Fig. 7. Schematic illustration of asymmetric cell-cell interactions mediated by gamones during conjugation in B. japonicum. When mature type I cells receive the food-deprivation stimulus (step 1), they start transcribing the gamone 1 gene and synthesizing gamone 1 (steps 2-3). Secreted gamone 1 is recognized by starved mature type II cells through the putative gamone 1 receptor (step 4), inducing them to synthesize gamone 2 from tryptophan (steps 5-6). The gamone 1 gene is inactive in mating type II cells. Secreted gamone 2 is recognized by type I cells through the putative gamone 2 receptor (step 7). Gamone 2 signal is transmitted to the nucleus, promoting further transcription of the gamone 1 gene (step 8). I, mating type I cell; II, mating type II cell; G1, gamone 1; G1-R, hypothetical receptor for gamone 1; G2, gamone 2; G2-R, hypothetical receptor for gamone 2; E, enzyme(s) for production of gamone 2; Trp, tryptophan.