Mechanism of Apomixis in Dichanthium annulatum (Forssk) Stapf

Abstract

The mechanism of apomixis in Dichanthium annulatum (Forssk) Stapf (2n = 40), was studied. Microsporogenesis was slightly irregular with the occurrence of a few univalents and multivalents at metaphase and bridges and laggards at anaphase of the first meiotic division. There was early degeneration of the megaspore mother cell in the ovules, and two types of embryo sacs (four-nucleate and eight-nucleate) developed from the aposporous embryo sac initials. Some of the ovules possessed multiple embryo sacs of either one or both types. Occasionally nucellar embryos were observed. The nature of fertilization was different in the two categories of embryo sacs. In the four-nucleate embryo sac the polar nucleus was invariably fertilized by one of the male nuclei, while the other had either entered the egg cell or remained outside. In the eight-nucleate embryo sac either one or both the polar nuclei were independently fertilized. In cases where a single polar nucleus was fertilized, the other degenerated. As a result of fusion between a polar nucleus with 40 chromosomes and the male gamete with 20, the chromosome number in the endosperm was 60. The elimination of syngamy occurred in five ways: (a) degeneration of the male nucleus inside the egg cell, (b) failure of the sperm nucleus to enter the egg, (c) nonavailability of the male gamete for fertilizing the egg cell in the eight-nucleate embryo sac where both the polar nuclei were independently fertilized, (d) restriction of the sperm nucleus to the cells of the suspensor, and (e) functioning of nucellar embryos.