UNILATERAL AND BILATERAL HYBRIDIZATION BARRIERS IN INTER-SERIES CROSSES OF 4X 2EBN SOLANUM STOLONIFERUM, S. PINNATISECTUM, S. CARDIOPHYLLUM AND 2X 2EBN S. TUBEROSUM HAPLOIDS AND HAPLOID-SPECIES HYBRIDS.

Authors: Hayes, Ryan; DINU, I - UNIV. OF MINNESOTA; THILL, C - UNIV. OF MINNESOTA

Submitted to: Plant Reproduction
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2004
Publication Date: 3/15/2005
Citation: Hayes, R.J., Dinu, I.I., Thill, C.A. 2005. Unilateral and bilateral hybridization barriers in inter-series crosses of 4x 2ebn solanum stoloniferum, s. pinnatisectum, s. cardiophyllum and 2x 2ebn s. tuberosum haploids and haploid-species hybrids. Sexual Plant Reproduction. v. 17. p. 303-311.

Interpretive Summary: Wild crop relatives are an important source of genetic variation for crop improvement. Pre-zygotic barriers that prevent male pollen tube growth in female plant styles can prevent fertilization of the egg and subsequent zygote formation. This can occur bilaterally when zygote formation is blocked in both direction of the cross or unilaterally when zygote formation is blocked in self incompatible (SI) x self compatible (SC) crosses. In several Solanaceae species, the S-locus for SI has been implicated in these barriers. According to the incongruity hypothesis however, these barriers result from a lack of genetic information needed for effective partner relationships, and are not related to SI. Other hybridization barriers are post-zygotic, occurring after fertilization; these include endosperm failure that can result in the absence of pre-embryo formation or embryo abortion. Many of these barriers can be overcome by manipulating plant ploidy between diploid (2x) and tetraploid (4x) level, altering the direction of the cross, or crossing to bridge species that can easily hybridize with both crop species and wild relatives. Wild Mexican potato species are an important untapped source of useful traits for potato improvement that are isolated from cultivated potato by pre- and post-zygotic barriers. The objectives of this research were to determine if: 1) S. stoloniferum can be used as a bridge species for introgression of Mexican 2x 1EBN species S. cardiophyllum and S. pinnatisectum and 2) pre- and/or post-zygotic barriers prevent hybridization among these Solanum inter-series crosses. Fruit formation and seed set was recorded for crosses between S. stoloniferum, 4x chromosome doubled S. cardiophyllum and S. pinnatisectum, and 2x cultivated potato haploids (tbr) or Haploid -Species hybrids (H-S). Pollen tube growth in plant styles was analyzed for each cross combination with fluorescence microscopy. Attempts to create bridge hybrids were not successful between S. stoloniferum, and S. cardiophyllum or S. pinnatisectum. Post-zygotic barriers prevented seed formation in S. stoloniferum x S. cardiophyllum crosses, while a bilateral stylar barrier was observed in crosses between S. pinnatisectum -S. stoloniferum and S. pinnatisectum -H-S/tbr. A non-stylar unilateral incongruity was discovered in H-S/tbr x S. stoloniferum crosses. Pollen rejection did not occur in the styles of SI genotypes and is not consistent with S-locus involvement in this cross. This may occur through lack of functional S. stoloniferum pollen genes needed for UI. Introgression strategies using these species would benefit potato breeding by introducing genetic diversity for both traits simultaneously through co-current introgression.

Technical Abstract: Wild Mexican potato species are an important untapped source of useful variation for potato improvement. Introgression methods such as 2n gametes, chromosome doubling, and crossing with disomic 4x 2 endosperm balance number (EBN) bridge species have been used to overcome post-zygotic endosperm failure according to the EBN hypothesis. Stylar barriers can prevent zygote formation, bilaterally when zygote formation is blocked in both direction of the cross or unilaterally when zygote formation is blocked in self incompatible (SI) x self compatible (SC) crosses. In several Solanaceae species, the S-locus for SI has been implicated in interspecific incompatibility. According to the incongruity hypothesis, interspecific incompatibility results from a lack of genetic information needed for effective partner relationships, and is not related to SI. The objectives of this research were to determine if: 1) disomic 4x 2EBN S. stoloniferum can be used as a bridge species for introgression of Mexican 2x 1EBN species S. cardiophyllum and S. pinnatisectum and 2) pre- and/or post-zygotic barriers limit hybridization among EBN compatible Solanum inter-series crosses. Fruit formation and seed set was recorded for inter-pollinations of S. stoloniferum, 4x 2EBN chromosome doubled S. cardiophyllum and S. pinnatisectum, and 2x 2EBN S. tuberosum haploids (tbr) or Haploid -Species hybrids (H-S). In vivo pollen tube growth was analyzed for each cross combination with fluorescence microscopy. Attempts to create bridge hybrids were not successful between S. stoloniferum, and S. cardiophyllum or S. pinnatisectum. Post-zygotic barriers prevented seed formation in S. stoloniferum x S. cardiophyllum crosses, while a bilateral stylar barrier was observed in reciprocal crosses of S. pinnatisectum -S. stoloniferum and S. pinnatisectum -H-S/tbr. Self compatibility in S. stoloniferum and S. pinnatisectum suggests that the S-locus does not contribute to this barrier. Alternatively, the presence of functional and non-functional (SC) S-alleles may explain interspecific incompatibility in intra- and inter-ploidy crosses. A non-stylar unilateral incongruity was discovered in H-S/tbr x S. stoloniferum crosses. Pollen rejection did not occur in the styles of SI genotypes. This may occur through lack of S. stoloniferum pollen active genes needed for UI, or through competitive interaction in S-locus heterozygous S. stoloniferum pollen. Introgression strategies using these species would benefit potato breeding by introducing genetic diversity for both traits simultaneously through co-current introgression.