In the intricate world of genetics, tandem repeat (TR) sequences stand out as a fascinating source of variation. These sequences consist of short DNA motifs repeated head-to-tail along chromosomes, playing a crucial role in shaping the genetic landscapes of many organisms. While traditional studies of TR evolution have primarily focused on large, randomly mating, haploid populations, new research reveals the profound influence of self-fertilisation—commonly known as selfing—on TR dynamics. This finding opens up fresh perspectives on how mating systems impact genetic diversity and evolutionary outcomes.
Selfing, an unusual but widespread reproductive strategy, significantly increases homozygosity in diploid organisms. This shift in genetic architecture can dramatically alter the evolutionary trajectory of tandem repeats, which are sensitive to changes in genetic variation and selection pressures. The groundbreaking study by Sudbrack and Mullon, published in Heredity, employs a blend of mathematical models and computer simulations to dissect the evolution of homologous TR sequences under varying degrees of selfing and different selective regimes. Their work sheds light on the nuanced ways selfing molds genetic variation and adaptation.
One of the core revelations of the study is that selfing amplifies homozygosity, which then intensifies the variance produced by unequal recombination events within individuals. Unequal recombination, a process that can alter TR copy numbers, contributes significantly to genetic variation within populations. Under selfing, this increased recombination variance inside individuals leads to more substantial differences in TR lengths, thereby pushing the evolutionary dynamics of these sequences into new territories. This insight overturns previous assumptions that predominantly considered outcrossing populations when modeling TR evolution.
Furthermore, the increased homozygosity brought about by selfing causes greater variability between individuals. This phenomenon arises because selfing reduces heterozygosity, making individual genetic profiles more uniform but distinctly varied from each other. As a result, genetic differences between individuals become more pronounced, and natural selection can act with enhanced efficacy. This enhanced selection pressure ultimately lowers the genetic load—the burden of deleterious mutations—in partially selfing populations compared to their fully outcrossing counterparts.
The study investigates four different modes of selection that are particularly relevant to tandem repeats. The first is additive purifying selection, where deleterious mutations have independent, cumulative effects on fitness. Under this regime, partially selfing populations experience stronger purifying selection due to elevated homozygosity, leading to more efficient removal of harmful TR variants. This results in a healthier genetic pool, contrasted with expectations that increased drift in selfing populations might weaken selection.
Next, the authors explore truncation-like purifying selection, a form of sharp threshold selection where TR lengths beyond a critical value are selected against rigorously. Selfing’s impact here is even more pronounced. The homozygosity intensifies the purging of deleterious variants exceeding the truncation threshold, effectively trimming extreme bubble-like expansions or contractions of tandem repeats, which might otherwise destabilize genomes. This stands as a powerful demonstration of selfing serving as a natural genome stabilizer.
Selection against heterozygotes, the third mode examined, dives into the less intuitive realm of “misalignment costs,” where heterozygotes face fitness penalties due to chromosomal mispairing during meiosis. Selfing, by increasing homozygosity, reduces these heterozygote states and hence the associated misalignment costs. The study finds that this reduction translates into an intriguing paradox: selfing lowers the genetic load by minimizing costly heterozygote mismatches, highlighting an evolutionary advantage to partial selfing in maintaining genome integrity.
Finally, stabilising selection acting in favor of intermediate TR lengths was scrutinized. This mode reflects a balance where TR sequences neither shrink nor expand excessively, preserving an optimal functional length. Here, partial selfing again enhances selection efficiency, ensuring that populations are more tightly clustered around this optimal length. Such stabilizing forces prevent runaway expansions that might result in diseases or genomic instability, underscoring the subtle evolutionary benefits of self-fertilization.
Beyond these selective regimes, the study also illuminates how selfing amplifies the interplay between genetic drift and selection. While drift tends to increase with selfing due to reduced effective population sizes, the compensatory boost in selection efficacy lowers deleterious mutation accumulation. This delicate balance places partially selfing populations in a unique evolutionary niche, differing substantially from classical genetic models that exclude inbreeding effects.
The implications of this research reverberate widely across evolutionary biology, genetics, and even applied fields like agriculture and conservation. Many crops and wild species reproduce through varying degrees of selfing, making this work critical for understanding how their genomes evolve under complex mating patterns. The findings suggest that breeding programs incorporating controlled selfing could harness TR dynamics to optimize genetic health and adaptability.
Moreover, this deeper understanding of TR evolution under selfing might illuminate the genetic bases of certain human diseases linked to tandem repeat expansions and contractions, such as Huntington’s disease and fragile X syndrome. Recognizing how selfing-like mechanisms or inbreeding affect these sequences could inspire new therapeutic strategies or diagnostic insights.
Sudbrack and Mullon’s study exemplifies the power of integrating theoretical models with computational experiments in unraveling genetic mysteries. Their approach moves beyond simplistic assumptions in population genetics, embracing the complexity of real-world mating systems and their impact on genome evolution. This paradigm shift encourages researchers to rethink long-held views about genetic variation and adaptation in natural populations.
As tandem repeat sequences continue to emerge as more than mere genetic curiosities, their evolutionary dynamics under diverse reproductive strategies gain increasing relevance. This study places selfing at the forefront of factors mediating TR variation, challenging the assumption that outcrossing dominates shaping genetic diversity. Going forward, integrating selfing into evolutionary theory promises richer, more accurate models with wide-ranging applications.
Ultimately, this work spotlights the intricate dance between mating systems, genetic mechanisms, and selection pressures in shaping genomes. The nuanced influence of partial selfing on tandem repeats underscores a larger narrative in evolution, where reproductive strategy fundamentally sculpts genetic landscapes. Such insights propel us closer to decoding the complex choreography governing life’s diversity and resilience.
In conclusion, the evolution of tandem repeat sequences in partially selfing diploid populations reveals a fascinating tapestry of genetic and evolutionary forces. The enhanced homozygosity stemming from selfing magnifies both within-individual and between-individual variation, thereby transforming how different selection modes operate on TRs. This heightened selection efficacy culminates in lower genetic load despite the counteracting effects of genetic drift, highlighting selfing as a pivotal factor shaping genetic variation. The research by Sudbrack and Mullon not only deepens our understanding of TR evolution but also opens avenues for exploring how mating systems influence genome stability and adaptation across a spectrum of life forms.
Subject of Research: Evolution of tandem repeat sequences under partial selfing and various selective regimes.
Article Title: The evolution of tandem repeat sequences under partial selfing and different modes of selection.
Article References:
Sudbrack, V., Mullon, C. The evolution of tandem repeat sequences under partial selfing and different modes of selection. Heredity (2026). https://doi.org/10.1038/s41437-026-00820-1
Image Credits: AI Generated
DOI: 29 January 2026
