Presentation Detail

Molecular Evolution

Chong, Rebecca A [1], Mueller, Rachel Lockridge [1].

Molecular evolution within dynamic mitochondrial genomes.

Mitochondria are intracellular organelles where oxidative phosphorylation (OXPHOS) is carried out to complete the process of ATP synthesis via cellular respiration. These organelles have their own genome; in metazoans, this is a small, circular DNA molecule encoding 13 peptides essential for electron transport, 22 transfer RNAs, and two ribosomal RNAs. Despite their evolutionarily conserved genomic content, metazoan mitochondrial genomes show a diversity of gene orders. Gene rearrangement in invertebrate mitochondrial genomes is correlated with high substitution rates; however, this correlation remains unexplored in vertebrates. Mitochondrial genes in some taxa also show spatial variation in substitution rates around the genome due to the mechanism underlying mtDNA replication. Regions persisting in the single-stranded state for longer are more prone to mutation, producing a mutation gradient. Although most vertebrates exhibit conserved mitochondrial gene order and genome size, some lineages have experienced genome expansion or gene rearrangement, which may shift genes to different positions along the mutation gradient. To date, the impacts of genome expansion and gene rearrangement on the molecular evolution of mitochondrial protein-coding genes remains untested in vertebrates. Salamanders include both normal vertebrate mitochondrial genomes and a range of independently derived gene rearrangements and genomic expansions. Because this genomic diversity is rare among vertebrates, salamanders provide a unique opportunity to compare the molecular evolution of dynamic verses non-dynamic vertebrate genomes. We first estimated genome composition for 62 salamander species to characterize the differences between normal and modified genomes. We compared synonymous and nonsynonymous substitution rates between the two groups to test for differences in rates of evolution. We also tested for differences in gene placement along the mutation gradient. Lastly, we tested for differences in functional constraint on protein-coding sequences between the two groups by comparing dN/dS, the strength of selection, for all mitochondrial genes. We show that (1) modified salamander genomes have a significant increase in both substitution rates. (2) Despite significant expansions, most genes in modified salamander genomes are not moved to a different position along the mutation gradient. Genes that do move are not substantially impacted by their new position; the mutation gradient in salamanders is weak. (3) Gene rearrangements and genome expansion events occur independent of levels of selective constraint acting on mitochondrial protein-coding sequences. Taken together, our results suggest that the same levels of selective constraint are acting on mitochondrial protein-coding genes in both dynamic and non-dynamic mitochondrial genomes, despite substantial differences in overall genome evolution.

1 - Colorado State University, Biology, Fort Collins, Colorado, 80523-1878, USA

Keywords:
gene rearrangement 
substitution rates
mutation gradient
selective constraint
salamanders.

Presentation Type: Regular Oral Presentation
Session: 145
Location: Alpine A and B/Snowbird Center
Date: Tuesday, June 25th, 2013
Time: 2:00 PM
Number: 145003
Abstract ID:1025
Candidate for Awards:W.D. Hamilton Award for Outstanding Student Presentation