Natural variation in Caenorhabditis briggsae mitochondrial form and function suggests a novel model of organelle dynamics

Abstract

Mitochondrial functioning and morphology are known to be connected through cycles of organelle fusion and fission that depend upon the mitochondrial membrane potential (ΔΨM); however, we lack an understanding of the features and dynamics of natural mitochondrial populations. Using data from our recent study of univariate mitochondrial phenotypic variation in Caenorhabditis briggsae nematodes, we analyzed patterns of phenotypic correlation for 24 mitochondrial traits. Our findings support a role for ΔΨM in shaping mitochondrial dynamics, but no role for mitochondrial ROS. Further, our study suggests a novel model of mitochondrial population dynamics dependent upon cellular environmental context and with implications for mitochondrial genome integrity.

Figure 1. C. briggsae natural isolates

Phylogenetic relationship, geographic origin and average nad5Δ heteroplasmy level (numbers above branches) of C. briggsae isolates included in this study (modified from Howe & Denver, 2008). nad5Δ heteroplasmy levels were determined using qPCR and describe the average percentage of nad5Δ-deletion bearing genomes within individual worms from each isolate (Howe & Denver, 2008). GL = global superclade; KE = Kenya clade; TE and TR = temperate and tropical subclades of GL; C(+) = isolates bearing the compensatory Ψnad5Δ-2 allele. Note that we assayed the natural HK104 isolate here rather than the inbred line reported in Estes et al. (2011), which evolved high nad5Δ levels in the lab (see Section 2.1).

Figure 2. Examples of bivariate relationships of mitochondrial phenotypes

Patterns of relationship between traits describing mitochondrial size, morphology, and within-individual variance are shown. All measurements were made on the same set of confocal images (see Section 2.2) and each point represents the bivariate phenotype for an individual nematode (N=167–170). A) Aspect ratio is negatively related to circularity within the functional mitochondria of individual worms (ρ=-0.806, P<=0.0001). Removing one outlier (black symbol) decreases the correlation slightly (ρ=-0.797, P<=0.0001). B) As the two-dimensional area of the total functional mitochondrial population increases, functional mitochondria become less circular (ρ=-0.509, P<=0.0001). C) As the two-dimensional area of individual nonfunctional mitochondria increases, these mitochondria become less circular (ρ=-0.698, P<=0.0001). D) Individual nonfunctional mitochondria become more variable with respect to circularity as their two-dimensional area increases (ρ=-0.409, P<=0.0001). When two outliers (black symbols) are removed, the correlation remains unchanged. Outliers were determined by calculating Mahalanobis distances for the correlation between traits of each sample and visually identifying extreme outliers.

Figure 3. A context-dependent model of mitochondrial dynamics in which mitochondria respond to the functional state of their intracellular environment

We propose that an organism has three types of mitochondrial populations, polarized, transiently depolarized, and persistently depolarized. The polarized population is capable of undergoing fusion (white arrows) while the persistently depolarized population is not (white blunted arrow). The transiently depolarized mitochondria are produced after a fusion-fission cycle and will either regain sufficient ΔψM and join the polarized/fusing population (gray arrow) or, if they are unable to regain ΔψM, join the persistently depolarized population (black arrow) (Twig et al., 2008). We propose that mitochondria in a more functional environment (higher ΔψM, at left) are more likely to regain ΔψM and join the polarized/fusing population. Normal fusion-fission cycles will maintain a majority of mitochondria in the canonical ovoid morph. Here, many depolarized mitochondria are destined to recover ΔψM and rejoin the fusing population. Conversely, mitochondria in a less functional environment (lower ΔψM, at right) are less likely to regain ΔψM and will therefore join the persistently depolarized/non-fusing population resulting in fewer polarized mitochondria. Here, lower than normal rates of fusion and fission will increase the shape heterogeneity of all mitochondria.