FUNDAMENTAL STUDIES OF DNA DIMINUTION (CHROMOSOME ELIMINATION) IN LOWER FISH DURING DIFFERENTIATION INTO SOMATIC CELLS.

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Four Japanese and one East Pacific Canadian hagfish in the order Myxinida, Eptatretus okinoseanus, Eptatretus burgeril Paramyxine atami, Myxine garmani and Eptatretus stoutil have been possess different chromosome numbers in their germ cells (spermatocytes and spermatogonia) from somatic cells (liver, blood, gill and kidney). The difference of the chromosome numbers between spermatogonia (54, 52, 48, 16 and 54) and somatic cells (34, 36, 34, 14 and 34) was 20, 16, 14, 2 and 20, in E. okinoseanus, E. burgeri, P. atami, M. garmani and E. stoutii, respectively. In E. okinoseanus, two types of germ cells, those having large amount of C-positive chromatin and those having small amount were observed. The relative amount of DNA eliminated from germ cells averaged 43.6%, (E. o. type A), 55.1% (E. o. type B), 20.9% (E. b.), 40.0% (P. a.), 29.8% (M. g.) and 48.7% (E. s.), respectively. These results clearly indicate that, in the 5 hagfish species in Myxinida which live in Japanese and Canadian P … More acific waters, chromosome elimination takes place during the early stages of cleavage except for the ancestral germ cells.DNA from germ line cells and somatic cells in E. okinoseanus (type A, type B), E. burgeri and M. garmani bad been isolated and digested with 30 kinds of restriction enzymes. Two enzymes (Bam H I, Dra I) generated weak bands limited to germ line DNA in both types of E. okinoseanus by agarose gel electrophoresis. DNA filter hybridization experiments proved that two DNA fragments by Bam HI, about 125 bp long, are contained only in the eliminated DNA of E. okinoseanus and may account about 1.25% and 1.32% of the total germ line genomic DNA. These values would correspond to approximate 8.5x10^5 and 9.0X10^5 copies in diploid, respectively. These fragments are hardly contained in the germ line genomes and somatic genomes of the other two species. In situ hyubridization experiments demonstrated that this DNA fragments are located on several small chromosomes which are limited in germ cells in both types and which are C-band positive chromatin in E. okinoseanus. Less