Kinetochore reproduction in animal evolution: Cell biological explanation of karyotypic fission theory Robin L. Kolnicki* Department of Veterinary and Animal Sciences, University of Massachusetts, Amherst, MA 01003-6410 Communicated by Lynn Margulis, University of Massachusetts, Amherst, MA, June 13, 2000 (received for review August 10, 1999) Karyotypic fission theory of Todd offers an explanation for the a diploid number range of 20–38. A secondary fission event in diverse range of diploid numbers of many mammalian taxa. The- a karyologically polymorphic population (2n ϭ 34, which either oretically, a full complement of acrocentric chromosomes can be includes one acrocentric pair or is comprised of all mediocentric introduced into a population by chromosomal fission. Subsequent autosomes) explains all karyotypes of Lemuridae (2n ϭ 44–62) inheritance of ancestral chromosomes and paired fission deriva- and Cheirogaleidae (2n ϭ 46 and 66), respectively. A separate tives potentially generates a diploid range from the ancestral secondary fission event in the ancestral stock of the Indridae condition to double its number of chromosomes. Although it is (2n ϭ 38, comprised of all mediocentric autosomes) explains all undisputed that both chromosomal fission and fusion (‘‘Robertso- indrid karyotypes (2n ϭ 40–70). A later independent fission nian rearrangements’’) have significantly contributed to karyologi- event in the ancestral stock of the Lepilemuridae (2n ϭ 20) cal diversity, it is generally assumed that independent events, the explains their karyotypes (2n ϭ 20–38). Karyotypic fission offers fission of single chromosomes or the fusion of two chromosomes, the most parsimonious explanation for the diverse chromosomal are the sources of such change. The karyotypic fission idea by arrangements found in lemurs. contrast posits that all mediocentric chromosomes simultaneously fission. Here I propose a specific cell biological mechanism for Karyotype Evolution in Animals. Mammalian diploid numbers Todd’s karyotypic fission concept, ‘‘kinetochore reproduction the- range from 6 to 92 (6, 7), and differences in chromosome number ory,’’ where a complete set of dicentric chromatids is synthesized arise primarily through reorganization of chromatin by means of during gametogenesis, and kinetochore protein dephosphoryla- fission or fusion. Low diploid numbers (2n ϭ 14–22) are found tion regulates dicentric chromatid segregation. Three postulates of in didelphid marsupials, considered to be the most primitive true kinetochore reproduction theory are: (i) breakage of dicentric mammals (8). The homology between marsupial mediocentric chromosomes between centromere pairs forms acrocentric deriv- chromosomes and smaller acrocentric pairs is shown by cross- atives, (ii) de novo capping of newly synthesized acrocentric ends species chromosome painting (9). Both Australian and South with telomeric DNA stabilizes these derivatives, and (iii) mitotic American marsupials exhibit a conserved complement of 2n ϭ checkpoints regulate chromosomal disjunction to generate fis- 14 with similar G-banding patterns (10). All higher marsupial CELL BIOLOGY sioned karyotypes. Subsequent chromosomal rearrangement, es- diploid numbers can be explained by fission. pecially pericentric inversion, increases the probability of genetic Karyological polymorphism is often best explained by fission isolation amongst incipient sympatric species polytypic for fission- theory, even in nonmammalian animal taxa. Chromosome mor- generated acrocentric autosomes. This mechanism obviates the phology and DNA banding in Diprion pini and D. similis (Hy- requirement for numerous independent Robertsonian rearrange- menoptera: Diprionidae) imply chromosome numbers doubled ments and neatly accounts for mammalian karyotype evolution as (from 2n ϭ 14–28) by fission (11). The range of diploid numbers exemplified in analyses of Carnivora, Artiodactyla, and Primates. in the Australian ant genus Myrmecia (2n ϭ 4–76) was inter- preted as the result of fission (12). Fission accounts for karyo- ϭ aryotypic fission theory explains diverse mammalian karyo- type diversity in reptiles; the iguana, Liolaemus monticola (2n ϭ Ktypes in taxa that include Canidae (2n ϭ 34–78), Artiodac- 38–44), and five species of colubrid snakes (2n 28, 34, and 36) tyla (2n ϭ 14–74), Old World monkeys and apes (2n ϭ 42–72), (13, 14). In the British Bay marine snail Nucella lapillus, typically ϭ and lemurs (2n ϭ 20–70) (1–4). The ancestral chromosomal monomorphic in diploid number (2n 26), a few populations complement for all mammals, in theory, was comprised of large display variation in chromosome morphology (i.e., acrocentric ϭ mediocentric (i.e., metacentric, submetacentric, and subtelocen- pairs or mediocentric homologs) along a cline (2n 26–36), tric) chromosomes (2n ϭ 14), from which episodes of chromo- which suggests fission as the best explanation. Trivalent forma- some fission generated karyotypes with higher diploid numbers tion during synapsis in N. lapillus hybrids involves 10 pairs of and smaller chromosomes (5). In populations with all medio- acrocentrics that correspond to 5 pairs of metacentric chromo- centric autosomes, a karyotypic fission event introduces a full somes (15). Homology between metacentric and subtelocentrics complement of homologous acrocentric derivatives (1). Varying in N. lapillus hybrids was confirmed by DNA fluorescence in situ retention of ancestral mediocentric linkages and incorporation hybridization studies (16). That chromosomal diversity of such of fission-generated acrocentric pairs potentially generates a distinct taxa is explicable by fission implies this mode of animal diploid range with up to twice the ancestral number of evolution is important. autosomes. On the basis of recent advances in cell biology, I postulate Karyotypic fission theory recently applied to lemurs (prosim- ‘‘kinetochore reproduction theory’’ as the mechanism for the ian primates) explains their karyotypic diversity (2n ϭ 20–70) simultaneous fission of an entire chromosome complement. with a minimum of four evolutionary steps, whereas prior Biochemical behavior of kinetochores potentially leads to chro- explanations required at least 100 independent chromosomal mosome polymorphism and offers the most parsimonious ex- mutations (4). The differential incorporation of fissioned chro- mosomes in five families (Lepilemuridae, Daubentoniidae, Le- *E-mail: [email protected]. muridae, Cheirogaleidae, and Indridae) accounts for extant The publication costs of this article were defrayed in part by page charge payment. This karyotypes of all living lemur species. A fission event in the article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. proposed ancestral karyotype for all lemurs (2n ϭ 20) generated §1734 solely to indicate this fact. PNAS ͉ August 15, 2000 ͉ vol. 97 ͉ no. 17 ͉ 9493–9497 Downloaded by guest on September 23, 2021 planation for fissioned chromosomes observed. I theorize that an chromosomes. Kinetochores are known to form independently additional round of kinetochore production followed by chro- of standard centromeric sequences (28). Chromosomes within a mosomal breakage between kinetochore pairs simultaneously karyotype even exhibit great variation in centromeric DNA, generates two acrocentrics for each mediocentric chromosome. which suggests that the essential base pair sequence with an A single plausible event: kinetochore reproduction that occurs affinity for centromeric protein binding is minimal. Both dupli- during gametogenesis affects all autosomes. Consequent chro- cation and epigenetic kinetochore formation provide likely basis mosomal rearrangement will produce normal offspring with no for vertebrate chromosomal evolution by Todd’s ‘‘karyotypic significant alteration of gene sequence, DNA quantity, or other fission’’ theory. phenotypic change. Clinical studies and long-term mammalian cell cultures in Kinetochore Protein Dephosphorylation Regulates Chromosome Seg- which multiple dicentric chromosomes spontaneously arise pro- regation. During anaphase II, sister chromatids separate; those vide evidence consistent with this theory (17). Transient multiple with duplicated centromeres must segregate from their mono- dicentric chromosomes were found in lymphocytes from popu- centric sisters to ensure that each daughter cell receives a full lations in the United States, the United Kingdom, and Japan, complement of chromosomes (Fig. 1A). Chromosomal fission at and in South America Indians (18). The actual frequency of this time results in partial aneuploidy. During division, a mech- formation of multiple dicentric chromosomes is likely to be anoprotein-based ‘‘mitotic checkpoint’’ surveys kinetochore at- higher than that reported. Although details of supernumerary tachment. The onset of anaphase is postponed until all kineto- kinetochore development remain to be elucidated, the occur- chores properly attach to spindle. A transient delay in cell-cycle rence of this frequently detected viable and heritable chromo- progression occurs while spindle attachment to chromosomes somal change is indisputable. My kinetochore reproduction that have additional kinetochores is surveyed (29). theory attributes crucial significance to changes in the kineto- Kinetochores are active in the mitotic checkpoint mechanism. chore reproduction timing that eventually lead to processes of Checkpoint proteins at the kinetochore communicate with the species diversification in animal evolution. ‘‘anaphase-promoting