text: Two-component systems are rare in eukaryotes. They appear in yeasts, filamentous fungi, and slime molds, and are relatively common in plants, but have been described as "conspicuously absent" from animals. Two-component systems in eukaryotes likely originate from lateral gene transfer, often from endosymbiotic organelles, and are typically of the hybrid kinase phosphorelay type. For example, in the yeast Candida albicans, genes found in the nuclear genome likely originated from endosymbiosis and remain targeted to the mitochondria. Two-component systems are well-integrated into developmental signaling pathways in plants, but the genes probably originated from lateral gene transfer from chloroplasts. An example is the chloroplast sensor kinase (CSK) gene in Arabidopsis thaliana, derived from chloroplasts but now integrated into the nuclear genome. CSK function provides a redox-based regulatory system that couples photosynthesis to chloroplast gene expression; this observation has been described as a key prediction of the CoRR hypothesis, which aims to explain the retention of genes encoded by endosymbiotic organelles.
  It is unclear why canonical two-component systems are rare in eukaryotes, with many similar functions having been taken over by signaling systems based on serine, threonine, or tyrosine kinases; it has been speculated that the chemical instability of phosphoaspartate is responsible, and that increased stability is needed to transduce signals in the more complex eukaryotic cell. Notably, cross-talk between signaling mechanisms is very common in eukaryotic signaling systems but rare in bacterial two-component systems.