The pattern of megasporogenesis and megagametogenesis in diploid Solanum species hybrids; its relevance to the origin of 2n-eggs and the induction of apomixis

Summary Megasporogenesis and megagametogenesis in diploid tuber-bearing Solanum species hybrids were studied and illustrated in detail. Megagametogenesis followed the normal or Polygonum type of embryo sac development. In megasporogenesis a few regularly occurring deviations from what is usually considered to be the normal course of megasporogenesis were detected.Firstly, the development of a surplus of adjacent or non-adjacent megaspore mother cells in a single ovule was frequently observed. As they eventually may give rise to normally reduced, sexual embryo sacs, it is concluded that the archesporium in potato cannot be delimited to a single cell or even a group of adjacent cells in the ovule. Secondly, the micropylar daughter cell which is formed after completion of the first division was often found to degenerate before the onset or completion of the second division giving rise to a triad instead of a tetrad of megaspores. The spatial arrangement of megaspores within the ovule was found to vary according to the variation in the relative orientation of second division spindles.It is concluded that these deviations should be considered random legitimate variations of megasporogenesis rather than systematic abnormal events.The implications of the overall pattern of megasporogenesis for the formation of 2n-eggs and the attempts to induce diplosporic or aposporic apomixis in potato are discussed.