While it’s a given that cats have kittens and dogs have puppies, imagine a creature that could birth both! A recent study published in the prestigious journal Nature, conducted by researchers at the University of Montpellier in France, has uncovered a biological marvel that defies such conventional understanding. They discovered that queens of the ant species, Messor ibericus, astonishingly produce sons from not one, but two distinct species: their own M. ibericus drones and those of M. structor.
The Ant Society: Queens, Workers, and Drones
In the intricate world of ants, an individual’s biological sex is fundamentally determined by its chromosome count. Ants are either haploid, meaning they possess a single set of chromosomes inherited exclusively from their mother’s egg, or diploid, carrying two sets—one from the mother’s egg and another from the father’s sperm.
Female ants, encompassing both the queen and worker castes, emerge from fertilized eggs. Males, known as drones, on the other hand, develop from unfertilized eggs. While queens engage in reproduction and produce offspring, worker ants are sterile and do not lay eggs.
An ant queen typically passes on just one chromosome from each of her pairs to an egg. After mating with a drone, she carefully stores his sperm in a specialized sac called the spermatheca. Later, during egg-laying, the queen has a remarkable ability to ‘decide’ which eggs will be fertilized by the stored sperm to develop into females, and which will remain unfertilized to become male drones.
Here’s where the Messor ibericus queen’s story takes an unusual turn. She mates with drones from both her own species, M. ibericus, and the distinct species, M. structor. Eggs fertilized by M. ibericus sperm give rise to new queens. However, eggs fertilized by M. structor sperm develop into M. ibericus and M. structor hybrids, destined to become worker ants. The unfertilized eggs, as expected, become M. ibericus drones.
The truly puzzling question, however, is this: how exactly do M. ibericus queens manage to produce M. structor drones?
Nature’s Cloners: Ant Queens and Genetic Duplication
The Montpellier researchers uncovered the astonishing mechanism: M. ibericus queens have a unique ability to eliminate their own chromosomes from certain eggs, either just before or after fertilization. When these ’empty’ eggs are then fertilized by M. structor sperm, they exclusively contain a single set of M. structor chromosomes. This genetic manipulation automatically directs their development into M. structor drones.
Without this peculiar chromosome elimination, the M. ibericus colony would be unable to produce M. structor drones. These specific drones play a crucial role, as they are essential for generating the next generation of workers (the ‘grandchildren’ in the colony’s lineage).
Adding another layer of intrigue, the study revealed that all the M. structor drones produced within the M. ibericus colonies were genetically identical – essentially, perfect clones. It appears these ants mastered the art of cloning far ahead of human scientific discovery.
Messor ibericus represents the first known example of an ant colony intentionally incorporating drones from an entirely different species. Researchers hypothesize that the M. ibericus genome might have evolved specific ‘caste-biasing’ genes, directing females carrying them towards queenhood. By producing M. structor drones, the colony creates inter-species hybrids in the following generation that effectively circumvent these caste-biasing genes, developing into workers. The hybrid nature of these workers also ensures their sterility, maintaining the colony’s unique social structure.
As the researchers noted in their paper, this ability to ‘produce the required species’ males in their own colonies’ grants M. ibericus a significant evolutionary advantage. It sustains a process called ‘obligate hybridization’ – a form of cross-breeding essential for the ants to maintain specific advantageous traits.
This groundbreaking research also sheds light on a long-standing mystery: why all the castes of M. ibericus (queens, workers, and drones) thrive across Spain, Portugal, and Greece, while M. structor individuals in these regions are exclusively male.
Typically, in virtually all other ant and bee species, every member of a colony – queens, workers, and drones – belongs to the same species. While workers are universally sterile, queens achieve their fertility through various species-specific mechanisms; for instance, honeybee larvae destined to become queens are fed a rich substance called royal jelly.
Unlocking the Mystery: Mitochondrial DNA Reveals the Truth
The crucial evidence confirming the M. structor drones’ origin from M. ibericus queens came from analyzing their mitochondrial DNA (mtDNA). Researchers found that M. structor drones born in M. ibericus colonies possessed M. structor nuclear DNA, but their mitochondrial DNA matched that of M. ibericus. In stark contrast, drones from pure M. structor colonies showed M. structor DNA in both their nuclei and mitochondria.
It’s important to remember that while the majority of an organism’s genetic material resides in the cell’s nucleus, a small but vital amount is found within the mitochondria – this is mtDNA. Crucially, mitochondria and their DNA are inherited exclusively from the mother’s egg; sperm cells do not contribute mitochondria to the offspring.
Further DNA sequencing confirmed that the nuclear DNA of M. structor drones from M. ibericus colonies was indeed most closely related to that of wild M. structor. Yet, their mitochondrial DNA perfectly matched that of their M. ibericus nestmates, definitively linking them to the M. ibericus queen.
Intriguingly, when investigating whether these ‘domesticated’ M. structor males could mate with wild M. structor females, an analysis of 45 M. structor genomes revealed no evidence of hybridization between these two lineages, suggesting a reproductive isolation.
A notable physical difference was observed: these domesticated M. structor males were comparatively hairless, distinguishing them from both their wild M. structor relatives and their M. ibericus male nestmates. This phenotypic divergence prompts a fascinating question: given their genetic isolation, should these domesticated males be recognized as a distinct species? This parallels the evolutionary path of dogs (Canis lupus familiaris), which, through genetic isolation from wolves (Canis lupus), evolved into a separate species.
However, the researchers proposed a more profound interpretation. They argued that the M. ibericus queen is essentially ‘directly cloning’ the M. structor species using her own egg cytoplasm. This remarkable replication of an ‘alien’ genome within her own cellular machinery evokes the concept of endosymbiosis – much like the ancient ‘domestication’ of mitochondria within eukaryotic cells. In this context, these clonal males could be viewed as functioning like organelles, but at the scale of an entire superorganism.
While this comparison between M. structor drones and cellular mitochondria is insightful, there’s a key distinction: an M. ibericus colony possesses the unique ability to replenish itself with ‘fresh’ M. structor genomes. Unlike cells, which cannot simply acquire new mitochondria, the ant colony can refresh its genetic pool.
This vital replenishment is achieved when M. ibericus queens mate with wild M structor drones. This strategic cross-mating allows the colony to effectively ‘purge’ any M. structor genomes that may have accumulated undesirable mutations over time, ensuring the genetic health and viability of their unique reproductive system.
D.P. Kasbekar is a retired scientist and author of this commentary.