Barriers for introgression of Solanum ochranthum into tomato via somatic hybrids

Solanum ochranthum Dunal is a nontuber bearing wild relative of the cultiva ted tomato (Lycopersicon esculentum Mill.), and a potential source of new g enes for disease and pest resistance. Because S. ochranthum is sexually iso lated from tomato, somatic hybrids between tomato (PI 367942; L. esculentum Mill. var. cerasiforme (Dunal) A. Gray VFNT cherry x L. peruvianum (L.) Mi ll. backcrossed to VFNT cherry) and S. ochranthum (LA2117) were developed p reviously to overcome these crossing barriers. Attempts to backcross these hybrids to tomato have been unsuccessful. Pollen fertility and mitotic and meiotic studies in tomato + S. ochranthum somatic hybrids determined the ca use of the sterility of the somatic hybrids and identified hybrids with mod erate fertility. Chromosome counts of dividing root tip cells delineated te traploid (2n = 4x = 48) and hexaploid (2n = 6x = 72) genotypes and aneuploi dy in these hybrids. Meiotic analysis of developing microspores confirmed t he presence of precocious division and laggard chromosomes at anaphase in b oth hexaploid and tetraploid hybrids. Bridges were observed in hexaploids a t anaphase I and II and multivalent configurations were observed at diakine sis. Multivalents and univalents; were evident in nearly all cells examined , proving that the two genomes are homoeologous. Aberrant microsporocytes w ith five to six developing microspores were noted in hexaploid hybrids. The occurrence of homoeologous; pairing between chromosomes of both fusion par ents is advantageous to effect recombination between these isolated species . However, the negative effects of multivalent formation and univalents lik ely contributed to observed sterility in these first generation fusion hybr ids. Low to moderate levels of pollen fertility (0% to 52%) were found in t etraploid hybrids, while little or no viable pollen (0% to 4%) was observed in hexaploid somatic hybrids.