Subtropical Lines Research Articles

Temperate testers’ efficiency in the screening of tropical and subtropical maize germplasm

Despite much effort, little exotic germplasm has been introduced in temperate maize breeding programs. Tropical and subtropical maize germplasm from CIMMYT is the most genetically diverse in the world and should be useful in widening the genetic base of temperate maize germplasm. Extracting the most suitable material from exotic germplasm is a tough challenge for plant breeding programs. This study aimed to determine the optimal number of testers to screen a maize line derived from tropical and subtropical CIMMYT germplasm. For this purpose, four temperate maize testers (MO17, B73, K47/3, and K18) and nine lines from CIMMYT were crossed. The 36 crosses along with two hybrid checks were assessed in a randomized complete block design with three replications at four maize growing regions in Iran. The GGE biplot procedure and methodology proposed by Fan et al. (2010) were used to analyze the results indicating a possibility of direct use of 2, 3, 4, and 9 CIMMYT lines to improve grain yield in temperate environments. Temperate maize testers B73 and K47/3, which belong to the Reid Yellow Dent heterotic group, were very efficient in selecting superior tropical and subtropical CIMMYT germplasm, whereas testers MO17 and K18 from the Lancaster Sure Crop heterotic group were fairly good and weak, respectively. Actually just one temperate tester could select most, if not all, of the top exotic tropical and subtropical lines, and a second tester would provide backup.

Comparing the efficiency of the three heterotic-group and traditional two heterotic-group classifications for the hybrid maize breeding

Increasing the efficiency of the hybrid-based maize breeding program has highly contributed to the heterotic group classification. The present study was aimed to compare the breeding efficiency of the three heterotic-group (TriHG) classification [Lancaster Sure Crop (LSC), Reid Yellow Dent (RYD), CIMMYT] system and usual two heterotic-group (DiHG) classification (RYD, LSC) system. To accomplish this, specific breeding efficiency (SBE) and general breeding efficiency (GBE) were estimated for the grain yield. The mating design was a line × tester scheme in which seven adapted tropical and subtropical lines were crossed to four testers. GBE increased by 128% in the TriHG classification system as compared to the DiHG system while no significant loss was observed in SBE. It seems that the TriHG system was advantageous over the DiHG system by improving the maize breeding efficiency. Therefore, using one tester from each of the three heterotic groups (RYD, LSC, CIMMYT) could be more efficient in hybrid-based maize breeding programs in temperate regions, including Iran.

Improving Breeding Efficiency of a Hybrid Maize Breeding Program Using a Three Heterotic‐Group Classification

Core Ideas Maize breeding efficiency can be greatly improved by adopting three heterotic groups in hybrid breeding. Using three heterotic groups improves maize breeding efficiency for using genetic resources. General breeding efficiency is used to measure efficiency of genetic resource usage, and specific breeding efficiency is to obtain maximum output with inputs. Suwan1 is an important heterotic group from tropical/subtropical and should be explore more worldwide. Heterotic group classification has a crucial impact on maize (Zea mays L.) breeding efficiency in a hybrid‐based breeding program. The objectives of this study were to: (i) investigate whether or not breeding efficiency could be improved using a three heterotic‐group (TriHG) classification (Reid and non‐Reid, and Suwan1 heterotic groups) system instead of a usual two heterotic‐group (DiHG) classification (Reid and non‐Reid heterotic groups) system; and (ii) estimate the impact of TriHG and DiHG systems on breeding efficiency, utilizing the same data in computing specific breeding efficiency (SBE) and general breeding efficiency (GBE) in a subtropical breeding program in southern China. Twenty‐five adapted tropical and subtropical lines were crossed to six testers using a line × tester mating design. Data on grain yield were used to calculate SBE and GBE for comparing DiHG‐ and TriHG‐based strategies. The TriHG classification system increased GBE by 77.8% over the DiHG system without a significant loss in SBE. We concluded that the TriHG system was better than the DiHG system for improving maize‐breeding efficiency. Therefore, maize breeders may re‐think their breeding strategy for improving breeding efficiency in long‐term breeding programs by adopting the suggested TriHG system. Hybrid maize improvement programs, especially in the early stages of breeding for identifying hybrids with high yield potential, can be become more efficient by using three testers with one from each of Reid, nonReid, and Suwan1 heterotic groups. This would improve the chances of developing high‐yielding hybrids in a breeding program with the available, diverse germplasm.

Utilization Potential of the Temperate Maize Inbreds Integrated with Tropical Germplasm

A diallel mating experiment was conducted using ten maize lines, which consisted of five tropical, subtropical maize inbreds and five temperate maize inbreds developed by consecutive backcross and introducing tropical germplasm. Totally, ninety crosses were made based on Griffing method III. The objectives of this study were to evaluate: (1) combining ability of the ten lines; (2) heterosis and heterotic performance of these crosses between the temperate lines integrated with tropical germplasm and the same tropical lines. Ninety crosses were evaluated in Kunming, Yunnan province in 2008 and 2009. One local elite hybrid Yunrui 6 was used in the trials as a check. The results showed that the inbreds integrated with tropical germplasm YML598 and YML58 had highly significant GCA effects in most traits, which should have high use potential in maize hybrid breeding programs. And it was found that combining ability for grain yield was closely related to combining ability for most of yield component factors, yield per plant had positive correlation with most of traits except anthesis silking interval and round tip. And it was showed that most of hybrids with relative heterosis H ≥10% and highly significant SCA effects for yield per plant were the crosses between tropical subtropical lines and temperate lines, which suggested that the five tropical sub-tropical inbreds and five temperate inbreds still belonged to their previous heterotic group respectively, and their heterosis did not be changed. Thus, using tropical subtropical lines as donor, backcrossing consecutively with temperate lines may be an effective way to improve the latter.

Genetic Characterization and Linkage Disequilibrium Estimation of a Global Maize Collection Using SNP Markers

A newly developed maize Illumina GoldenGate Assay with 1536 SNPs from 582 loci was used to genotype a highly diverse global maize collection of 632 inbred lines from temperate, tropical, and subtropical public breeding programs. A total of 1229 informative SNPs and 1749 haplotypes within 327 loci was used to estimate the genetic diversity, population structure, and familial relatedness. Population structure identified tropical and temperate subgroups, and complex familial relationships were identified within the global collection. Linkage disequilibrium (LD) was measured overall and within chromosomes, allelic frequency groups, subgroups related by geographic origin, and subgroups of different sample sizes. The LD decay distance differed among chromosomes and ranged between 1 to 10 kb. The LD distance increased with the increase of minor allelic frequency (MAF), and with smaller sample sizes, encouraging caution when using too few lines in a study. The LD decay distance was much higher in temperate than in tropical and subtropical lines, because tropical and subtropical lines are more diverse and contain more rare alleles than temperate lines. A core set of inbreds was defined based on haplotypes, and 60 lines capture 90% of the haplotype diversity of the entire panel. The defined core sets and the entire collection can be used widely for different research targets.

Genetic structure and diversity among maize inbred lines as inferred from DNA microsatellites.

Two hundred and sixty maize inbred lines, representative of the genetic diversity among essentially all public lines of importance to temperate breeding and many important tropical and subtropical lines, were assayed for polymorphism at 94 microsatellite loci. The 2039 alleles identified served as raw data for estimating genetic structure and diversity. A model-based clustering analysis placed the inbred lines in five clusters that correspond to major breeding groups plus a set of lines showing evidence of mixed origins. A "phylogenetic" tree was constructed to further assess the genetic structure of maize inbreds, showing good agreement with the pedigree information and the cluster analysis. Tropical and subtropical inbreds possess a greater number of alleles and greater gene diversity than their temperate counterparts. The temperate Stiff Stalk lines are on average the most divergent from all other inbred groups. Comparison of diversity in equivalent samples of inbreds and open-pollinated landraces revealed that maize inbreds capture <80% of the alleles in the landraces, suggesting that landraces can provide additional genetic diversity for maize breeding. The contributions of four different segments of the landrace gene pool to each inbred group's gene pool were estimated using a novel likelihood-based model. The estimates are largely consistent with known histories of the inbreds and indicate that tropical highland germplasm is poorly represented in maize inbreds. Core sets of inbreds that capture maximal allelic richness were defined. These or similar core sets can be used for a variety of genetic applications in maize.