Abstract
The transposon silencer piwi genes play important roles in germline determination and maintenance, gametogenesis, and stem-cell self-renewal, and the expression of certain piwi genes is indispensable for regeneration. Knowledge about piwi genes is needed for phylum Nemertea, which contains members (e.g., Lineus sanguineus) with formidable regeneration capacity. By searching the L. sanguineus genome, we identified six Argonaute genes including three ago (Ls-Ago2, Ls-Ago2a, and Ls-Ago2b) and three piwi (Ls-piwi1, Ls-piwi2, and Ls-piwi3) genes. In situ hybridization revealed that, in intact females, Ls-piwi2 and Ls-piwi3 were not expressed, while Ls-piwi1 was expressed in ovaries. During regeneration, Ls-piwi1 and Ls-pcna (proliferating cell nuclear antigen) had strong and similar expressions. The expression of Ls-piwi1 became indetectable while Ls-pcna continued to be expressed when the differentiation of new organs was finished. During anterior regeneration, expression signals of Ls-piwi2 and Ls-piwi3 were weak and only detected in the blastema stage. During posterior regeneration, no expression was observed for Ls-piwi2. To date, no direct evidence has been found for the existence of congenital stem cells in adult L. sanguineus. The “pluripotent cells” in regenerating tissues are likely to be dedifferentiated from other type(s) of cells.
Highlights
The ribbon worm Lineus sanguineus (Rathke, 1799) (Nemertea, Pilidiophora, Heteronemertea) is famous for its formidable regenerative ability
Four Argonaute proteins were found from the Notospermus geniculatus protein library, including three PIWI proteins (Ng-piwi1, Ng-piwi2, and Ng-piwi3) and one AGO protein (Ng-Ago2) (Figure S2, Supplementary Materials)
None of the three piwis are expressed in somatic tissues of intact worms, and only Ls-piwi1 is expressed in gonads of the female
Summary
The ribbon worm Lineus sanguineus (Rathke, 1799) (Nemertea, Pilidiophora, Heteronemertea) is famous for its formidable regenerative ability. By repeatedly amputating a single worm, over 200,000 worms could be obtained, each measuring less than one two-hundred-thousandth of the volume of the original individual [2]. In regenerating anterior structures, undifferentiated migratory cells beneath the newly formed epidermis give rise to a projecting regenerative bud or head blastema, which eventually gives rise to all the structures of the new anterior end [2,4,5].
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