The genetic control of a photoperiod sensitive response in sorghum

Abstract

The genetics of maturity in sorghum have been well documented for many years, and it has become a model for research concerning the factors that affect maturity or photoperiodic response in tropical cereals. Sorghum geneticists have used the four maturity loci, Ma,, Ma2l Ma3, and Ma, (Quinby, 1974), to explain most of the variation in maturity in sorghum. However, there are numerous lines and populations that show variation in maturity that can not be explained by the four characterized maturity loci. Recently, an extremely photoperiod sensitive hybrid was discovered from the cross of two photoperiod insensitive lines. Hybrids from this cross are extremely photoperiod sensitive. When planted in central Texas in early April, hybrids from this cross will not flower until mid October. The objectives of this research were: (i) to determine the genetic inheritance of the photoperiod sensitivity in these sorghum lines and the relationship between Ma,, Ma,, Ma3 and Ma4, and the gene(s) controlling this photoperiod sensitivity, (ii) to determine what frequency of U.S.-adapted sorghums will produce photoperiod sensitive hybrids with this source, (iii) to develop segregating population for use in molecular mapping of the genes responsible for this response. EBA-3 is a breeding line of Argentina descent that flowers in approximately 70- 75 days. The U.S.-adapted germplasms have flowering dates ranging between 65-85 days after planting. F,- populations from the cross of EBA-3 by different U.S. sorghum lines were evaluated for photoperiod sensitivity in College Station in summer 1996 and 1997. The individuals in the F, population were classified into two phenotypic classes; photoperiod insensitive (anthesis prior to August 15) or photoperiod sensitive (anthesis after September 20). F2 segregation ratios indicate that two independent gene loci interact in complementary dominant epistasis to control photoperiod sensitivity in these lines. These initial results were EBA-3 was also hybridized to a set of milo maturity isolines to deten-nine if the observed phenotype is a result of new to deten-nine if the observed phenotype is a result of new In all cases, F, hybrids were photoperiod sensitive, indicating that the currently characterized alleles are not involved in this response. In addition, hybridization of EBA-3 to 29 different U.S.-adapted germplasms resulted in photoperiod sensitivity in every case. Our conclusions are that this photoperiod-sensitive reaction is conditioned by complementary dominant epistasis at two loci and that these loci are most likely different from Mal, Ma2, Ma3, and Ma,. Therefore, the two loci involved in this response should be designated Ma5and Ma,.