Nonadditive gene expression in diploid and triploid hybrids of maize

Abstract

The molecular basis of hybrid vigor (heterosis) has remained unknown despite the importance of this phenomenon in evolution and in practical breeding programs. To formulate a molecular basis of heterosis, an understanding of gene expression in inbred and hybrid states is needed. In this study, we examined the amount of various transcripts in hybrid and inbred individuals (B73 and Mo17) to determine whether the quantities of specific messenger RNAs were additive or nonadditive in the hybrids. Further, we examined the levels of the same transcripts in hybrid triploid individuals that had received unequal genomic contributions, one haploid genome from one parent and two from the other. If allelic expression were merely the additive value in hybrids from the two parents, the midparent values would be observed. Our study revealed that a substantial number of genes do not exhibit the midparent value of expression in hybrids. Instead, transcript levels in the diploid hybrids correlate negatively with the levels in diploid inbreds. Although transcript levels were clearly nonadditive, transcript levels in triploid hybrids were affected by genomic dosage.

Figure 1.—

Diagram of the procedure to generate triploids. Day 1, females of the colorless version of an inbred line homozygous for the recessive r1 gene receive pollen from plants treated with trifluralin and that carry the dominant R1-scm2 allele, which conditions full color in the embryo and aleurone of the kernel. (A genome possessing the recessive alleles for color is signified by an r; a genome possessing dominant alleles for color is signified by an R.) On the left is depicted the female gametophyte, i.e., the embryo sac. The egg is portrayed as a single yellow circle toward the bottom and the central cell as a pair of yellow circles. On the right are the two types of pollen: one with a single diploid sperm and one with two normal haploid sperm. Day 2, the four embryo sacs on the left show the types of fertilizations that result from the pollinations on the previous day. Pollen grains possessing a single diploid sperm can effect only a single fertilization, either of the egg (far left) or of the central cell (second embryo sac). The normal pollen grains with two haploid sperm will accomplish double fertilization of both the egg and the central cell (third embryo sac). The pollination procedure uses a sparse application of pollen on day 1, so some embryo sacs remain unfertilized (fourth embryo sac). Double fertilization in the two left embryo sacs is accomplished by a second pollination. For this purpose untreated pollen carrying the recessive allele of r1 is used. Therefore, these fertilizations will result in tissue that does not produce anthocyanin pigment. The resulting kernels are portrayed at the bottom, indicating the pigment distribution in each.

Figure 2.—

Kernels resulting from trifluralin treatment. (A) The four classes of kernels diagrammatically depicted in Figure 1 are pictured here. The top left kernel resulted from a diploid sperm fertilizing the egg, resulting in a triploid embryo. The bottom left kernel resulted from a diploid sperm fertilizing the central cell, resulting in an abortive tetraploid endosperm. The bottom right-hand kernel resulted from a normal pollen grain with two haploid sperm fertilizing both the egg and the central cell; the embryo is diploid. The top right-hand kernel was the result of both the egg and central cells being fertilized on the second day by normal pollen carrying no color factors. Only the top left and bottom right types of kernels were used in this study. (B) Cytological analysis of a triploid seedling root tip. (C) Cytological analysis of a diploid sibling root tip.

Figure 3.—

Northern analysis of the transcript levels of the sh1 locus in the eight genotypes analyzed in this study. The bar graph shows the average expression level of the sh1 transcript in each genotype. Bars with light shading reflect expression levels in the diploids; Bars with dark shading reflect triploids. The error bars reflect the standard errors of four replicates. The sh1 mRNA shows a nonadditive expression in diploid and triploid hybrids. The two types of diploid hybrids differ only slightly from each other; however, the two types of triploid hybrids differ much more.