1. XENIA AND METAXENIA Fouzia Youseph M.Sc Botany(sem2)

2. Xenia and Metaxenia When, in course of hybridisation, the pollen changes the appearance of the hybrid seed still on the mother plant, the phenomenon is known as xenia. This is apparent in the case of endosperm characters. Formerly, it was thought to be a direct effect of the male on the female exerted through the pollen and the term ‘xenia’ was used by Focke as early as 1881 in this sense. While the zygote is formed by the union of a male and a female gamete so that it is diploid with two sets of genomes, the endosperm mother cell in Angiosperms is formed by the fusion of three nuclei—first two embryo- sac nuclei and then a male gamete. So the endosperm mother cell is triploid having three sets of genomes. When, in course of hybridisation, the pollen changes the appearance of the hybrid seed still on the mother plant, the phenomenon is known as xenia. This is apparent in the case of endosperm characters. Formerly, it was thought to be a direct effect of the male on the female exerted through the pollen and the term ‘xenia’ was used by Focke as early as 1881 in this sense. While the zygote is formed by the union of a male and a female gamete so that it is diploid with two sets of genomes, the endosperm mother cell in Angiosperms is formed by the fusion of three nuclei—first two embryo- sac nuclei and then a male gamete. So the endosperm mother cell is triploid having three sets of genomes.

3. The endosperm will then have three genes of an allelomorphic pair which may be symbolically represented as Aaa, Ttt, Ppp, etc. It is found that, in most cases where strict dominance is observed, if only one of these three genes is a dominant gene then the endosperm shows the domi­nant character inspite of the simultaneous presence of two recessive genes. The second important point in xenia is that endosperm characters are evident one generation earlier. In ordinary hybridisation the hybrid seeds and ordinary self- fertilised seeds on the mother plant all look alike. The hybrid character becomes apparent only when the hybrid seed grows into the F 1 plant. On the other hand, in the case of xenia, the hybrid seed become at once distinct from the self-fertilised seeds because of their endosperm characters. This is because the endosperm develops fully while it is still located on its mother plant, i.e., on the plant representing the previous generation. The endosperm will then have three genes of an allelomorphic pair which may be symbolically represented as Aaa, Ttt, Ppp, etc. It is found that, in most cases where strict dominance is observed, if only one of these three genes is a dominant gene then the endosperm shows the domi­nant character inspite of the simultaneous presence of two recessive genes. The second important point in xenia is that endosperm characters are evident one generation earlier. In ordinary hybridisation the hybrid seeds and ordinary self- fertilised seeds on the mother plant all look alike. The hybrid character becomes apparent only when the hybrid seed grows into the F 1 plant. On the other hand, in the case of xenia, the hybrid seed become at once distinct from the self-fertilised seeds because of their endosperm characters. This is because the endosperm develops fully while it is still located on its mother plant, i.e., on the plant representing the previous generation.

5. Similar endosperm characters are common specially in the different cereal crops which are mainly endosperm crops. Starchy (dominant) and glutinous (recessive) grains of rice, as also the starchy (dominant) and sugary (recessive) grains of maize, are similar pairs of endosperm characters which show xenia. Metaxenia is another phenomenon sometimes included within xenia. This means the effect of the pollen (or the post-pollination effect induced by the growing endosperm and embryo) on the maternal tissue enclosing the seed. This effect is specially visible in the development of apples. This type of development, however, is better explained differently than in the case of xenia. Similar endosperm characters are common specially in the different cereal crops which are mainly endosperm crops. Starchy (dominant) and glutinous (recessive) grains of rice, as also the starchy (dominant) and sugary (recessive) grains of maize, are similar pairs of endosperm characters which show xenia. Metaxenia is another phenomenon sometimes included within xenia. This means the effect of the pollen (or the post-pollination effect induced by the growing endosperm and embryo) on the maternal tissue enclosing the seed. This effect is specially visible in the development of apples. This type of development, however, is better explained differently than in the case of xenia.