Extraordinarily conserved chromosomal synteny of Citrus species revealed by chromosome-specific painting

Reliable identification of individual chromosomes in eukaryotic species is the foundation for comparative chromosome synteny and evolutionary studies. Unfortunately, chromosome identification has been a major challenge for plants with small chromosomes, such as theCitrusspecies. We developed oligonucleotide-based chromosome painting probes for all nine chromosomes inCitrus maxima(Pummelo). We were able to identify allC. maximachromosomes in the same metaphase cells using multiple rounds of sequential fluorescencein situhybridization with the painting probes. We conducted comparative chromosome painting analysis in six differentCitrusand related species. We found that each painting probe hybridized to only a single chromosome in all other five species, suggesting that the six species have maintained a complete chromosomal synteny after more than 9 million years of divergence. No interchromosomal rearrangement was identified in any species. These results support the hypothesis that karyotypes of woody species are more stable than herbaceous plants because woody plants need a longer period to fix chromosome structural variants in natural populations.