Critical Periods in Development of Ichthyophthirius multifiliis (Ciliophora) Populations1

ABSTRACT. Three periods in development that strongly influence population dynamics of Ichthyophthirius multifiliis were identified in experimental infections of channel catfish. The first occurred upon establishment within the host, 0 to 10 min postexposure (PE), when the parasite population that gained entrance declined 50%. Survival from 10 to 45 min PE, however, was constant. The second period identified came after I. multifiliis left the host and the free-living tomont encysted. The third occurred during reproduction. Although survival of encysting tomonts approached 100% among individuals departing after three to five days residence in the host, theront production varied significantly with parasite size, culture temperature during development, and length of residence by the trophont in the host. Theront production per tomont increased daily and on days three, four, and five PE was significantly higher for parasites developing at 24°C than for those at 21°C. At five days PE, mean production was 562 theronts/tomont and 240 theronts/tomont, respectively, and production by tomonts of equal size was greater for parasites maintained at 24°C.     

Lack of correlation between the coat protein composition of tobacco mosaic virus isolates and their ability to infect resistant tomato plants

The differing responses of a wide range of tobacco mosaic virus (TMV) isolates to various genes controlling resistance in tomato were not correlated with the amino acid composition of their coat proteins. Nor were they necessarily linked with ability to cause necrotic local lesions in White Burley tobacco, which is usually associated with the presence of methionine in the coat protein.     

Sex Relations and Gender Relations: Understanding Kwoma Conception

An analysis of Kwoma conception theory shows that the Kwoma regard men and women as mutually dependent equals, and that they regard women as the fertile sex that men must capture and keep so as to reproduce their patrilineages. The relationship between men and women is analogous to that between Kwoma and the bush spirits, sikilowas, the true source of all fertility and well-being.     

3,5-Dichlorophenoxyacetic acid as a growth retardant

Tomato seedlings, grown in the glasshouse, were sprayed with solutions of 3,5-dichlorophenoxyacetic acid as sodium salt at 2 times 10^-5, 4 times 10^-5 and 10^-4 M. The treated plants became dark-green, dwarfed, and compact. After 6–7 weeks normal growth was resumed. Measurements and analytical data on treated and control plants are presented.     

Fluctuations in populations of wheat bulb fly (Leptohylemyia coarctata) at Rothamsted

Several field experiments at Rothamsted have rotations including plots fallowed before sowing to winter wheat each year, which provide ideal conditions for maintaining infestations of wheat bulb fly (Leptohylemyia coarctata). In eight consecutive years of one experiment, a mean of 13% of the eggs laid on fallows in these rotations eventually survived to become adults. There was a considerable variation between years, ranging from a maximum survival of 28 % to a minimum of 7%. Most deaths occur in the larval stage, and although previous work showed that survival of the larvae increases with the number of shoots available for infestation, much of the variability in survival rates between seasons was apparently caused by other factors, possibly climatic. Eggs laid in fallows were sampled in most years from 1953 to 1972. They varied from 3.57 to 0.02 million/acre (8.81–0.05 million/ha), depending on the season and site; the mean was 1.08 million/acre (2.67 million/ha). Egg populations varied in synchrony at all the three sites studied; there were cycles lasting several years between peaks of abundance or scarcity but there was no consistent tendency for populations to increase or decrease. Fewest eggs were laid when the weather was cold and wet during July and August, i.e. when adults are active. Weather was more closely correlated with the number of eggs laid expressed as a fraction of those laid in the previous year (net reproductive rate) than with the number of eggs laid/acre.     

GENETIC AND CONCHOLOGICAL COMPARISON OF SNAILS (HELIX ASPERSA) DIFFERING IN SHELL DEPOSITION OF LEAD

Populations of snails inhabiting areas with different historiesof Pb contamination differed in their deposition of Pb in shellrelative to soft tissues. Genetic variation, measured usingisozymes, was not related to Pb history nor geographic distancebetween populations. Shell characteristics were significantlydifferent among sites; shell dry weight was strongly relatedto soil calcium levels. Shells of snails from areas with longhistories of Pb contamination were significantly more robust(greater shell width/shell height ratio) than snails from otherlocations. H. asprsa adaptation to Pb contamination may involvesignificant changes in shell characteristics but these do notcorrelate with genetic traits assessed with allozymes (Received 29 December 1994; accepted 15 October 1995)