1. Introduction
The continuous development of molecular biology and statistical tools to analyze the large amount of data obtained for different mapping populations is changing the approach to selecting plant materials for breeding [
1]. There is an increasing emphasis on the selection of multiple functional traits using, for this purpose, unrelated but genetically aligned plant materials. For maize, yield and its structural traits are the most important [
2,
3,
4]. Increasing the range of maize cultivation is linked to breeding progress, which includes exploiting heterosis (the vigor of the first generation of hybrids) and creating hybrids with lower climatic requirements [
5,
6,
7]. Access to increasingly modern cultivation technologies and breeding methods is also very important. The demand for new varieties of maize is constantly growing, making it the subject of intensive breeding and genetic research. The identification of molecular markers linked to genes determining not only the heterosis effect or grain yield but also other functional traits is now a priority in maize breeding programs [
1,
8].
To identify markers and their coupled genes, association mapping [
9,
10] and genomic selection [
11,
12] are currently the most commonly used. In association mapping, we can distinguish between candidate gene association and genome-wide association study (GWAS). In candidate gene association, correlations between DNA polymorphisms in a specific gene and a trait are checked. In the absence of detailed biochemical knowledge related to the sought-after trait, a GWAS analysis is justified. This approach searches for trait–marker associations across the genome and assumes that there are markers within the genome conditioning the expression of the trait that show coupling imbalances [
13]. It was on maize that the first association mapping was performed [
14] using simple RAPD and RFLP techniques. With the development of new biotechnological methods and statistical programs, the number of species studied has increased.
The breakthrough came in 2006 with Illumina’s launch of the Solexa (Genome Analyzer) Next Generation Sequencer. The sequencing technology used in this device is still being developed and Illumina now has several sequencing platforms, including two for ‘bulk’ analysis (NovaSeq and NextSeq), and the smallest is for sequencing small numbers of samples with shorter genomes (MiniSeq and iSeq). The platforms differ in the amount of data that they can obtain (from 1.8 GB for MiniSeq to 6000 GB on the NovaSeq platform) and the number of reads that they can generate in a limited time [
15]. Initially, the problem was the large amount of sequencing data preventing bioinformatics analyses on desktop computers; nowadays, analyses are performed on high-throughput servers with large RAM capacity, in either a Linux or an iOS environment [
16]. Many bioinformatics tools run algorithms that allow whole genomes and transcriptomes to be assembled from short sequences [
17]. As the results show, bioinformatics analyses currently pose the greatest difficulty in adapting NGS for diagnostics, as the result of improper folding is false positives and incorrectly folded genomes, making it difficult to correctly interpret the results obtained [
18].
One technology based on next-generation sequencing is DArTseq. This technology was used in the present study to identify single nucleotide polymorphism (SNP) and Silico DArT polymorphisms, for which associations with the heterosis effect were sought. DArTseq technology (in opposition to GBS) provides a large pool of both SNP and SilicoDArT markers (which are also characterized by dominance) [
19]. In this method, the complexity of the genome is reduced via restriction enzymes and sequencing short reads. DArTseq technology replaces the hybridization step, and sequencing is conducted using the Illumina system [
20].
Molecular analyses in maize are focused on the identification of new markers (SNP and SilicoDArT) and quantitative trait loci (QTLs) regions, as well as breeders’ interest in DNA analyses, which additionally includes the search for new methods to select the parental components for heterosis crosses [
21]. The idea here is to find a relationship between the yield of an F
1 hybrid and the heterogeneity of loci markers for its parental forms. It is well known that breeding success is determined by access to starting materials with significant genetic diversity because well-identified and heterotically partitioned starting material results in lower costs for the entire hybrid breeding process [
22].
The purpose of this study was to identify molecular markers (SNPs and SilicoDArTs) linked to the heterosis effects for yield and yield traits in maize (Zea mays L.).
3. Results
Next-generation sequencing yielded 81,602 molecular markers (28,571 SNPs and 53,031 SilicoDArTs). Of the 81,602, 15,409 (13,850 SNPs and 1559 SilicoDArTs) were selected for association analysis using the criteria given above. Due to statistically significant interactions, association analyses of genotypes–years, genotypes–locations, years–locations and genotypes–years–locations were performed independently for four environments (combinations of years–locations). The 105 molecular markers (8 SNPs and 97 SilicoDArTs) were associated with the heterosis effect of at least one trait in at least one environment. A total of 186 effects were observed (
Figure 3,
Table 1,
Table 2,
Table 3,
Table 4,
Table 5,
Table 6,
Table 7,
Table 8 and
Table 9).
The heterosis effect of the cob length was determined by five markers: four SilicoDArTs and one SNP (
Table 1). The effects of these markers varied from 2.494 (for 9693403—SilicoDArT marker in Łagiewniki 2014) to 3.579 (for SilicoDArT marker 2490222 in Smolice 2013), with an average of 2.977. The percentage variance accounted by individual markers varied from 59.4% (for 4584773—SilicoDArT marker in Smolice 2014) to 72.3% (for SilicoDArT marker 2548691 in Łagiewniki 2014), with an average of 66.76%. SilicoDArT marker 2490222 was associated with the heterosis effect of cob length in all four environments (
Table 1). The other four markers determined the heterosis effect only in single environments. The largest number (four) of associations was observed in Łagiewniki in 2014.
Table 1.
Molecular markers significantly (LOD > 3.0) associated with the heterosis effect of cob length (LC).
Table 1.
Molecular markers significantly (LOD > 3.0) associated with the heterosis effect of cob length (LC).
Type of Marker | Marker Name | Łagiewniki | Smolice |
---|
2013 | 2014 | 2013 | 2014 |
---|
Effect | R2 | Effect | R2 | Effect | R2 | Effect | R2 |
---|
SilicoDArT | 2490222 | 2.626 | 70.4 | 3.394 | 69.2 | 3.579 | 70.2 | 3.5 | 72 |
SilicoDArT | 2548691 | | | 2.964 | 72.3 | | | | |
SilicoDArT | 9693403 | | | 2.494 | 60.1 | | | | |
SilicoDArT | 4584773 | | | | | | | 2.754 | 59.4 |
SNP | 4776193 | | | 2.502 | 60.5 | | | | |
The heterosis effect of cob diameter was determined by 18 SilicoDArT markers (
Table 2). The effects of these markers varied from −0.3455 (for marker 4580744 in Smolice 2014) to 0.606 (for marker 2490222 in Smolice 2013), with a mean value of 0.367. R
2 for individual markers ranged from 58.3% (for marker 21697645 in Smolice 2013) to 74.5% (for marker 4769006 in Łagiewniki 2014), with an average of 63.91%. SilicoDArT markers 21697645 and 2529741 were associated with the heterosis effect of cob length in three of the four environments (except for Łagiewniki 2014 and Smolice 2013, respectively). The largest number (11) of associations was observed in Łagiewniki in 2014 (
Table 2).
Table 2.
Molecular markers (SilicoDArT) significantly (LOD > 3.0) associated with the heterosis effect of cob diameter (DC).
Table 2.
Molecular markers (SilicoDArT) significantly (LOD > 3.0) associated with the heterosis effect of cob diameter (DC).
Marker Name | Łagiewniki | Smolice |
---|
2013 | 2014 | 2013 | 2014 |
---|
Effect | R2 | Effect | R2 | Effect | R2 | Effect | R2 |
---|
9626263 | | | 0.4121 | 61.1 | | | 0.3547 | 72.1 |
2455130 | | | 0.4016 | 60.9 | | | | |
2380663 | | | 0.4261 | 69.7 | | | 0.3279 | 63.8 |
7057126 | | | 0.4267 | 66.2 | | | 0.3295 | 61 |
4769006 | | | 0.4387 | 74.5 | | | 0.3401 | 69.3 |
21697645 | 0.3414 | 63 | | | 0.432 | 58.3 | 0.3582 | 65.5 |
4584035 | | | 0.4293 | 59.5 | | | | |
4766435 | | | 0.4349 | 69.1 | | | | |
2529741 | 0.3228 | 62.6 | 0.4079 | 59.7 | | | 0.3248 | 59 |
7051437 | | | | | | | 0.3237 | 62 |
100000032 | | | | | | | 0.3255 | 62.8 |
100000044 | | | 0.3945 | 58.4 | | | | |
2490222 | 0.4276 | 60.1 | | | 0.606 | 71.8 | | |
4583359 | | | | | 0.444 | 62 | | |
9698148 | | | 0.419 | 67.1 | | | 0.326 | 63 |
4584773 | 0.3664 | 60.2 | | | 0.513 | 70 | | |
9679877 | | | | | | | 0.3455 | 60.3 |
4580744 | | | | | | | −0.3455 | 60.3 |
The heterosis effect of core length was determined by eleven markers: eight SilicoDArTs and three SNPs (
Table 3). The effects of these markers ranged from −2.241 (for 4767009—SilicoDArT marker in Łagiewniki 2014) to 3.377 (for 2490222—SilicoDArT marker in Smolice 2013), with an average of 2.033. The R
2 for particular markers ranged from 58.7% (for SilicoDArT marker 2383880 in Łagiewniki 2013 and SNP marker 4592834 in Łagiewniki 2014) to 78.1% (for SilicoDArT marker 7058267 in Smolice 2013), with an average of 63.57%. No marker was significant for the heterosis effect in Smolice in 2014. No marker was significant for the heterosis effect in more than two environments. Three markers (SilicoDArT markers 2490222 and 2548691, and SNP marker 4776193) were associated with the heterosis effect of core length in two environments (
Table 3). The largest number (eight) of associations was observed in Łagiewniki in 2014.
Table 3.
Molecular markers significantly (LOD > 3.0) associated with the heterosis effect of core length (LCO).
Table 3.
Molecular markers significantly (LOD > 3.0) associated with the heterosis effect of core length (LCO).
Type of Marker | Marker Name | Łagiewniki | Smolice |
---|
2013 | 2014 | 2013 | 2014 |
---|
Effect | R2 | Effect | R2 | Effect | R2 | Effect | R2 |
---|
SilicoDArT | 9624336 | | | 2.138 | 59.5 | | | | |
SilicoDArT | 2490222 | 2.446 | 62.7 | | | 3.377 | 65.3 | | |
SilicoDArT | 2548691 | 2.108 | 63.6 | 2.608 | 63.8 | | | | |
SilicoDArT | 2383880 | 1.763 | 58.7 | | | | | | |
SilicoDArT | 2425331 | | | 2.241 | 62.6 | | | | |
SilicoDArT | 4767009 | | | −2.241 | 62.6 | | | | |
SilicoDArT | 4764251 | | | 2.207 | 64 | | | | |
SilicoDArT | 7058267 | | | | | 3.13 | 78.1 | | |
SNP | 4592834 | | | 2.126 | 58.7 | | | | |
SNP | 4776193 | 1.895 | 61.5 | 2.409 | 65.5 | | | | |
SNP | 7049788 | | | 2.254 | 63.4 | | | | |
The heterosis effect of core diameter was determined by 27 markers: 24 SilicoDArTs and 3 SNPs (
Table 4). The effects of individual markers ranged from −1.1633 (for 2552284—SilicoDArT marker in Łagiewniki 2014) to 0.2323 (for 2495461—SilicoDArT marker in Smolice 2013), with an average of 0.1091. The R
2 for particular markers ranged from 58.3% (for 21698399—SilicoDArT marker in Smolice 2014) to 83.3% (for 4769006—SilicoDArT marker in Smolice 2014), with an average of 65.98%. The SNP marker 4590442 was associated with the heterosis effect of core diameter in all four environments (
Table 4). Two SilicoDArT markers (4769006 and 21697628) were associated in three of the four environments. The largest number (15) of associations was observed in Smolice in 2014.
Table 4.
Molecular markers associated with the heterosis effect of core diameter (DCO).
Table 4.
Molecular markers associated with the heterosis effect of core diameter (DCO).
Type of Marker | Marker Name | Łagiewniki | Smolice |
---|
2013 | 2014 | 2013 | 2014 |
---|
Effect | R2 | Effect | R2 | Effect | R2 | Effect | R2 |
---|
SilicoDArT | 4590044 | | | 0.1531 | 58.9 | | | | |
SilicoDArT | 9626263 | | | | | | | 0.1409 | 61.6 |
SilicoDArT | 2380663 | | | | | | | 0.1411 | 65.2 |
SilicoDArT | 4588725 | | | 0.1651 | 70 | | | 0.1423 | 66.5 |
SilicoDArT | 9680027 | | | | | 0.1816 | 62.1 | | |
SilicoDArT | 4769006 | 0.1595 | 65.3 | 0.1649 | 69.8 | | | 0.1573 | 83.3 |
SilicoDArT | 21698399 | | | | | | | 0.1377 | 58.3 |
SilicoDArT | 2602895 | | | | | | | 0.1426 | 66.9 |
SilicoDArT | 21697628 | 0.1668 | 60.6 | 0.1762 | 68.2 | 0.2039 | 67.8 | | |
SilicoDArT | 4766435 | | | 0.1605 | 62.1 | | | | |
SilicoDArT | 2529741 | 0.1603 | 62.4 | | | | | | |
SilicoDArT | 4583359 | 0.1744 | 67.1 | | | 0.2202 | 80.5 | | |
SilicoDArT | 9698148 | | | 0.1574 | 62.8 | | | | |
SilicoDArT | 4590586 | | | | | | | 0.1346 | 58.5 |
SilicoDArT | 2495461 | 0.1905 | 66.7 | | | 0.2323 | 74 | | |
SilicoDArT | 4778477 | | | 0.1655 | 66.6 | | | | |
SilicoDArT | 9679877 | | | | | | | 0.1503 | 63.2 |
SilicoDArT | 100000188 | | | | | 0.1858 | 65.3 | 0.1372 | 61.2 |
SilicoDArT | 9713962 | | | 0.1556 | 61.2 | | | | |
SilicoDArT | 4580744 | | | | | | | −0.1503 | 63.2 |
SilicoDArT | 2504629 | | | | | | | −0.1467 | 59.8 |
SilicoDArT | 2464213 | | | −0.1531 | 58.9 | | | | |
SilicoDArT | 2552284 | | | −0.1633 | 68.3 | | | −0.1369 | 60.9 |
SilicoDArT | 4591821 | | | −0.1589 | 60.7 | | | | |
SNP | 7058274 | | | | | | | 0.1467 | 59.8 |
SNP | 4777654 | | | | | | | −0.1426 | 66.9 |
SNP | 4590442 | 0.1642 | 65.9 | 0.1712 | 72 | 0.2075 | 79.3 | 0.1595 | 81.4 |
The 34 SilicoDArT markers and one SNP marker were associated with the heterosis effect of the number of rows of grain (
Table 5). The effects of the individual markers ranged from –1.412 (for 4765009—SilicoDArT marker in Łagiewniki 2014) to 1.597 (for 2544571—SilicoDArT marker in Łagiewniki 2014), with an average of 0.583. The R
2 for individual markers ranged from 58.2% (for 4764693—SilicoDArT marker in Smolice 2013 and 2393601 SilicoDArT marker in Łagiewniki 2014) to 79.9% (for 2544571 SilicoDArT marker in Łagiewniki 2014), with an average of 65.42%. No marker was significant for the heterosis effect in all four environments. Only one marker (SilicoDArT 2437173) was associated with the heterosis effect of the number of rows of grain in three environments (except for Łagiewniki 2013). The largest number (16) of associations was observed in Smolice in 2014 (
Table 5).
Table 5.
Molecular markers associated (LOD > 3.0) with the heterosis effect of the number of rows of grain (NRG).
Table 5.
Molecular markers associated (LOD > 3.0) with the heterosis effect of the number of rows of grain (NRG).
Type of Marker | Marker Name | Łagiewniki | Smolice |
---|
2013 | 2014 | 2013 | 2014 |
---|
Effect | R2 | Effect | R2 | Effect | R2 | Effect | R2 |
---|
SilicoDArT | 7057126 | | | | | | | 1.025 | 59.5 |
SilicoDArT | 2379644 | | | | | | | 1.01 | 60.9 |
SilicoDArT | 4778753 | 1.309 | 61.5 | | | | | | |
SilicoDArT | 2544571 | | | 1.597 | 79.9 | | | | |
SilicoDArT | 4764693 | 1.316 | 58.9 | | | 1.168 | 58.2 | | |
SilicoDArT | 2491739 | 1.491 | 69.4 | | | 1.326 | 69 | | |
SilicoDArT | 2437173 | | | 1.412 | 68.2 | 1.233 | 65.9 | 1.055 | 63.5 |
SilicoDArT | 4581103 | | | | | 1.156 | 60.2 | | |
SilicoDArT | 100000044 | | | 1.32 | 61.8 | | | 1.054 | 67 |
SilicoDArT | 100000049 | | | | | 1.201 | 62 | | |
SilicoDArT | 2444826 | | | | | | | 1.09 | 68.4 |
SilicoDArT | 2435684 | | | | | | | 1.201 | 61.5 |
SilicoDArT | 9668509 | 1.322 | 62.9 | 1.404 | 71.1 | | | | |
SilicoDArT | 9698148 | | | | | | | 1.02 | 62.2 |
SilicoDArT | 4588300 | | | 1.379 | 68.3 | | | 1.072 | 69.7 |
SilicoDArT | 4765474 | | | −1.379 | 68.3 | | | −1.072 | 69.7 |
SilicoDArT | 2462176 | 1.334 | 64.2 | | | | | | |
SilicoDArT | 9706913 | | | | | | | 1.08 | 67.1 |
SilicoDArT | 2386741 | | | | | 1.15 | 59.4 | | |
SilicoDArT | 9624718 | | | | | 1.181 | 63.1 | | |
SilicoDArT | 5584758 | 1.479 | 76.7 | | | | | | |
SilicoDArT | 2443558 | | | 1.325 | 58.9 | | | | |
SilicoDArT | 2611917 | | | | | | | 1.16 | 70 |
SilicoDArT | 2393601 | | | 1.286 | 58.2 | | | | |
SilicoDArT | 21696515 | | | 1.499 | 78.1 | | | | |
SilicoDArT | 7048312 | 1.384 | 69.8 | | | | | | |
SilicoDArT | 4591958 | | | | | | | −1.201 | 61.5 |
SilicoDArT | 4765009 | | | −1.412 | 68.2 | | | | |
SilicoDArT | 2425024 | | | | | | | −1.08 | 67.1 |
SilicoDArT | 2386050 | | | | | | | −1.16 | 70 |
SilicoDArT | 4578555 | | | −1.32 | 61.8 | | | −1.054 | 67 |
SilicoDArT | 4583930 | | | | | −1.187 | 63.9 | | |
SilicoDArT | 4578963 | | | | | | | −1.01 | 60.9 |
SilicoDArT | 4591690 | −1.297 | 60.2 | | | | | | |
SNP | 4764456 | | | | | −1.221 | 64.4 | | |
The heterosis effect of the number of grains in a row was determined by 15 SilicoDArT markers (
Table 6). The effects of individual markers ranged from −6.98 (for 2532754 marker in Smolice 2013) to 8.55 (for 2490222 marker in Smolice 2013), with an average of 5.725. The R
2 for individual markers ranged from 58.1% (for 4769448 marker in Łagiewniki 2014) to 76.7% (for 7058267 marker in Smolice 2013), with an average of 63.43%. Only one marker (2490222) was associated with the heterosis effect of the number of grains in a row in two of the four environments (Smolice in both years). The other 14 markers determined the heterosis effect of the number of grains in a row only in single environments. The largest number (eight) of associations was observed in Łagiewniki in 2014 (
Table 6).
Table 6.
Molecular markers (SilicoDArT) significantly (LOD > 3.0) associated with the heterosis effect of the number of grains in a row (NGR).
Table 6.
Molecular markers (SilicoDArT) significantly (LOD > 3.0) associated with the heterosis effect of the number of grains in a row (NGR).
Marker Name | Łagiewniki | Smolice |
---|
2013 | 2014 | 2013 | 2014 |
---|
Effect | R2 | Effect | R2 | Effect | R2 | Effect | R2 |
---|
2456387 | | | 6.16 | 61.2 | | | | |
2434942 | | | 6.35 | 58.3 | | | | |
4765710 | | | | | 6.98 | 67.9 | | |
4778447 | | | 5.89 | 58.5 | | | | |
2400466 | | | 6.59 | 63.3 | | | | |
2490222 | | | | | 8.55 | 61.6 | 7.99 | 65.8 |
2548691 | | | 7.32 | 65.5 | | | | |
4578219 | 3.422 | 59.5 | | | | | | |
4575644 | | | | | 6.25 | 59.4 | | |
2547687 | | | 6.18 | 65.2 | | | | |
2523729 | | | 6.15 | 64.5 | | | | |
2562473 | | | | | | | 6.65 | 61.5 |
4769448 | | | 6.02 | 58.1 | | | | |
2532754 | | | | | −6.98 | 67.9 | | |
7058267 | | | | | 8.07 | 76.7 | | |
The heterosis effect of mass of grain from the cob was determined by seven SilicoDArT markers (
Table 7). The effects of individual markers ranged from 35.98 (for 2548691 marker in Łagiewniki 2013) to 72.60 (for 2548691 marker in Smolice 2013), with an average of 52.682. The R
2 for individual markers ranged from 59.0% (for 2434942 marker in Łagiewniki 2014) to 86.7% (for 2490222 marker in Smolice 2014), with an average of 68.78%. SilicoDArT marker 2490222 was associated with the heterosis effect of mass of grain from the cob in all four environments, whereas marker 2548691 was associated in three of the four environments, except for Smolice 2013 (
Table 7). The largest number (four) of associations was observed in Łagiewniki in 2014 and in Smolice in 2014.
Table 7.
Molecular markers (SilicoDArT) significantly (LOD > 3.0) associated with the heterosis effect of mass of grain from the cob (MGC).
Table 7.
Molecular markers (SilicoDArT) significantly (LOD > 3.0) associated with the heterosis effect of mass of grain from the cob (MGC).
Marker Name | Łagiewniki | Smolice |
---|
2013 | 2014 | 2013 | 2014 |
---|
Effect | R2 | Effect | R2 | Effect | R2 | Effect | R2 |
---|
2434942 | | | 46.5 | 59 | | | | |
2397282 | | | 51.5 | 60.4 | | | 48 | 59.2 |
2490222 | 47.95 | 82.4 | 67.75 | 79.3 | 72.6 | 78.8 | 66.41 | 86.7 |
2548691 | 35.98 | 61.1 | 54 | 67.4 | | | 50 | 65 |
4584773 | | | | | | | 50.5 | 66.3 |
4778649 | 36.37 | 62.7 | | | | | | |
7058267 | | | | | 57.3 | 65.8 | | |
The heterosis effect of weight of one thousand grains was determined by ten SilicoDArT markers (
Table 8). The effects of individual markers ranged from –69.6 (for 4594009 marker in Łagiewniki 2013) to 69.7 (for 100000034 marker in Łagiewniki 2013), with an average of 20.2. The R
2 for individual markers ranged from 58.3% (for 4779015 marker in Łagiewniki 2014) to 74.0% (for 4594009 marker in Łagiewniki 2013), with an average of 62.66%. No marker was significant for the heterosis effect of WTGs in Smolice in 2014. No marker was significant for the heterosis effect of WTGs in more than two environments. Two markers (100000034 and 4594009) were associated with the heterosis effect of WTGs in two environments (
Table 8). The largest number (six) of associations was observed in Łagiewniki in 2014.
Table 8.
Molecular markers (SilicoDArT) significantly (LOD > 3.0) associated with the heterosis effect of weight of 1000 grains (WTGs).
Table 8.
Molecular markers (SilicoDArT) significantly (LOD > 3.0) associated with the heterosis effect of weight of 1000 grains (WTGs).
Marker Name | Łagiewniki | Smolice |
---|
2013 | 2014 | 2013 | 2014 |
---|
Effect | R2 | Effect | R2 | Effect | R2 | Effect | R2 |
---|
4773439 | | | 61.4 | 59.4 | | | | |
100000034 | 69.7 | 60.3 | | | 69 | 60.8 | | |
4779015 | | | 61 | 58.3 | | | | |
100000073 | | | −65.6 | 61.2 | | | | |
2429497 | | | | | 58 | 59.9 | | |
100000117 | | | 62.5 | 65.3 | | | | |
2526480 | | | 61.2 | 58.8 | | | | |
4594009 | −69.6 | 74 | | | −66.7 | 69.3 | | |
2449821 | | | −61.2 | 58.8 | | | | |
4587345 | 62.7 | 65.8 | | | | | | |
The heterosis effect of yield was determined by five markers: four SilicoDArTs and one SNP (
Table 9). The effects of individual markers ranged from 2.729 (for 9693146 SilicoDArT marker in Smolice 2013) to 6.850 (for 2490222 SilicoDArT marker in Smolice 2014), with an average of 4.830. The R
2 for individual markers ranged from 58.7% (for 2548691 SilicoDArT marker in Łagiewniki 2014) to 78.1% (for 2490222 SilicoDArT marker in Łagiewniki 2014), with an average of 68.7%. SilicoDArT markers 2490222 and 7058267 were associated with the heterosis effect of yield in three of the four environments, except for Smolice 2013 (
Table 9). The other three markers determined the heterosis effect of yield only in single environments. The largest number (three) of associations was observed in Łagiewniki in 2013.
Table 9.
Molecular markers associated with the heterosis effect of yield.
Table 9.
Molecular markers associated with the heterosis effect of yield.
Type of Marker | Marker Name | Łagiewniki | Smolice |
---|
2013 | 2014 | 2013 | 2014 |
---|
Effect | R2 | Effect | R2 | Effect | R2 | Effect | R2 |
---|
SilicoDArT | 9693146 | | | | | 2.729 | 61.7 | | |
SilicoDArT | 2490222 | 4.896 | 73.3 | 6.509 | 78.1 | | | 6.85 | 76.8 |
SilicoDArT | 2548691 | 3.804 | 58.7 | | | | | | |
SilicoDArT | 7058267 | 3.809 | 58.9 | 5.18 | 66.1 | | | 5.766 | 73.9 |
SNP | 7054002 | | | | | 3.926 | 70.8 | | |
Of particular note are three markers (2490222, 2548691 and 7058267) which were significant in 17, 8 and 6 cases, respectively. Two of them (2490222 and 7058267) were associated with the heterosis effects of yield in three of the four environments (
Table 9).
4. Discussion
It is estimated that by 2050 the planet will be inhabited by approximately 10 billion people and therefore there is increasing pressure to increase and balance food production. Breeders, with the support of scientific entities, are developing ever newer tools that guarantee ever greater accuracy in selection [
30,
31]. Current selection methods have been enriched by statistical models and advances in molecular biology that allow for both the identification of markers for individual traits that are the result of single genes and those that are determined by multiple QTLs to different degrees, explaining the phenotypic variation in a trait [
32,
33]. Recent selection methods allow work to be carried out using both Mendelian and population-based models. Each of the aforementioned models has a defined field of action that defines its use within specific experimental needs. Increasingly, molecular markers obtained by next-generation sequencing and association mapping are being used for selection [
34].
Association mapping is one tool for identifying novel markers and associated genes. Association mapping assumes that multiple plant materials (usually unrelated or distantly related with a high degree of alignment within lineages) are taken for study [
35]. Obviously, the size of the study population depends on the species and trait. In order to achieve the desired effect, the size of the mapping population can vary from about 200 to 800 individuals [
35]. Increasing the population size is usually not economically justified [
36]. The use of diverse plant materials means that recombinations occurring at earlier stages of derivation of materials are frequent, and the variation contained between them reflects the variation in the gene pool [
35]. Thus, the trait markers identified should be useful for broader plant material. The method assumes that all analyzed plant materials are screened for a specific trait and are profiled with the relevant DNA markers [
37]. In order to identify trait markers, it is necessary to have a large number of markers densely and evenly distributed across the genome. The density of such coverage is dependent on the value of the coupling imbalance, which will depend on the species and the trait [
38]. As a result, markers obtained by next-generation sequencing methods such as GBS [
25] or DArTseq [
39] and relatively high computing power [
38] are required for such studies. In this study, association mapping was used to identify molecular markers related with the heterosis effect. The results of our study indicate that the 105 molecular markers (8 SNPs and 97 SilicoDArTs) were associated with the heterosis effect of at least one trait in at least one environment. A total of 186 effects were observed. The number of statistically significant relationships between the molecular marker and heterosis effect varied from eight (for cob length) and nine (for yield) to forty-two (for the number of rows of grain).
Heterosis, as a genetic phenomenon, determines the beneficial consequence of crossbreeding, which is the vigor of the first generation of hybrids [
40]. Interest in hybrid vigor and its practical use started in 1880, when Beal found and described the existence of hybrid vigor after crossing two population varieties. Very often, the economic importance of the heterosis phenomenon can be demonstrated without statistical analyses, as its symptoms are sometimes striking [
41]. Although the phenomenon of heterosis has measurable economic benefits, unfortunately it cannot be perpetuated. One exception is vegetatively propagated plants, e.g., bananas, where production has been dominated by two vegetatively propagated hybrids [
42].
Very often, but not always, the phenomenon of heterosis concerns quantitative traits, e.g., plant yield. A major breeding goal is the heterosis of grain yield, although we know little about how heterosis effects affect yield components [
43]. Many authors argue that grain yield is a trait characterized by low heritability and that the lack of correlation between yield levels of parental lines and their F1 generation hybrids is a result of gene dominance and overdominance [
44,
45]. In some experimental combinations, despite the significance of additive effects, the percentage of non-additive gene effects to the genetic yield variance of F
1 generation hybrids often exceeded 80%. It appeared that environmental interaction effects influenced the expression of gene action [
46]. In the past decade, many researchers in their studies [
47,
48,
49] have applied molecular biology methods to detect and locate loci determining grain yield and yield structure traits in maize. However, intensive research is still underway to search for loci that are coupled to yield and its components [
50]. According to various studies, QTL regions associated with genes that determine grain yield and its components are distributed throughout the genome [
4]. Our results indicate that five molecular markers (four SilicoDArTs and one SNP) were associated with the heterosis effect of yield. Two (SilicoDArTs: 2490222 and 7058267) of them were associated with the heterosis effect of yield in three of the four environments.
In maize research, molecular analyses intensively focus on identifying QTL regions and new markers. Breeders with DNA analyses are also interested in developing methods to select parental components for heterosis crosses [
21] and to predict the effect of heterosis [
51]. An idea here is to find an association between the heterogeneity of loci markers for their parental forms and the yield of an F
1 hybrid [
52]. It is well known that starting materials with high genetic diversity make for breeding success, as well-identified and heterotically divided starting material results in lower costs for the entire hybrid breeding process [
22]. We can divide plant materials into heterotic groups according to the following criteria: genetic origin (pedigree), results of crossing in diallel systems, geographical origin and molecular markers. Unfortunately, the first three subdivision criteria presented are flawed. Therefore, in recent years, attempts have been made to select parental components for heterosis crosses on the basis of their genetic diversity, determined by molecular markers [
53]. In the research presented here, association analysis was used to detect relationships between the effect of heterosis and the genotype of hybrids. Similar studies were conducted on winter oilseed rape [
54].
Rapidly developing new genotyping methods based on hybridization markers or next generation sequencing are finding increasing application in basic research. The reproducibility of the results of DArT and DArTseq technologies and the availability of a very large number of SNP markers, as well as their declining costs, mean that these modern methods are finding increasing applications in identifying markers for quantitative traits and genome-wide selection in economically important plants. The use of these methods reduces work time significantly. DArTseq technology also proves itself as a powerful diagnostic tool for studying genotypic diversity [
55]. DArT and DArTseq technology have been successfully applied in Chinese common wheat (
Triticulum aestivum L.) to study the population structure and genetic diversity for 111 breeding lines and cultivars from northern China. The results obtained provided valuable information into China’s wheat breeding program for selecting parental forms [
56]. The DArT method has been widely used in parentage analysis, such as in oats (
Avena sp.), where groups corresponding to spring and winter forms were distinguished for 134 varieties [
57]. A low degree of differentiation was shown in 232 forms of Bengal pea (
Cajanus cajan) based on 696 DArT markers, of which only 64 were polymorphic; wild forms were the most diverse [
58]. Tomkowiak et al. [
53], using next-generation sequencing and association mapping, identified 15,409 silicoDArT markers and SNPs significantly associated with yield and yield structure traits in maize. From these markers, 18 SilicoDArT markers determined the quantitative trait, in all localities considered. A physical map was constructed based on these markers. Six of the eighteen identified markers (numbered 1818, 14506, 2317, 3233, 11657 and 12812) were located within the genes on chromosomes 8, 9, 7, 3, 5 and 1, respectively. Two markers (no. 15097—SilicoDArT—and no. 5871—SNP) linked to fusarium resistance in maize have been localized within genes [
59] on chromosomes 2 and 3, respectively. In their study, Tomkowiak et al. [
60], using DArTseq technology and association mapping, identified six markers associated with vigor and germination in maize. The existing literature indicates that four of these genes (phosphoinositide phosphatase sac7 isoform ×1 gene, grx_c8-glutaredoxin subgroup iii gene, sucrose synthase 4 isoform ×2 gene and putative SET domain-containing protein family isoform ×1 gene) determine the level of seed germination and seed vigor in maize.