CELL BEHAVIOUR AND MOLECULAR MECHANISMS OF CELL-CELL ADHESION

1. At the behavioural level, cell adhesion is generally non-specific. The search for molecular mechanisms of adhesion should be conducted on this basis.
2. Cells in general, be they from slime moulds or vertebrate epithelia, possess multiple molecular adhesive mechanisms. In epithelial cells this is shown by the number of their different ultrastructurally recognizable intercellular junctions. Elucidation of the structure and composition of such intercellular junctions will make a valuable contribution to the understanding of cell adhesion.
3. The measurement of cell adhesion is fraught with difficulties. Commonly used assays by aggregation cannot give a true representation of the normal adhesive interactions of cells in tissues, and the results they yield must be interpreted with caution. This is because it takes dissociated tissue cells up to 24 h to develop their full adhesiveness after making initial contact.
4. Cell-cell adhesion probably depends largely upon the interaction of complementary molecules on adjacent cell surfaces. Glycoproteins seem the most likely candidates but, as yet, there is no compelling evidence in any individual case and mechanisms of cell adhesion still remain obscure.     

The marine macrobenthic communities of Wallis and Smiths Lakes,New South Wales

The shallow macrobenthic communities of Wallis and Smiths Lakes on the central N.S. W. coast have been studied by replicate grab samples. Wallis Lake is a coastal marine lagoon constantly open to the sea, whereas Smiths Lake, also a coastal lagoon, is periodically closed to the sea for several months at a time. The benthic communities were described using indices of dominance and diversity. Between site communities were compared using MULTBET and a GOWER ordination. Generally, sites in Wallis Lake had a more diverse fauna than those in Smiths Lake. Weed beds supported more species and individuals than sandy substrates. The benthic fauna of these two lakes is compared with other coastal rivers and lagoons which have been studied in temperate eastern Australia.     

Mechanisms for tolerance of very high tissue phosphorus concentrations in Ptilotus polystachyus

Study of plants with unusual phosphorus (P) physiology may assist development of more P‐efficient crops. Ptilotus polystachyus grows well at high P supply, when shoot P concentrations ( [P] ) may exceed 40 mg P g^?1 dry matter (DM). We explored the P physiology of P. polystachyus seedlings grown in nutrient solution with 0–5 mM P. In addition, young leaves and roots of soil‐grown plants were used for cryo‐scanning electron microscopy and X‐ray microanalysis. No P‐toxicity symptoms were observed, even at 5 mM P in solution. Shoot DM was similar at 0.1 and 1.0 mM P in solution, but was ～14% lower at 2 and 5 mM P. At 1 mM P, [P] was 36, 18, 14 and 11 mg P g^?1 DM in mature leaves, young leaves, stems and roots, respectively. Leaf potassium, calcium and magnesium concentrations increased with increasing P supply. Leaf epidermal and palisade mesophyll cells had similar [P]. The root epidermis and most cortical cells had senesced, even in young roots. We conclude that preferential accumulation of P in mature leaves, accumulation of balancing cations and uniform distribution of P across leaf cell types allow P. polystachyus to tolerate very high leaf [P].     

A screen of worldwide wheat cultivars for hydroxamic acid levels and aphid antixenosis

In a screen of seedlings of a worldwide range of 47 cultivars of Triticum (mainly T. aestivum) the concentration of the hydroxamic acid DIMBOA ranged between 1 and 8 mmol/kg fresh wt. In a bioassay in which alatae of the aphid Sitobion avenae were released among replicated test seedlings, there were highly significant correlations between aphid ‘preference’ and DIMBOA levels in the seedlings. The value of these results in work leading to the production of aphid-resistant cultivars is discussed.     

Geographic and microgeographic genetic differentiation in two aphid species over southern England using the multilocus (GATA)4 probe

Samples of the grain aphid Sitobion avenue (F.) and the rose-grain aphid Metopolophium dirhodum (Walker) were collected in late March from wheat fields and adjacent road-side grasses at a number of locations in southern England. Unparasitized aphids were DNA fingerprinted using the multilocus (GATA)₄ probe. Over all locations, the fingerprints of individual S. avenue caught in wheat had lower overall average distances of band migration (ADBM) and shared a higher proportion of bands, than fingerprints of individuals caught in adjacent road-side grasses. The ADBM of fingerprints of S. avenue collected on road-side grasses altered significantly with geographical location, while the ADBM of fingerprints of S. avenue caught on wheat did not. A comparison of the fingerprints of individual M. dirhodum caught in wheat and neighbouring road-side grasses did not reveal any genetic differentiation. Fingerprints of M. dirhodum that were caught in the same host type did however, show significant variation in ADBM between different locations. With both S. avenue and M. dirhodum, spatial autocorrelation revealed that locations that were close together were no more likely to have individuals with similar ADBM than locations mat were far apart Our results suggest that (i) particular clones of S. avenue prefer to colonize wheat; and/or that (ii) particular clones of S. avenae perform better on wheat man other clones. It is unclear why M. dirhodum did not show any genetic structuring according to host type, but this species appears to engage in sexual reproduction much more frequently than S. avenae in southern England. M. dirhodum is likely to have displayed genetic heterogeneity between locations either because of founder effects, or because of genetic drift.     

The effect of hydoroxamic acid concentration at late growth stages of wheat on the performance of the aphid Sitobion avenae

From a previous worldwide screen of wheat cultivars as seedings, five were selected to encompass a wide concentraiton range of the hydroxaminc acid DIMBOA (0.99-8.07 mmol Kg ^-1 fr. wt). Compared with those in seedings, the concentrations of DIMBOA in flag leaves of the same cultivars were relatively low (0.06-0.87 mmol kg^-1 fr. wt). Concentraions of hydroxamic acids in whole ears at anthesis and the early milk stage were even lower, ranging from undetectable to 0.05 mmol kg^-1 fr. wt. The three-day mean relaive growth rate (MRGR) was determined for nymphs of Sitobion avenace on the flag leaf (GS 39) and on the ear (GS 60-60) of the five cultivars. No significant differences in MRGR were found between cultivatrs at the same growth stage, although MRGR was higher on ears on than on flag leaces. The results are discussed in realtion to the potential of hydroxamic acids in programmses aimed at breeding mature-plant aphid resistance into wheat cultivars.     

Molecular phylogeny of Iberian Fordini (Aphididae: Eriosomatinae): implications for the taxonomy of genera Forda and Paracletus

Abstract Mediterranean representatives of the galling aphid tribe Fordini (Hemiptera: Aphididae: Eriosomatinae) are usually grouped under the subtribe term Fordina. Aphids within Fordina display two‐year life cycles, alternating between Pistacia shrubs, where they induce conspicuous galls, and roots of Poaceae species. The high number of morphs present in a given species, the lack of knowledge of the complete cycle in some species, and the similarity between homologous morphs observed in different species pose many taxonomic problems in this group. We present results of a survey to elucidate the phylogenetic relationships among Fordini species present in the Iberian Peninsula and the Canary Islands. Sequences from the nuclear long‐wavelength opsin (LWO) and translation elongation factor 1α (EF‐1α) genes and from a region of the mitochondrial DNA containing the genes encoding the subunits six and eight of the F‐ATPase were used to infer relationships among 10 Fordina species, namely Smynthurodes betae, Aploneura lentisci, Baizongia pistaciae, two Geoica species (G. utricularia and G. setulosa), three species of Forda and two of Paracletus. Relationships between and within representatives of the genera Forda and Paracletus, both exemplifying taxonomic and ecological problems, were investigated in greater depth through extensive sampling and morphometric analysis. A total of eight, six and six samples from F. formicaria, F. marginata and F. riccobonii, respectively, were included in the survey, along with 40 and 4 samples classified initially as P. cimiciformis and P. donisthorpei, respectively. Our results are relevant to current studies on the evolution of host selection by aphids and on the evolution of gall morphology. Our phylogeny suggests that the group can be divided into two main clades. One clade is composed of aphids inducing small, low‐capacity galls on either P. atlantica or P. terebinthus (Smynthurodes betae and genera Forda and Paracletus). The second clade is composed of species inducing larger galls on P. lentiscus and P. terebinthus (Aploneura lentisci, Baizongia pistaciae and Geoica species). Our results suggest that revision of diagnostic characters used in the taxonomy of Paracletus is needed, and suggest Forda rotunda as a new synonym of Paracletus cimiciformis syn.n.