2n gametes in potato species and their function in sexual polyploidization

Abstract

The extent and pattern of polyploidy in the tuber-bearing Solanums varies among the many taxonomic series that have been identified in this subsection of Solanum . While several series appear to be entirely diploid, others exhibit a range of ploidy levels from 2x to 6x . and some contain only polyploid species .In many diploid . triploid and tetraploid species 2n gametes (gametes or gametophytes with the sporophytic chromosome number) have been detected . Both 2n eggs and 2n pollen occur . 2n gametes provide the opportunity for unilateral and bilateral sexual polyploidization . The genetic determination and consequences of sexual polyploidization strongly suggest that 2n gametes have been the major instrument in the polyploid evolution of the tuber-bearing Solanums . Somatic doubling of species and interspecific hybrids appears to be of very limited importance .New evidence for the occurrence of 2n eggs and 2n pollen in many species is reported, and data from the literature are added to illustrate the widespread distribution of 2n gametes throughout the subsection . A very high correlation is found between polyploidy and 2n gametes, and its significance is discussed . Proof is presented for the occurrence of alleles governing 2n pollen production in the cultivated tetraploids, providing additional evidence for the hypothesis that 2n gametes have been involved in their origin .Multiple unilateral and bilateral sexual polyploidizations are proposed for the origin of the cultivated tetraploids ; this accounts for the large variability encountered in this group, which closely resembles that of the related diploids . Similar evolutionary pathways are hypothesized for the other polyploid complexes in the subsection .A scheme is proposed in which participation of both 2n and n gametes link together all ploidy levels in the tuber-bearing Solanums, thus overcoming the ploidy barriers and providing for gene flow throughout the sympatric species of the subsection .

The importance of wild potato species resistant to the potato cyst nematode, Globodera pallida, pathotypes P4A and P5A, in potato breeding. II. The crossability of resistant species

“…The potential breeding of haploids for introducing alien germplasm into S . tuberosum directly at the diploid level has also been pointed out by Den Nijs & Peloquin (1977) and Peloquin (1981) . Even though crosses of the wild diploids with haploids were not as successful as crosses with diploid cultigens, nevertheless hybrid progenies resistant to G. pallida have been produced (Chavez et al ., 1988) .…”

The mechanism of 2n pollen formation in diploids of Costus speciosus (Koenig) J.E. Smith and role of sexual polyploidization in the origin of intraspecific chromosomal races.

“…The observed clear cut differences in n and 2n pollen size (diameter range 10.8 to 13.0 microns and 15.4 to 17.7 microns, respectiveely) in C. speciosus suggest the possibility of estimat ing the frequencies of n and 2n pollen based on their visual size discrimination. Such a criterion for estimating 2n pollen frequency has been used in Potato (Quinn et al 1974, Den Nijs and Peloquin 1977, Leue and Peloquin 1980, Jacobsen 1980, Veilleux and Lauer 1981 and in Medicago sativa (Vorsa andBringham 1979, McCoy 1982).…”

“…However, stable high 2n pollen production has been occasionally observed (Veilleux and Lauer 1981). The genetic background as well as the environmental factors have been reported to influence the frequency of 2n gamete formation (Stow 1927, Skiebe 1966, Stein 1970, Den Nijs and Peloquin 1977, Ramanna 1979, Veilleux and Lauer 1981, Van Breuklen 1981. The variation in 2n pollen frequencies among diploids of C. speciosus (Table 1) may be attributed mainly to their genotypic differences.…”

Use of 2n Gametes in Plant Breeding

“…Although 2n gametes have been documented in several genera, they have been extensively investigated in potato (Den Nijs & Peloquin, 1977;Mok & Peloquin, 1975a;Peloquin et al, 1999), rose Crespel et al, 2006;El Mokadem et al, 2002a;El Mokadem et al, 2002b;), lily (Barba-Gonzalez et al, 2005a, 2005bLim et al, 2001a;Lim et al, 2004), and alfalfa (Tavoletti et al, 1991a;Tavoletti et al, 1991b;Barcaccia et al, 2003). The 2n gametes have in turn contributed to these crops' breeding programmes.…”