Rachis fragility in F1 crosses between barley lines with alternative mutations

Wild barley (H. vulgare ssp. spontaneum) has a fragile rachis that allows seed dispersal. During its domestication process, at least two independent selections were made by early farmers in favour of tough rachis spikes, preventing spontaneous disarticulation of mature spikelets, and ensuring an efficient grain harvest. Rachis brittleness is controlled by two dominant, closely linked and complementary genes, Btr1 and Btr2, involved in the thinning and collapse of the cell walls under the rachis node. Independent recessive mutations in any of these genes, Non-brittle rachis 1 (btr1) or Non-brittle rachis 2 (btr2), turn the fragile rachis (brittle) of the wild form into the tough rachis phenotype (non-brittle). All cultivated barleys present a non-brittle phenotype, carrying either one or the other mutation. Hence, the cross of parents with alternative mutations in the non-brittle rachis genes (btr1btr1Btr2Btr2/Btr1Btr1btr2btr2) would lead to an F1 hybrid (Btr1btr1Btr2btr2) with trend to show a fragile rachis and, thus, that might present grain retention problems. The objective of the present work was to evaluate the potential agronomic problem that could arise in F1 crosses with different compositions at the non-brittle rachis genes, in the context of the emerging hybrid varieties cultivation. For this purpose, a controlled conditions greenhouse experiment composed of 23 hybrid crosses and 25 parents from a breeding program was performed. Rachis fragility was calculated as the percentage of number of rachis nodes disarticulated over total number of rachis nodes per spike, measured at two different times (2 and 4 weeks after ripening), through mechanical processing in an adapted single-spike threshing machine. Moreover, spontaneous disarticulation was assessed at three different times (2, 3 and 4 weeks after ripening) in a representative sample of genotypes exposed to natural conditions. Firstly, the non-brittle rachis genes genotype was checked in the hybrids and parents using specific KASP™ markers to ensure seed identity in the experiment. Subsequently, the statistical analysis of the phenotypic data showed significantly higher rachis fragility in crosses bearing alternative mutations in the non-brittle rachis genes (Btr1btr1Btr2btr2) compared to hybrids and inbred parents, carrying one of the deletions conferring the non-brittle phenotype (btr1btr1Btr2Btr2/Btr1Btr1btr2btr2), in the mechanic test, and even under natural conditions. This fact could jeopardize the efficient harvest of hybrids bearing alternative mutations; reduce the choice of possible crosses for hybrid barley breeding, and hinder the exploitation of potential heterotic patterns. Moreover, significant differences in rachis fragility were found within non-brittle genotypes. Non-brittle hybrids (btr1xbtr1 and btr2xbtr2) showed higher percentage of rachis disarticulation than inbred parents did. In addition, rachis brittleness variation was significantly higher within btr2 genotypes compared to btr1 genotypes. These findings, as well as previous studies, support the existence of further dominant genetic factors related to the control of rachis fragility, already detected in the literature, with higher prevalence in the btr2 pool. Finally, the influence of time after maturation on rachis brittleness was studied. For the first time, an increase in rachis fragility over time post-maturation was identified, only in the hybrid genotypes, both brittle and non-brittle types. This increase in rachis fragility with time, only observed by mechanical threshing, could mean an agronomic problem if it led to head shattering. In conclusion, this work encourages a deeper study of the genetic control of the rachis brittleness trait and urges the barley breeding for rachis tenacity with a view to hybrid varieties cultivation.