KARYOTYPES OF S-TYPE AND L-TYPE ROTIFERS BRACHIONUS-PLICATILIS MULLER,O.F.

A comparative study on the karyotypes of S and L types of the rotifer Brachionus plicatilis O. F. Muller was carried out to clarify their cytological differences. Embryos, stained with alcoholic hydrochloric acid-carmine, were made into squash chromosome preparations. Mitotic chromosomes of S type prometaphase cells and L type metaphase cells were studied. The two karyotypes differed in chromosome number and morphology. The female diploid embryo of the S type had 25 chromosomes (2n = 25), whereas that of L type had 22 chromosomes (2n = 22). The chromosome numbers of male haploid embryos of S and L types were n = 12 and 11, respectively. Differences in karyotypes suggest that S and L types are so differentiated they should be considered different species. The possible evolutionary steps of chromosomes changes leading to the two types are discussed.