Management Practices and Breeding History of Varieties Strongly Determine the Fine Genetic Structure of Crop Populations: A Case Study Based on European Wheat Populations

As the effects of climate change begin to be felt on yield stability, it is becoming essential to promote the use of genetic diversity in farmers’ fields. The presence of genetic variability in variety could fulfil this purpose. Indeed, the level of intra-varietal genetic diversity influences the spatio-temporal stability of yields and the disease susceptibility of crop species. Breeding history of varieties and their management practices are two factors that should influence intra-varietal genetic diversity. This paper describes the genetic diversity of eight wheat samples covering a gradient from modern single varieties to on-farm mixtures of landraces. This gradient discriminates between landrace, historical and modern varieties, considering the breeding history of varieties, between single-varieties and mixtures of varieties, and between ex situ and in situ de facto strategy in terms of management practices. Genetic diversity of these samples was analyzed with the help of 41 single nucleotide polymorphism markers located in neutral regions, through computing genetic indices at three different levels: Allelic, haplotypic and genetic group level. Population structure and kinship were depicted using discriminant analysis and kinship network analysis. Results revealed an increase in the complexity of the genetic structure as we move on the gradient of variety types (from modern single variety to in situ on-farm mixtures of landraces). For the landraces, the highest levels of genetic diversity have been observed for a landrace (Solina d’Abruzzo) continuously grown on-farm in the region of Abruzzo, in Italy, for many decades. This landrace showed an excess of haplotypic diversity compared to landraces or the historical variety that were stored in genebanks (ex situ conservation). Genetic analyses of the mixtures revealed that, despite a very high selfing rate in wheat, growing in evolutionary mixtures promotes recombination between different genetic components of the mixture, a second way to increase the level of haplotype diversity. When management practices such as growing in mixture and on-farm management are combined, they substantially increase the different levels of genetic diversity of the populations (allelic, haplotypic, genetic group diversity), and consequently promote their adaptability. Our results confirm the need to develop and manage evolving diversified large populations on-farm. These results invite crop diversity managers such as genebank curators, community seed bank managers and farmers’ organizations to adapt their management strategies to the type of variety they wish to manage, because we have shown that their choices have a strong influence on the genetic composition of the crop populations.