Chromosome elimination is a process in which some chromatins are discarded from the presumptive somatic cells during early embryogenesis. Eliminated chromatins in hagfish generally consist of repetitive sequences, and they are highly heterochromatinized in germ cells. In this study, we characterized four novel eliminated DNA families, EEPs1-4, from the Taiwanese hagfish Paramyxine sheni. Sequences of these four elements occupied 20-27% of eliminated DNA in total, and each family was arranged mainly in tandem in the germline genome with high copy numbers. Although most of these elements were eliminated, a minor fraction remained in somatic cells. Some eliminated DNA families are shared as eliminated sequences between Eptatretidae and Myxinidae. Fluorescence in situ hybridization (FISH) of these elements showed that not only heterochromatic chromosomes but also both ends of euchromatic chromosomes in germ cells are absent in somatic cells of P. sheni. It strongly suggests that chromosome terminus elimination, in addition to whole chromosome elimination, contributes to somatic chromosome differentiation. Telomere-FISH further showed that chromosome fragmentation and the subsequent de novo addition of telomeric repeats are the likely mechanisms underlying chromosome terminus elimination. These characteristics make it indispensable to study the evolution and mechanisms underlying chromosome elimination in hagfish.

Jorgensen, J. M.. Shichiri. M. & Geneser. F. A. 1998. Morphology of the hagfish inner ear.-Acra Zoologica (Stockholm) 79: 251-256.The inner ear of five species of hagfishes was examined with different light and electron microscopical techniques. In all species. the labyrinth contains a single macula and two cristae, in a single semicircular canal. The macula consists of a horizontal, a middle vertical and a posterior horizonral component. Each component is covered by numerous round statoconia. The ring-shaped cristae have very long kinocilia, but lack a proper cupula. The sensory epithelia show signs of regeneration. indicated by the presence of mitoses and apoptotic hair cells.

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