Genomic Structure of the Sweet Potato and Hexaploids in Ipomoea trifida (H.B.K.) DON.

Abstract

Ipomoea fnfida (H.B.K.) DON. forms a polyploid complex with a range from diploid to hexaploid. The synthesized hexaploids (SH, 2n=6x=90) with I. trlfida d{ploid (K221, 2n = 2x=30) and tetraplold (K233, 2n=4x=60) have been considered to have the same genomic structure (B1 B1 B2 B2 B2 B2) as the sweet potato (SP, 2n=6x=90). To determine the genomic structure of the derived hexaploids (DH) frorn intercrosses of I. trzfida triploid (K222, 2n=3x=45), and the genomic structure of I. trzfida hexaploid (K123, 2n=6x=90), F₁ hybrids from DH×SH, DH×SP, SH×K123, and DH×K123, and their backcross (BC₁) and double-cross progenies have been cytologically examined. When the genomes existing in quadruplicate in the parental hexaploids are non-homologous, such non-homology may cause failure of chromosome pairing in a BC₁ or double-cross progeny. Meiotic analyses of these hexaploid hybrids showed almost complete chromosome pairing at the matephase I (MI), suggesting that the hexaploids DH and K123 have the same genomic structure just as do SH and the sweet potato. Further, to clarify the genomic structure of the sweet potato and the I. tnfida hexaploids by estimating the degree of homology between the B₁ and B₂ genomes, meiotic pairing was observed in the tetraploid hybrids (B₁ B₁ B₂ B₂) from DH×K221 and tetraploid hybrids (B₁ B₂ B₂ B₂) from (DH×K221)×K233. Both tetraploid hybrlds demonstrated a reasonable frequency of tetravalents to support the autoploidy of their genomic constitution. These results led to the conclusion that the sweet potato and the I trifida hexaploids are autoploids with respect to the B genome of the I. trlfida diploid (K221). The use of wild germplasm of I. trtfida to incorporate its valuable traits into cultivars was undertaken and 'has brought some remarkable results in sweet potato breeding. A significant role of the autoploidy in possible gene flow through inter- and intraploidy hybridization between the sweet potato and I. trlfida was discussed.