Ultrastructure of the flame bulbs and protonephridial capillaries of Prorhynchus (Lecithoepitheliata, Prorhynchidae, Turbellaria)

The flame bulb of Prorhynchus is formed by a single cell. Its nucleus is not located in the cytoplasm at the base of the flame. Cilia of the flame have cross-striated hollow ciliary rootlets converging towards their tips. The maximum number of cilia counted was 13. The weir consists of a single row of longitudinal ribs that contain longitudinal filaments and possess regularly arranged protrusions along their surface appearing as transverse bands in horizontal section. A 'membrane' of extracellular material extends between the ribs. and loose material fills the places between the ribs, with a denser layer midway between adjacent ribs. Distally, the ribs fuse to form a continuous tube without a junction. Small protonephridial capillaries lack junctions, larger ducts have lateral flames and patches of long microvilli. Large excretory ducts open into a ciliated and lamellated cavity which is connected by a narrow canal to the excretory pore. The terminal part of the canal close to the pore possesses many cilia and microvilli. Phylogenetic implications of the findings are discussed.     

Preferential grazing of five varieties of spring barley by wild rabbits (Oryctolagus cuniculus)

Assessment of grazing damage to five varieties of spring barley (Hordeum vulgare L.) by wild rabbits in an enclosed site revealed varietal differences. The variety Atem was consistently least grazed and Triumph most grazed by rabbits. When variety effects were removed from the analysis significant negative correlations were observed between plant height and stem damage. Differences in sugar content of varieties did not fully explain varietal selection by grazing rabbits.     

A COMPARISON OF FOUR EAST AFRICAN CROCODILE (CROCODYLUS NILOTZCUS LAURENTI) POPULATIONS

Four crocodile populations have been examined by aerial photography with a view to assessing their length frequency distributions. The assumptions, and the limitations attached to such assumptions, necessary to convert length frequency information into information on the age structure, and hence the status, of the population are considered. Environmental conditions of value in understanding the status of the populations are listed. The Victoria Nile population is considered to show adequate recruitment, but dispersal of young crocodiles tends to give the converse impression. The population is probably stable and has not been heavily exploited in the past. The Grumeti River population may represent a local aggregation mostly of large males from Lake Victoria, probably retreating from human induced pressures. There is little recruitment through breeding, and the largest crocodile encountered in this study are members of this population. The Lake Rudolf population is a typical healthy population with high recruitment and high juvenile mortality. The population is probably stable, but may be expected to decline as fishing interests on the west shore expand. Growth is retarded in the Rudolf population. The Lorian swamp population is expanding rapidly probably after a period of heavy mortality wrought by hunters shooting for skins. The present conflicts with human/livestock interests is expected to increase, resulting in stabilization and decline of crocodile in the swamp. Some comments are made on the possible influence of various environmental parameters on present status and trends of the populations.     

Intragenic duplication and divergence in the spectrin superfamily of proteins

Many structural, signaling, and adhesion molecules contain tandemly repeated amino acid motifs. The alpha-actinin/spectrin/dystrophin superfamily of F-actin-crosslinking proteins contains an array of triple alpha-helical motifs (spectrin repeats). We present here the complete sequence of the novel beta-spectrin isoform beta(Heavy)- spectrin (beta H). The sequence of beta H supports the origin of alpha- and beta-spectrins from a common ancestor, and we present a novel model for the origin of the spectrins from a homodimeric actin-crosslinking precursor. The pattern of similarity between the spectrin repeat units indicates that they have evolved by a series of nested, nonuniform duplications. Furthermore, the spectrins and dystrophins clearly have common ancestry, yet the repeat unit is of a different length in each family. Together, these observations suggest a dynamic period of increase in repeat number accompanied by homogenization within each array by concerted evolution. However, today, there is greater similarity of homologous repeats between species than there is across repeats within species, suggesting that concerted evolution ceased some time before the arthropod/vertebrate split. We propose a two-phase model for the evolution of the spectrin repeat arrays in which an initial phase of concerted evolution is subsequently retarded as each new protein becomes constrained to a specific length and the repeats diverge at the DNA level. This evolutionary model has general applicability to the origins of the many other proteins that have tandemly repeated motifs.      

The nuclear envelope; its structure and relation to cytoplasmic membranes

An electron microscope study of thin sections of interphase cells has revealed the following:- Circular pores are formed in the double nuclear envelope by continuities between the inner and outer membranes which permit contact between the nucleoplasm and the cytoplasm unmediated by a well defined membrane. The pores, seen in sections normal to the nuclear envelope, are profiles of the ring-shaped structures described by others and seen in tangential section. The inner and outer nuclear membranes are continuous with one another and enclose the perinuclear space. The pores contain a diffuse, faintly particulate material. A survey of cells of the rat derived from the embryonic ectoderm, mesoderm, and endoderm, and of a protozoan and an alga has revealed pores in all tissues examined, without exception. It is concluded that pores in the nuclear envelope are a fundamental feature of all resting cells. In certain cells, the outer nuclear membrane is continuous with membranes of the endoplasmic reticulum, hence the perinuclear space is continuous with cavities enclosed by those membranes. There are indications that this is true for all resting cells, at least in a transitory way. On the basis of these observations, the hypothesis is made that two pathways of exchange exist between the nucleus and the cytoplasm; by way of the perinuclear space and cavities of the endoplasmic reticulum and by way of the pores in the nuclear envelope.