The dogrose (Rosa canina), the wild rose plant species, has a pentaploid genome with 35 chromosomes. It has an odd number of chromosomes, yet it can reproduce sexually courtesy a unique form of cell division called “Canina meiosis” in which only two sets of chromosomes undergo regular meiotic division while rest of 21 chromosomes are retained by the egg cell asymmetrically. Fertilisation of the tetraploid egg by the haploid pollen results in pentaploid offsprings. Mechanism behind this asymmetrical meiosis seen in dogroses was unknown. In a recent study, researchers found that centromere sizes play a decisive role in determining whether a chromosome will form bivalent or remain unpaired. Chromosomes with smaller centromere size form pairs and divide symmetrically while chromosomes with large centromere sizes remain unpaired and are retained by the egg cells. This is significant for plant breeding because making suitable changes in centromere sizes would mean inheritance of a desired chromosome.
The number of chromosomes in the offspring must remain the same as in their parents for continuity and integrity of gene pool. For ensuring this, gonads in most of the higher animals and plants undergo symmetrical meiotic cell division resulting in production of haploid gametes containing equal number of chromosomes (n) so that original diploid number (2n) is restored in the zygote following fertilisation of ovum by the male gamete. For, example, humans have 23 pairs (2n=46) of chromosomes. Meiosis division in gonads symmetrically produces haploid ova and sperms with (n=23) so that original diploid number (2n=46) is restored in the zygote following fertilisation. This is the case with most of sexually reproducing species of plants and animals. They have an even number of chromosomes. However, sexual reproduction in dogrose plant is unique.
The dogrose (Rosa canina), the common climbing, wild rose plant species, has a pentaploid genome viz., it has five sets of chromosomes, each set has 7 chromosomes. The plant has an odd number of chromosomes totalling 35, yet it can reproduce sexually courtesy an asymmetrical form of meiotic cell division called “Canina meiosis” in which only two sets (=14) of chromosomes form bivalents and undergo regular meiotic division while the rest of the chromosomes (=21) are retained by the egg cell.
In Canina meiosis, some chromosomes remain unpaired (univalents) while some form pairs (bivalents). The unpaired chromosomes are eliminated in spermatogenesis resulting in a haploid pollen grain with only the bivalent-derived chromosomes. In oogenesis, on the other hand, the egg cell receives one set of bivalent-derived chromosomes and all 21 unpaired chromosomes making the egg cell tetraploid. Fertilisation of the tetraploid egg cell by the haploid pollen (with the bivalent-derived chromosomes) restores the pentaploid (5n) genome in the offspring. Thus, the dogrose offsprings are majorly clone and partly reproduced sexually. This unique form of reproduction is known for a century, however its mechanism was unknown.
In a recent study, researchers investigated how targeted transport of the unpaired chromosomes into the egg cells take place during oogenesis in dogroses. They found involvement of centromere sizes of the chromosomes. Larger centromeres were mainly found in unpaired chromosomes while bivalent-forming centromeres were enriched with retrotransposons. Evidence suggest that the centromere size of a chromosome determines whether the chromosome will form bivalent or remain unpaired to be retained in egg cell during asymmetrical oogenesis. Chromosomes with small centromere sizes form bivalents and undergo symmetrical division while those with large centromere sizes remain unpaired and are retained asymmetrically in the egg cell. This is significant for plant breeding because making suitable changes in centromere sizes would mean inheritance of a desired chromosome.
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References:
- Lunerová J., et al 2020. Asymmetrical canina meiosis is accompanied by the expansion of a pericentromeric satellite in non-recombining univalent chromosomes in the genus Rosa. Annals of Botany, Volume 125, Issue 7, 4 June 2020, Pages 1025–1038, Published: 25 February 2020. DOI: https://doi.org/10.1093/aob/mcaa028
- Herklotz, V., Zhang, M., Nascimento, T. et al. Bimodal centromeres in pentaploid dogroses shed light on their unique meiosis. Nature (2025). Published 18 June 2025. DOI: https://doi.org/10.1038/s41586-025-09171-z
- Max-Planck-Gesellschaft. DRIVE TO SURVIVE: The seemingly impossible reproduction of dogroses hinges on a centromere trick. Posted on18 June 2025. Available at https://www.mpipz.mpg.de/pr-marques-2025-06-en
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