We initially assessed the effects of low temperature on meiotic chromosome synapsis and crossover formation in the hexaploid wheat (Triticum aestivum L.) variety ‘Chinese Spring’. At low temperatures, asynapsis and chromosome univalence have been observed before in Chinese Spring lines lacking the long arm of chromosome 5D (5DL), which led to the proposal that 5DL carries a gene (Ltp1) that stabilises wheat chromosome pairing at low temperatures. In the current study, Chinese Spring wild type and 5DL interstitial deletion mutant plants were exposed to low temperature in a controlled environment room during a period from premeiotic interphase to early meiosis I. A 5DL deletion mutant was identified whose meiotic chromosomes exhibit extremely high levels of asynapsis and chromosome univalence at metaphase I after 7 days at 13 °C, suggesting that Ltp1 is deleted in this mutant. Immunolocalisation of the meiotic proteins ASY1 and ZYP1 on ltp1 mutants showed that low temperature results in a failure to complete synapsis at pachytene. KASP genotyping revealed that the ltp1 mutant has a 4-Mb deletion in 5DL. Of 41 genes within this deletion region, the strongest candidate for the stabilisation of chromosome pairing at low temperatures is the meiotic recombination gene Dmc1. The ltp1 mutants were subsequently treated at 30 °C for 24 h during meiosis and exhibited a reduced number of crossovers and increased univalence, though to a lesser extent than at 13 °C. We therefore renamed our ltp1 mutant ‘ttmei1’ (temperature-tolerant meiosis 1) to reflect this additional loss of high temperature tolerance.
Figure 1: Map of wheat chromosome arm 5DL showing locations of deletions detected using 5DL-specific KASP markers (black or red text with BA prefix) or microsatellite markers (blue); markers are aligned with positions on the Chinese Spring IWGSC RefSeq v1.0 genome assembly (shown in Mb); yellow boxes mark the extent of the 5DL terminal deletions in lines with normal pairing at 13 °C; breakpoints of terminal deletion lines are indicated by blue arrows; pink boxes show positions of 5DL interstitial deletions in lines with normal pairing at 13 °C; orange boxes show the location of the 5DL deletion in the mutant line 22-F5 (ttmei1), which exhibits asynapsis at 13 °C; small grey box shows the ttmei1 deletion region and its flanking markers (in red) as detected in the initial KASP genotyping analysis with the only marker deleted in ttmei1 (BA00822801) shown in bold text; larger grey inset box shows fine mapping between flanking markers BA00334971 and BA00808441 (red text) using BA00822801 and 25 additional KASP markers. Markers shown in orange boxes are deleted in the ttmei1 mutant. The position of the candidate gene TaDmc1-D1 (green box) at 225 Mb is indicated with a black arrow