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Correspondence to: Ellen M. Rasch, Dept. of Anatomy and Cell Biology, J.H. Quillen College of Medicine, Johnson City, TN 37604. E-mail: emrasch@worldnet.att.net
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Summary |
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We studied the functional significance of marked differences in the DNA content of somatic cells and germ line nuclei by static FeulgenDNA cytophotometry for several species of microcrustaceans that exhibit chromatin diminution during very early stages of embryogenesis. Mature females and males showed many gonadal nuclei with elevated amounts of DNA that persist until dispersal of this "extra" DNA throughout the cytoplasm as fragments and coalescing droplets of chromatin during anaphase of the diminution division. (J Histochem Cytochem 49:795796, 2001)
Key Words: somatic cells, germ line nuclei, DNA, microcrustaceans
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Introduction |
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Chromatin diminution, the fragmentation and excision of particular heterochromatic chromosome segments during early cleavage stages, occurs in some species of cyclopoid copepods and results in marked loss of DNA from the genome of presumptive somatic cells but retention of this DNA in primordial germ cell nuclei (
Representative distributions of the DNA content of SC nuclei and prediminuted nuclei (PD) from a mature, egg-carrying female (Fig 1A) can be compared with the DNA levels found for sperm and PD nuclei from an adult male (Fig 1B), whose SC nuclei contained 2.96 ± 0.082 pg DNA (n=50). Sperm appeared to contain twice as much DNA as expected because half of the diploid SC genome would be about 1.5 pg DNA for this species (
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The heterogeneity of PD nuclear DNA levels, coupled with the absence of nurse cells and lack of mitosis preceding the appearance of large darkly stained nuclei in the gonadal regions of both males and females of M. edax, suggests that cycles of DNA endoreduplication may occur in germ cells during oogenesis (and perhaps spermatogenesis) well before egg maturation, fertilization, and the start of preparations for chromatin diminution that is routinely scheduled for an early cleavage division. The ready availability of multiple copies of DNA transcripts that can be used for protein biosynthesis during egg growth and maturation, plus a program for the providential removal of the "excess" DNA from presumptive SC nuclei by the process of chromatin diminution during a fourth or fifth cleavage division in M. edax, may constitute alternative mechanisms for more effective reproduction of this species in nature. Conversely, the elevated DNA levels of copepod germ cell nuclei may be more closely associated with nucleoskeletal DNA and/or genome evolution, as first proposed by
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Footnotes |
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Presented in part at the Joint Meeting of the Histochemical Society and the International Society for Analytical and Molecular Morphology, Santa Fe, NM, February 27, 2001.
Received for publication November 27, 2000; accepted February 16, 2001.
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