No evidence of DUI in the Mediterranean alien species Brachidontes pharaonis (P. Fisher, 1870) despite mitochondrial heteroplasmy

Abstract

Two genetically different mitochondrial haplogroups of Brachidontes pharaonis (p-distance 6.8%) have been identified in the Mediterranean Sea. This hinted at a possible presence of doubly uniparental inheritance in this species. To ascertain this possibility, we sequenced two complete mitogenomes of Brachidontes pharaonis mussels and performed a qPCR analysis to measure the relative mitogenome copy numbers of both mtDNAs. Despite the presence of two very similar regions composed entirely of repetitive sequences in the two haplogroups, no recombination between mitogenomes was detected. In heteroplasmic individuals, both mitogenomes were present in the generative tissues of both sexes, which argues against the presence of doubly uniparental inheritance in this species.

Figure 1

Genetic map of Brachidontes pharaonis mitochondrial genomes. The white arrows with orange bands represent protein-coding genes with predicted transmembrane domains. The dark arrows represent the rRNA and tRNA genes; the white boxes indicate the location of repetitive sequences. The figure and compositional indices were generated with MITOCONSTRICTOR as in^,.

Figure 2

The ratio of mtDNA to nDNA in seven heteroplasmic individuals of B. pharaonis. F represents female individuals; M represents male individuals. The yellow and green colored bars represent sequence copy number per nuclear DNA for type A mtDNA; The pink and blue bars represent sequence copy number per nuclear DNA for type B mtDNA. The target genes atp6 and nd4 are located on the opposite sites of the respective mitogenomes separated by repetitive sequences. Samples described as “Mantle” contained mainly gonadal tissues, not just the somatic mantle.

Figure 3

The phylogenetic position of Brachidontes pharaonis mitogenomes among mitogenomes of other mytilid mussels. The left tree represents Bayesian phylogenetic analysis done in BEAST2, and the right tree represents maximum likelihood analysis done in IQ-TREE. The nodes support values equaled 1 (Posterior probabilities, left) and 100% (bootstrap values, right), unless indicated differently.

Figure 4

The divergence between A and B type mitogenome of B. pharaonis. The blue color represents the p-distance for nucleotide sequences, and dark red color represents p-distance for translated protein genes.