A sample of 36 plants drawn at random from each of two further natural populations (R102 and R104) contained between them 30 and 26 different S-alleles respectively. In both samples, the frequencies of these alleles were significantly unequal. These results are compared with those presented in a previous paper (the R106 sample; Campbell and Lawrence, 1981b) and with those obtained by Emerson (1939) from the Oenothera population. Unequal S-allele frequencies appear to be characteristic of natural populations of P. rhoeas in the West Midlands and probably also elsewhere in the British Isles. The number of S-alleles found in these samples suggest that the number of alleles in the natural populations from which they were obtained is unlikely to be less than the number that Emerson eventually found in the Oenothera population, namely, 45. However, if this is indeed the case, it is no longer possible to regard P. rhoeas as a colonising species; on the contrary, on the evidence obtained from these three populations, it appears to be a permanent member of the flora of wayside places.

In the course of studies of immunity to experimental cholera in man, 10(5) or 10(6) Vibrio cholerae were given to 66 college students and other community volunteers under quarantine in an isolation ward. HLA antigen and blood group determinations were carried out to test the hypothesis that severity of clinical cholera is dependent in part upon genetically-determined host susceptibility. Fifty-five volunteers developed diarrhoea; 38 had mild illness and 17 had severe cholera (stool volume greater than or equal to 5.0 litres). HLA antigens were found in similar frequency in volunteers with severe, mild or no diarrhoea; antigen A1, A2, A3 and B7 were most common. Blood group O, however, was found in 64% of persons with severe cholera versus 36-38% of volunteers with mild or absent illness. Thus, while no correlation was found between HLA type and severity of cholera, these results do support the claims of other investigators that blood group O is found more frequently in patients with severe cholera than in the normal population.

SUMMARYThree methods of estimating the number of S-alleles in a population have been proposed in the literature (Bateman, 1947;Whitehouse, 1949;Paxman, 1963). These methods, all of which assume that genotype frequencies in the population are equal, are described briefly and are used to estimate the number of alleles in populations of Trifolium pratense (Williams and Williams, 1947), Oenothera organensis (Emerson, 1939) and Papaver rhoeas (Campbell and Lawrence, 1981 b;Lawrence and O'Donnell, 1981). The estimates yielded by Bateman's and Whitehouse's methods are similar to those given by Paxman's maximum likelihood method with the Trifolium and Oenothera data where there is little reason to suppose that the allele frequencies are other than equal. Bateman's method, however, breaks down when used on the Papaver data in which the S-allele frequencies are known to be unequal; and Whitehouse's and the maximum likelihood methods yield estimates which are biased downwards when used on these data.An attempt has been made, therefore, to devise two new estimators of the number of S-alleles in a population which do not assume that their frequencies are equal. The properties of these estimators has been investigated with data from eight populations generated on the computer in which the numbers and frequencies of alleles are known. One of these new estimators (E2) yields estimates which are less biased downwards than those given by Paxman's method when allele frequencies are unequal, but gives estimates which are biased upwards when these frequencies are equal. The other estimator (E1) is generally less satisfactory than the first, particularly when the number of alleles in the population is large. Though neither of these new estimators are wholly satisfactory, there is some justification for using E2 when allele frequencies are known to be unequal. Estimates given by E2 when used on the Papaver data range from 34 to 42 alleles which, bearing in mind that these estimates are still likely to be biased downwards, suggests that the number of alleles in natural populations of this species is likely to be between 40 and 45.A new procedure for calculating confidence intervals for maximum likelihood estimates, assuming equal allele frequencies, is also described and applied to the Oenothera and Papaver data.

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We conducted a double-blind, placebo-controlled, randomized crossover study to evaluate the safety and immunogenicity of a single 5 x 108-CFU dose of live oral recombinant cholera vaccine CVD 103-HgR in 94 North American adults. The vaccine was well tolerated without associated adverse reactions. Despite minimal fecal excretion of vaccine, 97% of subjects exhibited serum vibriocidal antibody and 72% had antitoxin responses.