Isozyme analysis, chromosome pairing, and fertility of Lycopersicon esculentum × Solanum lycopersicoides diploid backcross hybrids

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Abstract
The wild nightshade Solanum lycopersicoides possesses potentially valuable traits for tomato (Lycopersicon esculentum) improvement. Male sterility of the intergenic F1 hybrid and its stylar rejection of tomato pollen have until recently prevented introgression. In the work herein described, ovule fertility was detected in the F1 hybrid following pollinations with S. lycopersicoides. Pollinations of the F1 with L. pennellii derived bridging hybrids resulted in a BC1 population of 65 plants. BC2 seed was obtained from most BC1 individuals. Male fertility of the BC1 plants ranged from 0 to 66% acetocarmine-stainable pollen. Of the 43 plants scored, 41 were diploids (2n = 24) and 2 were aneuploids (2n = 25 and 2n = 26). Chromosome pairing of the BC1 at diakinesis and metaphase I averaged 11.1 bivalents per pollen mother cell. BC1 and BC2 plants were scored for 19 isozyme and morphological markers covering 10 of the 12 S. lycopersicoides chromosomes. Significant deviations from Mendelian segregation were observed for seven loci, with an excess of homozygotes (+/+) in all cases. Wa, which controls the white anther trait of S. lycopersicoides, was mapped to within 15 centiMorgans (cM) of Got-4, while Mdh-4 was closely linked to Pgi-1 (< 2 cM). Plants heterozygous for the S. lycopersicoides allele (+/s) at Dia-2, Skdh-1, Wa, and Aco-1 showed significantly lower pollen stainability than homozygotes (+/+). Skdh-1 and Got-3 heterozygotes had less chromosome pairing at meiosis than homozygotes. These hybrids will permit the introgression of novel traits with S. lycopersicoides into tomato.