1. Introduction
Maize (
Zea mays L.) is the world’s most important agricultural crop in terms of the area cultivated, the quantity of grain harvested, and the value of production [
1]. It is the main food source in the diets of marginalized populations in Latin America, sub-Saharan Africa, Asia, and the Caribbean [
2], providing the most calories (65–85%) and often representing the only source of protein [
3,
4].
Blue corn is important in Mexican cuisine, as it is the basic ingredient of many traditional dishes, due to its favorable nixtamalization characteristics, its culinary quality, and, of course, its attractive and appetizing color [
5].
The blue coloring of the corn grain, with different intensities and shades, is caused by anthocyanins, plant pigments located mainly in the pericarp, the aleurone layer, or in both structures of the grain (2). Anthocyanins are polyphenols that constitute a large group of secondary metabolites with diverse organoleptic effects.
Recently, anthocyanins were reported to have nutraceutical, free radical scavenging, and anti-inflammatory properties, as well as aiding in the regulation of cell division; in addition, they help in the treatment of cardiovascular and neurodegenerative diseases [
6,
7], a property which has also led to a greater interest in consuming maize. This consumer preference has caused the actual production, based on creole genotypes, to not satisfy the growing demand for this type of pigmented maize. It is estimated that the consumption demand of this grain in traditional foods exceeds the 450 thousand tons per year that are currently produced. It is, therefore, appropriate to develop hybrids and improved varieties of blue maize, with higher yields and better plant and grain characteristics. To identify the best lines and crosses for generating hybrids with the desired characteristics, it is important to determine the effects of the combining abilities in blue corn lines from the highlands of Mexico. Sharing the knowledge gained from this study with other countries could lead to an expansion in the use of the genetic diversity of blue corn to develop new hybrids. These new hybrids could open up the possibility of producing foods with new physical and nutraceutical properties.
In the Central Highlands of Mexico, the genetic diversity of blue corn is extensive; however, there are few studies related to genetic improvements in this variant of corn to increase its yield and resistance to lodging, or to optimize its grain texture. Native creole varieties of blue corn yield from 2.7 to 6.6 t ha
−1 [
8], and it is feasible to exceed this yield by developing hybrids that are suitable for rainfed and irrigated areas. The formation of hybrids that will be commercially released after being evaluated in multiple environments involves identifying populations or varieties with broad genetic bases, developing lines, defining their general and specific combining abilities, and selecting superior lines [
9].
The general combining ability (GCA) and specific combining ability (SCA) concepts are essential for developing hybrids. The first defines the average behavior of a line in its hybrid combinations, and the second evaluates hybrid combinations with respect to the average behavior of the lines. GCA results from additive gene action, while SCA depends on dominance and epistasis [
10]. When the effects of SCA predominate, it is recommended to exploit them through hybridization [
11]. Information on combining ability and heterosis is imperative in a genetic improvement program to develop hybrids or synthetic varieties [
12].
Studies on the effects of combining ability and heterosis on yield in tropical germplasm lines indicated that GCA was more important than SCA [
13,
14,
15,
16,
17], while other authors [
18] found that only the effects of SCA were significant, while it has also been determined that both GCA and SCA were significant for grain yield [
19].
It is known that the variances and reciprocal effects of SCA predominate over those of GCA [
20], which suggests that, with the determination of reciprocal effects, it is possible to define the order of the female parental lines according to their productive capacity [
21].
In corn populations from the High Valleys of Mexico, high GCA values were associated with high yield [
22]. Lines from the same region, derived from the Mich 21 and Tlax 151 populations, showed GCA values of 0.92, and their SCA cross values ranged from 1.1 to 1.7 [
23]. Furthermore, lines from these same populations showed SCA values of 1.98 [
24], and combinations between lines of the Mich 21 population, with different levels of inbreeding, showed GCA values of 0.79 and SCA values of 2.4 [
25].
Meanwhile, temperate–subtropical maize lines, evaluated in high valleys, had GCA values of 1.5 and SCA values of 2.0 [
26]. Maize lines with different pericarp colors, derived from native populations, had a GCA value of 1.2 and an SCA value of 0.85 for yield [
27].
There are few studies on inbred lines of blue corn from the Central Highlands of Mexico that show the genetic effects of combining abilities and reciprocal effects, as well as their application in a commercial hybrid development program. Therefore, the objectives of this study were as follows: (1) to determine the effects of general and specific combining abilities, as well as the reciprocal effects for the yields of 10 elite lines of blue corn from the Central Highlands of Mexico; and (2) to select lines with outstanding genetic effects to form hybrids and synthetic varieties of blue corn.
4. Discussion
The variability in grain yield was concentrated in hybrids and SCAs, since they accounted for 68% and 62% of the total sum of squares, respectively. This finding is consistent with the results reported by the authors of [
17]. Additionally, the variability in SCAs was found to be 39% higher than that of GCAs. The significance of GCA × Loc, SCA × Loc, and RE × Loc indicated that these effects varied between locations, which was also noted by the authors of [
17,
25].
The performance of the corn hybrids was mainly determined by the genetic effects of dominance or interaction, agreeing with the findings reported by several authors [
16,
18]. The high and significant values of GCAs in the lines studied corroborated that lines developed from populations with a broad genetic base showed favorable additive effects in their combinations, and the significant values of SCA indicated the possibility of establishing a favorable heterotic pattern through which to develop commercial hybrids [
38]. The significant results detected for GCA × Loc, SCA × Loc, and RE × Loc indicated that the values for these effects were not consistent across locations. Due to the differences in environmental conditions that prevailed between localities, an intense genotype × environment interaction was observed, as documented in the literature [
19]. Of the reciprocal crosses, 93% were not statistically significant. Therefore, it can be interpreted that no maternal effects manifested in most of the lines studied. However, the significance of REs and MEs could be attributed to some female progenitor lines, whose performance could have a cytoplasmic influence [
21]. Although the reciprocal crosses had higher yields than the direct crosses, this does not necessarily indicate that the differences in the lines’ behaviors are due to the cytoplasmic effects observed in their hybrids [
39]; therefore, the determinations of reciprocal and maternal effects on yield were not conclusive, coinciding with the common axiom in the analysis of diallel crosses of corn that established the absence of maternal effects for this trait [
40].
Per the proposal to designate heterotic groups [
37] and based on the significance of the effects of SCAs, it can be noted that the main heterotic pattern of this research was represented in the lines (L1, L2, L4, and L5) derived from the “Chalqueño” variety population, and its combination with the L10 line derived from the combination of the “Chalqueño” and “Bolita” populations. The effects of SCAs in these combinations were positive and significant, which implied that the lines were in opposite heterotic groups, as it has been documented that, when the effects of SCA are negative, the lines belong to the same heterotic group [
37,
41].
5. Conclusions
Considerable effects of general and specific combining abilities were observed for lines and crosses, which infers the existence of additive and dominance genetic effects in lines that allowed the generation of high-yielding experimental hybrids from the High Valleys of Mexico.
The combinations of lines generated from the germplasm sources of the populations of the Chalqueño and Bolita varieties showed the possibility of a favorable heterotic pattern for generating blue corn hybrids with good yield and plant height, as well as excellent intense blue coloration, present in the aleurone layer of the grain.
Combining ability studies in corn have mainly been carried out on white and yellow corn. However, the present study was conducted with blue corn due to the significant genetic diversity of this type of corn in Mexico. There is a lack of knowledge about the genetic effects of combining abilities in blue corn, and the present study aimed to shed light on them. The findings of this study can open up the possibility of expanding the use of the genetic diversity of blue corn in other countries in order to generate new hybrids and cultivars. This, in turn, will allow for the diversification of foods, with better nutraceutical properties due to their anthocyanin contents.