Genetic Study on Variation among Strains of the Rotifer Brachionus plicatilis.

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We collected 67 rotifer strains ( Brachionus plicatilis ) from many localities all over the world, including Japan. At first, we divided these strains into two types (L,30; S,37) by judging morphologically with the shape difference of anterior spines in lorica. Then, for each strain, 20 individuals which multiplied from one individual were measured morphometrically in 7 body parts and 5 varieties, when they grew up until laying the first eggs. The cluster analysis indicated that 67 strains can be divided into two large clusters, each of which consist only of group of L or S strains.Isozyme variations of 67 strains were detected by horizontal starch gel electrophoresis with a little amount of drip thawed from frozen stock as a crude enzyme. Six loci on 6 enzymes were interpretable as isozyme. Large differences in isozyme variation on 23 allele on 6 loci were recognized between L and S types. From genetic distances according to gene frequencies of 46 allele, the dendrogram among 67 strains was drawn. The strains were divided into two groups, each of which consists only of S type or L type strains. This result indicates the great genetical differences between L and S type strains.Among 37 S type strains, four strains showed isozyme patterns different from other S type strains. In these strains, three loci were particular for isozyme patterns. Morphometric studies showed that lorica sizes were much larger than the mean of 37 S type strains. The four strains might be triploid.