**3. The mechanism of polyembryonic seed set in facultatively apomictic ovules**

### **3.1 The dominantly developmental process of the embryo sac formed in micropylar end by parthenogenesis**

At 0 DAA, most of the embryo sac located in micropylar end became typical ones. In the accessions tested, the percentage of the typical embryo sacs in micropylar end was 90.9–100% higher than that in the other end (40.4–86.0%) (**Table 2**, [43]). If the typical one is S type, it consists of 8-nucleates of egg cell, 2 synergids and 3 antipodal. If the typical one is PN or PS type, it consists of 4-nucleates of egg cell, synergid and 2 polar nuclei.

At 6 h after anthesis, polar nuclei firstly started to divide to mean 9.0 cells in 55 of 59 ovules of Competitor. At 1 DAA, first division of egg cell was observed while the endosperm had reached mean 38.8 cells. This phenomenon is same to *P. maximum* that after the fertilization between the sperm and polar to form endosperm, with stimulation of the fertilization, the egg cell automatically divides into an embryo by parthenogenesis [20, 21]. Here, the endosperm always appeared as free-nuclear endosperm during 0–2 DAA. From 1 to 4 DAA, the mean numbers of egg embryo and endosperm cells were increased ca. 4 times per day, respectively. The number of ovules containing developed egg embryo and endosperm were increased as the days after anthesis increased. However, the ovules unfertilized remained in a certain number showed in **Table 2**. At 2–4 DAA, formation of endosperm cell wall started from the position farthest and nearest to the embryo, respectively. At 4 DAA, egg cell has undergone 6 cycles of division and became an embryo containing mean >64 cells. The endosperm was well-developed and almost occupied the whole space of the sac where it is in. After 4 DAA, the endosperm and young embryo developed so fast that the numbers of both cells could not be counted under the microscope. On the other


**Table 2.**

*Distribution of matured embryo sacs at anthesis in facultatively apomictic varieties of* P. notatum*.*

hand, during 1–4 DAA, nucellar cells were changed to be vacuolated, as endosperm cells hanged from free-nuclear to cell-wall-formed with well-developed cytoplasm. No morphological differences were observed between the developed sacs either derived from sexual sac or apomictic sac.

In polyembryonic ovules, the embryo sac in micropylar end developed dominantly when compared with that in the other ends. **Figure 3** showed the developments of

#### **Figure 3.**

*Multiple embryo formation in the same embryo sac in* P. notatum *flowers at 7–10 d after anthesis (DAA). A1. Two embryos with well-developed endosperm appeared in the chalazal end. A2. A globular-stage embryo around by endosperm in micropylar end at 7 DAA. (A1 and A2 are the same specimen). B) One embryo around by endosperm in micropylar end and one embryo in the side of ovule tissue were observed at 8 DAA. C) Two embryos side by side are located in the micropylar end. e = egg cell, p = polar cells, s = synergid, em = embryo, en = endosperm, mi = micropylar end. Bar = 45 μm.*


**Table 3.** *Development of embryo sac (ES) in the flowers 4 d after anthesis in facultatively apomictic varieties of* P. notatum*.*

*The Cytological Mechanism of Apospory in* Paspalum notatum *Analyzed by Differential… DOI: http://dx.doi.org/10.5772/intechopen.104575*

both embryo and endosperm of the sac in micropylar end, and the other sacs were crowded out to chalazal end, with dividing egg embryo and polar nuclei [36].

The ovules containing dominantly developed embryo sacs were investigated at 4 DAA in 4 varieties. The results showed in **Table 3** indicated that, 56–87% of the ovules contained developed sacs in micropylar end in all the accessions, and in contrast, on other ends, 0% in 3 varieties and 3% in one variety.

## **3.2 The developmental process of other types of embryo sacs formed in chalazal end**

Some other cases appeared different from the above. 1) A single embryo sac in micropylar end contains a well-developed embryo and unfertilized 2 nuclei. 2) A single embryo sac contains 2 embryos and well-developed endosperm. 3) The embryo sac degenerated in micropylar end, and in chalazal end, the sac with developed embryo and unfertilized 2 nuclei. And the data of other types could be known from **Table 3**. Among the 4 varieties, the number of the embryo sac in micropylar end containing only embryo was 1 of 27 ovules in Nanou, and the numbers containing only endosperm were 4 of 30 ovules in Competitor, 2 of 30 ovules in Tifton, respectively. In contrast, in the other end, the numbers of the sacs containing only embryo were 4–17% in all the varieties, and the number containing only endosperm was 0% in the varieties.
