Habitat characterization of the morphologically similar mayfly larvae,Caenis and Tricorythodes (Ephemeroptera)

The larvae of Caenis (Caenidae) and Tricorythodes (Tricorythidae), once considered to be confamilial, have notable morphological and behavioural similarities. Univariate and multivariate (discriminant analysis) techniques were used to determine which environmental variables best characterized the larval habitats of Caenis and Tricorythodes at 40 sample sites on 29 rivers within the Interior Plains of Alberta, Canada. River width, depth and substrate type distinguished riverine habitats of the two genera. Larvae of Tricorythodes occurred in wide rivers of varying depths that possessed coarse substrates. Although larvae of Caenis occurred in a variety of habitats, they were found more frequently on stable substrates in narrow, deep rivers.     

A hybridization and enzyme blocking method to enrich minor cDNA sequences

A method is presented which allows for the enrichment of low frequency cDNA sequences. The crucial step in the procedure is the hybridization of a pool of cDNA to homologous or heterologous RNA to a Rot value which will leave minor sequences in a single strand cDNA form while the major sequences form cDNA:RNA hybrids. This allows subsequent enzymatic differentiation between major and minor sequences resulting ultimately in the degradation of the major sequences. The procedure is general and simple as it requires no column chromatography step. The method is designed to integrate into a widely used cDNA cloning protocol and results either in double-stranded cDNA which can be used for molecular cloning or as a source of probes for hybridization.     

The role of microtubules in establishing nuclear spatial patterns in multinucleate green algae

Summary In uninucleate cells, cytokinesis follows karyokinesis, thereby reestablishing a specific nucleus-to-cytoplasm ratio. In multinucleate cells, cytokinesis is absent or infrequent; no plasmalemma boundary defines the cytoplasmic territory of an individual nucleus. Several genera of large multinucleate green algae were examined with epifluorescence light microscopy to determine whether the patterns of cytoplasmic organization establish nuclear cytoplasmic domains. Randomly spaced nuclei, singular mitotic events and cytoplasmic streaming characterize the organization of two genera,Derbesia andBryopsis (Caulerpales). The cells ofValonia, Valoniopsis, Boergesenia, Ventricaria (Siphonocladales), andHydrodictyon (Chlorococcales) display regularly spaced nuclei which undergo synchronous divisions in a stationary cytoplasm. In the cytoplasm of genera with regularly spaced nuclei, microtubules radiate from all nuclei in late telophase-early interphase. These internuclear microtubule arrays are not found in algal genera with randomly spaced nuclei. It is hypothesized that these microtubule arrays play a role in establishing the cytoplasmic domain of each nucleus in genera with regularly spaced nuclei. Loss of microtubule arrays during the events of mitosis correlated positively with the increasing randomization of nuclear patterns in algae grown in microtubule inhibitors. Cytoplasmic domains were maintained when cells were grown in the same media in the dark. This suggests that, after a round of division, regular nuclear spacing in certain multinucleate algae is reestablished by internuclear microtubule arrays, which are not, however, required to maintain spacing during interphase.Dedicated to the memory of Professor Oswald Kiermayer     

Connecting the Retina to the Brain

The visual system is beautifully crafted to transmit information of the external world to visual processing and cognitive centers in the brain. For visual information to be relayed to the brain, a series of axon pathfinding events must take place to ensure that the axons of retinal ganglion cells, the only neuronal cell type in the retina that sends axons out of the retina, find their way out of the eye to connect with targets in the brain. In the past few decades, the power of molecular and genetic tools, including the generation of genetically manipulated mouse lines, have multiplied our knowledge about the molecular mechanisms involved in the sculpting of the visual system. Here, we review major advances in our understanding of the mechanisms controlling the differentiation of RGCs, guidance of their axons from the retina to the primary visual centers, and the refinement processes essential for the establishment of topographic maps and eye-specific axon segregation. Human disorders, such as albinism and achiasmia, that impair RGC axon growth and guidance and, thus, the establishment of a fully functioning visual system will also be discussed.     

MOLECULAR DELINEATION OF SPECIES AND SPECIES RELATIONSHIPS IN THE RED ALGAL AGAROPHYTES GRACILARIOPSIS AND GRACILARIA (GRACILARIALES)1

Delineation of species in the economically important agarophyte genera Gracilaria and Gracilariopsis has proven extremely difficult using available morphological characteristics. In this study, we examine the usefulness of two transcribed spacers for molecular systematic studies of these genera. The polymerase chain reaction was used to amplify the internal transcribed spacers (ITSs) and the intervening 5.8S ribosomal DNA of the nuclear ribosomal repeat region. In addition, a plastid spacer region and flanking regions of coding genes were amplified from the RUBISCO operon. Both regions were sequenced for individuals and populations of Gracilariopsis lemaneiformis (Bory) Dawson, Acleto, et Foldvik to determine the usefulness of these spacers in delimiting populations. These studies reveal that there is as much variation among individuals of a population as there is between individuals of geographically separate populations. In addition, the ITS spacer regions were compared between different species of Gracilariopsis and Gracilaria. The nuclear ITS spacer region is conserved at a species level in both genera and provides phylogenetically informative characters that can be used to examine species interrelationships among relatively closely related taxa. However, because of the difficulties of aligning this entire region among species from the two genera, the ITS region is not useful for examining intergenera relationships. ITS interspecies sequence comparisons indicate that Gracilariopsis lemaneiformis from California is significantly different from G. lemaneiformis from China and that a species of Gracilariopsis from Peru is more closely related to G. lemaneiformis from North Carolina than it is to the other Gracilariopsis species examined. In addition, these studies indicate that Gracilaria chilensis Bird, McLachlan, et Oliveira from New Zealand and Gracilaria tenuistipitata Chang et Xia from southeast Asia are as closely related as are Gracilaria verrucosa (Hudson) Papenfuss, G. pacifica Abbott, and Gracilaria robusta Kylin. Phylogenetic analysis of aligned plastid spacer sequences from Gracilaria and Gracilariopsis taxa provide similar conclusions about species relationships.     

BIOCHEMICAL TAXONOMY OF MARINE PHYTOPLANKTON BY ELECTROPHORESIS OF ENZYMES. II. LOSS OF HETEROZYGOSITY IN CLONAL CULTURES OF THE CENTRIC DIATOMS SKELETONEMA COSTATUM AND THALASSIOSIRA PSEUDONANA1, 2

An electrophoretic survey of 12 new isolates of Thalassiosira pseudonana Hasle & Heimdal and 25 new isolates of Skeletonema costatum (Grev.) Cleve revealed several heterozygote genotypes at malate dehydrogenase (MDH) and phosphohexose isomerase (PHI) loci. The new clones were maintained in culture for 6 mo and then reasayed at these two loci. All MDH heterozygotes and halj of the PHI heterozygotes had become homozygous. This resulted in a collection of clones that are largely homozygous but that are samples of polymorphic species. The physlogical implications of this loss of heterozygosity in clonal cultures has not been analyzed. Hawever, any change in a clone that is the result of culturing conditions reduces the usefulness of that clone as a laboratory test organism for ecological correlations.     

Apoptosis in lactating and involuting mouse mammary tissue demonstrated by nick-end DNA labelling

Mammary involution after cessation of milk removal is associated with extensive loss of secretory epithelial cells. Ultrastructural changes and the appearance of oligonucleosomal DNA laddering in ethidium bromide-stained gels indicates that cell loss during involution occurs by apoptosis. In this study, a technique for nick end-labelling of genomic DNA with radiolabelled deoxynucleotide has been used to monitor the induction of programmed cell death in mice after litter removal at peak lactation. This technique proved more sensitive than conventional ethidium bromide staining, and results suggested that apoptosis was induced rapidly by milk stasis, before extensive tissue re-modelling had begun. Oligonucleosomal DNA laddering on agarose gels was detected within 24 h of milk stasis, and increased progressively for at least 4 days. Nick-end labelling also detected laddering before litter removal, suggesting that programmed cell death is a normal feature of the lactating tissue. The DNA end-labelling technique was also adapted for in situ visualisation of apoptotic cells in tissue sections. By this criterion, apoptotic cells were identified in both the secretory epithelium lining the alveoli of the gland and, increasingly with prolonged milk stasis, amongst those sloughed into the alveolar lumen. The results demonstrate the utility of these techniques for study of mammary cell death and suggest that, whilst apoptosis is rapidly induced by milk stasis, it is also a normal physiological event in the lactating mammary gland.