All-female broods and mimetic polymorphism in Acraea encedon (L.) (Lepidoptera: Acraeidae) in Tanzania

In some populations of the African butterfly, Acraea encedon, there are two kinds of females, one producing offspring in a normal 1:1 sex ratio, the other producing females only; in other populations the sex ratio is apparently normal. All-female broods had hitherto been mainly associated with populations in which field sampling revealed an excess of females. The all-female brood trait is described from a population at Dar es Salaam which field sampling suggested was normal, and this indicates that the trait may be much more widespread and common than had previously been supposed. This discovery also extends the known distribution of the trait across Africa from Sierra Leone to eastern Tanzania. The butterfly is also a polymorphic Müllerian mimic of Danaus chrysippus, which is a highly unusual phenomenon as Müllerian mimicry is almost invariably monomorphic. The relative frequencies of two corresponding colour forms of the two species of butterflies at Dar es Salaam adds support to the hypothesis that they are indeed Mullerian mimics. The results of breeding experiments suggest that the polymorphic forms in Acraea encedon are allelic with dominance.     

Anisophylly of Lateral Shoots

The inequality shown by anisophyllous leaves in plants withdecussate phyllotaxis can be measured as the difference in sizebetween the two leaves at a node, expressed as a percentageof their mean size. This is referred to as the degree of anisophylly. In Acer anisophylly of two types with different causation occurs.

1. 1. Primary anisophylly which is shown only by first year shootsdeveloped from axillary buds and which is due to the inequalityof the leaf primordia. Primary anisophylly is thus determinedby events occurring during the formation of the bud and is irreversible.
2. 2. Secondary amsophylly which is shown chiefly by the second(and subsequent) year's extension growth of lateral shoots frombuds which are isophyllous and which is due to the positionin which the shoot develops. This type of anisophylly is reversibleif the position of the developing shoot be changed.

Effects of secondary anisophylly may be superimposed on thoseof primary anisophylly. The recognition of these two causally distinct types of anisophyllyoccurring in one plant does much to resolve the apparently conflictinginformation available relating to lateral anisophylly.     

Control of clubroot by soil partial sterilization with particular reference to the treatment of small natural foci of infection

Experiments were made on field plots with four soil partial sterilants applied at different dose rates or as combination treatments. The effects of treatments on clubroot were assessed by growing cabbages on the field plots or in a glasshouse in soil removed from the plots. In two experiments, no plants grown on plots or seedlings grown in soil samples from plots treated with Dowfume M.C.2 or Telone were infected with clubroot, whilst only one infected seedling was found in soil samples from those plots treated with chloropicrin. Although relatively few infected plants were recovered from Basamid treated plots, many infected seedlings were found in soil samples from those plots. The effectiveness of these treatments as indicated by seedling survival was Dowfume M.C.2 < chloropicrin < Telone < Basamid. At five sites a combination of Dowfume M.C.2 at 98 g/m² with 40 g/m² Basamid resulted in all cabbage seedlings grown in soil samples being disease free.     

EGG LAYING BEHAVIOUR OF BULINUS OCTOPLOIDUS BURCH (BASOMMATOPHORA: PLANORBIDAE)

Egg laying behaviour of Bulinus octoploidus in the laboratorycomprises three phases: (1) grazing, (2) deposition and (3)sweeping. The grazing phase lasts about 60–90 min andcan be recognized by the slowness of forward movement, movementin a restricted area, extended pseudobranch, and regular radularmovements against the substratum. Radular movements stop onlyduring turns and have a maximum frequency of about 45 min^–1.Egg mass deposition occurs at the end of the grazing phase andis accomplished in 1 min or less. Egg mass sweeping is a post-depositionphase lasting 8—12 min, during which the snail "sweeps"back and forth across the egg mass, apparently to anchor andflatten it. ¹ Authors' present address: Museum of Zoology, The Universityof Michigan, Ann Arbor, Michigan 48109, U.S.A. (Received 23 January 1979;     

A defensive endosymbiont fails to protect aphids against the parasitoid community present in the field

1. The value of protective mutualisms provided by some facultative endosymbionts has been well demonstrated in the laboratory, yet only recently has their effectiveness in the field been studied. ‘Candidatus Hamiltonella defensa’ is known to defend aphids from parasitoid wasps in laboratory trials. However, the efficacy of this defence varies among parasitoids, suggesting that protection will vary spatially and temporally depending on parasitoid community composition. 2. This demonstrated specificity and a dearth of studies on Hamiltonella in the field prompted the authors to quantify parasitism rates of Hamiltonella‐infected and ‐uninfected Aphis craccivora Koch aphid colonies in a manipulative field study. 3. It was found that A. craccivora in central Kentucky alfalfa were parasitised by Lysiphlebus testaceipes (Cresson) and Aphelinus sp. Surprisingly, Hamiltonella infection did not lower successful parasitism by the naturally occurring parasitoid wasps. Whether Hamiltonella was effective against L. testaceipes was subsequently tested in a controlled laboratory assay, and no effect on parasitism rate was found. 4. This study emphasises the fact that defensive symbionts sometimes provide no tangible defensive benefits under field conditions, depending on parasitoid community composition. It is hypothesised that the protective mutualism may be beneficial in geographically localised areas. When the symbiosis is effective against a local parasitoid community, aphid clones may experience eruptive population growth and rapidly disperse across a large area, allowing spread to habitats with different parasitoid communities where the mutualism is an ineffective defence.     

Microbial community structure and biogeochemistry of Miocene subsurface sediments: implications for long-term microbial survival

Thirty closely spaced cores were obtained from Miocene-aged fluvial, lacustrine and palaeosol subsurface sediments ranging in depth from 173 to 197 m at a site in south-central Washington to investigate the size and composition of the microbial community in relation to sediment geochemical and geophysical properties. Total phospholipid fatty acid (PLFA) analysis indicated that the greatest concentrations of microbial bio-mass were in low-permeability lacustrine sediments that also contained high concentrations of organic carbon. Community structure, based on lipid analyses and on in situ hybridization of bacterial cells with 16S RNA-directed DNA probes, also revealed the presence of metabolically active bacteria that respire sulphate and/or Fe(III) in the lacustrine sediments. Concentrations of pore water sulphate were low (4–8 mg/L) and HCI-extractable Fe was predominantly Fe(II) in the same samples where total biomass and organic carbon were highest. The low hydraulic conductivity (10^-6 to < 10^-9 cm/s) of these sediments has likely contributed to the long term maintenance of both bacteria and organic carbon by limiting the supply of soluble electron acceptors for microbial respiration. These results suggest that the current subsurface microbial population was derived from organisms that were present during lake sedimentation = 6–8 million years ago.