Progressive Heterosis in Tetraploid Maize

Progressive heterosis and inbreeding depression can be markedly higher in tetraploid hybrids than in their diploid counterparts. Here, we examine progressive heterosis, i.e. the additional substantial heterosis seen in 4x double-cross hybrids that is not seen in 2x double-crosses of the same genotype. Using artificially induced Maize tetraploids, we compared single and double-cross hybrids for heterotic phenotypes at the diploid and tetraploid levels. A robust progressive heterosis was found in 4x double-cross hybrids when compared to their 4x single-cross hybrid parents, but this heterosis was not found in the same comparisons at the 2x level. Based on the complementation model of heterosis, there should be recessive alleles in inbred A that are complemented by B and other recessives in B complemented by A; as well as recessives in C complemented by D and vice versa to account for single-cross heterosis. However, to explain double-cross (or progressive) heterosis, the recessives complemented in A/B cannot match recessives in C/D, otherwise A/B/C/D would have some homozygotes that would lower the heterotic effect. Furthermore, to account for strong progressive heterosis in 4x A/B/C/D, A and B must both contain the same recessives which are not found in either C or D and the reciprocal must also occur with C and D containing similar recessives not found in either A or B. By analyzing whole genome re-sequencing data from the four inbred lines we do not find the necessary spectrum of alleles to explain tetraploid progressive heterosis under the complementation of complete recessives model.