The strictly maternal inheritance (SMI) is a pattern of mitochondrial inheritance observed across the whole animal kingdom. However, some interesting exceptions are known for the class Bivalvia, in which several species show an unusual pattern called doubly uniparental inheritance (DUI) whose outcome is a heteroplasmic pool of mtDNA in males. Even if DUI has been studied for long, its molecular basis has not been established yet. The aim of this work is to select classes of proteins known to be involved in the maintenance of SMI and to compare their features in two clam species differing for their mitochondrial inheritance mechanism, that is, the SMI species Ruditapes decussatus and the DUI species Ruditapes philippinarum. Data have been obtained from the transcriptomes of male and female ripe gonads of both species. Our analysis focused on nucleases and polymerases, ubiquitination and ubiquitin-like modifier pathways, and proteins involved in autophagy and mitophagy. For each protein group of interest, transcription bias (male or female), annotation, and mitochondrial targeting (when appropriate) were assessed. We did not find evidence supporting a role of nucleases/polymerases or autophagic machinery in the enforcement of SMI in R. decussatus. On the other hand, ubiquitinating enzymes with the expected features have been retrieved, providing us with two alternative testable models for mitochondrial inheritance mechanisms at the molecular level.

The stick-insect genus Pijnackeria includes four diploid bisexual and two polyploid (3n, 38 4n) parthenogenetic species. Earlier analyses of the tetraploid parthenogen P. hispanica 39 using mitochondrial markers allowed to trace its maternal ancestry to Pijnackeria 40 originis, while no maternal nuclear contribution was found, thus suggesting an 41 androgenetic and hybrid origin. The recently described Pijnackeria recondita-42showing, among other features, a specific antennal structure linking it to the tetraploid 43 parthenogen-prompted us to check whether the new species could be P. hispanica 44 unknown paternal ancestor. In this work we use karyology and of molecular analysis of 45 the mitochondrial gene cytochrome c oxidase subunit 2 (cox2), and the nuclear gene 46 elongation factor 1 subunit α (ef1-α) to investigate the origin of such a complex 47 tetraploid hybrid parthenogen. 48The molecular analysis supported P. recondita as being a paternal ancestor of the P. 49 hispanica, but also suggested that two more fathering species have to be taken into 50 account: P. barbarae and the unknown paternal ancestor of the triploid hybrid P. 51 masettii. Therefore, P. hispanica is apparently a polyphyletic chimeric androgen, which 52 we propose to indicate as an androgenetic complex. Our data also revealed that P. 53 hispanica is between 1.96 Myr and 3.31 Myr old, making it the oldest parthenogenetic 54 taxon discovered among insects.