In the bramble thickets of Sicily, mainland Italy, and France, a group of stick insects has been quietly rewriting the evolutionary rulebook—trading sexual reproduction for something far stranger. Researchers analyzing genomic data from more than 500 wild Mediterranean stick insects of the genus Bacillus have uncovered a stepwise evolutionary pathway that challenges everything we thought we knew about how sex is lost in nature. The discovery, published in the Proceedings of the National Academy of Sciences, reveals a surprisingly complex journey from hybrid mating to "virgin birth," a reproductive transition so rare in nature that scientists have struggled to study it directly.
The story begins about 8,000 years ago, shortly after the last ice age, when two different stick insect species met and produced hybrid offspring—creatures carrying one genome copy from each parent. Normally, such hybrids are evolutionary dead ends, sterile misfits like mules. But these Bacillus hybrids found an escape route through an extraordinary reproductive strategy called hybridogenesis. In this unusual mode, only the maternal genome is passed down clonally to offspring, while the paternal genome is somehow removed from developing eggs each generation. To restore their hybrid status, these insects must periodically mate with males of the paternal species, whose sperm is essentially hijacked by the hybridogenetic lineage.
What makes this system remarkable is not just the strangeness of hybridogenesis itself, but what came next. Many generations later, hybridogenesis gave rise to something even more radical: true parthenogenesis, or "virgin birth," where females reproduce without males entirely, passing both genomes to their offspring as perfect clones. The evolutionary story grew even more elaborate when some parthenogenetic lineages added a third genome by mating with yet another parental species, creating asexual stick insects that carried genetic material from three different ancestral sources.
To verify this evolutionary narrative wasn't simply theoretical, researchers from the Universities of Lausanne, Lund, and Rostock recreated the key transitions in laboratory crosses—a painstaking effort spanning several years, since stick insects have a one-year generation time. Every single transition inferred from wild populations could be replicated experimentally, suggesting each step along the way was biologically plausible and, more intriguingly, that each transition prepared the ground for the next.
The findings overturn a long-standing assumption in evolutionary biology: that asexual reproduction represents a dead end. Researchers had reasoned that creatures reproducing without males accumulate harmful mutations faster and adapt more slowly than sexual reproducers, condemning them to evolutionary obsolescence. Yet here, in the stick insect genus Bacillus, hybridization and the loss of sex instead created opportunities for further innovation. Rather than collapsing, these asexual lineages persisted and even diversified, suggesting that the initial reproductive upheaval opened doors to evolutionary possibilities that wouldn't have existed in strictly sexual populations.
The work offers a rare glimpse into how one of evolution's most fundamental transitions actually unfolds—not as a single dramatic switch, but as a series of incremental steps, each building on the last. For scientists studying the evolution of sex itself, these Mediterranean stick insects have become invaluable teachers, revealing pathways in nature that evolution had hidden for thousands of years.