The Complete Maternally and Paternally Inherited Mitochondrial Genomes of a Freshwater Mussel Potamilus alatus (Bivalvia: Unionidae).

Doubly uniparental inheritance (DUI) of mitochondrial DNA, found only in some bivalve families and characterized by the existence of gender-associated mtDNA lineages that are inherited through males (M-type) or females (F-type), is one of the very few exceptions to the general rule of strict maternal mtDNA inheritance in animals. M-type sequences are often undetected and hence still underrepresented in the GenBank, which hinders the progress of the understanding of the DUI phenomenon. We have sequenced and analyzed the complete M and F mitogenomes of a freshwater mussel, Potamilus alatus. The M-type was 493 bp longer (M = 16 560, F = 16 067 bp). Gene contents, order and the distribution of genes between L and H strands were typical for unionid mussels. Candidates for the two ORFan genes (forf and morf) were found in respective mitogenomes. Both mitogenomes had a very similar A+T bias: F = 61% and M = 62.2%. The M mitogenome-specific cox2 extension (144 bp) is much shorter than in other sequenced unionid mitogenomes (531-576 bp), which might be characteristic for the Potamilus genus. The overall topology of the phylogenetic tree is in very good agreement with the currently accepted phylogenetic relationships within the Unionidae: both studied sequences were placed within the Ambleminae subfamily clusters in the corresponding M and F clades.

Byssogenesis in the juvenile pink heelsplitter mussel, Potamilus alatus (Bivalvia: Unionidae).

The North American pink heelsplitter (Potamilus alatus) differs from most freshwater mussels in China by the ability to secrete an ephemeral byssus during its juvenile stage. In the present study, light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to investigate this ephemeral byssal structure, and amino acid composition was analyzed and compared with that of other species. The results revealed that the byssus consists of a long byssal thread and a few adhesive plaques which are randomly set up along the thread and assembled by petioles. There is a thin but distinctive cuticle with a continuous homogeneous matrix surrounding the byssal thread. Structural variation occurred when the byssal thread was differentially stretched. Four-stranded helical primary fasciculi, which form a stable rope-like structure, become evident after removal of the cuticle. The primary fasciculi consist of bundles of hundreds of parallel secondary fasciculi, each measuring about 5 µm in diameter. All evidence indicates that the byssus of the pink heelsplitter has a significantly different macrostructure and microstructure than the permanent byssus of the marine mussel Mytilus. Byssogenesis ceases when juveniles exceed 30 mm in length, although it varies greatly even among juveniles of similar size. Byssus formation is influenced by substrate type. The unique characteristics of the byssus have important advantages for survival, transition, and aggregation during the early life history. This study not only provides first insight into the structure of the ephemeral byssus and its relationship to freshwater mussel development and growth, but also suggests possibilities for the synthesis of novel biopolymer materials particularly useful in freshwater ecosystems.