Every cell cycle the genomic content must be replicated in order to be equally distributed into two identical daughter cells in the form of chromosomes. Faithful chromosome segregation during mitosis is essential for life and relies on the activity of hundreds of proteins. Arguably, to understand the molecular and structural principles behind mitotic spindle assembly and function in mammals, the simpler the system, the better. Diploid chromosome number among mammalian species is generally well restrained, typically ranging from 36 to 60. However, rare exceptions do exist, such as the red viscacha rat (Tympanoctomys barrerae), whose genome is distributed among 102 chromosomes [1], and on the other extreme, the Indian muntjac (M. muntjak) with only 6 or 7 chromosomes in females or males, respectively [2]. Asian muntjacs have drawn attention to many geneticists and biologists because they exhibit the greatest chromosomal diversity within related species. The genus Muntiacus underwent an extreme karyotype diversification with chromosome numbers extending from the thought last common ancestor of all Cervidae with 2n=70 [3], to 2n=46 (M. reveesi) [4], 2n=13♀/14♂ (M. feae) [5], 2n=8♀/9♂ (M. crinifrons) [6] and 2n=6♀/7♂ (M. muntjak) [2]. The latter is believed to be the result of a repeated series of tandem and centric fusions [7, 8, 9], giving rise to large and morphologically distinct chromosomes, with one pair of acrocentric chromosomes (chromosome 3+X) containing an unusually large compound kinetochore (2 µm linear length) [10, 11]. This represents a unique advantage for micromanipulation and high-resolution live-cell studies of mitosis, if combined with state-of-the-art molecular manipulation [12]. Here we describe 65 RNA interference (RNAi) phenotypes that take advantage of the unique cytological features of the Indian muntjac, a small deer whose females have the lowest known chromosome number in mammals. This library contributes with a detailed phenotypic analysis of mitosis in Indian muntjac, offering a powerful resource open to the cell division community. Ultimately, this systematic analysis discloses overlooked functional roles for known mitotic proteins, while opening questions that were not addressable in more complex eukaryotic model systems.
When referring to this online resource, please cite:
Almeida, A.C., Soares-de-Oliveira, J., Drpic, D., Cheeseman, L.P., Damas, J., Lewin, H.A., Larkin, D., Aguiar, P., Pereira, A.J., and
Maiato, H. (2022) Augmin-dependent microtubule self-organization drives kinetochore fiber maturation in mammals.
Cell Reports, 39, 110610. DOI: 10.1016/j.celrep.2022.110610.
References:
1. Contreras L.C., Torres-Mura J.C., and Spotorno A.E., The largest known chromosome number for a mammal, in a South American desert rodent. Experientia, 1990. 46(5): p. 506-8.
2. Wurster D.H. and Benirschke K., Indian muntjac, Muntiacus muntjak: a deer with a low diploid chromosome number. Science, 1970. 168(3937): p. 1364-6.
3. Bogenberger J.M., Neitzel H., and Fittler F., A highly repetitive DNA component common to all Cervidae: its organization and chromosomal distribution during evolution. Chromosoma, 1987. 95(2): p. 154-61.
4. Yang F., et al., Chromosomal evolution of the Chinese muntjac (Muntiacus reevesi). Chromosoma, 1997. 106(1): p. 37-43.
5. Soma H., et al., The chromosomes of Muntiacus feae. Cytogenetic and Genome Research, 1983. 35(2): p. 156-158.
6. Ma S.L., Wang Y.X., and Shi L.M., A new species of the genus Muntiacus from Yunnan, China. Chinese Zool., 1990. Res. 1147.
7. Hsu T.C., Pathak S., and Chen T.R., The possibility of latent centromeres and a proposed nomenclature system for total chromosome and whole arm translocations. Cytogenet Cell Genet, 1975. 15(1): p. 41-9
8. Chi J.X., et al., Defining the orientation of the tandem fusions that occurred during the evolution of Indian muntjac chromosomes by BAC mapping. Chromosoma, 2005. 114(3): p. 167-72.
9. Mudd A.B., et al., Analysis of muntjac deer genome and chromatin architecture reveals rapid karyotype evolution. Commun Biol 3, 480 (2020).
10. Rattner J., and Bazett-Jones D., Kinetochore structure: electron spectroscopic imaging of the kinetochore. The Journal of cell biology, 1989. 108(4): p. 1209-1219.
11. Drpic D., et al., Chromosome Segregation Is Biased by Kinetochore Size. Curr Biol, 2018. 28(9): p. 1344-1356 e5.
12. Almeida A.C., et al., Functional Dissection of Mitosis Using Immortalized Fibroblasts from the Indian Muntjac, a Placental Mammal with Only Three Chromosomes. Methods Mol Biol. 2020;2101:247-266.