Knell, R.J., Begon, M. and Thompson, D.J. (1996) Transmission dynamics of Bacillus thuringiensis infecting Plodia interpunctella: a test of the mass action assumption with an insect pathogen. Proceedings of the Royal Society of London Series B (Biological Sciences), Vol.263 pp 75-81. Go
Knell, R.J., Begon, M. and Thompson, D.J. (1996) Comparative transmission dynamics of two insect pathogens. In: Frontiers of Population Ecology (Floyd, R.B., Sheppard, A.W. and De Barro, P.J., eds.), CSIRO publishing. pp 269-276.
Knell, R.J., Begon, M. and Thompson, D.J. (1998) Host-pathogen population dynamics, basic reproductive rates and threshold densities. Oikos Vol. 81 pp 299-308.Go
Knell, R.J. (1998) Generation cycles (review paper). Trends in Ecology and Evolution Vol. 13 No. 5 pp 86-190.Go
Knell, R.J., Begon, M. and Thompson, D.J. (1998) Transmission of Plodia interpunctella Granulosis Virus does not conform to the mass action model. Journal of Animal Ecology Vol. 67 pp 592-599.Go
Tomkins, J.L., Simmons, L.W., Knell, R.J. and Norris, K.A. (1999) Correlates of ball size and rolling speed in the dung beetle Kheper nigroaeneus (Coleoptera: Scarabaeidae). Journal of Zoology Vol 248 pp 483-487.Go
Knell, R.J., Fruhauf, N. and Norris, K.A. (1999) Conditional expression of a sexually selected trait in the stalk-eyed fly Diasemopsis aethiopica. Ecological Entomology Vol 24 pp 323-328.Go
Knell, R.J. (1999) Sexually transmitted disease and parasite mediated sexual selection. Evolution Vol 53 pp 957-961.Go
Boots, M. and Knell, R.J. (2002) The evolution of risky behaviour in the presence of a sexually transmitted disease. Proceedings of the Royal Society of London Series B (Biological Sciences), Vol.269 pp 585-589. Go
Proceedings of the Royal Society of London Series B (Biological Sciences), Vol.263 pp 75-81.
Robert. J. Knell, Michael Begon and David J. Thompson
Population Biology Research Group, Department of Environmental & Evolutionary Biology, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX, U. K.
SUMMARY
Central to theoretical studies of host-pathogen population dynamics is a term describing transmission of the pathogen. This usually assumes that transmission is proportional to the density of infectious hosts or particles and of susceptible individuals. We tested this assumption with the bacterial pathogen Bacillus thuringiensis infecting larvae of Plodia interpunctella, the Indian meal moth. Transmission was found to increase in a more than linear way with host density in fourth and fifth instar P. interpunctella, and to decrease with the density of infectious cadavers in the case of fifth instar larvae. Food availability was shown to play an important part in this process. Therefore, on a number of counts, the usual assumption was found not to apply in our experimental system
Journal of Animal Ecology Vol. 67 pp 592-599.
Robert. J. Knell, Michael Begon and David J. Thompson
Population Biology Research Group, Department of Environmental & Evolutionary Biology, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX, U. K.
SUMMARY
1) Transmission of insect pathogens is traditionally described by a term which states that transmission is proportional to the densities of the susceptible hosts and the infectious units, multiplied by a constant, the transmission coefficient. Theoretical studies suggest that deviations from this can be important in host-pathogen population dynamics, but little is known of how commonly pathogen transmission conforms to the conventional model.
2) We describe a test of the traditional assumption for the Indian meal moth, Plodia interpunctella (Lepidoptera: Pyralidae) (H¸bner) and its granulosis virus using a modification of the previous methods, which allows for unpredictable declines in the amount of infectious material present.
3) The estimated transmission coefficient increased with the density of susceptible hosts, and showed a marked decline with density of infectious cadavers. This suggests that the usual assumption does not adequately describe transmission in this system.
4) The reasons for this deviation from the usual assumption are likely to be a combination of behavioural and physiological changes at high host density and differential susceptibility to the pathogen leading to an effect analogous to pseudointerference in parasitoids.
KEY WORDS density-dependence, host-pathogen interactions, parasite-host models, transmission coefficient.
Oikos Vol. 81 pp 299-308.
Robert. J. Knell, Michael Begon and David J. Thompson
Population Biology Research Group, Department of Environmental & Evolutionary Biology, University of Liverpool, P.O. Box 147, Liverpool, L69 3BX, U. K.
ABSTRACT
Populations of the Indian meal moth, Plodia interpunctella (H¸bner) were maintained in the laboratory in the presence of 2 pathogens, P. interpunctella Granulosis Virus (PiGV) and Bacillus thuringiensis, for between 9 and 12 generations. Neither pathogen had any discernible effect on the population dynamics of the host, either individually or in combination.
The dynamics of the two pathogens were very different, with PiGV persisting within the host population and B. thuringiensis usually becoming extinct. This behaviour can be explained by the host threshold density and the relationship between host density and the basic reproductive rate (R0) for each pathogen. The host threshold density was considerably less for PiGV than for B. thuringiensis, indicating that PiGV would be able to persist in much lower density populations than B. thuringiensis.
None of the populations of P. interpunctella showed the generation-length cycles which have previously been described from similar experiments. This may be due to the quality, and possibly also the quantity of food available to the larvae.
Trends in Ecology and Evolution Vol. 13 No. 5 pp 86-190.
Robert J. Knell
Department of Zoology, University of the Witwatersrand, PB3, WITS 2050, Johannesburg, South Africa
Keywords: Generation cycle, intraspecific competition, parasitoid, pathogen.
Generation cycles are population cycles with a period of roughly one generation. They have been observed in tropical pest populations and in laboratory populations. Theory suggests that they can arise both from intraspecific competition and from the influence of natural enemies, and ecological studies of populations of insects showing these cycles are starting to shed light on the mechanisms that maintain them.
Journal of Zoology Vol 248 pp 483-487.
Joseph L. Tomkins1*, Leigh W. Simmons 1, Robert J. Knell 2 and Keith A. Norris2.
1 Department of Zoology, The University of Western Australia, Nedlands 6009, WA, Australia. email jtomkins@cyllene.uwa.edu.au
2 Department of Zoology, University of the Witwatersrand, Johannesburg, Private Bag 3, WITS 2050, South Africa.
* Author for correspondence
Keywords: Developmental instability, fluctuating asymmetry, performance, body size.
Abstract
1. Ball rolling in dung beetles is an energy expensive activity associated with elevated thoracic temperatures. The ability of individuals to engage in such energy expensive behaviours may be dependent on condition.
2. Bilateral asymmetries arising from the interaction between an individual's genes and its environment have been hypothesised to reflect an organisms quality. A number of studies have shown that individuals with elevated levels of asymmetry, perform less well under stress. We tested this hypothesis by measuring correlates of dung ball rolling speed in males and females of Kheper nigroaeneus (Coleoptera: Scarabaeidae).
3. We found that larger beetles produced larger dung balls and rolled them faster down an experimental track. However, we found no relationship between ball rolling speed and the asymmetry of fore and hind tibia, or the pooled asymmetry of both legs. There was also no relationship between the asymmetry of the beetle and the number of mites that it carried. The level of mite infestation did not influence ball rolling speed.
R. J. KNELL, N. FRUHAUF and K.A. NORRIS
Ecological Entomology Vol 24 pp 323-328.
Department of Zoology, University of the Witwatersrand, South Africa
Abstract.
1.An important assumption of many models of the evolution of sexually-selected ornaments is that such ornaments should be conditionally expressed. This was investigated by rearing larvae of the stalk-eyed fly Diasemopsis aethiopica (Diptera: Diopsidae) (Rondani) on two food treatments: normal and low-quality food.
2. Female flies responded to better food by increasing both their body size and the relative length of their eyestalks.
3. Male flies responded by increasing only the length of their eyestalks.
4. This demonstrates that food quality can change the allometric relationship between body size and the size of a sexually-selected character in this species, and gives an insight into the different developmental priorities of each sex in this species.
Key words. Allometry, conditional expression, Diasemopsis aethiopica, diet quality, Diopsidae, sexual selection.
Robert J. Knell
Evolution Vol 53 pp 957-961.
Department of Zoology, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa. Email robk@gecko.biol.wits.ac.za (NB this email address doesn't work anymore: try R.Knell@qmw.ac.uk instead)
ABSTRACT
Few studies have investigated the consequences of parasite-mediated sexual selection on the parasites involved. In some cases parasite-mediated sexual selection could lead to increased virulence, but I develop a simple model that shows that, if a parasite is sexually transmitted (i.e. is a sexually transmitted disease, or STD) and if mating success of the host is adversely affected by the parasite then less virulent STDs will be selected for because transmission of the STD depends on the mating success of the host. This selection for reduced virulence could have important consequences for the role of STDs in sexual selection.
Keywords: parasite, parasite-mediated sexual selection, sexually transmitted disease, STD, sexual selection, virulence
Michael Boots1 and Robert J. Knell2.
1 Institute of Biological Sciences, University of Stirling, Stirling FK9 4LA,
UK.
2 School of Biological Sciences, Queen Mary, University of London, Mile End
Road, London E1 4NS.
Proceedings of the Royal Society of London Series B (Biological Sciences), Vol.269 pp 585-589.
Summary
Sexually transmitted diseases (STDs) are widespread in nature, often sterilizing
their hosts or causing other pathogenic effects. Despite this there is a widespread
occurrence of behaviours that are likely to increase the risk to an individual
of contracting a STD. Here we examine the evolution of behaviours such as promiscuity
or mate choice that increase the risk of contracting an STD but also provide
a fitness benefit. As might be expected, the balance between risk and fitness
benefit defines the optimal strategy, but this relationship is not straightforward.
In particular we often predict the coexistence of highly risky and highly risk
averse individuals. Surprisingly, very safe strategists that only suffer a small
cost will tend to coexist with highly risky strategists rather than out compete
them as might have been expected. Rather than selecting for monogamy or for
reduced mate choice, therefore, the presence of an STD may often lead to variability
in either promiscuity or mate choice.
If you would like reprints of any of these please feel free to mail me.