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

Abstract

Prior to 1980, experimental induction of polyploidy led to the release of several tetraploid Brassica rapa ssp. rapa as turnip 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. We collected all available B. rapa lines listed as tetraploid from germplasm banks, and subsequently investigated chromosome karyotypes, meiotic chromosome behaviour and fertility. Unexpectedly, all accessions showed unstable meiosis: the average tetravalent frequency per meiosis per plant ranged from 4.8 to 6.4 per line. Using chromosome-specific fluorescence in situ hybridisation 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 21 individuals sequenced (one per accession), approximately half (9/21) 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 may 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.

Keywords: Brassica; aneuploidy; autopolyploid; cytogenetics; meiosis; tetravalents.