Crossover rate between homologous chromosomes and interference are regulated by the addition of specific unpaired chromosomes in Brassica

Abstract

SummaryRecombination is a major mechanism generating genetic diversity, but the control of the crossover rate remains a key question. In Brassica napus (AACC, 2n = 38), we can increase the homologous recombination between A genomes in AAC hybrids. Hypotheses for this effect include the number of C univalent chromosomes, the ratio between univalents and bivalents and, finally, which of the chromosomes are univalents.To test these hypotheses, we produced AA hybrids with zero, one, three, six or nine additional C chromosomes and four different hybrids carrying 2n = 32 and 2n = 35 chromosomes. The genetic map lengths for each hybrid were established to compare their recombination rates.The rates were 1.4 and 2.7 times higher in the hybrids having C 6 or C 9 alone than in the control (0C). This enhancement reached 3.1 and 4.1 times in hybrids carrying six and nine C chromosomes, and it was also higher for each pair of hybrids carrying 2n = 32 or 2n = 35 chromosomes, with a dependence on which chromosomes remained as univalents.We have shown, for the first time, that the presence of one chromosome, C 9 , affects significantly the recombination rate and reduces crossover interference. This result will have fundamental implications on the regulation of crossover frequency.

Understanding and Manipulating Meiotic Recombination in Plants

“…When plants carrying one extra C 9 chromosome (AA + 1C 9 ) were examined, the recombination rate was 2.7-fold higher than that of the AA diploid genome. This finding suggests that the number of unpaired C chromosomes and the presence of specific C chromosomes in the hybrids modulated the frequency of COs on the AA genome (Leflon et al, 2010;Suay et al, 2014).…”

Homoeologous Chromosome Sorting and Progression of Meiotic Recombination inBrassica napus: Ploidy Does Matter!

“…However, given the limited extent to which synaptic multivalents persisted to pachytene in B. napus euploids, it appears that most interhomoeolog recombination intermediates abort early and are probably redirected into intersister or noncrossover pathways (Hunter and Kleckner, 2001). Evidence for very short nonreciprocal exchanges between homoeologous sequences, which possibly originated from meiotic noncrossovers, was recently obtained in cotton (Gossypium hirsutum), another bivalent-forming allopolyploid species (Salmon et al, 2010;Flagel et al, 2012). Further studies are needed to demonstrate that similar events occur in B. napus.…”

“…The five BACs were labeled using Biotin-Nick Translation Mix (Roche) and DIG Nick Translation Mix (Roche) as described by Lysak et al (2006) or random priming with Alexa 488-5-dUTP and biotin-14-dUTP (Invitrogen, Life Technologies) (Suay et al, 2013). The labeled BACs were then precipitated and the dry pellet dissolved in 10 mL HB50 (50% deionized formamide, 23 SSC [0.30 mM NaCl and 0.030 M Na3-citrate], and 50 mM sodium phosphate, pH 7.0) and 10 mL SD20 (20% dextran sulfate in HB50).…”

The Fate of Chromosomes and Alleles in an AllohexaploidBrassicaPopulation

“…A number of QTL with effects on meiotic behavior have been identified in B. napus (Liu et al 2006), including the major locus PrBn (Jenczewski et al 2003) with quantitative effects on crossover frequency between nonhomologous chromosomes (Nicolas et al 2009). Recently, presence of chromosomes C6 and C9 and retention of additional univalent chromosomes were also found to increase crossover frequency between homologous A-genome chromosomes in AA + C hybrids (Suay et al 2014). Little is known about nonhomologous chromosome pairing control in B. juncea and B. carinata, although some evidence for a genetic locus in B. juncea with a major effect on nonhomologous pairing has been obtained (Prakash 1974).…”