Effects of the Two-early-flowering-time Genes, Ef1 and m-Ef1, Founded in Low Latitude were Reexamined in One of the Origins of Donor Parent under Fluctuating Field Environments as to Earliness Effect and Fitness

Aims: Early flowering 1 (Ef1) and modifier of Early flowering 1 (m-Ef1) genes were firstly detected in the cultivars fitted to northern margins of rice cultivation. Hence, it has been inferred that the genes might be essential for adaptation to the regions. However, the genes have been identified in low latitudes and the effects on earliness and fitness in the origins have not been tested. To know genetic shift in response to climatic change, the present study was performed as to earliness effect and fitness.
Study Design: The six near-isogenic lines (NILs) associated with Ef1 and m-Ef1 genes and the recurrent parent were used. In each genotype, twelve plants were divided into two replicates in a paddy field. In each replicate, genotypes were placed at random.
Place and Duration of Study: The place of this study was National Agricultural Research Center in Sapporo (43 °N) in Japan. Duration of the study was summer season between year 2003 (cool summer) and 2004 (warm summer).
Methodology: In each plant, flowering time was recorded every day and days to flowering from germination to (DF) were calculated. Earliness effect of a plant of a NIL was defined as subtraction of DF of a NIL from that of the recurrent parent in same ID of same replicate. After harvest, maximum fertility of a panicle (Fmax), minimum fertility of a panicle (Fmin), total number of panicles with more than a fertile grain (Effective panicle number: EPN) and total number of panicles (TNP) were recorded in each plant. Fitness in a plant was conventionally estimated from the formulation, 1/2 (Fmax + Fmin) x (EPN / TNP).
Results: Compared with the previous study, which evaluated the effect of Ef1 and m-Ef1 genes in low latitude, effect of epistasis between Ef1 and m-Ef1 on earliness qualitatively changed between low and high latitudes. Furthermore, it was estimated that one of the epistasis might accelerate flowering time in response to low temperatures. At phenotype level, Individuals with earlier flowering time tended to show higher fitness. However, at genotype level, the tendency was broken in low-temperature year. As a result, correlation of fitness between two years showed no significance.
Conclusion: Epistasis-by-environment interaction might reveal significant role in earliness effects between latitude and between years. In addition, even in the case that earliness effects were preserved among genotypes between years, there is a possibility that fitness might not always be conserved under fluctuating field conditions in the northern limit of rice cultivation.