## 2 Materials and Methods  
After analyzing 176 datasets from 38 articles [25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63], data were collected on:  
1. heterosis effects: \[\hat{H}_{ij}=\frac{\overline{P}_{ij}-\overline{P}_{p}}{\overline{P}_{p}},\] (1) where \(H_{ij}\) denotes the heterosis effect of the \(ij\)-th hybrid, \(\overline{P}_{ij}\) denotes the mean value of the \(ij\)-th hybrid, \(\overline{P}_{p}\) denotes the mean value of the \(i\)-th and \(j\)-th parental forms.
2. SCA effects: \[\widehat{SCA}_{ij}=\frac{1}{2}\big{(}y_{ij}-y_{ji}\big{)}-\hat{g}_{i}-\hat{g} _{j}-\hat{h},\] (2) where \(SCA_{ij}\) denotes specific combining ability effect of the \(ij\)-th hybrid, \(y_{ij}\) denotes the mean value for the genotype obtained by crossing the \(i\)-th and \(j\)-th parental forms, \(g_{i}\) (\(g_{j}\)) denotes the effect of general combining ability of the \(i\)-th (\(j\)-th) parental form, \(\mu\) denotes the grand mean.
3. Phenotypic differentiation of parental forms was assessed by Mahalanobis distance: \[d_{ij}=\left[\left(\mathbf{y}_{i}-\mathbf{y}_{j}\right)^{\prime}\mathbf{S}^{-1}\Big{(} \mathbf{y}_{i}-\mathbf{y}_{j}\Big{)}\right]^{\frac{1}{2}},\] (3) where \(d_{ij}\) denotes Mahalanobis distance between the \(i\)-th and \(j\)-th hybrids, \(\mathbf{y}_{i}\) and \(\mathbf{y}_{j}\) denote the \(t\)-dimensional vectors of means of individual traits for the \(i\)-th and \(j\)-th parental forms, respectively, and \(\mathbf{S}\) denotes the covariance matrix.
4. Genetic differentiation or similarity of parental forms: \[GS_{ij}=\frac{2N_{ij}}{N_{i}+N_{j}},\] (4) where \(N_{i}\) denotes the number of alleles in the \(i\)-th parental form, \(N_{j}\) denotes the number of alleles in the \(j\)-th parental form, \(N_{ij}\) denotes the number of alleles present in both the \(i\)-th and \(j\)-th parental forms.  
The data collected included five crop species: maize [25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45], rapeseed [46; 47; 48; 49; 50; 51; 52], pea [53; 54], barley [55; 56; 57], and wheat [58; 59; 60; 61; 62]. Datasets that met the following conditions were selected for analysis: (i) the effect of heterosis was calculated only in comparison with the average of the parental forms [63], (ii) Mahalanobis distances [64] were calculated for at least nine traits (\(t>8\)), (iii) the diversity or genetic similarity of the parental forms was calculated according to the formula proposed by [PERSON] and [PERSON] [65]. When the data were on genetic similarity (GS), it was converted to genetic diversity (GD) according to the formula:  
\[\text{GD}=1-\text{GS}. \tag{5}\]  
In total, 1980 sets of observations were analyzed. Relationships of heterosis and SCA effects from variation in parental forms were analyzed using Pearson's linear correlation coefficients:  
\[r_{XY}=\frac{cov(X,Y)}{\sigma_{X}\sigma_{Y}}, \tag{6}\]  
where \(cov(X,Y)\) denotes the covariance between \(X\) and \(Y\), \(\sigma_{X}\) and \(\sigma_{Y}\) denote the standard deviations of \(X\) and \(Y\), respectively. Significance of coefficients of correlations (\(r\)) was tested using a two-sample \(t\)-test:  
\[t=\frac{r\sqrt{n-2}}{\sqrt{1-r^{2}}}. \tag{7}\]  
All these analyses were conducted using the GenStat v. 22 statistical software package [66].