By Katie Au, Class of 2029
Figure 1. Image of the Eurasian common shrew
The Eurasian common shrew (S. araneus) has two rare traits. One is seasonal brain size plasticity, where the brain shrinks during winter and regrows in spring. The other is extensive chromosomal rearrangements, or structural differences in chromosomes, within the species. In a study examining S. araneus, William R. Thomas and his colleagues hypothesized that specific genes and pathways involved in brain growth, metabolism, and DNA repair would show evidence of evolutionary adaptation in the shrew’s lineage and become more active during Dehnel’s phenomenon.
To evaluate their hypothesis, the researchers used a chromosome-scale genome assembly, a digital representation of DNA, of S. araneus to compare with other mammals. Then, the cortex and hippocampus were specifically analyzed for seasonal RNA expression, which was recorded during periods of shrinking and regrowth. Evolutionary models were also used to match genes in the species’ lineage. Genes that had evolved in the shrew were found to be involved in the Fanconi anemia pathway, which is crucial for repairing damaged DNA. This finding offers an explanation for the shrew’s ability to continue reproduction despite the species’ chromosomal rearrangements. Thomas and colleagues also found important brain development genes that showed either evolutionary changes, varied activity during the seasons, or both. Additionally, genes related to metabolic functions were positively selected, suggesting a correlation between changes in energy expenditure from brain shrinkage in the winter and regrowth in the spring. Several of these genes were discovered to be located near breakpoint regions, areas where chromosomes frequently break and rearrange, supporting the connection between the shrew’s genomic structure and its unique traits of brain plasticity and chromosomal rearrangement.
The findings of Thomas and his colleagues indicate that Dehnel’s phenomenon found in the Eurasian common shrew is not simply a matter of activating or repressing genes according to seasonal changes; the abnormal characteristic is supported by larger evolutionary changes in the DNA and the chromosomal structure of the shrew. Obtaining a better understanding of how SA can alter its neurological makeup can help researchers better understand brain regeneration and neuroplasticity, especially in humans. Furthermore, the shrew’s ability to maintain successful reproduction despite frequent genetic shuffling can teach scientists how the common shrew’s cells can manage DNA instability, leading to important discoveries in other fields such as oncology. Continued research on the specific genes could ultimately shed light on how the brain repairs and rebuilds itself.
Work’s Cited:
[1] W. Thomas, et al., Genomic comparisons and the adaptive basis of brain size plasticity and chromosomal instability in the Eurasian common shrew. Molecular Biology and Evolution 43, msag006 (2026). doi: 10.1093/molbev/msag006
[2] Image retrieved from:https://www.flickr.com/photos/twoody291/7127854669/in/photolist-bRS7S2
