Cell type determines plastid transmission in tomato intergeneric somatic hybrids

Mesophyll (M)- and suspension culture (S)-derived protoplasts of both Lycopersicon esculentum, tomato, and its wild relative Solanum lycopersicoides were fused as S + M, M + M and S + S combinations, respectively, to resolve the role of parental cell types in determining cpDNA transmission to intergeneric somatic hybrid plants. The mesophyll cpDNA was preferentially transmitted to 96% of the plants, each regenerated from a separate callus, in M + S and S + M fusion combinations. In contrast, for the M + M combination there was an equable distribution of either tomato cpDNA or that of S. lycopersicoides among the 34 hybrid plants. The number of plastids or proplastids in mesophyll or suspension protoplasts was not a factor regulating cpDNA transmission. Mesophyll or suspension protoplasts of both fusion partners had comparable frequencies of either plastid type with a mean of 23. The biased transmission of plastids from the mesophyll parent in somatic hybrid plants of S + M and M + S combinations appears to be due to differential multiplication of plastids, possibly conditioned by an unequal input of the nucleoids found in plastids versus proplastids. In the M + M fusion, plastid and nucleotide input and subsequent plastid multiplication are apparently equal, and when combined with random sorting out leads to an equal distribution of parental cpDNAs in the regenerated somatic hybrid plants. For the S+S combination, 22 somatic hybrid plants have exclusively tomato cpDNA, an outcome that is not readily explained by donor cell input.