Cell and molecular biology of bryophytes: ultimate limits to the resolution of phylogenetic problems

Ultrastructure, biochemistry and 5S rRNA sequences link tracheophytes, bryophytes and charalean green algae, but the precise interrelationships between these groups remain unclear. Further major clarification now awaits primary sequence data. These are also needed to determine directionality in possible evolutionary trends within the bryophytes, but are unlikely to overturn current schemes of classification or phylogeny. Comparative ultrastructural studies of spermatogenesis, sporogenesis, the cytoskeleton and plastids reinforce biochemical and morphogenetic evidence for the wide phyletic discontinuities between mosses, hepatics and hornworts, and also rule out direct lines of descent between them. Direct ancestral lineages from charalean algae to bryophytes and to tracheophytes are also unlikely. EM studies of gametophyte/sporophyte junctions, plus immunological investigations of bryophyte cytoskeletons, are likely to accentuate the differences between mosses, hepatirs and hornworts. Other priorities for systematics include elucidation of oil body ultrastructure, analysis of the changes in nuclear proteins during spermatogenesis and a detailed comparison of bryophyte and charalean plastids. The combined evidence from ultrastrueture, biochemistry, morphology and morphogenesis warrants general acceptance of the polyphyletic origin of the bryophytes. Ultrastructural attributes should be more widely used in bryophyte systematics.     

Gibberellins, Amylase, and the Onset of Heterosis in Maize Seedlings

Rood, S. B. and Larsen, K. M. 1988. Gibberellins, amylase, andthe onset of heterosis in maize seedlings.—J. exp. Bot.39: 223–233. The possible involvement of gibberellins and amylase in heterosisof maize seedlings was investigated in two parental inbreds,CM7 and CM49, and their single cross F₁ hybrid, CM7xCM49. Germinationof all three genotypes was complete within 36 h after the onsetof imbibition. By 48 h, heterosis (hybrid vigour) for increasedshoot and root length was consistently observed. The endogenousconcentration of gibberellin A₁ (GA₁) was measured in 48 h seedlingsby gas chromatography-mass spectrometry with selected ion monitoring(GC-SIM) using [²H₂]-GA₁ as an internal standard. The GA₁ concentrationwas highest in the hybrid (59 ng g^–1 dry wt.), intermediatein CM49 (9.0 ng g^–1), and lowest in CM7 (<5.0 ng g^–1).Amylase activities in all three genotypes were very low at 24h, but increased during the next 24 h, after which time amylaseactivity in the hybrid was significantly higher than that ofeither parental inbred. Inhibitors of gibberellin (GA) biosynthesis,AMO-1618 or CCC, inhibited germination, shoot and root growth,and amylase activity in all three genotypes. Conversely, exogenousgibberellic acid (GA₃) increased amylase activity, particularlyin the inbred CM7. Amylase isozymes were separated through polyacrylamidegel electrophoresis and generally similar profiles of starchdegrading enzymes were observed in the three genotypes. SinceGA is known to control a-amylase biosynthesis in some cereals,these results are consistent with the hypothesis that GAs areinvolved in the regulation of heterosis in maize. A higher endogenousGA₁ concentration in the hybrid could result in increased amylaseactivity in the hybrid seedlings and consequently, more rapidstarch hydrolysis which fuels heterosis for early growth. Key words: Amylase, germination, gibberellic acid, Gibberellin A₁, heterosis, hybrid vigour, Zea mays     

Studies on the transmission of velvet tobacco mottle virus by the mirid, Cyrtopeltis nicotianae

The minimum acquisition period of velvet tobacco mottle virus (VTMoV) by its mirid vector Cyrtopeltis nicotianae was about 1 min, with an increase in the rate of transmission (i.e. proportion of test plants infected) for acquisition periods up to 1000 min. Pre-acquisition starvation periods up to 18 h did not affect the rate of transmission. After an acquisition access period of 2 days, the minimum inoculation period was between 1 and 2 h and the rate of transmission increased with increasing inoculation time; when the acquisition access period was 1 h, or if vectors were fasted for 16 h after the 2 day acquisition, the rate of transmission was significantly lower. When mirids were transferred sequentially each day to a healthy plant after a 24 h acquisition feed, they transmitted intermittently for up to 10 days. Up to 50% of mirids transmitted after a moult and this was not due to the mirids probing the shed cuticles or exudates of infective insects. Mirids transmitted after a moult, following acquisition periods of 10, 100 or 1000 min. C. nicotianae transmitted solanum nodiflorum mottle virus (SNMV), sowbane mosaic virus (SoMV) and southern bean mosaic virus (SBMV), but not subterranean clover mottle virus (SCMoV), lucerne transient streak virus (LTSV), tobacco ringspot virus (TRSV), galinsoga mosaic virus (GMV), nor nicotiana velutina mosaic virus (NVMV). Tomato bushy stunt virus (TBSV) was transmitted to 1/58 test plants.     

Spinal Mechanisms in the Control of Lamprey Swimming

SYNOPSIS. The lamprey, an anguilliform fish, swims using lateralundulatory movement; a transverse wave passes backward, fromhead to tail, the amplitude of the wave increasing as it movestailward. The wave of muscle activity producing this movementtravels down the body faster than the mechanical wave. The wayin which certain features of anguilliform movement contributeto its efficiency have been described. The neural activity underlyingswimming is characterized by: 1) rhythmical alternation betweenthe two sides of a single segment; 2) a burst duration thatremains a constant proportion of the cycle time and is independentof the cycle frequency; 3) rostrocaudal phase lag that is constantand also independent of the cycle frequency. Local circuitsin the lamprey spinal cord can generate this locomotory patternin the absence of sensory feedback following activation of excitatoryamino acid receptors; the pattern is centrally generated. Ithas been hypothesized that the spinal central pattern generatorfor locomotion consists of a series of segmental burst generatorscoupled together by an intersegmental coordinating system. Theintersegmental coordinating system functions to keep the frequenciesof the oscillators along the cord constant and to provide theappropriate rostrocaudal phase lag. Mechanosensitive units withinthe spinal cord are sensitive to movement of the spinal cord\notochordand movement of the spinal cord/notochord can entrain the burstpattern. Entrainment occurs through movement-related feedbackonto neurons at the local level. The possible roles this movement-relatedfeedback plays during locomotion are discussed.     

Genetics and Molecular Biology of Mouse Pigmentation

The formation of mouse coat color is a relatively complex developmental process that is affected by a large number of mutations, both naturally occurring and induced. The cloning of the genes in which these mutations occur and the elucidation of the mechanisms by which these mutations disrupt the normal pigmentation pattern is leading to an understanding of the way interactions between gene products lead to a final phenotype.     

Light (Blt), a Mutation That Causes Melanocyte Death,Affects Stria Vascularis Function in the Mouse Inner Ear

The Light mutation (B^lt) is a dominant allele of the b-locus on mouse chromosome 4 which causes progressive dilution of coat colour. Melanocytes within the hair follicles of mutant mice develop normally but later degenerate, due to the accumulation of a toxic product, so that the hair becomes lighter with age. Previous studies on W-locus spotting mutants, from which melanocytes are absent, have shown that melanocytes in the stria vascularis of the inner ear are essential for the development and/or maintenance of the endocochlear potential (EP) which is normally around 100 mV. In this study, physiological recordings from the ears of Light mutants were correlated with strial ultrastructure. EPs recorded from all b/b controls and young homozygous and heterozygous mutants (20–22 days old) were normal (77 to 113 mV), but were reduced (19 to 59 mV) in about 30% of ears from older mutants (B^lt/B^lt and B^lt/b). Strial function therefore appears to develop normally but later degenerates in some mutants. This suggests that strial melanocytes are affected by the Light allele and that the continued presence of melanocytes is necessary for strial function. There was no obvious association between the recorded EP value and the ultrastructural appearance of the stria. No structural abnormalities of the stria were noted in control or mutant mice aged 20 days to 4 months including those which had a reduced EP. Strial atrophy was common in old controls and mutants (1–2 years), and appeared to be an age-related process rather than an effect of the Light mutation. Similarly, pigment build-up was common in all strial cells of old mice. However, the accumulations of lipofuscin-like pigment were much larger and more abundant in aged brown non-agouti mice than those observed in old agouti mice, which suggests that this age-related process also has a genetic component.