288 Parasitology Today, vol. 8, no. 8, t 992

In future, novel techniques for con- structing and screening gene-enriched DNA libraries by PCR-driven DNA- DNA competitive hybridization t9 or subtraction procedures 2° will be useful to isolate complete repertoires of sex- specific genes from various schistosome species and life cycle stages. Also, quan- titative PCR 21'22 could be used to study developmentally regulated expression of female-specific genes in the different life cycle stages of schistosomes.

Laboratory studies have shown that the duration of infection in the mam- malian host has an influence on the sex ratio of the next parasite generation 2, and that there is a preponderance of male schistosomes in chronic infections, which may be due to greater infectivity of male miracidia for snails, greater numbers of male cercariae in infective doses and/or a higher degree of survival of male schistosomula and adult male worms ~, The PCR technique Is has shown that 50-day-old infections in mice produce 45% male eggs and infections of greater than 100 days produce approximately 80% male eggs. These preliminary results are similar to those achieved by others using the propagation techniques ~'2. Most recent studies suggest immunological mechan- isms for the selective destruction of female eggs in infected mice 23, and it may be that genes linked or associated with the W-chromosome in the hetero- gametic females are involved in the expression of antigens responsible for this selective destruction. There is, as

yet, no direct evidence to support these hypotheses, which leaves this exciting field open to investigation at a mol- ecular level. In future, it should be possible to determine quantitatively the sex ratios of eggs in histological sections of infected livers by in situ-hybridiz- ation 24 or modifications of PCR pro- cedures for paraffin-embedded sec- tions 2s. Hybridization experiments by Gasser et al. (unpublished) have shown that the radioactively labelled W l PCR product can discriminate between two different populations of S. rnansoni eggs in infected mouse livers, which may be females and males. However, further studies are essential to evaluate this technique.

Acknowledgements I thank Graham Mitchell (Royal Melbourne Zoological Gardens), Don McManus (Queensland Institute of Medical Research) and Nell Chilton (University of Melbourne) for comments on the manuscript. Financial support by the Australian National Health and Medical Research Council and the Rockefeller Foundation is gratefully ac- knowledged (G. Mitchell).

References I Liberatos, J.D. (1987) Exp. Parasitol. 64,

165-177 2 Mitchell, G.F. et of. (1990) Parasitology 101,

27-34 3 Liberatos, J.D, and Short, R.B, (1983)

J. Parasitol. 69, 1084-1089 4 Short, R.B, et al. (1989)J, Parasitot, 75,

920-926 5 Webster, P. et al. (1989) Mol. Biochem,

Parasitol. 36, 217-222

6 Bobek, L. et of, (I 986) Proc NatlAcad. Sci. USA 83, 5544-5548

7 Davis, A,H. et al. (1986)Proc Natl Acad, Sci. USA 83, 5534-5538

8 Simpson, A.J.G. and Knight, M. (1986) Mol. Biochem. Parasitol, 18, 25-35

9 Johnson, K.S. et al, (1987) Mol. Biochem. Parasitof, 22, 89- 100

I 0 Reis, M.G, et at. (I 989) Mot. Biochem. Parasitol. 32, 113-120

II Spotila, L.D, et al. (i987) Mol, Biochem. Parasitot. 26, 17-20

12 Simpson, A.G.J. et al. (1982) Mol. Biochem, Parasitol. 6, 125-137

13 Walker, T.N, et al. (1989) Mol. Biochem. Parasitol. 33, 93-100

14 Wright, M.D. et al. (1990) J. Immunol. 144, 3195-3200

15 Gasser, R.B. et al. (1991) Mol. Biochem. Parasitol. 47, 255-258

16 Saiki, R,K. et al. (1988) Science 239, 487-494 17 Erlich, H.A. et at. (1991) Science 252,

1643-1651 18 Kemp, D.J. et al. (I 990) Gene 94, 223-228 19 Lebeau, M.C. et al. (I 991 ) Nucleic Acids Res.

19, 4778 20 Swaroop, A. et al. ( 1991 ) Nucleic Acids Res. t 9,

1954 21 Wang, A,M. and Mark, D.F. (1990)in PCR

Protocols (Innis, M.A. et of., eds), pp 70-75, Academic Press

22 Gilliland, G. et al. (1990) in PCR Protocols (Innis, M.A. et al., eds), pp 60-69, Academic Press

23 Mitchell, G,F. et al. (1991) Adv. Parasitol. 30, 167-200

24 Hirai, H, et at. (1989) Exp. Parasitol. 69, 175-188

25 Wright, D.K, and Manos, MM, (1990)in PCR Protocols (Innis, M.A. et al., eds), pp 153-158, Academic Press

Robin Gasser is at the Faculty of Veterinary Science, The University of Melbourne, Princes Highway, Wembee, Victoria 3030, Australia.

Schistosomes: Development,

Reproduction and Host Relations

by Paul F. Basch, Oxford University Press, 1991. £45.00 (ix + 248 pages)

ISBN 0 19 505807 0

Paul Basch, a leading schistosomologist of three decades' standing, goes about his task with admirable gusto, writing, for example, that 'a pair of schistosomes in copulo roughly resembles an ana- conda in a canoe'! This book is for those enthusiasts who believe, with the author, that 'As a key part of the eggmaking apparatus... (Mehlis' gland)

... is deserving of concentrated atten- tion' or that 'The conjugal relations between male and female schistosomes and the natural history of their sexual development represent one of the most interesting chapters in all of para- sitology'.

There are five chapters, beautifully illustrated with drawings and lithographs from classical papers, scanning and transmission electron micrographs, and extensive tabulations of data. The fol- lowing areas are covered: a general overview of schistosomes; develop- ment in the mammal host; structure and physiology of adults and eggs; repro- ductive biology; development in the snail host. The author's laudable aim is 'to facilitate the work of those investi- gators who wish to apply advanced techn!ques to schistosomes, but who

lack familiarity with the fundamental biology of these organisms'. However, competition is already fierce in this area. Basch's monograph covers his chosen topics fully and accurately but does not address schistosome biochemistry and molecular biology, immunology, patho- genesis, epidemiology, chemotherapy or control, all of which are discussed in the comprehensive and competitively priced Biology of Schistosornes: From Genes to Latrines, edited by Rollinson and Simpson and published in 1987. And for clinical parasitologists, there is also the authoritative 1987 publication Schistosomiasis, edited by Mahmoud.

Martin Taylor Department of Medical Parasitology London School of Hygiene and Tropical Medicine Keppel Street, London, UK WC I E 7HT