Cельское xозяйство/ 5. Растениеводство, селекция и семеноводство

Doktor biology Seilova L.B.

National Pedagogical University named after Abai, Almaty, Kazakhstan

APOMIXIS: USE IN SUGAR BEET BREEDING

Amphimixis is usual sexual reproduction in angiosperms plants: a zygote is formed by fusion of chromosomally reduced female (egg) and male (sperm) gametes produced during meiosis and develops into an embryo. Sexual reproduction results in genetic recombination during both microsporogenesis and megasporogenesis and allow crossing of compatible plants to produce new gene combinations. Apomixis is naturally occurring mode of reproduction that results in embryo formation without the involvement of meiosis or fertilization of the egg. Apomictic plants reproduce through seeds having an embryo which is formed without reduction of the chromosome number and fertilization - directly from a chromosomally unreduced megaspore mother cell or from a somatic cell of the nucellus or ovule. Such vegetative or asexual reproduction by means of seeds is called apomixis.

Apomixis is found mostly in polyploid species of the families Gramineae, Rosaceae, Asteraceae and often confers fertility to hybrid genotypes which otherwise would have been sterile. Most successful apomictic species are facultative with sexual reproduction and apomixes being in equilibrium. Such species usually comprise sexual and apomictic entities, often with several ploidy levels and are called agamospecies or agamocomplexes. Y.Savidan [1] explaned the evolutionary adaptability and multitude of microspecies recognizable in successful agamocomplexes by their dual ability to occasionally sidestep sexual reproduction and to multiply the successful combinations asexually.

Apomictic reproduction can be recognize because the offspring of a single mother plant is often more uniform than expected and similar to the mother plant. Such offspring is called maternal progeny. In a broad sense apomixis may include asexual reproduction through purely vegetative organs such as rhizomes, bulbils, etc., as well as many modern biotechnological methods of asexual reproduction [2].

Apomixis in plants occurs in three major forms: diplospory, apospory and adventitious embryony. In diplospory, a non-reduced embryo sac develops from an archespore cell (embryo-sac mother cell) through omission or restitution of meiosis; the egg cell develops parthenogenetically into an embryo, or another cell of the embryo sac divides and develops into an embryo. The latter route is called apogamety. In apospory, the megaspore mother cell may or may not enter meiosis and develops into a reduced embryo sac, but in either case, one or more nucellar cells develop into 2n embryo sacs with either seven cells (Hieracium type) or four cells (Panicum type) from a somatic cell of the nucellus or the integuments instead of the embryo sac mother cell. In facultative aposporous apomicts embryo sacs of meiotic and mitotic origin may occur in the same nucellus.

In many apomictic species high frequencies of multiple or twin seedlings occurs. In obligate apomicts multiple seedlings are always of apomictic origin and identical to the mother plant, whereas in facultative apomicts non-identical twins are possible.

Adventitious or nucellar embryony: the embryo develops directly from the sporophytic tissue, without formation of a gametophyte. This most closely equals pure asexual reproduction [3,4].

If no fertilization of the central nucleus is necessary for seed development, apomixis is called autonomous and the resulting endosperm will have double the somatic or the somatic chromosome number. However, in many apomictic species pollination is obligatory for the formation of endosperm and the development of the egg cell – this is pseudogamy. The pollen quality of pseudogamous species is better that of species with autonomous apomixis [5,6].

Apomixis is a unique reproductive mechanism, genetically governed event capable to provide a maternal inheritance of characters, uniform offspring with high seed yield in unlimited number of generation. Apomictic cultivars are the great potential for advancing crop production. Heterosis could be maintained over generations of seed production. The apomictic hybrids displaying valuable pest resistance environmental adaptability, or other traits would be immediately available

for seed increase and replicated testing, while of selfing or backcrossing of lines

before replicated testing being [7,8].

A good example for practical using of facultative apomixis is diploid sugar beet (Beta vulgaris L., Centrospermae) – the important technical cross-pollination culture [9]. In our experiments, for selection the apomictic genotypes two following ways were used:

1) emasculation of flowers without subsequent pollination;

2) emasculation of flowers and marker pollen pollination.

Apomixis was displayed through dominant marker method and appearing of normal seeds in non-pollinated emasculated flowers according to the suggesting scheme:

- differentiation of sugar beet population on line spectrum and selection the self-fertile genotypes (sf);

- crossing the sf-lines with dominant markers. Presents in offspring the recessive homozygotes is an evidence of apomixis;

- emasculation of flowers the proposed apomicts without subsequent polli-nation. Formation seeds under isolators will serve as confirmation an apomictic status of plants;

- embryological analysis of plants to study mechanisms led to apomixis. Genotypes, recovered through such way should be replicated by sibs method to obtain apomictic lines.

The detailed embryological investigations allow to ascertain in this crop all known apomictic mechanisms – apogamy, diplospory, apospory and adventitious embryony. In first case, the embryos are arised from synergid or antipode cells and considered as a modification of the parthenogenesis from the egg cell. These embryos were of two types – loose with giant nucleuses, and elongated with small nucleuses. Both embryos always have aberrant type and sooner or later degenerated [10]. In diplospory and apospory, the chromosomally unreduced (2n) embryo sacs were produced. But diplosporous embryos did not develop further multiple stages because of absence or early degeneration of endosperm. Really functional mechanisms of apomixes in diploid sugar beet are only apospory and adventitious embryony. Generative cell, that produced aposporous embryos as a rule were located in the middle of micropilar part of ovule or its halazal compartment. They had very dense cytoplasm, large nucleuses and sharpen borders. Micropyle, necessary for fertilization was absent here as well as in other similar cases. In aposporous embryo sacs the egg cell developed parthenogenetically, the embryos may or may not have suspensors, but polyploidal endosperm always presented.

Adventitious embryony in form of nucellar embryony took place in later stage of ovule development. There were two ways for the formation of nucellar embryos (with and without suspensors): through the detachment of embryonal cell complex arising from nucellus in any part of the ovule or directly from a single initial cell. Very often nucellar embryos were been a sourse of polyembryony.

The normal development of nucellar embryos are not depend on presence or absence the endosperm. The phenomenon of normal development of such type of embryos it is explaned by the presence in sugar beet ovuls and other species of Centrospermae - Piperaceae, Caryophyllaceae, Urticaceae - the specific nutrition tissue – perisperm, forming through accumulation the starch grains in nucellar cells surrounding the embryo sac on 7-8 days later after anthesis beginning. Thus perisperm has the equal status with endosperm in a period of development the apomictic progeny.

Apomictic lines with high seed productivity were obtained. As a rule they are self-fertile, that is why the multiplication of them does not need an isolation. The apomictic lines of sugar beet are of potential use as new materials for the selection scheme, after control of their productivity, and as the ancestor of a new cultivars.

Литература

1. Savidan Y.H., Carman J.G., Dresselhaus T. (2001). Classification of apomictic mechanisms. The flowering of apomixis: from mechanisms to genetic engineering. Mexico, D.E.: CIMMYT, JRD. 243 p.

2. Vijendra Das L.D. (2006). Genetics and Plant Breeding // Types of Plant Reproduction: Vegetative, Apomixis and Sexual. P.66-74.

3. Bicknell R.A., Koltunov A.M. (2004). Understanding Apomixis: recent advances and remaining conundrums // Plant Cell. V.16, n.1. P.228-256.

4. Наумова T.Н. (2008). Апомиксис и амфимиксис у цветковых растений // Цитология и генетика. Т.42, № 3. С.51-63.

5. Hanna W.W. (1995). Use of apomixes in cultivar development // Advances in Agronomy. V.54. P.333-350.

6. Nogler G.A., van Dijk P.J. (2001). How to avoid sex: the genetic control of gametophytic apomixis // Plant Cell. V.13. P.1491-1498.

7. Crane C.F. (2001). Apomixis: the plant breeders dream // Seedling. V.8, n.3. P.8-11.

8. Богомолов М.А. (2005). Особенности использования апомиксиса у сахарной свеклы при создании исходного материала // Сахарная свекла. № 8. С.19-21.

9. Jassem B. (1990). Apomixis in the genus Beta // Apomixis Newsletter, n.2. P.7-23.

10. Batygina T.B. (2006). Embryoidogeny // Embryology of flowering plants. Terminology and concepts. V.2. Seed. Enfield, Plymouth: Sci. Publ. P.403-409.

На II Международную научно-практическую конференцию «Дни науки - 2010» (Чехия, Прага)