Hybrid species formation in Brassica
Plant Genetics and Genomics
Final Report Abstract
Several exciting and novel results were observed in this project. In particular, the Brassica A and C genomes were observed to come back together to form a new, single, stable genome under the right circumstances. In this case, these circumstances are the hybridization between allotetraploid species B. juncea (2n = AABB) and B. carinata (2n = BBCC), followed by self-pollination with selection for fertility over six generations. This finding is extremely novel in the speciation literature: such a pathway has previously been proposed for evolution of some of the subgenomes in the wheat group allopolyploids but has never been experimentally observed or verified. In parallel, major, ongoing karyotypic rearrangements were detected in several large populations of allohexaploid Brassica. These were shown to influence fertility, and a strong relationship between irregular meiosis and seed fertility was also observed (r 2 = 0.84). Several genomic loci potentially contributing to stabilisation of meiosis were also identified, some of which are associated with copynumber variation, and some with putative variation in underlying meiosis gene alleles. Fertility of these allohexaploid populations was also highly variable, with some individuals showing parent species-level fertility while others were sterile, suggesting further selection may be possible to restore fertility and genomic stability in these lines. These results shed light on the interplay between different genetic and genomic factors in the establishment of new, stable species. Completely novel Brassica allohexaploids from five different genotype combinations were also generated from the cross B. nigra × B. napus, taking advantage of embryo rescue and colchicinedoubling techniques. Newly produced allohexaploids show strong adult plant resistance to blackleg disease, comparable to that found in the resistant B. nigra parent. As well, frequent A-C/B chromosome exchange was observed during meiosis in these lines, suggesting that this material would be perfect as a bridge to introgress blackleg resistance into B. napus (rapeseed) as well as for building a diverse genetic and phenotypic basis for a new allohexaploid crop species. An unexpected result from the first project period was the discovery of a novel quartet mutant phenotype in one of the allohexaploid populations: such a mutant is hypothetically extremely useful for genetic analyses. However, upon closer inspection via phenotyping under glasshouse and controlled temperature environments, the quartet phenotype was found to exhibit low penetrance and variable, genotype-specific temperature responses, and as such was unsuitable for further work. Results and material from this project have already led into several new projects and project directions, with planned exploitation of the B. napus by B. nigra hybrid material for introgression of blackleg resistance into rapeseed, and ongoing investigation of meiosis genes which might be associated with the transition to stable meiosis in novel polyploids.
Publications
- (2015) “Polyploidy and Hybridization for Crop Improvement”, Science Publishers (CRC Press), Enfield, NH, USA
Mason A.S. (Ed.)
(See online at https://doi.org/10.1201/9781315369259) - (2018) A quartet pollen phenotype identified in a population of Brassica interspecific hybrids shows incomplete penetrance and variable response to temperature. Plant Biology 20(5):894-901
Gohar M., Gäbelein R., Mason A.S.
(See online at https://doi.org/10.1111/plb.12854) - (2018) Allohexaploids in the genus Brassica. Critical Reviews in Plant Science 37 (5): 422-437
Gaebelein, R. and Mason, A.S.
(See online at https://doi.org/10.1080/07352689.2018.1517143) - (2018) The long and short of doubling down: polyploidy, epigenetics, and the temporal dynamics of genome fractionation. (2018) Current Opinions in Genetics and Development 49: 1-7
Wendel J.F., Lisch D., Hu G., Mason A.S.
(See online at https://doi.org/10.1016/j.gde.2018.01.004) - (2019) Hybrids between Brassica napus and B. nigra show frequent pairing between the B and A/C genomes and resistance to blackleg. Chromosome Research 27(3):221-236
Gaebelein R., Alnajar D., Koopmann B., Mason A.S.
(See online at https://doi.org/10.1007/s10577-019-09612-2) - (2019) Inherited allelic variants and novel karyotype changes influence fertility and genome stability in Brassica allohexaploids. New Phytologist 223(2):965-978
Gaebelein R., Schiessl S.V., Samans B., Batley J., Mason A.S.
(See online at https://doi.org/10.1111/nph.15804) - (2019) Interspecific hybridization for Brassica crop improvement. Crop Breeding, Genetics and Genomics 1:e190007
Katche E., Quezada-Martinez D., Katche E.I., Vasquez-Teuber P., Mason A.S.
(See online at https://doi.org/10.20900/cbgg20190007) - (2019) The role of genomic structural variation in the genetic improvement of polyploid crops. The Crop Journal 7 (2), 127-140
Schiessl S.-V., Katche E., Ihien E., Singh Chawla H., Mason A.S.
(See online at https://doi.org/10.1016/j.cj.2018.07.006) - (2020) Homoeologous exchanges, segmental allopolyploidy and polyploid genome evolution. Frontiers in Genetics 11:1014
Mason A.S., Wendel J.F.
(See online at https://doi.org/10.3389/fgene.2020.01014) - (2021). Stable, fertile lines produced by hybridisation between allotetraploids Brassica juncea (AABB) and B. carinata (BBCC) have merged the A and C genomes. New Phytologist 230(3): 1242-1257
Katche E., Gaebelein R., Idris Z., Vasquez‐Teuber P., Lo Y., Nugent D., Batley J., Mason A.S.
(See online at https://doi.org/10.1111/nph.17225)
