GENOME WIDE ASSOCIATION OF HEAT TOLERANCE LOCI OF WHEAT IN HOTSPOTS OF SUDAN AND SYRIA

ABSTRACT

One hundred and eighty-nine (189) wheat genotypes were evaluated in multi-environments (Tel-hadya (Syria), Dongola (Sudan) and Wadmedani (Sudan)) for heat tolerance from 2011 to 2012. Genomic mapping of the quantitative trait loci underlying heat tolerance in the crop was also performed. The field experiment was laid  out in an alpha lattice design. The data obtained were subjected  to restricted  maximum  likelihood (REML) for generation of best linear unbiased estimates (BLUEs). The heat tolerance study in the two seasons (early and late) in Tel-hadya,  Syria showed  that  days  to heading,  days to maturity and grain filling duration, plant height and grain  yield were significantly (p <0.05) reduced in late season when compared with their performance in the early season. The grain loss due to heat stress in the late season compared to the early season was in the range of 58 to 88%.The effect of heat stress on days to heading, days to maturity and grain filling duration, plant height and grain  yield  of  the  crop  varied  across  the  sites  (Late  season  in Tel-hadya,  Dongola  and Wadmedani). The decreased effect of heat stress was most pronounced in late season in Tel- hadya  than  in  the  two  sites  in  Sudan.  The  Additive  Main  effects  and  Multiplicative Interaction (AMMI) estimates showed 20 top yielding genotypes in all the sites with a grain yield range of 2.593 t/ha in Gen135 to 2.893t/ha in Gen117. In terms of their AMMI stability values ranking, Gen 60, Gen 68, Gen101, Gen155 and Gen118  were ranked first, second, third, fourth and fifth, respectively.  The broad sense  heritability estimates for grain yield ranged  from 0.297 (Dongola)  to 0.449 (Late  season in Tel-hadya).  Other traits like grain filling duration, plant height and one thousand kernel weight showed moderate to low broad sense heritability across the sites. The path coefficient analyses across the three sites showed that days to heading, canopy temperature and grain filling duration had reduced direct effect on the grain yield, while biomass and harvest index showed positive direct effect on the grain yield. Days to maturity showed negative direct influence on the grain yield in the late season in Tel-hadya, but positive direct effect was observed in the two environments in Sudan. The broad sense heritability of days to heading ranged from 0.804 (Wadmedani) to 0.908 (Late season in Tel-hadya),  while days to maturity ranged  from 0.68 (Dongola)  to 0.793 (Late season in Tel-hadya). The structure analysis revealed that the wheat germplasm studied had seven sub-populations.  The linkage disequilibrium  (LD)  analysis obtained showed that the LD  decay  was  approximately  at  25cM.  The  genome  wide  association  mapping  of  the quantitative  trait  loci  associated  with   heat  tolerance  showed  that  few  markers  were consistently detected in the three environments, while many were environment- specific.

INTRODUCTION

Bread wheat (Triticum aestivum L.) is unarguably one of the world’s most important and widely consumed cereal crop (Asif et al., 2005; Bushuk, 1998). The flour is used for making bread, biscuits,  confectionary products, noodles,  wheat  gluten  among others. The world population is expected to reach about 9 billion by the end of the 21st  century,

and it has been predicted that the demand for cereals, especially wheat, will increase by approximately 50% by 2030 (Borlaug and Dowswell, 2003).Wheat production attracts increasing attention globally owing to its importance as a staple food crop, such that the availability of wheat  and wheat  products  are seen as a food  security  issue  in many countries. This has led to growing of wheat in many parts of the world even where it was not formerly grown. Paliwal et al. (2012) indicated that wheat is one of the most broadly adapted  cereals.  Although  wheat  is  a  thermo  sensitive  long  day crop  that  requires relatively low temperature for its optimal  yield, it is  being grown in the tropics and subtropics  despite  the  relatively  high  temperature  that  is  associated  with  the  areas (Rehman et al., 2009). In spite of the growing attention on the crop globally, Ali (2011) reported that its production in many regions of the world is below average because of adverse environmental conditions. High temperature which imposes heat stress on wheat is a major  limitation  to  its  productivity  in arid,  semi-  arid,  tropical  and  subtropical regions of the world (Ashraf and Harris, 2005). It affects the different growing stages of the crop  especially during anthesis and grain filling (Rehman et al., 2009) leading to poor   grain  yield  and  quality.  This  is  exacerbated  by  the  increasing  temperature associated with global warming, thus breeding for high temperature tolerance in wheat is a major challenge globally.

A detailed understanding of the genetics and morpho-physiology of heat tolerance and use of effective breeding strategies to address the situation would be ideal. Sikder and Paul (2010)  reported  that identification  of wheat  varieties  suitable  for  heat  stressed condition would be an important step toward achieving high yield potentials in wheat. Major gains have been achieved in the improvement of economic traits of wheat through conventional breeding, and more recently; through marker assisted selection (MAS) that has transformed plant breeding. Advances in molecular technologies have resulted in the mapping  and  identification  of  quantitative  trait  loci  (QTLs)  controlling  traits  of importance  in  wheat,  thereby  permitting  improvement  beyond  the  upper  limit  of conventional   breeding  approaches.  The  two  most  commonly  used   approaches  in mapping  and  identification  of QTLs  are  bi-parental  and  association  mapping  (AM). Association  mapping,  which  is more  recent,  has  been  utilized  in overcoming  some limitations  associated   with  bi-parental   mapping  approach   in  exhaustive   genomic dissection of putative QTLs of  interest in plants. These limitations that are associated with  bi-parental  mapping  approach  are  time  consuming  in  generation  of  mapping population from a cross between two parents, low recombination events in the mapping population which  leads to poor mapping resolution,  and detection of only few QTL, among others. AM has the potential to identify a single polymorphism within a gene that is responsible for phenotypic differences (Braulio et al., 2012).

Although  significant  variation  for  heat  tolerance  exists  among  wheat   germplasm (Reynolds  et  al.,  1994;  Joshi et  al., 2007a,  b),  no  direct  selection  criteria  for  heat tolerance are available (Paliwal et al., 2012). This is probably because of lack of detailed understanding of the morphological, physiological and genetic bases for heat tolerance in wheat. Phenotypic selection for heat tolerance has been performed using grain filling duration (Yang et al., 2002); one thousand grain weight, canopy temperature depression (Reynolds et al., 1994a; Ayeneh et al., 2002) and grain yield. Despite these attempts, Ortiz et al. (2008) and Ashraf  (2010) reported that breeding for heat tolerance using trait- based selection is still in its infancy stage and warrants more attention.

Wheat  developmental   phases  such  as  ear  emergence,   anthesis  and  maturity  are controlled by three groups of vernalization (Vrn), photoperiod (Ppd), and the earliness per se genes (Kosner and Pankova, 1998) and their expression plays a significant role in wheat adaptation to different locations (Gororo et al., 2001). These three sets of genes together influence flowering time, and the suitability of genotypes for flowering under particular  environmental  conditions  (Snape  et  al.,  2001;  Dubcovsky  et  al.,  2006). Differences  in  flowering  time  could  be  of  vital  physiological  implication  in  heat tolerance in wheat crop. The paucity of knowledge of the underlying physiological basis of heat tolerance as well as the genomic regions associated with heat tolerance in wheat prompted  this research.  This study was carried  out to characterize  the physiological bases of heat tolerance and identify QTLs/genomic regions underlying these traits in a

collection of 189 elite wheat germplasm in diverse multi-environmental  regions.  The information   gathered   from  the  characterization   would   enable  wheat   breeders  to effectively combine, accumulate or pyramid several desirable traits into locally adapted germplasm leading to the development of wheat varieties with high yield, stability and enhanced heat tolerance. This research was initiated to achieve the following objectives:

      to evaluate the grain yield performance and stability of 189 elite ICARDA wheat genotypes in multi-heat stressed environments.

      to determine  the morphological  and physiological  bases for heat tolerance  in wheat.

    to map the putative quantitative trait loci (QTLs) underlying heat tolerance in wheat.

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