INTRODUCTION

In the cultivated potato, meiotic polyploidization by means of 2n-gametes has received considerable attention, both in relation to the development of more efficient alternatives to current breeding programmes (Mendiburu et al. 1974; Peloquin 1982; Hermsen 1984b) and the production of well-performing and relatively uniform hybrid progenies for the new technology of growing potatoes from true seeds (Peloquin 1983). As to the latter, the possibility of using 2n-egg formation in attempts to induce gametophytic apomixis has recently been contemplated (Hermsen 1980; Iwanaga 1982; Hermsen et. al. 1985).

In gametophytic apomixis an unreduced embryo sac is formed that can be of either diplosporic (sexual) or aposporic (somatic) origin (Rutis hauser 1967). As there are strong suggestions from earlier literature (Powers 1945; Petrov 1970; Asker 1980; Hermsen 1980; Matzk 1982) that both aposporic and diplosporic apomixis comprise distinct and genetically regulated elements, the experimental induction of gametophytic apomixis might well be achieved by a combination of them.

The elements of diplosporic apomixis that can be distinguished are a strongly reduced crossing over in megasporogenesis, the formation of unreduced megaspores and embryo sacs, and parthenogenetic development of the unreduced egg cell. In aposporic apomixis, parthenogenetic development should be combined with the development of a somatic cell of the ovule into an unreduced embryo sac. In potato, the genetically controlled elements of displosporic apomixis are available (Hermsen 1980; Hermsen et al. 1985). With apospory, however, the aforementioned development of a somatic cell, as claimed to occur in diploid potato hybrids (Iwanaga 1980, 1982), has not yet been reported (Jongedijk 1985).