A cohort of newly settled Mytilus edulis (L.) (Mollusca: Bivalvia) spat (mean shell length 530 µm) was collected from red filamentous algae at a site on the North Wales coast. After a period of growth in the laboratory, cellulose acetate and starch gel electrophoresis were used to investigate genotype frequencies at the Gpi, Odh, Lap, Pgm, Idh and Mpi loci. Individual testing of each locus, using the X2 test revealed significant deficiencies of heterozygotes at the Pgm, Mpi and Idh loci. However, further testing using a sequential Bonf erroni test, designed to assess the probability of observing at least one significant result from a number of tests by chance alone, revealed that only the deficiency of heterozygotes at Pgm and Mpi loci was significant at the table-wide 95% level. Since post-collection mortality was <3%, these heterozygote deficits must have existed at the time of collection. The conclusion drawn is that heterozygote deficiencies in mussels are generated during the larval stage or at early settlement.

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Cohorts of meiosis I and meiosis II induced triploid Mytilus edulis were produced from mass matings in the laboratory and reared alongside normal diploid cohorts during 1990 and 1991. Diploid cohorts generally exhibited a significant positive correlation between multilocus heterozygosity and size. This correlation was absent or much more weakly expressed in cohorts.of triploid mussels and this supports the 'associative overdominance' hypothesis rather than the 'direct involvement' hypothesis as the major explanation for the correlation. For both diploids and triploids, no correlation between heterozygosity and the coefficient of variation of size was evident and heterozygotes at single loci were not larger than homozygotes. Triallelic triploid heterozygotes were no larger than diallelic triploids. The diploid cohorts were generally in agreement with the Hardy-Weinberg model and there was no trend towards heterozygote deficiency.

A bioassay method for the polyene antibiotics nystatin and amphotericin B is proposed based on the measurement of the efflux of rubidium ions from a rubidium-loaded yeast culture challenged with the antibiotics. For this purpose a major proportion of the intracellular K+ ions in a Saccharomyces cerevisiae culture has been substituted by Rb+ ions. The rubidium leakage is measured by atomic absorption spectrophotometry, and a straight-line, dose-response correlation has been obtained for both antibiotics.It has been recognized since the early 1960s that one of the principal effects of polyene antibiotic action on susceptible microorganisms is the leakage of vital cytoplasmic constituents from the cells (8-11). The work of Sutton et al. (11) and Marini et al. (10) highlighted the leakage of K+ ions and NH4+ ions particularly. Since these early papers, many authors have examined induced potassium efflux from yeast cells by polyenes (4,5,7,(12)(13)(14).From some of the literature referred to above a correlation between polyene concentration and potassium efflux can be identified, and the prognosis for a possible rapid bioassay of this class of antibiotics, by measuring ion release from cells, is good.Smithler To overcome this problem, an alternative ion, which could be introduced into the yeast cells without affecting their viability or membrane function but which had a very low level of natural occurrence, was sought.To reproduce as closely as possible the characteristics of K+ ion leakage across the nystatin-impaired yeast cell membrane, it is necessary to find an ion of like charge and ionic radius. K+ ions exist in solution in a hydrated form, and the hydrated ionic radii of K+ (0.232 nm) and Rb+ (0.228 nm) are very similar, as are the ionic mobilities of these two ions.Rubidium ions are known to be well tolerated by erythrocytes, and an examination of the literature revealed a number of papers reporting the uptake of Rb+ ion by the yeast S. cerevisiae (1-3). Rubidium can also be determined at low levels with precision by atomic absorption spectrophotometry. It was therefore decided to examine the feasibility of using Rb+ uptake and subsequent release from yeast cells by the action of the polyene antibiotics nystatin and amphotericin as a bioassay system. The work reported here clearly shows such a system to be feasible in that a linear dose-response relationship can be obtained. MATERIALS AND METHODSYeast culture. S. cerevisiae SQ.1600 (from the Squibb Culture Collection) was used throughout.

A method has been developed for the determination of chlorhydroxyquinoline (Halquinol) in medicated pig feeds. Because of the interference by feed constituents in simple spectrophotometric and polarographic assay procedures, a spectrofluorimetric procedure is recommended. Spectrofluorimetric measurements are taken in a methanolic solvent containing 5 per cent. of chloroform, and the fluorescence of chlorhydroxyquinoline, as its magnesium chelate, is measured a t 500 nm, with an excitation wavelength of 402 nm. Cyanide is used to suppress interference from copper and zinc salts that are commonly added to these feeds. The procedure is not affected by the presence of other feed additives, such as dimetridazole and arsanilic acid.