Negative Evidence for Sex-Linked Heteroplasmy in the Nemertean Worm Notospermus geniculatus (Delle Chiaje, 1822)

Doubly Uniparental Inheritance (DUI) is considered one of the major exceptions to the common rules of eukaryotic cell biology and germline formation. DUI is known in bivalves, which belong to the phylum Mollusca; conversely, no DUI evidence was found in some gastropod species. Investigating the presence of DUI in Nemertea is of particular interest given the fact that these spiralian animals are suggested to be a sister group of Neotrochozoa (i.e., Mollusca and Annelida). DUI species are normally detected as having two sex-associated mtDNAs, which can be highly divergent. In this work, the presence of sex-associated mitochondrial genomes was investigated in the nemertean species Notospermus geniculatus and no evidence was found for DUI. Even if these are preliminary results, negative evidence is still interesting because of the high importance of the DUI phenomenon in many research fields, where it plays a pivotal role in understanding eukaryotic evolution. For this reason, further research on DUI species detection should be highly encouraged, as well as the publication of negative results beside positive ones, as is the case for the present study, improving the knowledge on the biology and ecology of a broad spectrum of marine species.