Copyright 2003 by the Genetics Society of America The Role of 2n Gametes and Endosperm Balance Number in the Origin and Evolution of Polyploids in the Tuber-Bearing Solanums

Polyploidization has played a major role in the origin and evolution of polyploid species. In this article we outline the unique characteristics of 2n gametes and implications of their participation in the evolution of polyploid Solanum species. The genetic consequences of 2n gametes indicate that sexual polyploidization results in greater variability, fitness, and heterozygosity than does somatic doubling. Further, the mecha-nisms of 2n gamete formation and the frequency of 2n gamete-forming genes in present polyploids and their ancestral species provide additional evidence of their involvement. Equally important is the endosperm, via the endosperm balance number (EBN) incompatibility system, in complementing the role of 2n gametes. In fact, the EBN system acts as a screen for either 1n or 2n gametes, depending on the EBN and chromosome numbers of parental species. EBN in combination with 2n gametes maintains the ploidy integrity of diploid ancestral species, while providing the flexibility for either unilateral or bilateral sexual polyploidization. GAMETES with the somatic chromosome number S. juzepczukii and of a 1n gamete from tetraploid S.tuberosum, the cultivated potato. The significance of 2n(2n gametes) are the results of modified meiosis affecting specific stages of micro- and megasporogen- gametes in polyploid evolution of plants has also been emphasized in a classic article on the origin of poly-esis. The incidence of 2n gametes is frequent in the plant kingdom, and their occurrence has been reported ploids by Harlan and De Wet (1975), and more re