Table 1.

Hierarchical levels for the study of heteroplasmy. 1. The population level: different individuals might contain different mtDNA haplotypes. 2. The individual level: the individuals are heteroplasmic, but the tissues are homoplasmic for alternative haplotypes. The different organs are an oversimplified representation for the different tissues. 3. The tissue level: The tissue can be heteroplasmic, but its cells can be homoplasmic for different haplotypes. 4. The cell level: the cell can be heteroplasmic, but its mitochondria can be homoplasmic for alternative haplotypes. 5. The mitochondrion level: the mitochondrion can contain different mtDNA haplotypes.

Levels of Heteroplasmy	Description	Experimental Evidence for b
1. Heteroplasmic population	Individuals are (a) heteroplasmic or (b) homoplasmic for alternative haplotypes	All organisms
2. Heteroplasmic individual	Tissues are (a) heteroplasmic or (b) homoplasmic for alterative haplotypes	Bivalves (ref. [22])
3. Heteroplasmic tissue	Cells are (a) heteroplasmic or (b) homoplasmic for alternative haplotypes	Indirect evidence and refs. [26,27]
4. Heteroplasmic cell	Mitochondria are (a) heteroplasmic or (b) homoplasmic for alterative haplotypes	Indirect evidence and (ref. [28])
5. Heteroplasmic mitochondrion	Mitochondria are heteroplasmic.	Direct observation in ref. [29]