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 level of inter- and infra-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, we 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. Contents Summary 1 I. Introduction 2 II. Methods used to detect the presence and frequency of 2n gametes 3 III. Frequency of 2n gamete production 5 IV. Mechanisms of formation and the influence of external factors 6 V. The genetic consequences of First Division Restitution (FDR) and Second Division Restitution (SDR) 12 VI. 2n gametes and the unilateral and bilateral sexual polyploidization of crop plants 13 VII. The evolutionary significance of 2n gamete production 15 Acknowledgements 18 References 18.