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By tracing the genomes of nine brown algal species, researchers have shed light on the ancient origins, remarkable stability, and unexpected transformations of U/V sex chromosomes. This reveals how sex determination can evolve, shift, and even be replaced in these aquatic organisms.

Sex determination is a cornerstone of biology, yet many mysteries remain about how sex chromosomes evolve, especially outside familiar mammal and bird systems. Unlike mammals, where sex is set at fertilization by XX/XY chromosomes, brown algae determine sex during spore formation based on U (female) or V (male) sex chromosomes. Despite their importance, the rise, evolution, and demise of U/V sex chromosomes have remained an enigma.

This new research, in Nature Ecology & Evolution, demonstrates that these specialized sex chromosomes arose between 450 and 224 million years ago. A key finding is the identification of a pivotal male-determining gene, MIN, as well as six other core sex-linked genes, which have astonishingly remained almost unchanged across these vast evolutionary timescales.

The study also reveals that the extent and gene content of the sex-determining region in these chromosomes have diversified uniquely across brown algal lineages, often expanding through chromosomal rearrangements and acquiring new, lineage-specific genes. These structural changes may be linked to the increasing morphological and reproductive complexity observed among different brown algal species.

The dynamic nature of sex determination systems

The study also highlights that "orphan" or taxonomically restricted genes (genes unique to certain lineages) evolve with unexpected frequency in U and V chromosomes, underscoring their dynamic nature.

"The U/V sex chromosomes have unique genomic characteristics that make them prone to evolving orphan genes. This evolutionary pattern may provide important clues about how unique brown algal genes originated," said Josué Barrera-Redondo, first author of the paper and former postdoctoral researcher at the Max Planck Institute for Biology Tübingen.

The international team, led by researchers from the Max Planck Institute for Biology Tübingen (Germany), in collaboration with the CNRS and the Genoscope (France), uncovered two surprising cases where the ancestral U/V sex chromosome system has radically changed or even disappeared, transforming into autosomes.

In two hermaphroditic species, formerly male individuals acquired key female-specific genes, enabling them to develop the reproductive structures of both sexes. Meanwhile, the marine seaweed genus Fucus lost the U/V system when its lineage became completely diploid. The researchers found that new sex-determining genes supplanted the ancestral V chromosome gene MIN, marking a major shift in how sex is controlled genetically.

"Our research reveals that brown algal sex chromosomes manage a delicate balance between stability and surprising flexibility. This demonstrates how dynamic and diverse sex determination can be across the tree of life," explains lead researcher Dr. Susana Coelho, Director of the Department of Algal Development and Evolution at the Max Planck Institute for Biology Tübingen.

These findings show how sex-determining systems can collapse and be replaced, demonstrating that even something as fundamental as "male" and "female" is not fixed forever. It highlights a dynamic evolutionary landscape where sex-determining systems can evolve in unexpected ways—how they originate, reinvent themselves to persevere, and sometimes transform into autosomes. This offers fresh insights into sexual reproduction as one of the most fundamental aspects of biology.

This comprehensive investigation into brown algal sex chromosomes increases our understanding of U/V chromosome evolution and positions brown algae as a powerful model to explore the genetic and evolutionary basis of sex determination.