Polyploid evolution via 2n gametes

Abstract

Polyploid evolution is evident in several important taxonomic series of the tuber-bearing Solanums. Polyploids can result from the functioning of 2n gametes following both intra and inter-ploidy matings. The bilateral and unilateral sexual polyploidizations (BSP and USP, respectively) are greatly facilitated by the existence of genetically determined 2n gametes. Many species, in series containing polyploids, have individuals with 2n gametes. The genetic basis of 2n pollen formation has been established in Phurejahaploid Tuberosum hybrids, and the genetic basis of 2n egg formation is being investigated in these hybrids andS. chacoense. Preliminary results indicate that relatively few genes may be involved. FDR and to a lesser extent SDR 2n gametes avoid the inbreeding in the polyploid that is maximal with somatic doubling. The vigor, fertility and competitive ability of a founder polyploid are dependent on minimizing inbreeding depression. Even more important, polyploids, particularly from FDR 2n gametes, inherit the beneficial epistatic interactions of the parent(s). Genetically determined 2n gametes make systematic sexual polyploidization a recurring phenomenon in two ways. For example, initial BSP of two diploids gives rise to a tetraploid, and thereafter USP ensures continuing contact between ploidy levels resulting in unidirectional introgression. Triploids can also be involved in polyploid evolution if they produce 2n gametes by FDR. Artificial hybridizations indicate that 2n gametes, by overcoming crossing barriers between various ploidy levels, provide the opportunity for continuous gene flow.