Unequal inheritance
Normally you get half of your chromosomes from your mum and the other half from your dad. With a diploid genome, in which of each chromosome there are only two copies, this is relatively easy. It gets more complicated when an organism as multiple genomes. But even then, all chromosomes form a pair. And when there are unexpectedly an uneven number of chromosomes, then no sexual reproduction takes place. The organism is sterile. But against all odds the dog rose, Rosa caninae, who has five copies of each chromosome, reproduces sexually. German and Czech researchers show in Nature hoe the dog rose manages this.
The first thing the researchers did was sequencing the dog rose’s genome. This showed the researchers that there were seven groups of five chromosomes. In each group two of the five chromosomes are for 99-100% identical. The other three are for 95-98% identical. There are therefore four subgenomes present, of which only one occurs as a diploid (twofold), and the other three as a haploid (singular).
Subsequently the researchers compared the dog rose genome with that of other roses. For this it is useful to know that roses genetically can be grouped in two groups. The Rosa group and the Synstylae group. Two of the subgenomes showed similarity with the in Synstylae grouped R. chinensis. The researchers gave those subgenomes the names S1 and S1, with S1 corresponding to the diploid subgenome. The other two subgenomes were coming from the in the Rosa group residing R. rugosa, and got the designation R3 and R4.
Pollen
During the reproduction of the dog rose 14 chromosomes form a pair, while the other 21 chromosomes stay single. This results in that dog rose gets seven chromosomes from his dad and 28 chromosomes from his mum. The researchers expected that those seven chromosomes it gets from its dad are coming from the S1 subgenome.
To confirm this the researchers studied the chromosomes present in the pollen. These contained seven chromosomes. All coming from subgenome S1. Confirming the hypothesis of the researchers.
The last question the researchers hoped to answers was: how does the dog rose makes sure that only chromosomes from S1 pair up and not for example those of S2 with S1. To investigate this the researchers studied the centromeres of the chromosomes. This is where the chromosomes come together, and form a kind of X.
A different structure
The researchers specifically looked at the parts of DNA that was repeated at the centromeres. Those repetitions can be split in two groups. One group with transposons, also called jumping genes. And a second group with tandem repeats.
They noticed that on the S1 and in a lesser extend on R4 chromosomes there were a lot of ATHILA type jumping genes. This was in contrast with S2, R3, and in a lesser extend R4 chromosomes which had more CANR4 tandem-repeats. The researchers speculate that the difference in centromere structure contributes to the disproportional inheritance of the subgenomes.
So even though dog rose has an uneven number of chromosomes it still manages to sexually reproduce itself. But it uses a trick. It only admits the subgenome that occurs in diploid in its pollen. The other three subgenomes are only inherited via mum.
Literature
Herklotz, V., Zhang, M., Nascimento, T. et al. Bimodal centromeres in pentaploid dogroses shed light on their unique meiosis. Nature 643, 148–157 (2025). https://doi.org/10.1038/s41586-025-09171-z
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