LAUSR

What is the minimum number of chromosomes that a eukaryotic species can tolerate? This question has long fascinated biologists as chromosomal rearrangements are associated with both speciation and various genetic diseases. Nature already has an answer, at least for mammals. In the 1970s, researchers at the Kunming Institute of Zoology (KIZ), Chinese Academy of Sciences (CAS), performed a comparative cytogenetic study of Indian muntjac (Muntiacus muntjak, deer species with 2n=6♀/2n=7♂ chromosomes), Chinese muntjac (M. reevesi, 2n=46 chromosomes), and their F1 hybrids to infer karyotypic evolution (Shi et al., 1980). The dramatic reduction in chromosome number from the ancestral state of 2n=70 in water deer (Hydropotes inermis) occurred in less than 10 million years following a series of Robertsonian (centromere-to-centromere, Rb) and tandem (centromere-totelomere, Td) fusions (Yin et al., 2021). How were these chromosomal fusions generated? What advantages were favored by natural selection? What were their functional impacts on the genome? As intriguing as they are, muntjacs are not a feasible experimental model for addressing these questions, whose answers will provide important insights into the principles of genome evolution and regulation at the chromosomal level. The challenge of engineering chromosome numbers to a minimum was first conquered in yeast four years ago (Luo et al., 2018; Shao et al., 2018). Most recently, two groups of researchers at CAS independently achieved artificial chromosome fusions in mice (Wang et al., 2022; Zhang et al., 2022; Figure 1). The technical hurdles are obviously immense: in yeast, simultaneous deletions of centromeres and telomeres of any two of the total 16 chromosomes followed by their linkage through recombination are required, resulting in as few as n=1 or 2 (Luo et al., 2018; Shao et al., 2018), or even one circular chromosome (Shao et al., 2019). In mice, the sophisticated manipulation of yeast-like haploid embryonic stem cells (haESCs) may be accompanied by unexpected accidents. For example, in Wang et al. (2022), after the telomere of chr1 (the largest chromosome) was engineered to tandemly fuse with the centromere of chr2 (chr1+2), the fused chromosome broke into a partial chr1, which, in turn, fused with chr2. This accident lead to valuable discoveries. A striking common outcome of minimalist trials to reduce chromosome numbers in yeast and mice, despite their great differences, is that chromosome fusions have little impact on global gene expression and result in relatively mild phenotypic changes. As few as 0.5% of yeast genes and less than 10% of mouse genes show significant differences in gene expression in response to chromosome fusions. This must be interpreted under the context that immediately after fusion, dramatic alterations in the three-dimensional (3D) genome architecture, e.g., topologically associated domains (TAD) and novel interchromosomal contacts, have already occurred compared to unfused chromosomes. These counterintuitive results have several important implications: first, chromosome fusions may occur frequently and reach fixation as slightly/mildly deleterious mutations during evolution, particularly when deleterious consequences can be counteracted or overwhelmed. For example, in humans and chickens (Daniel, 2002; Dinkel, 1975), Rb-fused chromosomes can be preferentially transmitted during asymmetric female meiosis (the “centromeric drive hypothesis” (Henikoff & Malik, 2002; Malik & Bayes, 2006)). This is also related to the considerable karyotypic diversity of muntjac deer, in which chromosome numbers are mainly shaped by fusion (Yin et al., 2021). Second, 3D genome architecture appears to play a more structural than a regulatory role in the nucleus. TADs are hypothesized to regulate gene expression by restricting specific interactions between cis-regulatory elements and their target genes. However, complex chromosomal rearrangements (e.g., inversions and segmental duplications) that disrupt TADs in Drosophila do not result in corresponding