# Unraveling Mitochondrial Genome Evolution in Puccinia striiformis f. sp. elymi, the Elymus Stripe Rust Fungus

## TL;DR

This study reveals how the mitochondrial genome of a specialized stripe rust fungus evolved through massive intron loss, leading to a smaller genome.

## Contribution

The study identifies a 748-bp SNP cluster in nad4 as a potential marker for differentiating Pse lineages.

## Key findings

- The Pse mitogenome is 28.34% smaller than the wheat stripe rust mitogenome due to intron loss.
- Pse maintains all 14 core protein-coding genes and shows a negative GC-skew.
- Phylogenomic analysis places Pse as a sister to the Pst clade with 100% bootstrap support.

## Abstract

Puccinia striiformis f. sp. elymi (Pse) is a specialized forma specialis of stripe rust infecting Elymus dahuricus, yet its mitochondrial evolution remains poorly understood. In this study, we assembled the complete mitogenome of Pse using PacBio HiFi sequencing, yielding a circular mitogenome of 72,952 bp. This reveals a striking asymmetric evolutionary pattern with a 28.34% genomic contraction compared to the wheat stripe rust P. striiformis f. sp. tritici (Pst-CYR32). Our analysis demonstrates that this streamlining is strictly driven by a massive and systematic loss of mitochondrial introns. The Pse mitogenome exhibits negative GC-skew (−0.0184) consistent with strand-asymmetric mutational pressure, while maintaining a strictly conserved and syntenic complement of all 14 core protein-coding genes (PCGs), alongside 24 tRNAs and 2 rRNAs. Phylogenomic analysis positions Pse as sister to the Pst clade with strong support (100% bootstrap). A 748-bp SNP cluster within nad4 (14.2% sequence divergence versus 3.1% genome-wide average) provides a candidate molecular marker for lineage differentiation, pending population-level validation. This study establishes a genomic foundation for investigating mitochondrial reductive evolution in host-specialized rust lineages, highlighting the dynamic role of introns in driving organellar genome size variation.