MITOCHONDRIAL INHERITANCE AND GERMLINE DETERMINATION IN A BIVALVE SPECIES WITH DOUBLY UNI PARENTAL INHERITANCE (DUI).

Metazoan mitochondria are inherited maternally, but a noteworthy exception is Doubly Uniparental Inheritance (DUI). In DUI males, during zygote segmentation, sperm mitochondria (M) aggregate and end up in the primordial germ cells, thereafter being transmitted through sperm, while they degrade in females, which transmit egg mitochondria only (F). The molecular dynamics of this segregation are unknown, but they are thought to be similar to those selecting mitochondria to enter the germline of all metazoans (i.e. the “mitochondrial bottleneck”). Moreover, a role of M mitochondria in germline determination has been proposed for DUI. Using in vivo staining, we confirmed the M displacement pattern in Venerupis philippinarum, a well-known DUI species. We also evidenced a role of microtubules in the movement of M mitochondria in male embryos: actually, a microtubule bunch is formed next to the cleavage furrow, where sperm mitochondria are localized. Moreover, we searched by TEM for a cytoplasm cloud-like material called nuage, which is transferred to germ cells only and includes specific proteins (i.e. Vasa-like proteins), miRNAs and other post-transcriptional regulatory elements. The nuage is normally located in the chromatoid body (Cb) in spermatocytes, and in the Balbiani body (Bb) or mitochondrial cloud in developing oocytes. There are evidences that some of the Bb-associated mitochondria are taken up by primordial germ cells, suggesting a selection for a specific sub-population of mitochondria to be transmitted to offspring. We identified both Cb and Bb by TEM analyses on V. philippinarum gonads, but, surprisingly, two Bb were found. Further analyses are planned to verify whether this unusual characteristic is related to DUI or not. All that considered, DUI is an useful model system to study mitochondrial inheritance, because of the unique chance to analyze separately two distinct mitochondrial lineages, understanding where they localize and which ones enter the germline.