalebone whale) — genomes has a repeat length of 1,760 bp and the three clones analysed showed a high degree of conformity. The repeat contains a 72 bp sequence with dyad symmetry and striking intrastrand complementarity. The rest of the repeat comprises a unique sequence. The repeat unit of the heavy satellite of the blue whale is 422 bp. Also this component is tandemly organized. About half the length of the repeat constitutes a unique sequence and the other half is made up of subrepeats with TTAGGG as a frequent motif. The light satellite has not been sequenced and its basic repeat unit has not yet been identified. The chromosomal localization of the three components was determined by in situ hybridization using ³H-labelled cloned fragments as probes. The common cetacean component was located in most interstitial and terminal C-bands. The heavy satellite occurred primarily in terminal C-bands. When the two components hybridized to the same terminal C-bands, the localization of the heavy satellite was distal to that of the common cetacean component. Neither component shared localization with the light satellite which is located in centromeric C-bands in just a few chromosome pairs.     

The blastomere periphery ofXenopus laevis,with special reference to intercellular relationships

Summary The peripheral region of theXenopus laevis blastomere is considered to be comprised of the extracellular material, the plasma membrane and the subsurface cytoplasm. These regions were examined with the electron microscope during cleavage and blastula stages (stages one to seven, Nieuwkoop and Faber, 1967). The cell contact relationships varied according to the location of the cell in the embryo. Superficially, a dense terminal junction occurred, possessing point contacts where the membranes approached to within 30 Å. More deeply, a variety of relationships appeared: wide intercellular spaces bridged by pseudopodia, long regions of unbridged parallel membrane or complex interdigitation. Tight junctions were found in limited numbers and developed at about stage seven. Extracellular material was examined using histochemical techniques on dissociated andin situ cells. The latter had appreciable amounts of such material, but dissociated cells reacted inconsistently to different techniques. The cytoplasm subjacent to the membrane possessed a filamentous network at all stages examined, but extensive microfilament tracts and microtubules appeared only at gastrulation.     

The ultrastructure of the marine blue green alga,Trichodesmium erythraeum,with special reference to the cell wall,gas vacuoles,and cylindrical bodies

Summary The marine blue green alga, Trichodesmium erythraeum, was studied with electron microscopy in an attempt to elucidate the structural basis for its rapid lysis when removed from its marine environment. In this connection, it was found that a thining of the electron-dense layer of the longitudinal wall at the site adjacent to transverse wall attachment was responsible for lysis. The underlying biochemical basis for this change has not been elucidated because of the extreme difficulties of maintaining and growing the alga in culture under defined conditions. Several other features of considerable interest also were found. Especially interesting is the very regular array of gas vacuoles in the form of a hollow cylinder which shields most of the photosynthetic system. It was suggested that the gas vacuoles might possibly function optically, having adaptive value in protecting the free-floating alga from excessive radiation. In addition, a detailed structure of the cylindrical bodies was presented, and its structure with the photosynthetic lamellae was compared. On the basis of sectoring to form fragments of double lamellar units from the cylindrical body which are identical in structure to the photosynthetic lamellae, it has been postulated that the cylindrical body may be the site of synthesis for the photosynthetic system in Trichodesmium erythraeum.