TANSLEY REVIEW NO-78 - GAMETES WITH THE SOMATIC CHROMOSOME-NUMBER - MECHANISMS OF THEIR FORMATION AND ROLE IN THE EVOLUTION OF AUTOPOLYPLOID PLANTS

The production of 2n gametes in plants, i.e. gametes with a somatic chromosome number, is considered to be the dominant process involved in the origin of polyploid plants. In this review, we provide a synthesis of current knowledge concerning the production of 2n gametes. Firstly, we describe the different methods used to detect and quantify the production of 2n gametes in plants, which include morphological and flow cytometry screening of the occurrence of 2n pollen, the analysis of crosses among diploid and tetraploid parents and the instigation of micro- and mega-sporogenesis. Secondly, the high lever of inter- and intra-specific variation in 2n gametes production is described. Thirdly, the various cytological anomalies responsible for the production of 2n gametes are reviewed, with particular reference to the relative genetic consequences of the first and second restitution divisions that give rise to 2n gametes. Fourthly, the significance of 2n gametes in crop plant improvement is discussed, in relation to somatic chromosome doubling to obtain new polyploid varieties. In particular, Ne compare the genetic and yield consequences of methods based on unilateral and bilateral sexual polyploidization. Finally, we outline how knowledge of the variety of mechanisms involved in 2n gamete production have increased our understanding of the evolutionary significance of polyploidy and the population biology of polyploid plants.