The green rice leafhopper is never alone. When a female’s egg and a male’s sperm fuse into a new cell, that cell is already infected with bacteria. As the newly conceived leafhopper grows from one cell into millions, its internal bacteria—its endosymbionts—go along for the ride. Right from the start, the leafhopper isn’t an individual in its own right, but a collection of animal and microbes that live together.

Many insects and other animals inherit endosymbionts from their parents, but almost all of them do so from their mothers. There’s good reason for this. An egg cell is big. Its central nucleus, which contains its DNA, is surrounded by a spacious and roomy cytoplasm, which can house lots of bacteria. But