Submitted to: Journal of Theoretical and Applied Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/25/2004
Publication Date: N/A
Interpretive Summary: Endosperm is a unique regulatory and nutritive tissue required for successful in vivo seed production. Many wild and cultivated potato species are sexually isolated and can not hybridize due to rapid endosperm failure. In vitro embryo rescue and ovule culture can be used to successfully obtain hybrids of many potato species by bypassing the need for endosperm in these crosses. Additionally, identification of bridging species which hybridize with both species of an incompatible inter-species combination may aid in introgression efforts. Development and utilization of sexual introgression methods using in vitro embryo rescue and bridging crosses would benefit potato breeding efforts for increased pest resistance and quality by expanding the genetic diversity available to potato breeders. In particular, the Mexican species S. pinnatisectum Dunal (2x, 1EBN) contains many genes for pest resistance and improved quality, but is currently inaccessible to potato breeders through sexual inter-species hybridization. This research analyzes the post-zygotic reproductive barriers among isolated wild potato species and proposes strategies to overcome such incompatibilities. S. pinnatisectum (2x, 1EBN), chromosomally doubled derivatives (4x, 2EBN) of S. pinnatisectum, Mexican bridging species S. verrucosum Schlechtd (2x, 2EBN), and diploid adapted potato genotypes were tested for inter-species hybridization with and without the aid of in vitro ovule culture and phytohormone application. Sexual diploid and triploid inter-species hybrids from S. verrucosum x S. pinnatisectum were generated by direct auxin treatment followed by in vitro embryo culture. Cell division timing and tissue collapse of hybrid endosperm was investigated using confocal microscopy. Application of phytohormones in inter-ploidy S. pinnatisectum 4x x 2x cultivated potato crosses did not delay endosperm collapse and embryos were not formed. In vitro cultured ovules of S. pinnatisectum x cultivated potato crosses did not regenerate, indicating the additional post-zygotic barriers exist in this cross that are not present in S. verrucosum x S. pinnatisectum crosses.
Technical Abstract: Distant hybrids of Solanum species can undergo endosperm failure, which represents a post-zygotic barrier in their inter-species hybridization. The Mexican 1EBN species S. pinnatisectum Dunal (series Pinnatisecta, Api Api) and recent autotetraploids of this species and the Mexican bridging species S. verrucosum Schlechtd. (series Tuberosa, AA, 2EBN), haploids (2x, 2EBN) of the South American 4x S. tuberosum L. (series Tuberosa, A1 A1 A2 A2, 4EBN)and F2 haploid-species hybrids with South American 2x 2EBN A genome species S. berthaultii Hawkes series Tuberosa, S. sparsipilum (Bitter.) Juz. and Bukasov series Tuberosa and S. chacoense Bitter. series Yungasensa were tested for inter-cross ability. The development of hybrid endosperms was investigated for these combinations with confocal microscopy with regard to cell division timing and tissue collapse. Sexual diploid (AApi) and triploid (AApi Api) inter-series hybrids from intra-and inter-EBN crosses of S. verrucosum and S. pinnatisectum were generated by using auxin treatment. F1-embryos were rescued in vitro. Their hybrid status was verified by microsatellite marker analysis, and the ploidy was determined by chromosome counting. Application of phytohormones in inter-ploidy S. pinnatisectum 4x x S. tuberosum 2x crosses did not delay endosperm collapse and embryos were not formed. Other diploid, 1EBN species also tested in remote hybridizations with Group Tuberosum were S. cardiophyllum Lindl., S. trifidum Correll and S. tarnii Hawkes & Hjert., from series Pinnatisecta and S. bulbocastanum Dunal., from series Bulbocastana. This research analyzes the post-zygotic reproductive barriers among isolated species of the section Petota and proposes strategies to overcome such incompatibilities.