MATHIEU ANATOLE TELE AYENAN
GENETIC STUDIES OF HEAT TOLERANCE IN TOMATOES (Solanum lycopersicum L.)
ABSTRACT
Heat stress is a threat to global agricultural production. Cultivated tomatoes are particularly sensitive to day and night temperature above 29.5°C and 21°C, respectively, especially during the reproductive stage. Heat stress constrains off-season tomato production in coastal countries in West Africa. To extend tomato production seasons in the region, the development of heattolerant tomato varieties adapted to local growing conditions is crucial. This study aimed at (i) identifying new sources of heat tolerance in tomatoes, (ii) assessing the diversity of tomato germplasm based on single nucleotide polymorphism (SNP) and heat tolerance traits, (iii) assessing the combining ability of selected lines and developing heterotic groups, and (iv) unraveling the genetic architecture of heat tolerance traits in a recombinant inbred lines population. A combination of field and laboratory experiments were undertaken to evaluate 42 tomato lines/accessions. Four new sources of heat tolerance, namely BJ01 and BJ02 from Benin, WAC1, and ATS020 from Ghana were identified based on their pollen viability, fruit set percentage, number of fruits per plant, and fruit yield under heat stress. These genetic resources had equal or superior performance to known sources of heat tolerance like “Nagcarlang” (LA2661), Saladette (LA2662) or CLN1621L. A combined phenotypic and genomic analysis clustered the lines into four groups, namely highly tolerant (e.g.: BJ01, BJ02, LA2661), moderately tolerant (e.g.: ATS020, LA2662), moderately sensitive (e.g.: CLN2498D, CLN3125L), and highly sensitive (e.g.: Pectomech, Tropimech, P008). Parental lines were selected from the clusters to generate hybrids. A factorial mating design (North Carolina mating design II) was used with five (two highly tolerant and three highly heat-sensitive locally collected lines) and 19 female parents (moderately tolerant and heat-sensitive lines from World Vegetable Center and University of Florida) to generate 55 hybrids. A second set of hybrids (54) was developed from a partial diallel between 15 lines selected from each cluster. The first set of hybrids was evaluated along with their parents in Ze in 2020 and Abomey-Calavi in 2021 3(Experiment 1) while the second set of hybrids was evaluated in Zopah, and IITA Station (Abomey-Calavi) in 2021 (Experiment 2) for fruit setting percentage (FS), the number of fruits per plant (NFP), fruit yield (FWP), individual fruit weight (FW). High better and mid-parent and standard heterosis were recorded for most of the hybrids showing their potential in outperforming best commercial checks. Both general combining ability (GCA) and specific combining ability (SCA) were important in determining the performance of the hybrids. The best crosses identified from Experiment 1 were Fla.7171 x P005 and CLN2026D x ATS020, combining positive SCA effects and high per se performance for FW and FWP. In Experiment 2, BJ01 showed high and positive GCA for all the traits except FW. In Experiment 2, the comparison between the breeding efficiency of heterotic grouping based on GCA of multiple traits (HGCAMT) and heterotic grouping based on SNP markers revealed that the SNP-based method was 28% more efficient. The SNP-based method grouped the 15 parental lines into three heterotic groups composed of five lines each. A highly tolerant (BJ01, male) and moderately tolerant (CLN2498, female) were selected to develop F3:4 recombinant inbred lines (RILs). The RILs were evaluated for NFP and FS under heat stress and genotyped using SNP markers. Thirteen promising heat-tolerant RILs were identified. The quantitative trait loci (QTL) analysis revealed three QTLs, one on chromosome 9 (explaining 9.6% of the total variation) and two on chromosome 11 for NFP. The two QTLs on chromosome 11 explained 29.98% of the total variation and overlapped on a region rich in stress response peroxidase and tRNA isopentenyl transferase, which are involved in adjusting adaptive plant responses to biotic and abiotic stresses. The findings provide important information regarding sources of heat tolerance, the genetic architecture of heat tolerance traits, and heterotic groups to fast track the improvement of heat tolerance in tomatoes