Apomixis in Brachiaria decumbens Stapf

Abstract

Apomixis is asexual reproduction leading to a seed. It is the predominant mode of reproduction in grasses of the genus Brachiaria.Brachiaria decumbens is one of the widely cultivated species of grasses in the tropical areas. The knowledge about the mechanisms of apomixis and the possibility of controlling this system will extend the possibilities of introducing variability in Brachiaria and will maintain the apomictic character desirable for its propagation.A comparison of gametophyte development between a sexual diploid (D4) and an apomictic tetraploid (D58) genotype of Brachiaria decumbens was made.A calendar made based on the development in time show the differences between sexual and apomictic development. The aposporic embryo sac of the Panicum type grows faster than the Polygonum meiotic one. The apospore initials in Brachiaria decumbens were observed very early in the ovule, next to archespore or to the meiocyte. During microspore and ovule development, the pattern of callose deposition was different in sexual and apomictic plants.The distribution of the nuclei, positioned in one pole in the aposporic embryo sac allowed to differentiate them from the bipolar distribution of the nuclei observed in meiotic embryo sacs of the sexual plants. Cellularisation took place early in aposporic embryo sac and by the time of anthesis embryogenesis had already started in the apomict. In ovules of apomictic plants many embryo sacs developed. Apomixis expressed in the female side, had also an influence on the male side.Histochemical tests developed to localise the in situ sucrose synthase or invertase activity were used during ovule development in an apomictic and a sexual genotype of Brachiaria decumbens . A delay of activity of both enzymes was observed in apomictic compared to the sexual development. This delay was also reflected by the amount of carbohydrates detected in a HPLC assay. Furthermore antibodies were used to localise the enzyme in the cell. The higher level of carbohydrates detected in the early stages of development in the sexual plant is in line with the amount of enzyme and its activity. The retarded metabolism of carbohydrate in the apomict could be related to the entrance of the apospore directly in gametophytic pathway leading to a faster embryo sac development.Total mRNA pattern as observed by labelling poly A tails did not detect the differences in expression in sexual and apomictic plant ovules. But the difference found in the in situ amount of total RNA showed a change in metabolism of cells that are differentiating from nucellar cells in the chalazal side of the archespore or meiocyte during aposporic development. Such cells the first initial apospores also showed ribosome population that differ from other nucellar cells. In the sexual plant, the presence of some particular cells in the nucellus at chalazal side of the meiocyte suggested a silent aposporic capacity of this tissue. If such cells could develop into apospore initials under other conditions, like polyploidisation, is not known. In our observations the diploid plants never developed apospores in any stage of ovule development. Only studies with artificial polyploids could give a clue to these questions.Differential display of mRNA was used for identifying and cloning differentially expressed genes related to apomictic and sexual ovule development. The in situ pattern of expression of two selected sequences showed that the differential bands were in reality not exclusively expressed in one stage and one type of reproductive development. Yet, one of the sequences (4-29), similar to a membrane protein, had a pattern of expression that differs in ovules of sexual and apomictic plants and during the stages of ovule development.An in situ pattern of expression of a somatic embryogenesis receptor-like kinase gene isolated from Arabidopsis thaliana (AtSERK1) is presented on the sexual and apomictic plant. This gene was expressed in early stages of somatic and zygotic embryogenesis. It is though to play important function in cells with embryonic capacity. In an apomictic and sexual genotypes of Brachiaria decumbens , distinct pattern of expression of this gene was observed in mature ovules before and after anthesis. The occasional expression of this gene before anthesis in the egg apparatus of the apomict pointed to briefly and transiently expression of this gene, that in contrary might be necessary for longer time in the development of the zygotic embryo.Finally, the data found in the sexual and apomictic plant of Brachiaria was compiled to characterise the reproduction processes. The existing theories about the origin of apomixis are summarised and some consideration was made based on the data observed in Brachiaria .</p