An Evolutionary Scandal
In freshwater ponds, puddles, and even on damp moss across the globe, lives an organism that defies a fundamental rule of animal evolution. The bdelloid rotifer is a microscopic invertebrate, typically 0.1 to 0.5 mm long, that has persisted for an estimated 80 million years without any form of sexual reproduction. This all-female lineage, with no males ever discovered, reproduces solely through parthenogenesis, where mothers lay eggs that hatch into genetic copies of themselves. This long-term asexuality was famously called an "evolutionary scandal" because sexual reproduction is generally important for long-term survival. Sex shuffles genes, allowing populations to adapt to changing environments and purge harmful mutations. Without it, lineages are expected to accumulate detrimental mutations and be unable to keep pace with evolving parasites, leading to eventual extinction.
The bdelloids have thrived, diversifying into over 400 species. Their secret weapon is not sex, but theft. Bdelloid rotifers routinely steal genes from other organisms in their environment through Horizontal Gene Transfer (HGT). Analysis of their DNA reveals that up to 10% of their active genes have been acquired from bacteria, fungi, plants, and algae. These foreign genes provide new functions, from producing antibiotics to fight fungal infections to creating enzymes that break down toxins.