“…By contrast, environmental sources of mitochondrial variation may have more transient or localized effects, such as damage to mitochondria in tissue affected by a toxin or exposure to radiation (Zhang et al, ). This is important to note because mutations that accrue in one tissue may not be present in all tissues (in the case of heteroplasmy, where an individual possesses more than one mitochondrial haplotype; Barr, Neiman & Taylor, ; Kmiec, Woloszynska & Janska, ; Gandolfi et al, ), and the density and functions of mitochondria vary among tissues and may be differently affected by changing conditions (Hulbert et al, ; Benard et al, ; Holmström et al, ; Salin et al, ). Measuring mitochondrial function in avian red blood cells, for example, may not be an accurate substitute for measuring mitochondrial performance in neural tissue in cases where damage may be localized, although such relationships can be validated (Stier et al, ).…”