More than meets the eye: A multi-year expressivity analysis of tomato sterility in ps and ps-2 lines

Functional male sterility in tomato (Solanum lycopersicum L.) positional sterile ps and ps-2 lines provides a good facultative sterility system for F1 hybrid seed production. In order to elucidate the observed variability in the trait stability, expressivity was observed in a comparative multi-year study (2008 – 2012) across seven ps and two ps-2 tomato lines. Tomato lines undergoing the study were cultivated from seeds obtained in the preceding year and grown in spring-summer season in greenhouse with constant temperature monitoring. Flowers (normally-shaped or ps-shaped), fruit number, and seed number per fruit were counted for the lowest first six branches of each plant, every year of the study. Lapses in sterility (expressed as percentage of seedless fruits or as number of seeds per seeded fruit produced) were attributed to dramatic greenhouse temperature fluctuations during the flowering period. Analyses of independent descendant F2 hybrid populations of ps × fertile crosses indicated a repeated presence of hitherto unreported phenotypic classes, ps-flowered plants with low or normal seed counts, strongly suggesting the gene responsible for the ps phenotype is distinct from tomato sterility gene. Segregation analyses of all independent crosses showed that either of these traits (ps flower phenotype or fruit sterility) was conferred by a single recessive gene. The calculated genetic distance between these two genes was 13.4 cM, based on the calculation of occurrence of the segregants across the investigated independent F2 populations. All F2 segregants classes produced viable pollen, pointing that the occurrence of novel segregants classes was not due to changed pollen viability. The distinction of ps from tomato sterility gene was further confirmed with molecular analyses of F2 sub-populations using a recently developed ps-linked DNA marker, resulting in a calculated genetic distance of 12 cM between it and the sterility gene.