Estimation of Polyploidy of Embryos and Gametes Based on Polyploidy of Endosperm Remaining in the Citrus Seed

Abstract

Investigation of the tissue-specific ploidy levels of mature complete seeds produced by the cross-breeding of citrus varieties demonstrated that the ploidy level of the inner seed coat is consistent with that of the endosperm. We carried out another investigation using seeds obtained by natural crossing of a diploid ‘Otachibana’ to examine changes in the tissue-specific ploidy levels and structures over time, and found that the endosperm exhibited triploidy and retained a jelly-like structure until mid-August, which then transformed into a membrane structure lining the inner surface of the inner seed coat (except where the chalaza is present) by late August. By late October, the membrane-like endosperm had been assimilated into the inner seed coat. As the seeds matured, the inner seed coat that had initially exhibited diploidy eventually displayed no polyploidy at all. The endosperm, on the other hand, exhibited polyploidy even after late October. Although it was previously thought that the endosperm degenerates and disappears over time, our study demonstrated that it actually transforms into a membrane structure and attaches itself to the inner surface of the inner seed coat. In fact, the tissue that was believed to constitute the inner seed coat actually turned out to be the transformed endosperm that has been assimilated into the inner seed coat. Our study also showed that, by measuring the ploidy level of this tissue, we can estimate the ploidy levels of the embryos and its gametes without damaging the embryos.