PERIZONIUM AND INITIAL VALVE FORMATION IN THE DIATOM NAVICULA CUSPIDATA (BACILLARIOPHYCEAE)1

This paper describes the perizonium and initial valve formation in Navicula cuspidata Kütz., based on light microscope (LM) and scanning electron microscope (SEM) observations. The perizonium consists of concentric over-lapping bands, laid down sequentially at the tips of the expanding biconical auxospore during its elongation. The central perizonial band has fimbriate edges and is considerably more rigid than the more distal bands. During auxospore elongation and the band secretion, the chloroplasts continuously oscillate between the two ends of the cell; this oscillation ceases once the elongation is complete. The initial valves, formed within the perizonium, are molded into the basically biconical shape of the perizonium except for a central flattening of each valve face. In contrast to the raphes in gametangial and vegetative valves which are surrounded by a smooth axial area, the raphes in initial valves lie within a raised ridge running along the apical axis of the valve. The regular pattern of apically oriented ridges on the outer surface of vegetative valves is also lacking on initial valves. Comparison of pore–pore spacing within striae of gametangial valves, initial values and post-initial valves (first division and vegetative cells) reveals that the pore–pore distance within striae is conserved at all sexual stages. However, the distance between striae is considerably larger in initial valves than in gametangial and post-initial valves. Vegetative interstriae spacing as well as the planar morphology of the valve face is regained at the first division of the initial cell. This suggests that the spacing between striae is dependent on the sexual stage of the cell during valve formation (i.e. not directly dependent on the cell size) and can be altered independently of the pore–pore spacing.     

Mutual support and selection between branches of damaged plants

Snow (1931) showed that the fate of a branch of a plant that had suffered local herbivory could be determined by correlative effects of other branches. This neglected work was continued in the current study, herbivory being simulated by the removal of leaves of different ages from pea plants with two branches. A damaged branch was suppressed when an undamaged alternative branch was present; otherwise the damage never prevented continued development. The removal of mature leaves had a smaller effect than the removal of immature, expanding leaves. When leaves were removed from both branches it was the branches that suffered less damage to their immature leaves that continued shoot development. Branches from which all photosynthetic leaves were repeatedly removed developed only when they retained their immature leaves and remained dominant, inhibiting the development — but not the photosynthesis — of the other branch on the same plant. Accounting for these results requires mechanisms that compare the different branches of the plant and select for development the ones that have the greatest potential for future, rather than present, photosynthesis. It is concluded that a compartmentalized or sectorial distribution of essential substrates can be modified by correlative relations that are probably mediated by hormones.     

Integrating conservation and development: incorporating vulnerability into biodiversity-assessment of areas

Protection of regional biodiversity requires that priority for protection of individual areas be based on both the contribution the area can make to representing overall biodiversity and the degree to which the area, in the absence of action, is vulnerable to loss of its biodiversity. Attempts to apply these criteria together largely have been ad hoc. A solution to this problem is presented for environmental surrogate data, based on environmental diversity (ED). ED uses a standard ecological continuum model to link environmental pattern to species-level biodiversity, so that a set of areas can be characterized by its relative expected biodiversity. This allows explicit incorporation of estimates of area-vulnerability, interpreted as the relative probability that any member species will not persist, into biodiversity assessments. The contribution of a given area to regional expected biodiversity is influenced not only by its own vulnerability value, but also by the vulnerability of other areas. Increasing the degree of protection of any area (reducing its vulnerability) will increase expected biodiversity: however, expected regional biodiversity sometimes may be maximized when limited resources for protection are directed to an area with lower vulnerability rather than to one with higher vulnerability.The allocation of land uses in a region need not be viewed as an all-or-nothing assignment of protection. The effect of a particular management regime on the biodiversity of a given area can be equated with some consequent degree of vulnerability; viewed positively, a management regime that offers some degree of biodiversity protection can make a measurable contribution to the protection of the biodiversity of a region.     

Maximum-likelihood models for mapping genetic markers showing segregation distortion. 2. F2 populations

In F₂ populations, gametic and zygotic selection may affect the analysis of linkage in different ways. Therefore, specific likelihood equations have to be developed for each case, including dominant and codominant markers. The asymptotic bias of the classical estimates are derived for each case, in order to compare them with the standard errors of the suggested estimates. We discuss the utility and the efficiency of a previous model developed for dominant markers. We show that dominant markers provide very poor information in the case of segregation distortion and, therefore, should be used with circumspection. On the other hand, the estimation of recombination fractions between codominant markers is less affected by selection than is that for dominant markers. We also discuss the analysis of linkage between dominant and codominant markers.     

The adaptive significance of colour pattern polymorphism in the Australian scincid lizard Lampropholis delicata

Females of Lampropholis delicata are dimorphic for colour pattern, the difference between morphs being the presence or absence of a distinct white mid-lateral stripe. A less distinct striped morph occurs also in males. We evaluated alternative hypotheses for the maintenance of this polymorphism by examining temporal and spatial variation in morph frequency, testing for differential selection among morphs using data on body size and reproductive traits from preserved specimens, and experimentally manipulating colour pattern in free-ranging lizards of both sexes, to assess the influence of the lateral stripe on survival rates. We found that the relative frequency of striped individuals varied among populations and decreased from north to south in both sexes, coincident with an increasing incidence of regenerated tails. Morph frequencies did not change through time within a population. Striped gravid females appeared to survive better and produced larger clutches than did non-striped females. In our experimental study, the relationship between survival and colour morph differed between the two sexes; males painted with a white lateral stripe had lower survival than control (brown stripe) males, but survival did not differ between striped and control females. The different response in the two sexes may be due partly to differences in temperature and microhabitat selection. We propose that the white lateral stripe decreases susceptibility to predators in gravid females but increases risk of predation in males, especially in combination with low temperatures. The polymorphism might be maintained by: (1) opposing fitness consequences of the stripe in males and females; (2) sex-specific habitat selection; and (3) gene flow in combination with spatial variation in relative fitness of the two morphs.     

Decrease in tolerance to fenvalerate, in resistantHelicoverpa armigera after pupal diapause

Pyrethroid resistant and susceptible adults ofHelicoverpa armigera (Lepidoptera: Noctuidae) were screened for tolerance to pyrethroids after 6 wk or 12 wk pupal diapuse. Resistant larvae and F₂ larvae from a cross between resistant and susceptible parents (two replicates), were reared under conditions to induce pupal diapause. After eclosion, adults were tested in glass vials coated with the pyrethroid fenvalerate, at a dose (DD) that is known to discriminate between susceptible and heterozygous resistant individuals. In all diapause experiments, the frequency of resistance was lower in the test groups that had experienced diapause compared with the non-diapausing control group. The underlying cause of the decline is not certain but selective mortality of resistant versus susceptible individuals could not account for all the difference in two of the three experiments — tolerance to the pyrethroid, fenvalerate, is most likely to have declined either as a consequence of diapause or from the extended time of development associated with diapause. These results indicate that monitoring programs could underestimate pyrethroid resistance frequencies when usingH. armigera adults emerging from diapause.     

Feeding behaviour of Cerastoderma edule in a turbid environment: physiological adaptations and derived benefit

Resuspension of bottom sediments by wind andtide-driven currents often occur in shallow waters,coastal embayments and estuaries. These processes maylead to dramatic variations in the concentration andorganic richness of suspended particle assemblages.Since resuspended matter is mainly inorganic,decreasing organic contents are usually associatedwith higher seston loads.Under this environmental context the feeding behaviourof bivalves shows a wide degree of plasticity, whichhas been interpreted as having high adaptive value. Inorder to evaluate benefits derived from thisbehaviour, we have used functional relationshipsobtained in previous studies, relating feedingparameters to characteristics of suspended food, topredict the effect that different feeding responseswould have exerted. In cockles, main processesdetermining energy acquisition are feeding rates andpreingestive food selection. Thus, the procedurefollowed in the present work consisted of simulatingrates of food absorption under alternative feedingbehaviours characterised by: (a) no preferentialingestion of filtered organic matter and (b) maintenance ofconstant clearance rates. In theabsence of selection of organic matter at thepreingestive level, ingestion rate of organics (OIR)would decline with increasing seston loads to 30% ofvalues predicted by functions fitted to experimentaldata; difference in absorption rate (AR) would be evengreater, falling to 10%, due to the strong effectthat the organic content of ingested matter exerts onabsorption efficiency. On the other hand, had theclearance rate (CR) kept constant despite theincreasing seston load, OIR and AR would have fallento values respectively 30% and 49% lower than actualvalues.From these results it is concluded that the ability ofsorting particles before ingestion and the capabilityof adjusting clearance rate are key elements in thefeeding behaviour that enable cockles to be welladapted to cope with changes in the water columncaused by resuspension events.     

Cryptic coloration and choice of escape microhabitats by grasshoppers (Orthoptera: Acrididae)

Cryptic coloration is found in many Orthoptera, especially in Acrididae, showing a great variety of forms. In a grasshopper assemblage in southeastern Brazil, preferences for escape places were detected; grasshoppers tended to escape to backgrounds in which they seem to be more cryptic. Coloration was measured using the Simpson diversity index, to quantify 'aspect diversity' (diversity of colours and shapes of patches along the insect's body). A weak positive correlation was found between grasshoppers' aspect diversity and diversification in use of escape places (use of many backgrounds to escape). Grasshoppers with similar colour patterns tended to use the same structures (leaves, sandy soil, stones) to escape.     

Density- and frequency-dependent predation of artificial bird nests

Predators are often expected to vary their relative predation rates according to the frequency of prey types in the environment (frequency-dependent predation). The underlying cause for this must lie in some dependency of absolute predation rates on the density of prey types in the environment (density-dependent predation). However, frequency-dependent predation may either be caused by 'simple' density-dependent predation, in which the absolute predation rate on a given prey type depends purely on the density of that type, or by more complex responses in which absolute rates depend also on the density of other prey types. It is usually difficult to distinguish the underlying cause of frequency-dependent predation, because frequencies tend to change as densities change. Here, we describe the results of an experiment conducted to disentangle these phenomena under two prey richness (low and high) conditions. We used artificial bird nests (placed on shrubs and on saplings) baited with quail eggs placed in natural forests as models of natural bird nests. Our results indicate that both the absolute and relative predation rates on the prey types may vary in complex ways. Predation rates depend on a complex interaction between the prey's own density, other prey density and the diversity of prey in the environment. Neglecting to include, or consider, these complexities into analyses may lead to erroneous conclusions in studies of absolute or relative predation rates.     

Selection of a proteolytic enzyme to solubilize lean beef tissue

Many proteases are available for the hydrolysis of various protein substrates. The qualitative effect of most experimental variables on reaction progress is known, so it is possible to devise a rational procedure for selecting the best enzyme. Reaction time and enzyme concentration should be chosen in the region where they have little effect on reaction progress. Substrate concentration should be low to avoid possible product inhibition. Each enzyme should be tested at its optimum pH, and at a range of temperatures around (mainly below) the reported temperature optimum. Enzyme cost and other relevant factors should also be considered in the enzyme selection. Using this selection procedure Alcalase was chosen as the most appropriate enzyme for solubilizing lean beef tissue.