A naturally heteroplasmic clam shows the effects of genetic bottleneck on paternal mtDNA
Mitochondrial DNA (mtDNA) is present in multiple copies within an organism.Since these copies are not identical to each others, a single individual carries a large andheterogeneous population of mtDNAs, a condition known as heteroplasmy.Several factors play a role in the dynamics of the within-organism mtDNA population: among them genetic bottlenecks and strictly maternal inheritance of mitochondria are known to deeply shape the levels of heteroplasmy across mtDNAs.The only exception to the strictly maternal inheritance of mitochondria is called doubly uniparental inheritance (DUI), found in ~ 100 bivalve species. In such species, two separate lineages of mtDNA exist: one is inherited through mitochondria of the female oocyte (F-type) and the other through mitochondria of male sperm (M-type). Females usually posses only the F-type, while males are homoplasmic for the M-type in gonads, and heteroplasmic in somatic tissues. Since bivalve sperm posses four mitochondria, M-type mtDNA faces an incredibly tight bottleneck, possibly the smallest bottleneck size investigated so far.In this study, we analyzed the M- and F-type mtDNA variability within individuals of the DUI species Ruditapes philippinarum. This work provides new insights into the genetic bottlenecks of mtDNA, giving for the first time the opportunity to investigate the effects of the smallest known bottleneck affecting mtDNA populations. As a consequence of this narrow bottleneck, male individuals of R. philippinarum show huge variability in the M-type mtDNA populations coming from different tissues, a condition so pronounced that leads genotypes coming from the same individual to never cluster together.