Faculty of Science, School of Sciences, Natabua Campus Lautoka BIO706: Embryology Lecture 28: Apomixis

Apomixis Apomixis, derived from two Greek word "APO" (away from) and "mixis" (act of mixing or mingling). Long back, Winkler (1908) defined apomixis as "the substitution for sexual reproduction or another asexual reproductive process that does not involve nuclear or cellular fusion (i.e. fertilization)".

In other words apomixis is a type of reproduction in which sexual organs of related structures take part but seeds are formed without union of gametes. Seeds formed in this way are vegetative in origin.

Types of apomixis

Vegetative Reproduction Propagation by parts other than seed such as bulbis, bulbs, runner, sucker etc. These propagules are formed by sporophyte only. Three types of vegetative reproduction 1. Propagules are formed outside the floral regions and despite the occurrence of functional sex organ, no fertilization and seed setting take place Eg. Agave, Globba

2. The propagules are formed outside the floral parts, produce bulbils or bulblets as a means of reproduction Eg. Fritillaria. 3. The propagules are formed on the floral branches either in addition to flowers or in place of them. This phenomenon called vegetative vivipary, quite common in grasses Eg. Lilium, Poa, Festuca.

Recurrent Apomixis (Agamospermy) Embryo sac either arise from a cell of archesporium (generative apospory) called Diplospory Or from some other part of nucelleus (Somatic apospory) The embryos may either arise from the egg (diploid parthenogenesis) or from some other cell of the embryo sac (diploid apogamy)

Generative Apospory (Diplospory) A diploid embryo sac is formed from megaspore mother cell without a regular meiotic division. Diplospory divides into three types on the basis of division of MMC Taraxacum type Ixeris type Eupatorium type

Taraxacum type MMC with restitution nucleus undergoes mitotic division to form two cells, of which one degenerates and other forms the embryosac. Eg Taraxacum albidum, polyploidy.

Ixeris Type Semihetertypic division followed by restitution nucleus. No pairing of chromosomes during MMC division, unreduced number of chromosomes. Further divisions are entirely mitotic and lead to formation of eight nucleate embryo sac.

Eupatorium type The nucleus of MMC directly undergoes the normal mitotic divisions (three nuclear divisions) to form unreduced 8-cell embryo sac, as an in Eupatorium glandulosum

Somatic Apospory Somatic cell i.e. nucellus and chalzal function as the initial of embryo sac Somatic apospory frequently preceded by degeneration of legitimate mmc. Sometimes aposporous and normal embryo sac develop together Two types: Hieracium type and Panicum type

Hieracium type

Panicum Type One or more nucellar cells develop into an aposporous embryo sac. After vacuolation there are two divisions producing four nuclei all of which remain in the micropylar region. Wall formation occurs at this stage giving rise to two synergids, one egg and one polar nucleus. Eg. Panicoidae members

Adventive Embryony No gametophytic generation The embryo originates from diploid cell of ovule lying outside the embryo sac and belonging either nucellus or integument. Cells concern in such development become richly cytoplasmic and actively divide to form small group of cells which eventually pushes their way into embryo sac and grow further to form a true embryo. Adventive embryony usually lead to form polyembryony Eg. Citrus

Development of adventive embryos in Citrus

Development of adventive embryos in Nigritella nigra

Benefits of apomixis 1. Rapid multiplication of genetically uniform individuals can be achieved without risk of segregation. 2. Heterosis or hybrid vigour can permanently be fixed in crop plants, thus no problem for recurring seed production of F 1 hybrids. 3. Efficient exploitation of maternal effect, if present, is possible from generation to generation. 4. Homozygous inbred lines, as in corn, can be rapidly developed as they produce sectors of diploid tissues and occasional fertile gametes and seeds.

Questions are welcome