Assessing the phenotypic and molecular selection indices for grain yield improvement in maize (Zea mays L.)

Introduction
Maize as a tropical cereal is a main source of food for humans and livestock, as well as biofuels and fiber in some regions of the world. Increasing maize production is one of the main priorities of the country, Iran. One of the columns of increasing production is development of new high-yielding cultivars. To improve a complex trait such as grain yield that has low heritability, indirect selection by other traits or developing a suitable index based on several traits can be used. In this study, linear phenotypic selection index (LPSI) and linear molecular selection index (LMSI) were prepared using the combination of morphological traits and informative ISSR molecular markers. The objective of the present study was to prepare appropriate selection indices in maize to improve grain yield.
Materials and methods
The plant materials of this research were 97 maize genotypes that were cultivated in a randomized complete block design with six replications in the research field of the Faculty of Agriculture, Urmia University, Urmia, Iran. Morphological traits were measured from the tasseling stage to the physiological maturity. Sixty ISSR primer combinations were also used to prepare the molecular profile of the studied maize genotypes. To select the suitable genotypes, two indices including linear phenotypic selection index and linear molecular selection index were used, and the efficiency of the indices was compared with the estimation of different parameters such as the rate of genetic gain and response to selection.
Research findings
The results of the linear phenotypic selection index showed that the highest rate of genetic gain based on the index (DG) was observed for chlorophyll content (99.15) and the lowest one for number of ears per plant (0.01). The expected genetic gain for all studied traits (DH) and response to selection was estimated at 163.2234 and 0.774, respectively. Based on the linear molecular selection index, the highest rate of genetic gain (DG) was observed for leaf area (99.31) and the lowest one was observed for number of ears per plant (0.02). The expected genetic gain for all studied traits (DH) and response to selection was also estimated at 50.972 and 0.774, respectively. The results showed that the correlation between index and breeding value (rHI) in the LPSI index was relatively favorable (less than one), and in the LMSI index was optimal (one), but both correlations were significant at 0.05 probability level according to the t-test. However, the efficiency of selection based on the index (ΔH) was 163.22 for the LPSI index and 50.97 for the LMSI index. On the other hand, the degree of genetic gain of trait (DG) was different depending on the type of index. For example, the ratio of genetic gain (DG) derived from molecular to phenotypic index for the number of ears per plant and grain yield (2.00 and 1.28, respectively) was higher than the other traits. Also, the best genotype based on both indices was genotype number of 61.
Conclusion
According to the results obtained from the present study and the review of sources in this field, it seems that it is possible to benefit from the advantages of development of the LMSI index in the breeding programs in early generations, but in advanced generations, it is better to select genotypes using the LPSI index, in which case the cost of molecular evaluations will be reduced.