Nonmethylated transposable elements and methylated genes in a chordate genome

Area of Science:

• * Genomics and epigenetics
• * Comparative genomics of invertebrate chordates

Background:

• * DNA methylation patterns vary significantly across animal species.
• * The genome defense model posits methylation primarily targets transposable elements for genome stability.
• * Understanding methylation in basal chordates like Ciona intestinalis offers insights into evolutionary methylation dynamics.

Purpose of the Study:

• * To investigate the genome-wide DNA methylation landscape in the invertebrate chordate Ciona intestinalis.
• * To determine the distribution of methylation across different genomic elements, including genes and transposable elements.
• * To assess the compatibility of observed methylation patterns with the prevailing genome defense model.

Main Methods:

• * Analysis of whole-genome bisulfite sequencing data to map cytosine methylation.
• * Identification and annotation of genes and various types of repetitive elements (transposable elements).
• * Comparative analysis of methylation levels between genes and repetitive elements.

Main Results:

• * The Ciona intestinalis genome exhibits a mosaic structure with both methylated and nonmethylated domains.
• * Active long terminal repeat retrotransposons and long interspersed elements are predominantly nonmethylated.
• * A significant fraction of abundant short interspersed elements are also methylation-free, contrasting with genes, which are mostly methylated.

Conclusions:

• * The observed methylation pattern in Ciona intestinalis contradicts the genome defense model.
• * Cytosine methylation in this urochordate may preferentially target genes rather than endogenous transposable elements.
• * These findings suggest alternative roles or evolutionary trajectories for DNA methylation in early chordate genomes.