Abstract
Mitochondria have an active role in germ line development, and their inheritance dynamics are relevant to this process. Recently, a novel protein (RPHM21) was shown to be encoded in sperm by the male-transmitted mtDNA of Ruditapes philippinarum, a species with Doubly Uniparental Inheritance (DUI) of mitochondria. In silico analyses suggested a viral origin of RPHM21, and we hypothesized that the endogenization of a viral element provided sperm mitochondria of R. philippinarum with the ability to invade male germ line, thus being transmitted to the progeny. In this work we investigated the dynamics of germ line development in relation to mitochondrial transcription and expression patterns using qPCR and specific antibodies targeting the germ line marker VASPH (R. philippinarum VASA homolog), and RPHM21. Based on the experimental results we conclude that both targets are localized in the primordial germ cells (PGCs) of males, but while VASPH is detected in all PGCs, RPHM21 appears to be expressed only in a subpopulation of them. Since it has been predicted that RPHM21 might have a role in cell proliferation and migration, we here suggest that PGCs expressing it might gain advantage over others and undertake spermatogenesis, accounting for RPHM21 presence in all spermatozoa. Understanding how foreign sequence endogenization and co-option can modify the biology of an organism is of particular importance to assess the impact of such events on evolution.
Highlights
All sexually reproducing organisms arise from gametes, cells coming from meiosis and a differentiation process
The first representatives of this cell lineage to appear in the embryo are the primordial germ cells (PGCs), a generally small population of early segregating cells that are committed to germinal fate
The experimental design followed two steps: first we identified by Real-Time qPCR the developmental stage in which the first germ cell proliferation event occurs in juveniles, and compared the transcriptional dynamics of F-type and M-type mitochondrial targets with that of gametogenic adults; we used specific antibodies to localize the R. philippinarum VASA homolog (VASPH) and RPHM21 in juvenile and adult specimens.dBased on the obtained results we conclude that male germ cells express both the proteins, from their first appearance in juveniles to their proliferation during gametogenesis of fully grown adults, but while VASPH was detected in all visible germ cells, RPHM21 appeared to be expressed only in a subpopulation of them
Summary
All sexually reproducing organisms arise from gametes, cells coming from meiosis and a differentiation process. Vasph and cytb_F transcription levels showed a strong correlation in adults but not in juveniles, while both the M-type mitochondrial targets did not show any correlation with vasph (S3 Fig, S4 Table).
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