A Preliminary Study of Apomictic Characteristics of ‘Mianli’ (Pyrus sinkiangensis Yü)

Abstract

This study investigated the ploidy of ‘Mianli’ with flow cytometry and the traditional chromosome squash technique. Its pollination biology and the occurrence and formation of embryo sacs before and after flowering were observed in paraffin sections to characterize its embryo sacs. The intersimple sequence repeat (ISSR) marker technique was used to test the uniformity of progeny of ‘Mianli’ treatments. The chromosome number of ‘Mianli’ is 2n = 2x = 34. The ploidy results were consistent with those identified by flow cytometry. ‘Mianli’ is male-sterile, and the anatropous ovule has double integuments. ‘Mianli’ can bear fruit normally and produce fertile seeds under the treatments of emasculation with bagging or no emasculation with bagging, but the seed yield is very low and significantly lower than that under artificial pollination or natural pollination. The developmental process of embryo sacs under natural pollination showed that most megasporocytes develop into mature sexual embryo sacs through meiosis and a few megasporocytes degenerate. Some sexual embryo sacs continue to develop into embryos after fertilization, and some sexual embryo sacs are aborted. In addition, new aposporous initial cells are generated irregularly at each stage from the emergence of megasporocyte to the end of sexual reproduction or abortion. The observation of the development of embryo sacs under emasculation with bagging showed that after pollination is blocked, mature sexual embryo sacs degenerate, and aposporous mononucleate embryo sacs appear around the degenerated sexual embryo sacs or in the peripheral tissues. Then, the process of proembryonic masses developing into spherical embryo was observed. A genetic uniformity analysis of progeny of ‘Mianli’ using ISSR was performed. The results showed that the progeny population under emasculation with bagging has high consistency at the molecular level, with some plants having full consistency with the female parent’s banding pattern, demonstrating consistency with the maternal genetic characteristics. The progeny under artificial pollination or natural pollination do not have the same banding pattern as the female parent. Because there is no pseudogamy, all of the progeny are true hybrids. In summary, it seems that ‘Mianli’ only has sexual reproduction in the presence of pollen, and only a few ovules are stimulated to undergo apomixis after pollination is blocked.