Highlight: The “Wild West” of Mitochondrial Genome Evolution in the Gastrotricha

Genome architecture reflects an organism's evolutionary history. Properties such as gene order, gene orientation on each DNA strand, and repeat content can have a crucial impact on gene regulation and, ultimately, on adaptation. Thus, a central goal of evolutionary genomics is to understand the processes that promote genome stability and/or change. The tiny genomes of mitochondria (i.e. mitogenomes) illustrate this very well. Mitochondrial genomes, particularly in animals, are traditionally depicted as highly conserved in terms of size, gene content, and structure across deeply divergent lineages. However, as more studies expand our knowledge of mitochondrial genetic diversity beyond well-known animal taxa, astounding levels of mitogenome architectural variation are being uncovered (Smith and Keeling 2015; Lavrov and Pett 2016). Yet, some animal groups remain critically underexplored including the Gastrotricha, known colloquially as “hairybellies” (Fig. 1). The sister group to the (relatively) more famous flatworms (Platyhelminthes), Gastrotricha is a lineage of very small (80 μm to 3.8 mm long) worms found in marine and freshwater environments. Until last year, complete mitochondrial genomes had only been sequenced for two species—out of a total of over 900 species described so far. Mitogenome evolution in hairybellies (Gastrotricha). The Gastrotricha comprise over 900 species divided into two major monophyletic lineages, the Chaetonotida (light blue, photos of representative species on the top) and the Macrodasyida (pink, photos of representative species at the bottom). The alignment of color blocks at the right of the phylogenetic tree represents synteny (gene order) of mitochondrial genes in the 23 species studied by Kosakyan et al. (2026), illustrating conserved mitogenome structure in the Chaetonotida and the highly variable patterns among species of Macrodasyida. The two circular plots illustrate the stark differences in mitogenome organization between one species of Chaetonotida and one species of Macrodasyida. Figure by M. Antonio Todaro, based on data presented in Kosakyan et al. (2026). Overlooked as gastrotrichs may be, that did not discourage Anush Kosakyan (Fig. 2), a researcher at the Laboratory of Meiofauna, led by M. Antonio Todaro, of the University of Modena and Reggio Emilia (Italy). She explains that: “The Gastrotricha represent one of the least explored animal phyla, despite being widespread and ecologically important in the food web of aquatic ecosystems.” For researchers such as Kosakyan, this lineage represents an enticing opportunity to investigate mitogenome evolution across animals. As she points out: “Their poorly understood evolutionary history makes them an ideal group for uncovering patterns that would remain invisible in well-studied model organisms.” GBE author spotlight: Anush Kosakyan. Anush Kosakyan is a researcher in the Laboratory of Meiofauna, at the Department of Life Sciences, University of Modena and Reggio Emilia, Italy. She focuses on understudied animal lineages and their mitochondrial diversity. Her work integrates molecular phylogenetics, comparative genomics, and biodiversity research to uncover hidden evolutionary patterns across the phylum Gastrotricha. Anush completed her doctoral studies in protistology at the University of Neuchâtel in Switzerland and is currently involved in an EU-funded, large Italian biodiversity initiative within the National Biodiversity Future Centre (NBFC). Her research aims to broaden our understanding of genomic innovation in nonmodel organisms. In a new article published in Genome Biology and Evolution, Kosakyan et al. (2026) have lifted the veil on mitogenome evolution in the Gastrotricha. Kosakyan et al. (2026) sequenced the mitochondrial genomes of 21 species from the two traditionally recognized lineages of the Gastrotricha: the mostly freshwater Chaetonotida (nine species sequenced), and the mostly marine Macrodasyida (12 species). The authors’ initial phylogenetic analyses of these 21 species (plus the two previously available mitogenomes) confirmed the presence of two distinct lineages (Fig. 1). However, when the authors examined the organization of various genomic features in these species’ mitogenomes, they uncovered an intriguing pattern. “We discovered that Gastrotricha exhibit two radically different mitochondrial genome architectures in the two major lineages,” Kosakyan reveals, as she explains that “Chaetonotida mitogenomes are conserved in genome structure, resembling the ‘classical’ animal mitochondrial organization, while Macrodasyida mitogenomes feature extensive rearrangements and variability in gene content, arrangement, strand polarity, and repeat abundance.” A detailed look at some of the variability within the Macrodasyida exemplifies this contrast (Fig. 1). Mitogenome lengths vary by over 5 kb in the Macrodasyida (while less than 200 bp in the Chaetonotida). The number of tandem repeats per mitogenome varied between 0 and 9 in the Macrodasyida, compared to the smaller range of 0 to 1 in the Chaetonotida. Perhaps even more strikingly, gene order and the direction of transcription of genes were entirely rearranged within the marine group, even among species belonging to the same genus. Why do these two sister lineages feature such starkly different patterns of mitogenome conservation? Kosakyan et al. (2026) argue that the contrasting pattern between highly ordered Chaetonotida mitogenomes and the structurally dynamic mitogenomes of the Macrodasyida could be a legacy of fundamentally different evolutionary pressures acting on each of the two hairybelly lineages: “Although genetic drift remains the most parsimonious explanation, we are not excluding that the more stable mitogenomes may be associated with life histories that favor genomic efficiency, while the highly rearranged mitogenomes may reflect relaxed constraints or ecological conditions that tolerate or even promote structural innovation.” Determining whether life history traits drive mitogenome evolution remains an important open question. The high variability in mitogenome organization among Gastrotricha illustrates the dynamic nature of genome evolution. For Kosakyan, “the ‘standard’ animal mitogenome is only one of many possible solutions, since while many animals maintain a conserved mitochondrial architecture, others can tolerate and even sustain dramatic structural changes.” More broadly, Kosakyan et al. (2026) highlight how continued exploration of biological diversity beyond traditional model organisms is necessary to comprehend evolutionary processes shaping genome organization. Want to learn more? Check out these other articles on mitogenome evolution recently published in SMBE journals: Nhat Nam N, Pham Anh Thi N, Do HD. New insights into the diversity of mitochondrial plastid DNA. Genome Biol Evol. 2024:16(9):evae184. https://doi.org/10.1093/gbe/evae184. Ozerova I, Fallmann J, Mörl M, Bernt M, Prohaska SJ, Stadler PF. Aberrant mitochondrial tRNA genes appear frequently in animal evolution. Genome Biol Evol. 2024:16(11):evae232. https://doi.org/10.1093/gbe/evae232. Wang H, Wang D, Shao B, Li J, Li Z, Chase MW, Li J, Feng Y, Wen Y, Qin S, Chen B. Unequally Abundant Chromosomes and Unusual Collections of Transferred Sequences Characterize Mitochondrial Genomes of Gastrodia (Orchidaceae), One of the Largest Mycoheterotrophic Plant Genera. Mol Biol Evol. 2025:42(4):msaf082. https://doi.org/10.1093/molbev/msaf082.