University of Missouri

Darker orange endosperm color associated with haploid embryos: Y1 dosage and the mechanism of haploid induction

—Coe, EH; Neuffer, MG

The mechanism by which haploids are induced at increased frequency by pollinations with Stock 6 and similar strains is a property of the parent upon which the pollen is borne. The mechanism may be specific to the sporophyte or to the gametophyte. In either case the mechanism by which haploids are induced is dependent on properties of the pollen grain itself, according to these alternative events and consequences:

  1. If pollen grains having a single 1n sperm are responsible, the endosperm will be 3n.
  2. If pollen grains having a single 2n sperm are responsible, the endosperm will be 4n.
  3. If pollen grains have two 1n sperms, one of which fails to fuse with the egg, the endosperm will be 3n.
  4. If pollen grains have two 1n sperms, both of which fertilize the polar nuclei, the endosperm will be 4n.

We have observed that kernels with haploid embryos arising from crosses with Y1 Y1 haploid-inducer pollen parents have darker, more-orange endosperm color than sibling kernels on the same ear. A dosage effect of Y1 in the endosperm, both to the eye and chemically, is well established (Mangelsdorf, PC and Fraps, GS, 1931. Science 73:241–242; Randolph, LF and Hand, DB, 1940. J. Agric. Res. 60:51–64). We have not yet assessed crosses of Y1 Y1 males onto y1 y1 ear parents, where a dosage effect of 2 vs. 1 would be expected to be even more apparent than that for 4 vs. 3.

This is a useful observation for selection of kernels having haploid embryos, and it can be taken to mean that these haploid-embryo kernels have received an extra dose of Y1 in the endosperm, i.e., that the endosperm is 4n, and that the mechanism of haploid induction involves either a single 2n sperm or a fertilization of the polar nuclei by two sperm. Possibly the mechanism simply involves close proximity or adhesion of the two sperm at the time they enter the embryo sac. Physical association of sperm nuclei, akin to adhesion, has been documented (M. T. Chang and M. G. Neuffer, Maydica 37:233–243, 1992). Alternatively, a 3n endosperm associated with a haploid embryo may be physiologically affected in such a way that pigmentation is intensified. We consider this unlikely because the endosperm is not otherwise morphologically altered. The absence of morphological alteration in fact raises a complementary issue: because kernels from the cross of 2n × 4n plants, having 4n endosperm with a 3n embryo, are substantially affected morphologically (Randolph, LF, 1935, J. Agric. Res. 50:591–605; Sarkar, KR and Coe, E, Jr. 1971, Crop Sci 11:539–542), the expectation is that 4n endosperms with 1n embryos would also be affected, but this is not the case for the putatively 4n endosperms associated with 1n embryos. The contradiction could possibly arise from differences in paternal imprinting.

Because of differences in color intensity of the endosperm, sensory equipment might well be used to sort out haploid seeds mechanically. This should offer an efficient, effective, and economical way to sort out haploids on a large scale.

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