Development of High Tryptophan Maize Near Isogenic Lines Adapted to Temperate Regions through Marker Assisted Selection - Impediments and Benefits

Abstract

Breeding program aimed at converting standard maize inbred lines to their quality protein maize (QPM) counterparts for growing in temperate climate is being conducted at Maize Research Institute (MRI). The objective of the research presented herein was to develop QPM versions of two commercial ZP inbreds through marker assisted selection (MAS) with opaque2 specific molecular markers, while maintaining their good agronomic performances and combining abilities. Donor line was a tropical QPM line CML 144. After two backcross and three selfing generations, six near isogenic lines (NILs) with 93% recovery of the recurrent parent genome were created from one cross. Average increments of 30% in tryptophan content and 36% in quality index were obtained, as well as kernels with less than 25% opaque endosperm. Grain yield was increased by 11–31% and combining abilities of the improved lines were on a par with the original line. Correlations between biochemical and agronomic parameters revealed that selection for plant height, ear length and kernel row number together with tryptophan content could be recommended for development of QPM with this material. However, several impediments emerged during selection. Major drawbacks in NIL development were small number of opaque2 recessive homozygotes (4.5% and 7.6% in BC2F2 of two crosses) and poor seed set throughout selection, which led to the loss of one cross. Moreover, in the other cross many plants in different generations had to be omitted from further selection due to the insufficient number of kernels. This phenomenon could be explained by incompatibility between pollen and style, possibly due to the exotic donor germplasm. Overall, it could be expected that the use of NILs, which are adapted to temperate climate and have high percentage of domestic germplasm, would outbalance the noted impediments and increase MAS efficiency in different breeding programs.

Generation and Evaluation of Modified Opaque-2 Popcorn Suggests a Route to Quality Protein Popcorn

“…As one way to avert protein malnutrition, many QPM conversion studies were carried out previously for dent corn (Babu et al, 2005; Gupta et al, 2009, 2013; Sofi et al, 2009; Jompuk et al, 2011; Krishna et al, 2017; Surender et al, 2017). Common challenges in QPM conversion programs included incomplete recovery of vitreous endosperm (modification), discontinuation of certain crosses from poor seed set and failure to improve lysine and tryptophan content (Moro et al, 1996; Prasanna et al, 2001; Vivek et al, 2008; Kostadinovic et al, 2016). Since popcorn kernels naturally have a very high proportion of vitreous endosperm (Figure 3), and this is the kernel region in which starch can melt during the popping process, complete endosperm modification is likely of paramount importance in QPM conversion into a popcorn background.…”

“…Poor seed set has been observed in other QPM conversions throughout selection and can lead to the loss of targeted crosses (Kostadinovic et al, 2016). This phenomenon may be attributable to incompatibility between pollen and style.…”

“…Breeding for QPM requires introgression of both the o2 allele and multiple unlinked modifier quantitative trait loci (QTLs). Availability of O2 in-gene and flanking markers can facilitate the introgression process of the o2 allele (Babu et al, 2005; Babu and Prasanna, 2014; Surender et al, 2014; Kostadinovic et al, 2016; Krishna et al, 2017). The difficulties encountered in QPM conversion projects result from restoring kernel vitreousness because of the complexity of modifier QTLs.…”

“…A previous QPM breeding program adopted both foreground (for o2 ) and background selection using MAS (marker-assisted selection) and developed QPM conversion lines within two backcrossing generations (Babu et al, 2005; Kostadinovic et al, 2016). MAS has not been widely used in selection of modifier genes because regions delineated as modifier QTLs tend to be large genetic intervals and highly variable in effect.…”

Production and Selection of Quality Protein Popcorn Hybrids Using a Novel Ranking System and Combining Ability Estimates

“…To alleviate these setbacks, CIMMYT employed a large-scale breeding program involving multiple opaque-2 varieties and selected moderately improved vitreousness levels through back-crossed generations. Along with the opaque-2 mutation, CIMMYT observed the necessary introgression of unknown amino acid and endosperm vitreousness restorer genes through phenotypic selection for the biofortified, vitreous QPM end product (Babu et al, 2005;Sofi et al, 2009;Panda et al, 2010;Mbuya et al, 2011;Babu and Prasanna, 2014;Surender et al, 2014;Kostadinovic et al, 2016;Krishna et al, 2017). Though most amino acid and endosperm modifier genes remain unidentified, QTL studies have suggested that endosperm restorer genes are located on Chromosomes 1, 5, 7, and 9 (Holding et al, 2008(Holding et al, , 2011Babu et al, 2015).…”