Symbiotic partnerships with beneficial microbes drive many metabolic and defensive adaptations in animals and plants. While intracellular symbionts often show genome erosion and co-evolution with their hosts, the consequences of extracellular lifestyles are less understood despite being widespread and functionally important. Using insect-bacteria symbioses as a model, this review highlights diverse routes of extracellular symbiont transfer, unified by the bacteria's ability to survive outside hosts, imposing different genomic, metabolic, and morphological constraints. The evolutionary implications of transmission routes (intracellular versus extracellular) do not necessarily match those of transmission mode (vertical versus horizontal), a key distinction for understanding genomic and physiological outcomes for both partners.
The psychedelic mushroom Psilocybe cubensis, first described from Cuba in 1906, was long thought to have been introduced to the Americas with cattle from Africa and Europe around 1500 CE. A new cryptic species, Psilocybe ochraceocentrata, has been discovered in sub-Saharan Africa as the closest wild relative of P. cubensis. DNA analysis of African Psilocybe specimens and molecular clock dating show the two species last shared a common ancestor about 1.5 million years ago, well before cattle domestication. Both species grow on large herbivore dung, indicating a predisposition to cattle dung. Ecological niche modeling suggests the common ancestor had suitable habitat across Africa, Asia, and the Americas over the last 3 million years, shedding light on the wild origins of domesticated P. cubensis.