The cytogenetics of the tamarillo, Cyphomandra betacea (Cav.) Sendt., and its wild relatives

Abstract

The genus Cyphomandra (Solanaceae) is a morphologically diverse group of over thirty species of shrubs and trees, native to tropical regions of South and Central America. There are two main centres of species diversity - the Andes and southeast Brazil. Only one species is cultivated to any extent (the tamarillo or tree tomato, C. betacea), and this is for its fruit. Ten species were collected and studied for this work. The chromosomes of the Cyphomandra taxa studied had several distinguishing features. These were large size (among the largest in the Solanaceae), an invariable somatic chromosome number (2n = 2x = 24) and a large proportion of metacentric or submetacentric chromosomes. Most C-bands were located at the telomeres with small interstitial bands present in most species. Significant differences existed between the karyotypes of the species. Flow cytometric measurement of nuclear DNA confirmed the extraordinary size of the genomes of Cyphomandra and showed a nearly fourfold variation in magnitude between the species. Interspecific hybridization was attempted and it was found that most crosses appeared to fail after fertilization. However, one species hybrid was obtained with relative ease, and this was between the two Brazilian species C. corymbiflora and C. diploconos. The morphology and fertility of the hybrid was described and backcross progeny were raised. Reciprocal differences occurred in the compatibility of most species combinations. The information available from these crosses suggest non-involvement of the S locus in the control of interspecific incompatibility. Spontaneous triploid and tetraploid seedlings were found at low frequency among commercial plantings of the diploid tamarillo and were thought to arise from the union of unreduced gametes. Tetraploids were induced by colchicine treatment of germinating seed and triploids and aneuploids were synthesized by interploidy crosses. Some morphological traits, and pollen viability, were described for all chromosome levels. Although the euploids were judged to be of no commercial value in their present form, the aneuploids showed some potential as vegetatively propagated varieties. Three species, including the tamarillo, were found to be self-compatible whereas the others showed features typical of gametophytic self-incompatible plants. Aspects of pollen, seed and fruit biology were investigated. Some diploid chromosomal and genetic mutants were described. One species, C. luteoalba, was found to be quite distinct from all other species examined in terms of its morphology, C-banding pattern and interspecific compatibility. This species should be separated at sectional or generic level from all other species examined. Species relationships were discussed in relation to the variety of methods used to assess them. Comment was made on the value and control of indirect indicators of genome size, and there appeared to be little if any nucleotypic control over these characters. Implications of these results for basic and applied genetics were discussed.