Tetraploid turnips ( <i>Brassica rapa</i> ssp. <i>rapa</i> ) are highly tolerant of tetravalent formation and aneuploidy

Experimental induction of polyploidy in turnip (Brassica rapa ssp. rapa) prior to 1980 led to the release of several tetraploid cultivars. Most experimentally induced polyploids are meiotically unstable and show reduced fertility, but we hypothesized that these commercially propagated turnip lines should have restored fertility and stabilized meiosis. Here, we collected all available B. rapa lines listed as tetraploid from germplasm banks, and investigated chromosome karyotypes, meiotic chromosome behaviour, and fertility. Unexpectedly, all accessions showed unstable meiosis: the mean tetravalent frequency per meiosis per plant ranged from 4.6 to 6.4 per line. Using chromosome-specific fluorescence in situ hybridization probes, we found that most chromosomes showed similar frequencies of tetravalent formation except for chromosomes A3 and A6, which predominantly formed tetravalents (>90%). Of the individuals sequenced (one per accession), approximately half (9 out of 21) were aneuploid (loss or gain of a whole chromosome), and two displayed additional chromosomal rearrangements. We nevertheless observed no significant phenotypic abnormalities or reductions in fertility, although all accessions were self-incompatible. Our findings indicate that stabilizing meiosis might not always be necessary to produce relatively fertile and homogeneous polyploid populations and point at self-incompatibility as a possible mechanism helping prevent fixation of undesirable aneuploid karyotypes.