Morphological characterization and genetic diversity analysis of Tunisian durum wheat (Triticum turgidum var. durum) accessions

Abstract

Background: Tunisia is considered a secondary center of diversification of durum wheat and has a large number of abandoned old local landraces. An accurate investigation and characterization of the morphological and genetic features of these landraces would allow their rehabilitation and utilization in wheat breeding programs. Here, we investigated a diverse collection of 304 local accessions of durum wheat collected from five regions and three climate stages of central and southern Tunisia.

Results: Durum wheat accessions were morphologically characterized using 12 spike- and grain-related traits. A mean Shannon-Weaver index (H') of 0.80 was obtained, indicating high level of polymorphism among accessions. Based on these traits, 11 local landraces including Mahmoudi, Azizi, Jneh Khotifa, Mekki, Biskri, Taganrog, Biada, Badri, Richi, Roussia and Souri were identified. Spike length (H' = 0.98), spike shape (H' = 0.86), grain size (H' = 0.94), grain shape (H' = 0.87) and grain color (H' = 0.86) were the most polymorphic morphological traits. The genetic diversity of these accessions was assessed using 10 simple sequence repeat (SSR) markers, with a polymorphic information content (PIC) of 0.69. Levels of genetic diversity were generally high (I = 0.62; He = 0.35). In addition, population structure analysis revealed 11 genetic groups, which were significantly correlated with the morphological characterization. Analysis of molecular variance (AMOVA) showed high genetic variation within regions (81%) and within genetic groups (41%), reflecting a considerable amount of admixture between landraces. The moderate (19%) and high (59%) levels of genetic variation detected among regions and among genetic groups, respectively, highlighted the selection practices of farmers. Furthermore, Mahmoudi accessions showed significant variation in spike density between central Tunisia (compact spikes) and southern Tunisia (loose spikes with open glume), may indicate an adaptation to high temperature in the south.

Conclusion: Overall, this study demonstrates the genetic richness of local durum wheat germplasm for better in situ and ex situ conservation and for the subsequent use of these accessions in wheat breeding programs.

Keywords: Durum wheat; Genetic diversity; Landrace characterization; Local landraces; Phenotypic diversity; Population structure.

Fig. 1

Principal component analysis plot depicting 11 durum wheat landraces within 304 Tunisian accessions using 12 morphological traits under GenAlEx (version 6.501) [25]; (a) Projection of the 12 variables on the PCA plot axes. SS: spike shape, SL: spike length, AL: awn length, SC: spike color, NS: number of spikelets/spike, GlC: glume color, GN: number of grains/spikelet, GSp: grain shape, GSz: grain size, GC: grain color, AC: awn color, SD: spike density; (b) Projection of the 304 accessions on the PCA plot axes. Accessions were color-coded according to their landraces nomenclature, as identified with the morphological characterization

Fig. 2

Population structure analysis of 302 Tunisian durum wheat accessions genotyped with 10 SSR markers: (a) Plot of mean posterior probability (ln P(D)) values per cluster (K); (b) delta-K analysis of Ln P(D), based on 10 replicates per K, for K ranging from 1 to 20, with a burn-in period of 100,000 and Monte Carlo Markov Chain replicates of 100,000 iterations; (c) Membership coefficient bar plot displaying population structure at K = 11 for 302 Tunisian durum wheat accessions genotyped with 10 SSR markers inferred from STRUCTURE [26]. Each MLG is represented by a vertical line and they are ordered by color-coded genetic group (G1 to G11). For each genetic group, corresponding durum wheat landrace is mentioned. * Azizi landrace was divided into two genetic group G1 and G8

Fig. 3

Minimum spanning network using Bruvo’s distance of 302 durum wheat accessions genotyped with 10 SSR markers, performed under R software. Each node represents a multilocus genotype (MLG) and the size of the node is proportional to the number of accessions representing the MLG. MLGs were color-coded according to their membership to a genetic group (G1 to G11) as defined by STRUCTURE [26] at K = 11. Admixed individuals were color-coded in grey. Edge widths represent relatedness

Fig. 4

Geographic distribution of the 11 genetic groups (G1-G11), defined by STRUCTURE (version 2.3.4) [26] on 163 geo-localized durum wheat accessions genotyped with 10 SSR markers, over the regions of origin and the bioclimatic stages in Tunisia (https://www.d-maps.com/)