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The existence of ant colonies was a mystery even to the famous naturalist Charles Darwin. How could evolution produce workers that cannot reproduce? Darwin assumed that workers increase the reproductive success of their queen, thereby explaining the so-called reproductive division of labor: males provide sperm, which the queens store and use to fertilize eggs throughout their lives. Workers search for food, care for the brood and defend the nest, but do not reproduce.

It has been known since the 1960s that Darwin was right: siblings can pass on gene copies to future generations just as effectively as their own offspring. However, it was previously unclear how a complete restructuring of the ant genome took place to simultaneously contain the genetic blueprints for reproductive individuals and sterile workers. A new international study provides a number of answers to this fundamental question.

Much of the structure of the ant genome can only be explained in relation to the emergence and subsequent changes in the queen and worker castes, which jointly realize a "superorganismal" form of colonial organization. After ants emerged more than 150 million years ago, natural selection continued to diversify their genomes. This led to a series of evolutionary innovations, such as massive increases in colony size and increasing differences between queens and workers.

At the genetic level, this means that after the original ant genome had stabilized evolutionarily in the early Cretaceous period, ant genes were recombined to an unusually high degree. This rearrangement was particularly extensive in the most species-rich ant subfamilies today.

At the same time, smaller groups of linked genes remained excluded from such rearrangements for more than 100 million years, especially those that mediate the reproductive division of labor between queens and workers. This illustrates how fundamental and crucial a well-functioning caste differentiation was and is in ants, the team concludes.

The results of this interdisciplinary research, which have now been in the journal Cell, are based on almost 10 years of collaboration between more than 50 international scientists from more than 25 countries. Researchers from Germany (Dr. Lukas Schrader/University of Münster), Denmark (Prof. Dr. Jacobus Boomsma/University of Copenhagen) and China (Prof. Dr. Guojie Zhang/Zhejiang University, Hangzhou, China) coordinated the study.

The team ("Global Ant Genomics Alliance") sequenced and compared the genomes of more than 140 ant species, covering a large part of the biological diversity of these insects. The study covered ants with regular colonies, such as army ants with millions of workers per colony, and species whose queens are a hundred times larger than the smallest workers, but also species without queens whose workers clone themselves, and social parasites that do not need any workers at all.

"The publication is a milestone in our understanding of the molecular and genetic foundations of ants and probably also other social insects such as honeybees," emphasizes Schrader.

The researchers used numerous biological methods, such as genome sequencing, gene expression analysis, analysis of phenotypic ("visible") traits, functional analysis of individual genes, pharmacological manipulation and morphological investigations.