[Paper Review] Relation between Gene Content and Taxonomy in Chloroplasts

Bashar Talib Al-Nuaimi, Christophe Guyeux+3 more|arXiv (Cornell University)|Sep 20, 2016

Genomics and Phylogenetic Studies2 references3 citations

TL;DR

This study analyzes the relationship between chloroplast gene content and evolutionary taxonomy using de novo annotation of 845 complete chloroplast genomes. It identifies core and pan genomes to construct a well-supported phylogenetic tree, revealing that gene content evolution—particularly gene loss and duplication—correlates with major evolutionary events like endosymbiosis, with pan-genome analysis showing distinct patterns across lineages such as Viridiplantae and red algae.

ABSTRACT

The aim of this study is to investigate the relation that can be found between the phylogeny of a large set of complete chloroplast genomes, and the evolution of gene content inside these sequences. Core and pan genomes have been computed on extit{de novo} annotation of these 845 genomes, the former being used for producing well-supported phylogenetic tree while the latter provides information regarding the evolution of gene contents over time. It details too the specificity of some branches of the tree, when specificity is obtained on accessory genes. After having detailed the material and methods, we emphasize some remarkable relation between well-known events of the chloroplast history, like endosymbiosis, and the evolution of gene contents over the phylogenetic tree.

Motivation & Objective

• To investigate the relationship between chloroplast gene content and evolutionary taxonomy across a large set of complete chloroplast genomes.
• To develop and apply novel computational tools for de novo annotation, core and pan genome computation, and phylogenetic inference in chloroplasts.
• To identify evolutionary patterns in gene content changes—such as gene loss and duplication—along the chloroplast phylogenetic tree.
• To explore whether specific gene content changes correlate with major evolutionary events like endosymbiosis or lineage-specific radiations.
• To lay the groundwork for ancestral genome reconstruction by ensuring accurate phylogenetic inference and gene content evolution tracking.

Proposed method

• Downloaded 845 complete chloroplast genomes from NCBI as of March 2016, representing diverse lineages including Viridiplantae, Rhodophyta, and Stramenopiles.
• Performed de novo genome annotation using DOGMA, with custom scripts to automate annotation across the entire dataset.
• Computed core and pan genomes using gene presence/absence across all 845 genomes, with core genome used for phylogenetic tree construction.
• Constructed a well-supported phylogenetic tree based on core genome sequences to ensure accurate species relationships.
• Conducted pairwise genome comparisons using custom scripts to detect gene losses, insertions, and deletions across closely related species.
• Mapped gene content changes onto the phylogenetic tree to correlate evolutionary events with genomic changes.

Experimental results

Research questions

• RQ1 How does chloroplast gene content vary across major lineages such as Viridiplantae, Rhodophyta, and Stramenopiles?
• RQ2 To what extent do gene loss and duplication events correlate with known evolutionary events like endosymbiosis in chloroplast history?
• RQ3 Are there statistically significant associations between the presence or absence of specific gene subsets and particular taxonomic clades?
• RQ4 Can gene content evolution patterns be reliably mapped onto a phylogenetic tree derived from core genome sequences?
• RQ5 What role do repeated genes and indel mutations play in shaping chloroplast genome size variation across species?

Key findings

• Chloroplast genomes range from 73 to 271 genes, with Viridiplantae species showing the most variable gene counts, while red algae and some stramenopiles have larger genomes.
• Gene content variation is primarily driven by gene loss and duplication, not large-scale recombination, as confirmed by manual inspection of gene positions and sequences.
• The pan-genome analysis revealed that most green lineage organisms (e.g., Viridiplantae) have fewer genes (73–120) compared to red algae (up to 271 genes), indicating lineage-specific gene loss.
• In pairwise comparisons, gene loss events were frequent: for example, in the Camelineae clade, up to 173 genes were lost between Barbarea and Hibiscus, with matching ratios as low as 28.26%.
• The ACCA gene showed repeated loss across multiple independent branches, suggesting vulnerability to deletion mutations.
• Genome size variation is largely due to the number of paralogous genes, not structural rearrangements or recombination, as confirmed by detailed gene-by-gene comparison.