Conventional and molecular genetic analysis of factors contributing to variation in the timing of heading among spring barley (Hordeum vulgare L.) genotypes grown over a mild winter growing season

Factors contributing to variation in heading date in spring barley were examined in several studies commencing with a survey of developmental variation in a large collection of genotypes and concluding with the molecular genetic analysis of 7 doubled haploid populations. Genotypes varied considerably in their specific responses to photoperiod and vernalisation, and in the duration of a pre-inductive (or juvenile) phase defined in this paper as a 'basic vegetative period'. The latter includes differential genotype responses to ambient temperature and their interaction with photoperiod. Combinations of these largely independent environmental variables account for variation in heading date associated with differences in growing season conditions, particularly geographic region, sowing dates, and cultivar adaptation. Under extended and natural (short) photoperiods, in both summer and winter field plantings, conventional genetic analysis was characterised by simple Mendelian segregation combined with considerable transgressive segregation within distinct early and late flowering subpopulations. Equivalent transgressive segregation characterised molecular genetic analysis that identified 16 quantitative trait loci (QTLs) with contributions ranging from >50% of the variation recorded to <10%. These were dominated by 2 QTLs located on chromosome 2, one of which on 2HS was associated with response to extended photoperiod and the other, located near the centromere, with variation in the duration of the basic vegetative period. As only one population segregated for response to vernalisation, all analyses were restricted to parents and progeny homozygous for no response. Three other QTLs on 1HL, 3HL, and 5HL were primarily associated with vernalised parents and progeny characterised by prostrate seedling growth habits, which questions any assumption of a pleiotrophic association between genes for vernalisation and growth habit. The potential for exploiting markers for selection is considered to be limited by the considerable transgressive segregation observed in lines homozygous for parental alleles, and the limited understanding of the causes of variation in the phenotypic expression of the QTLs identified. Such markers would be useful in the selection of backcrossed progeny and in developing materials for investigating fundamental mechanisms contributing to developmental variation.