What is the source of the striking yellow coloration in pea aphids?

Genes acquired from fungi via horizontal gene transfer

The striking yellow coloration found in certain pea aphid populations, specifically the yellow morphs of *Acyrthosiphon pisum*, does not stem entirely from pigments derived from ingested host plants, although diet can influence phenotype. Instead, the evidence points to a fundamental evolutionary event: the acquisition of specific genes from fungi through a process known as horizontal gene transfer. This incorporation of genetic material from another kingdom of life provided the necessary biochemical machinery for the aphids to synthesize carotenoid pigments, which result in the observable yellow hue. This mechanism demonstrates that the aphid's color palette is not strictly limited by parental inheritance or diet alone, but can be dramatically expanded by scavenging genetic traits from entirely different organisms.