The Fate of Chromosomes and Alleles in an AllohexaploidBrassicaPopulation

Abstract

Production of allohexaploid Brassica (2n = AABBCC) is a promising goal for plant breeders due to the potential for hybrid heterosis and useful allelic contributions from all three of the Brassica genomes present in the cultivated diploid (2n = AA, 2n = BB, 2n = CC) and allotetraploid (2n = AABB, 2n = AACC, and 2n = BBCC) crop species (canola, cabbages, mustards). We used highthroughput SNP molecular marker assays, flow cytometry, and fluorescent in situ hybridization (FISH) to characterize a population of putative allohexaploids derived from self-pollination of a hybrid from the novel cross (B. napus 3 B. carinata) 3 B. juncea to investigate whether fertile, stable allohexaploid Brassica can be produced. Allelic segregation in the A and C genomes generally followed Mendelian expectations for an F 2 population, with minimal nonhomologous chromosome pairing. However, we detected no strong selection for complete 2n = AABBCC chromosome complements, with weak correlations between DNA content and fertility (r 2 = 0.11) and no correlation between missing chromosomes or chromosome segments and fertility. Investigation of next-generation progeny resulting from one highly fertile F 2 plant using FISH revealed general maintenance of high chromosome numbers but severe distortions in karyotype, as evidenced by recombinant chromosomes and putative loss/duplication of A-and C-genome chromosome pairs. Our results show promise for the development of meiotically stable allohexaploid lines, but highlight the necessity of selection for 2n = AABBCC karyotypes.T HE Brassica genus contains the largest number of cultivated crop species of any plant genus (Dixon 2007). Six major crop species are B. rapa (Chinese cabbage, turnip), B. oleracea (cabbage, cauliflower, broccoli), B. nigra (black mustard), B. napus (canola, rapeseed), B. juncea (Indian mustard, leaf mustard), and B. carinata (Ethiopian mustard). These six species share a unique genomic relationship: progenitor diploid species B. rapa (2n = AA), B. oleracea (2n = CC), and B. nigra (2n = BB) gave rise to the allotetraploid species B. juncea (2n = AABB), B. napus (2n = AACC), and B. carinata (2n = BBCC) through pairwise crosses (Morinaga 1934;U 1935). However, despite the fact that each pair of genomes coexists in an allotetraploid species, no naturally occurring allohexaploid species (2n = AABBCC) exists. In general, interspecific hybridization and polyploidy in plants are potent evolutionary mechanisms, allowing formation of new species with adaptation to a wider range of climatic conditions and greater "hybrid vigor" than their progenitor species (Otto and Whitton 2000;Leitch and Leitch 2008). Hence, production of an artificial allohexaploid in the agriculturally important Brassica genus could potentially give rise to new crop types with greater intersubgenomic heterosis ) and tolerance of a wider range of environmental conditions than preexisting Brassica crops (Chen et al. 2011).

Centromere Locations inBrassicaA and C Genomes Revealed Through Half-Tetrad Analysis

“…The Brassica model is unusually amenable for generating interspecific hybrids in many combinations of A, B, and C genomes (Chen et al 2011), with largely regular chromosome pairing and segregation between homologous chromosomes derived from different species (Leflon et al 2006;Mason et al 2014b). However, even in this system we found an unusual example of failure of homologous Brassica chromosomes to segregate normally.…”

A quartet pollen phenotype identified in a population of Brassica interspecific hybrids shows incomplete penetrance and variable response to temperature

“…), followed by self‐pollination for two generations (Mason et al . ). In the present study, 162 healthy seeds from the allohexaploid hybrid ‘SP_097’ (described in Mason et al .…”

“…In the present study, 162 healthy seeds from the allohexaploid hybrid ‘SP_097’ (described in Mason et al . ) were selected and sown (10 June 2015) under greenhouse conditions; 156 out of 162 seeds germinated, and 144 healthy seedlings were moved to pots to obtain the final population for further investigation.…”

“…Parent genotypes (Brassica napus, B. carinata and B. juncea) used in this study to produce an allohexaploid plant population were 'Surpass400_024DH', '95923.3.2_01DH' and 'JN9-04'; hereafter referred to as S4, C1 and J1, respectively; origins of these cultivars are detailed in Nelson et al (2009) and. An allohexaploid hybrid plant population was obtained from the cross (B. napus 9 B. carinata) 9 B. juncea (Mason et al 2012), followed by self-pollination for two generations (Mason et al 2014). In the present study, 162 healthy seeds from the allohexaploid hybrid 'SP_097' (described in Mason et al 2014) were selected and sown (10 June 2015) under greenhouse conditions; 156 out of 162 seeds germinated, and 144 healthy seedlings were moved to pots to obtain the final population for further investigation.…”

“…An allohexaploid hybrid plant population was obtained from the cross (B. napus 9 B. carinata) 9 B. juncea (Mason et al 2012), followed by self-pollination for two generations (Mason et al 2014). In the present study, 162 healthy seeds from the allohexaploid hybrid 'SP_097' (described in Mason et al 2014) were selected and sown (10 June 2015) under greenhouse conditions; 156 out of 162 seeds germinated, and 144 healthy seedlings were moved to pots to obtain the final population for further investigation.…”