Chromosome Research

, Volume 14, Issue 8, pp 817–830 | Cite as

Peculiar behavior of distinct chromosomal DNA elements during and after development in the dicyemid mesozoan Dicyema japonicum



The dicyemid mesozoans are obligate parasites that inhabit the cephalopod renal appendage. Dicyemids have a simple body, consisting of approximately 30 cells: one long cylindrical axial cell contains intracellular stem cells (called axoblast), from which embryos are derived, and is surrounded by some 30 peripheral somatic cells. Somatic cells divide at most eight times in their life span, and never divide after differentiation. During early somatic cell development, numerous unique DNA sequences are first amplified and then eliminated, in the form of extrachromosomal circular DNA, leading to genome reduction. In this study we demonstrate that the remaining sequences, single-copy genes and repetitive sequences, have very different fates. Single-copy genes, such as β-tubulin, are initially amplified, presumably via endoreduplication, but subsequently decrease in copy number through development, suggesting that the whole genome is initially amplified and then the amplified DNAs are simply diluted in successive cell divisions, with little DNA replication. In contrast, repetitive sequences are maintained even in terminally differentiated somatic cell nuclei. Considering the increasing intensity of in-situ hybridization, incorporation of BrdU, and a general correlation between nuclear content and cell size, those repetitive sequences must be selectively endoreplicated in the peripheral cell nucleus, concomitant with the increase of cell size. The biological significance of this mechanism is discussed as a unique dicyemid adaptation to parasitism.

Key words

chromatin elimination Dicyema japonicum Dicyemidae DNA excision endoreplication repetitive sequences 


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Copyright information

© Springer 2006

Authors and Affiliations

  1. 1.Division of Life Science, Graduate School of Natural Science and TechnologyKanazawa UniversityKanazawaJapan
  2. 2.Hayama Center for Advanced Studies (HCAS)Graduate University for Advanced Studies (Sokendai)KanagawaJapan
  3. 3.IMBA–Institute of Molecular Biotechnology of the Austrian Academy of SciencesViennaAustria

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