A team led by CIRAD (Centre de coopération internationale en recherche agronomique pour le développement) recently completed a vast study of a collection of more than 3000 sorghum accessions representative of the genetic resources of the species. Some forty molecular markers were used to analyse the genetic structure of the collection and determine how sorghum diversity is organized. The study was conducted under the CGIAR Generation Challenge Programme (GCP), and is one of the most wide-ranging studies ever undertaken into the genetic diversity of a cultivated plant.
Sorghum is the fifth most widely grown cereal worldwide. Its centre of diversity is in the northwestern quarter of Africa, where it was probably domesticated more than 5000 years ago. Cultivated sorghums are classified into five basic races (bicolor, caudatum, durra, guinea and kafir) and ten intermediate races, based on the morphological characteristics of their grains.
There are many sorghum collections worldwide, but their large size and variety of objectives make it difficult to study the genetic diversity of the species. It was therefore necessary to obtain an overview of that diversity and define a reference collection that could be used for future studies of genetic resources, and also for their management and use. This is what prompted researchers from CIRAD and their partners to build up a large sorghum collection, which they then studied using microsatellite markers.
In all, 3367 accessions were studied: 280 lines and cultivars from breeding programmes, 68 wild and weedy accessions, and more than 3000 local varieties, from collections held by CIRAD and International Crops Research Institute for the Semi-atid Tropics (ICRISAT), plus Chinese material supplied by the Chinese Academy of Agricultural Sciences (CAAS).
The 41 genetic markers used were part of a reference SSR (simple sequence repeats) kit developed by CIRAD for sorghum, which provides good coverage of the sorghum nuclear genome.
The researchers were able to split the sorghum accessions into 13 groups of varying sizes, within which the accessions were structured according to their geographical origin, and within each region, to their race. The peripheral groups in West Africa, southern Africa and East Africa were the most homogeneous and most clearly differentiated. Conversely, with an average of 19 alleles per locus, the central groups from central Africa and East Africa had the greatest allelic diversity. These groups were much more diversified, with more intermediate forms and wild types linked to cultivated forms.
The researchers did not observe any correspondence among the cultivated sorghum accessions between race and the group defined using markers, except for the kafir race from southern Africa. The bicolor, caudatum, durra and guinea races were each found in at least three groups, which suggests that they are morphotypes rather than "races". The wild and weedy accessions were highly diversified. They were classed among cultivated sorghums, which proves that there is intense gene flow between cultivated and wild sorghum varieties.
The geographical distribution of diversity, the limited congruence between marker-based classifications and that of races, and the probably existence of intense gene flow were the main results of the study. Sorghum diversity is thus apparently largely determined by the geographical distribution of the species in several directions from its centre of origin, through gene exchanges between races and wild forms, and by selection for race-related trait associations, which is responsible for the phenotypical convergence of genetically differentiated sub-populations.
This study is a preliminary exploration of sorghum genetic material, which could serve as the basis for analyses of the major collections worldwide. The marker kit can be used to characterize any new material, which can then easily be compared to known resources. To facilitate this type of study, the researchers identified within the collection a much smaller, representative sub-set that can be used to identify the regions and racial sub-groups with a good degree of variability for a given trait. The set comprises 383 sorghum accessions and includes lines from breeding programmes, wild sorghum forms and a mini-collection of genetically diverse accessions for which considerable data is already available. The five basic morphological types, ten intermediate types and wild forms from virtually every geographical origin are also in the set.
The grains, which are being held by ICRISAT, are available on request and all the data relating to these genetic resources are on Genesys, the global information system on genetic material held ex-situ. This mini-collection and the analysis should foster research on sorghum and boost international collaboration in this field.
This article was adapted from a piece entitled Sorghum genetic resources: a reference collection to study how diversity is organized published on the CIRAD Website.
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