Doubly uniparental inheritance (DUI) of mitochondrial DNA is a rare phenomenon occurring in some freshwater and marine bivalves and is usually characterized by the mitochondrial heteroplasmy of male individuals. Previous research on freshwater Unionida mussels showed that hermaphroditic species do not have DUI even if their closest gonochoristic counterparts do. No records showing DUI in a hermaphrodite have ever been reported. Here we show for the first time that the hermaphroditic mussel Semimytilus algosus (Mytilida), very likely has DUI, based on the complete sequences of both mitochondrial DNAs and the distribution of mtDNA types between male and female gonads. The two mitogenomes show considerable divergence (34.7%). The presumably paternal M type mitogenome dominated the male gonads of most studied mussels, while remaining at very low or undetectable levels in the female gonads of the same individuals. If indeed DUI can function in the context of simultaneous hermaphroditism, a change of paradigm regarding its involvement in sex determination is needed. It is apparently associated with gonadal differentiation rather than with sex determination in bivalves.

We determined the nearly complete mitochondrial genomes of the Arctic Calanus glacialis and its North Atlantic sibling Calanus finmarchicus, which are key zooplankton components in marine ecosystems. The sequenced part of C. glacialis mitogenome is 27,342 bp long and consists of two contigs, while for C. finmarchicus it is 29,462 bp and six contigs, what makes them the longest reported copepod mitogenomes. The typical set of metazoan mitochondrial genes is present in these mitogenomes, although the non-coding regions (NCRs) are unusually long and complex. The mitogenomes of the closest species C. glacialis and C. finmarchicus, followed by the North Pacific C. sinicus, are structurally similar and differ from the much more typical of deep-water, Arctic C. hyperboreus. This evolutionary trend for the expansion of NCRs within the Calanus mitogenomes increases mitochondrial DNA density, what resulted in its similar density to the nuclear genome. Given large differences in the length and structure of C. glacialis and C. finmarchicus mitogenomes, we conclude that the species are genetically distinct and thus cannot hybridize. The molecular resources presented here: the mitogenomic and rDNA sequences, and the database of repetitive elements should facilitate the development of genetic markers suitable in pursuing evolutionary research in copepods.

Zebra mussels Dreissena polymorpha (Veneroida, Dreissenidae) are known for their invasive behavior. Despite numerous studies dealing with this species, no results of large sequencing projects have been published to date, hampering marker development. In this study, we present a relatively large novel transcriptomic dataset obtained by Illumina MiSeq technology from mantle and male gonad of D. polymorpha sampled in Poland. The transcriptomic data were typical for the tissue analyzed. Moreover, they showed the expression of a single mitochondrial genome, indicating that this species do not have doubly uniparental inheritance of mitochondria. The sequences of mitochondrial transcripts were used to design primers and obtain nearly complete sequence of the zebra mussel mitochondrial genome by PCR and Sanger sequencing. This mitogenome has unique gene order, with the genes split into two blocks encoded in opposite directions. The closest mitogenome available in GenBank belongs to the marine clam Mya arenaria (Myoida, Myidae). The average divergence of the sequences of these mitogenomes is quite high, in the range of 20%. Both mitogenomic and transcriptomic resources should prove very useful for elucidating population genetics and conservation issues involving this important species.