High Mitochondrial Changes Linked to Healthier Cells in Snails

Greg Howard
14th June, 2024

High Mitochondrial Changes Linked to Healthier Cells in Snails

Image Source: Natural Science News, 2024

Key Findings

  • The study, conducted at the University of Nottingham, focused on the terrestrial snail Cepaea nemoralis and its sister species C. hortensis
  • Researchers found a high rate of single nucleotide polymorphism (SNP) heteroplasmy in Cepaea species, which was negatively correlated with mtDNA copy number
  • The study revealed that purifying selection acts on non-synonymous mutations, especially in genes coding for essential components of the mitochondrial respiratory chain
Mitochondrial DNA (mtDNA) in molluscan species has long intrigued scientists due to its unusual characteristics, such as wide variations in size, frequent genome rearrangements, and high intraspecies variability. A recent study conducted by researchers at the University of Nottingham delves into this phenomenon using whole genome sequencing of the terrestrial snail Cepaea nemoralis, its sister species C. hortensis, and other snail species to explore the origins of mtDNA variation[1]. The study's primary finding is the high rate of single nucleotide polymorphism (SNP) heteroplasmy in somatic tissues of Cepaea species, which was negatively correlated with mtDNA copy number. Heteroplasmy refers to the presence of more than one type of mitochondrial genome within a cell. Specifically, individuals with fewer than ten mtDNA copies per nuclear genome exhibited more than 10% heteroplasmy across all positions, with evidence suggesting that this heteroplasmy is transmitted through the germline. This finding aligns with earlier research showing that mtDNA can exhibit significant variability within species, particularly in groups with high genome plasticity and fast nucleotide substitution rates[2]. Further analysis revealed that purifying selection acts on non-synonymous mutations even when the rare allele frequency is low. This selection was particularly evident in genes coding for cytochrome oxidase subunit 1 and cytochrome b, which are essential components of the mitochondrial respiratory chain. This aspect of the study builds on previous findings that mtDNA in various animal groups, including tunicates, experiences rapid evolutionary rates affecting both protein-coding and ribosomal RNA genes[3]. Interestingly, the study also discovered length heteroplasmy in the mtDNA of some Cepaea nemoralis individuals, characterized by multiple direct repeat copies of tRNA genes. For instance, some snails had up to 12 direct repeat copies of tRNA-Val, while another species, Candidula rugosiuscula, exhibited 24 copies. This phenomenon was also observed with tRNA-Thr repeats in C. hortensis. These repeats likely arise from error-prone replication but were not correlated with mitochondrial copy number in C. nemoralis. This finding is consistent with previous observations that animal mtDNA can vary significantly in gene content and organization, challenging the traditional view of mtDNA as a small, circular, and conserved molecule[4]. The study's findings provide valuable insights into the mechanisms of replication, mutation, and evolution in molluscan mtDNA. For instance, the negative correlation between mtDNA copy number and SNP heteroplasmy supports the hypothesis that mtDNA copy number may influence evolutionary rates by affecting the availability of templates for homologous recombination repair, a mechanism previously suggested to play a role in mtDNA evolution in plants[5]. By analyzing a six-generation matriline of Cepaea nemoralis, the researchers could track how mtDNA variation is inherited and maintained across generations. This approach is particularly useful for understanding the evolutionary dynamics of mtDNA at short evolutionary distances, as previously highlighted in studies comparing mtDNAs of congeneric species[2]. Overall, this study from the University of Nottingham advances our understanding of mtDNA variation in mollusks and provides a framework for future research on the biology and evolution of mtDNA across different animal phyla. The findings highlight the complex interplay between replication mechanisms, mutation rates, and evolutionary pressures in shaping mitochondrial genomes.

GeneticsAnimal ScienceEvolution

References

Main Study

1) High heteroplasmy is associated with low mitochondrial copy number and selection against non-synonymous mutations in the snail Cepaea nemoralis

Published 13th June, 2024

https://doi.org/10.1186/s12864-024-10505-w

Related Studies

2) Evolution of the mitochondrial genome of Metazoa as exemplified by comparison of congeneric species.

https://doi.org/10.1038/hdy.2008.62

3) Tunicate mitogenomics and phylogenetics: peculiarities of the Herdmania momus mitochondrial genome and support for the new chordate phylogeny.

https://doi.org/10.1186/1471-2164-10-534

4) Animal Mitochondrial DNA as We Do Not Know It: mt-Genome Organization and Evolution in Nonbilaterian Lineages.

Journal: Genome biology and evolution, Issue: Vol 8, Issue 9, Sep 2016

5) Genome copy number predicts extreme evolutionary rate variation in plant mitochondrial DNA.

https://doi.org/10.1073/pnas.2317240121


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